Calibration and acquisition problems on Tektronix TDS 340 oscilloscope

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During one of my experiments with the TDS 340 oscilloscope made by Tektronix, I suddenly noticed that the AUTOSET button does not set the correct parameters for most signals, including the 1kHz calibration signal. Although it worked fine for a long time, the button now sets the oscilloscope to 5V/div, 25ms/div with wrong trigger settings, obviously not optimal to display a 1kHz square wave:

Failed self-calibration and diagnostics test

Thinking that the oscilloscope is out of calibration, I ran a self-calibration using the UTILITY menu, only to find out that things have gotten worse. The calibration process failed after 4 minutes and the oscilloscope reported problems upon power on:

WIth all input signals removed, I ran a diagnostics test from the UTILITY menu and sure enough, acquisition and calibration errors were reported:

Error log

The error log provided some more details – the calibration issues may have been due to the acquisition problems affecting certain tests during the calibration process. Problems with trigger and signal path of Channel 1 (error codes diagAcq_ch1Trigger, diagAcq_ch1SigPath and diagAcq_holdoff) were reported:

No errors were reported with channel 2. Are these errors related to my autoset problems? To answer this,  I performed a simple test by turning off channel 1 waveform display, feeding a signal to channel 2 and press AUTOSET. Surprisingly, autoset worked just fine and selected the correct settings to display a stable waveform on channel 2:

The suspect: faulty acquisition board

So some problems with channel 1 prevented the oscilloscope from detecting the correct settings in autoset mode. Determined to find the issue, I opened up the oscilloscope and located the acquisition board:

To my disappointment, unlike other Tektronix oscilloscope models, this acquisition board is also the mainboard of the oscilloscope and is very compact with mostly surface-mounted components. There are no through-hole electrolytic capacitors on the board which may cause problem as the capacitors become dry and fail with time. Although I did find some posts here and here referring to a problem similar to mine, my limited time and the nature of the problem prevented me from putting in more efforts to fix the issue. Except for the autoset feature, the oscilloscope still seems to be stable and works well even at high frequencies. I decided to live with the issue and manually select the correct settings for each input signal until some other more serious problems occur.

UPDATE (Jan 2018): You may find the service manual, schematics, and various other interesting technical documents for the TDS 520 and other similar oscilloscopes by Tektronix here.  Thanks to PrecisionAnalytic for providing the files.


See also:

Programming the Tektronix TDS 340 100MHz digital storage oscilloscope
Exploring Tektronix TDS 340 100MHz digital storage oscilloscope

4.00 avg. rating (84% score) - 1 vote


A tough developer who likes to work on just about anything, from software development to electronics, and share his knowledge with the rest of the world.

58 thoughts on “Calibration and acquisition problems on Tektronix TDS 340 oscilloscope

  • February 3, 2014 at 2:20 am

    I've been searching for (affordable) option 14 boards because of the huge advantage it has regarding data capture, but so far my quest hasn't yielded anything, although I'm getting the idea that the board is simply the convenient interface connectors that connect directly to the 50 pin header on the main board, am I correct? I was actually hoping to create a "make shift" version to be able to connect with at least rs232 serial and maybe also VGA but I'm afraid to touch the connector as the scope isn't actually mine. Could you maybe provide some clarity as to how the option 14 board is connected? Is it a "simple" as I think it is? And if so, would you be so bold to divulge some of the pin layout? The model that is currently under my care is the tds 340 (non a), although the floppy drive holds little interest to me anyway. It would be a great help if I would be able to store the captured information.

    Very nice article by the way, already very enlightning.

  • February 3, 2014 at 10:52 am

    Hi PaulFW,

    You can read my other article that has detailed information on the Option 14 board here if you haven't done so.

    There are a bunch of ICs on the Option 14 boards that are connected to the 50-pin male IDC connector which will be connected to the 50-pin male IDC port reserved for the Option 14 card that you see on the oscilloscope motherboard. There is also another 6-pin video cable that must be connected to a dedicated slot on the oscilloscope motherboard in order to have VGA output on the 9-pin VGA port on the Option 14 board. The oscilloscope can tell whether or not the Option 14 board is installed even with no peripherals connected to the board ports. For example, when no Option 14 board is installed, pressing HARD COPY will show instructions on installing the board. When it is installed, pressing HARD COPY will show HARD COPY DEVICE NOT RESPONDING unless a hard copy device is properly configured. There are some processing and handshaking involved for the Option 14 board to report its presence to the oscilloscope before the oscilloscope sends any data to it. So to answer your question, I don't think you can just solder some adapter to the 50-pin IDC port and hope to get any kind of output without installing an actual Option 14 card. You will at least need to duplicate the processing circuitry on the card.

    You can purchase Option 14 boards for cheap price if you search eBay hard enough – either as individual items as as part of another faulty Tek oscilloscope. Sometimes asking the seller directly will help – I got mine this way. Option 14 boards can be safely exchanged between TDS340/360/380 oscilloscopes.

    Hope this helps. Let me know if you have any other questions.

  • February 3, 2014 at 2:00 pm

    Not the answer I was hoping for of course, but that would almost be too easy. Can you give any indication as to how cheap cheap exactly is? I've seen the scopes running for nearly $2k. Is it worth the effort, or is it better for me to invest in one of those cheap portable DSO's? This scope is really something else as far as my experience goes so I'd like to keep experimenting with it but spending my entire student loan for a scope that's well.. more fun, is unfortunatly not an option and although they're not very appealing, for my needs it might be a better idea to just buy a DSO quad which might not be so accurate but it's sufficient enough for what I need. Thanks for the lightning fast reply by the way! :)

  • February 3, 2014 at 2:30 pm

    Hi PaulFW,

    I got my TDS 340 as a used oscilloscope but in otherwise working condition for around 150 USD from a local electronics equipment seller. The Option 14 card was purchased several months later in 2013 from another eBay seller for around 70 USD inclusive of shipping (there were several cheaper offers earlier that year on eBay, just that I did not search at the right time ;) ). I think you can find similar cheap offers for the Option 14 card if you spend some time searching on eBay and wait for the best offer possible.

    In my opinion, I don't think the DSO Nano, DSO Quad or other similar portable digital oscilloscopes are a good value for money if you want something that serves both as a learning tool and for long term use. Like you said, they are pretty inaccurate with highly speculative specifications and function terribly at high frequencies, with tiny and clumsy user interface to manipulate waveforms. Their build quality may also vary – I heard some which failed just after a few months of use. However, these devices can be useful once you have more experience with a mainstream oscilloscope and just want something else to experiment.

    If you can't find the Option 14 card for data output on this TDS 340 oscilloscope and have to purchase another oscilloscope, I'd recommend the Rigol DS1052E 50MHz digital storage oscilloscope that can be purchased for around 350 USD from some sellers. It has very good build quality with USB connections for waveform (both raw data and screen capture) output and large sample memory. There are several good reviews of that oscilloscope on the eevblog website – you can have a look. I have used one for the past 3 years and it serves me well so far.

  • March 10, 2017 at 11:20 am

    Does the VGA output connect directly to the motherboard? If so, what are the pinouts on the board going to on the VGA connector? Can you provide a photo of the Option 14 board also please.

  • May 24, 2017 at 2:51 pm

    Actually the VGA output is wired to the motherboard. The actual connector is mounted to a small PCB that is part of the option 14 hardware but it is nothing but a passthrough. You can easily make a cable to feed VGA from the 6 pin connector on the motherboard to a VGA connector on the back panel.

  • ToughDev
    May 25, 2017 at 10:57 am

    Thanks. Never know that the VGA port is just a pass through. I have always thought that some processing on the Option 14 board is required to produce the VGA output.

  • October 9, 2017 at 10:14 am

    The TDS520 I have does not have a VGA connector, only GPIB. I haven’t opened the case up yet as I want to research so I know what I am doing before. Is the 6 pin VGA on all the motherboards?

  • October 9, 2017 at 10:35 am

    The more I think about this from reading. I assume the TDS5XX/6XX/7XX are the same as the TDS3XX series where the 6 pin is just a pass through on the Option 14 board. Tektronix doesn’t have the TDS520 when I went to register. Only shows TDS520A and my serial number is showing may or may not be valid??? I just emailed support and hope to hear back from them.

    I was also thinking since the CRT is dead, using a arcade video card adapter that can be found cheap on eBay since has auto detection and conversion of the input to the VGA output plus multiple VGA outputs so can run a new LCD in front and output in back. Any experience with the later approach?

  • ToughDev
    October 9, 2017 at 11:25 am


    I am not sure about the TDS520A. If it indeed has the VGA connector header on the motherboard you might be able to purchase a cheap 7 inch LCD from eBay with VGA input and mount it in place of the original CRT. On my unit (TDS 340), the VGA output is standard (640×480 31kHz), so any modern VGA display should be able to display without the need for a multisync monitor or a scan converter (which is required, for example, on an old Amiga or Atari that only outputs 15 KHz VGA signal).

    If your unit does not have the VGA header, another method is to study the inputs to the CRT circuitry. On the monochrome units that I have experimented (e.g. Macintosh SE) this is simply a set of 3 pins (HSYNC, VSYNC and VIDEO). You can use a Raspberry PI to process the input and display the picture onto a cheap LCD panel connected to the PI’s HDMI port. Once done, remove the original CRT & supporting circuitry and mount the Raspberry PI and LCD as a replacement. This should work reasonably well and produce an acceptable frame rate.

    On the arcade video adapter/converter idea, it’s either a hit or miss. I have not much luck with those – either they are very sensitive to the inputs or produce outputs that are marginal and can only work with more tolerant displays. I have a CGA to VGA adapter whose output can only be displayed on an old CRT monitor and freezes all my other LCD monitors!

  • October 9, 2017 at 12:44 pm

    Thanks for your feedback. I’m finding before I even open up this TDS520, this repair is going to be a project with a range of possibilities. I need to go through all the documentation I have to find the troubleshooting section to see if there is more than the CRT that went bad.

    I found your site ToughDev which is the best for LCD replacement potential in regards to feedback (thanks a bunch), “Mr. Carlson’s Lab”, “tekhobbycap”, “feedbackloop” and some other Youtube videos to be helpful in regards to preparing for the potential for replacing the capacitors as well as other components or upgrades as reads like the caps could go bad and some advise to replace before you turn on the machine if they’re a certain model serial number or older just in case. Looks like that’s the TDS X00 series.

    Looks like from what I’ve watched and read we can upgrade to a better tantalum grade that are found in the TDS7XX series. Seemed risky, though I may be thinking years ago when tantalum had a bad reputation from certain suppliers.

    I also should be able to turn on the FFT Math option since that looks like a great option to have.

    I was thinking before I get into reworking the components, I can use the GPIB with the NI PCI-GPIB 488.2 board to determine what is going on during startup to help troubleshoot. Same goes for modification, i.e. turning on FFT. Have you used the GPIB before with your TDS? Any advice?

    Thanks again in advance for your time!

  • ToughDev
    October 9, 2017 at 2:19 pm

    In my experience, it’s the electrolytic capacitors (especially the SMD electrolytics) that usually fail, possibly without any physical indications. Tantalum (and ceramic) capacitors seldom fail, and those that do fail most likely have been subjected to abuse (e.g. high voltage from a faulty power supply), and often with obvious indications (burn marks etc.). When you are about to recap the board, replace the electrolytics first. Tantalum and ceramic capacitors should only be replaced if you are sure they have failed. You can use an ESR meter to test these capacitors before replacement.

    A few troubleshooting ideas for you to tell if it’s indeed the CRT that failed before any physical modifications:

    1. Is the power LED on? If there are fans inside the unit, are they on? This indicates at least the 5V and 12V portion of the power supply is working.
    2. Check if the buttons are responsive. In particular, pressing the AUTOSET button should trigger a set of relay clicks (to set various voltage, timebase, and trigger settings). If you hear clicks after pressing the button, a large portion of the electronics should be working.
    3. Check if there is a 1kHz output signal on the probe test point.

    With regards to the GPIB idea, I am not sure if a device of this age would output meaningful startup information at boot up via GPIB. I think it’s more for sending commands to the oscillocope to set various configurations and to retrieve the sample memory. Let me know if you manage to capture anything.

    If all fails, I would open up the device and probe the clock line and various data lines on the processor board. There might also be a JTAG or miniature serial connector somewhere on the board that should give you an idea what is working and what is not working.

  • October 10, 2017 at 7:19 am

    I was thinking replacing the electrolytic caps with tantalum since are better quality and especially use a better quality brand that is reputable. Ceramic may not last and be as stable for as long I’ve read. I thought there was a military grade capacitor though I have to check to see if they’re polar and I forget what they are at the moment… maybe silver mica. I want to upgrade the scope to be better in quality if I can.

    I read somewhere, and I can’t find the post as may be removed, that some if not all the capacitors need to not be a higher resistance and not too low ESR for some reason… so I need to keep that in mind if required.

    Also, I uploaded a video that is the first power on of the TDS-520. I made this spring when I was moving so I can document what I am doing from start to finish.

    I did reference this link if that is ok as you have a great reference site and are great support in general.

    When reviewing the video I notice that the lights come and the fan powers on.

    I’ll do more troubleshooting next once I find out for certain regarding the GPIB as I might try to power on with that connected to the NI PCI to GPIB card if there is utility.

    Since you mentioned JTAG and the serial mini connectors on the board… I can test the values on the TDS-520B that is working and compare.

    Thanks again, kindest regards!

  • October 10, 2017 at 7:22 am

    Typo on the above comment… I meant some of the capacitors need to be a higher resistance.

  • ToughDev
    October 11, 2017 at 2:34 pm

    Thanks for the link to my site from the video. :)

    From what I can see, most likely your oscilloscope hangs during the POST (power-on self test) process at boot up, evident from the many LEDs that remain on for a long time. For a working scope, those LEDs should only be on for a short time after power on, followed by a set of relay clicks, and then only a few LEDs for the channels and timebase should be on permanently.

    Before you go ahead and replace the capacitors, try to open it up and reseat any socketed chips and/or expansion cards. I have observed this symptom on one of my oscilloscopes with a loose acquisition board (the board where the channel inputs are connected).

    Keep me updated on the progress.

  • November 22, 2017 at 1:59 pm

    Finally, got around to opening up the case. Also, I read on pg 6-53 of the “TDS-520 SERVICE MANUAL” in the flow chart that after going through the Figure 6-22 Primary Troubleshooting Procedure where DS1 flash .8, then displays pausing to flash “.d” which held for about 60 seconds then went to “.e”… that I have to “Replace the A11 DRAM Processor/Display module”

    I did like you noted and removed A11 DRAM Processor/Display module (the top board) board first checking all the cable and connections where I found the J39 board not connected.

    I reconnected and powered back on with success in the CRT powering up though with a gray vertical middle section and on each side a lighter gray/white vertical section. None of the relays turned on and all the LEDs’ stayed on after 60 seconds.

    I went through the Figure 6-22 Troubleshooting again, that referenced to the Figure 6-26 Display Troubleshooting Procedure. I unplugged J5 cable, powered on the scope and tested PIN 1 (silk screened arrow next to board) and PIN 2 (pin above PIN 1) to see if @ +25V and PIN 3 (next to board) & PIN 4 (above PIN 3) if @ +5.1V. The PIN 1 & 2 read +25.7V and PIN 3 & 4 read +4.9V. The same readings were found on J27 PIN 1 and PIN 17 respectively.

    I next have to test J18 PINS 1 through 3 video signals to see what the video signal looks like. I am guessing those are the through hole test locations on the board coming out of the U190 chip for 2 and 3 and 1 comes out of the UR1 chip. Though I am also seeing that the previous owner may have replaced the J5 connector on the board since there are initials on the board also and the new J5 connector covered up the J18 pins that may have been removed/clipped though on the bottom of the board I do see the trace and PIN locations.

    Any idea on what the J18 PINS 1, 2 and 3 are? I am thinking my later observation.

    I’ll set up the TDS-520B to read the J18 PINS next and go from there.

    Either way, looks like the issue is something to do with either the A11 DRAM Processor/Display module that notes to be replaced or replace the A20 Display Assembly… though I may be able to repair those boards if I find the schematics specifications in more detail.

  • November 22, 2017 at 2:47 pm

    Wondering if maybe before I do anything, I can replace the electrolytic capacitors first on the A11 and A20 board and see what that does. Probably still worth checking J18 signals first to see of those two, which board to work on replacing components.

  • ToughDev
    November 22, 2017 at 10:49 pm


    Do you have a high resolution of the circuit boards and a photo of the CRT display showing the patterns you described? I might be able to troubleshoot in more details with the photos. I have searched but could not find the component-level circuit diagram of the TDS520, only the module-level troubleshooting manual.

    From your description most likely the analog part of the oscilloscope (low voltage rails, CRT horizontal/vertical deflection & high voltage power supplies) are working fine; otherwise you won’t see the CRT display and the voltages measured will not match the manual. The problem therefore should be with the digital logic circuit.

    Are you able to see anything from the patterns on the CRT, e.g. any distorted text? Or are only solid bars displayed on the CRT?

    If you can see something, it’s possible the video DRAM memory is malfunctioning. This may be caused by old electronics capacitors like what you suspect, or because of DRAM failure due to aging. Probing the DRAM lines with another scope may yield some useful information. See this for further information.

    If you cannot see anything and the patterns are solid, maybe the video controller circuitry is somehow put into ‘test mode’, hence outputting gray patterns on the CRT. This may happen due to the lack of synchronization signals (e.g. J18 pin 1/2) or, for some video controllers, because a dedicated TEST pin is somehow shorted to ground. You can post a photo of the board and I can help to locate J18 and identifying the next steps. If you can locate where the horizontal & vertical sync signals are, an idea would be to compare the voltages around these pins to find which components are at fault. See the comment section of my other post here where another reader encountered a similar problem with a vintage laptop LCD panel. Also check out this for some hints.

    As for the initials, it might be from the previous owner who attempted to repair the board, but could also be from the quality testing process itself. Some companies randomly test its boards prior to production release, with the person responsible for the testing writing his initials on it.

    Also, since the LEDs remain on and there are no relay clicks, your oscilloscope probably hangs during power-on-self-test (POST). Try the following if you haven’t tried before:

    1. Probe the low voltage rails (3.3V & 5V) with another oscilloscope. They may look fine with a multimeter, but could actually be very noisy due to aging capacitors, thus preventing the digital logic circuit from working properly.

    2. Try to identify any crystal oscillators on the board. There should be at least one around the main processor. If you find it, probe with your other scope to see if it’s oscillating.

    3. The TDS520 maintains its calibration constants on a NVRAM memory chip, which has an internal battery whose shelf life has probably expired by now. Usually the oscilloscope will not totally stop working just because the NVRAM has failed, but you can always try to locate it on the board, and if it is socketed, try to boot the scope with the NVRAM removed and see if anything changes.

    A trick I often use is to leave the oscilloscope powered for at least 2 minutes and turn it off. After that, try to identify the temperature of various components. This is more convenient if you have a thermal camera or an infrared thermometer, but can be done by feeling the components with your hands (be careful not to touch the high voltage power supply). You can usually tell if some components are not running as expected (e.g. not hot at all) or are having a short (hot to the touch).

    Let me know how it goes.

  • November 23, 2017 at 12:34 am

    Hi, thanks for all the detailed feedback. I still have to read some more and do some other chores outside before Winter starts freezing the ground (transplant perennials, drainage and landscaping). I will perform the tests later this evening and reply back with details.

    I’ve uploaded the videos and image to this link:

    Board photo:

  • ToughDev
    November 24, 2017 at 10:38 am


    Thanks for the video and the photos. It looks like the first thing you can try is to boot the scope with the RTC/NVRAM chipset removed (it is the DALLAS module at bottom right of your photo). The chip is most likely in a socket and can be removed by carefully lifting it off on both sides to avoid benting the pins.

    The next thing to try is to use rubbing alcohol and clean the legs of all the chips, especially those that look a bit dirty and those around the electrolytic capacitors. If that still doesn’t work, you may want to replace the electronics with brand new ones.

    Look forward to your updates :)

  • November 24, 2017 at 11:33 am

    Hi, thanks for the feedback.

    I’m still probing the board Cable Pins testing voltage at the moment (J5 Display Module, J2 Front Panel Module, J26 and J27) while the board is installed to narrow down location in board where issues.

    I re-read the TDS-520 Service Manual “Figure 6-22 Primary Troubleshooting Procedure” and realized I skipped the “Figure 6-30 Processor/Front Panel Troubleshooting Procedure.”

    I just completed the “Figure 6-30 Processor/Front Panel Troubleshooting Procedure” and found that when J2 is unplugged the ON/STANDBY doesn’t power on the scope and there is no 3.125Mhz clock on J2 Pin 25. Therefore, the flowchart shows to Replace A11 DRAM Processor/Display Module.

    This lines up with the Figure 6-26 Display Troubleshooting Procedure where J5 Pins 1 & 2 are +25.7V and Pins 3&4 are +4.9V. J27 Pin 1 is +25.7V and Pin 17 is +4.9V.

    I also tested J2, J26 and J27 plugged in (I can only test on the back of the cable socket when cable is plugged in, not sure how to test lower pins unless unplugged). I am thinking that the pins are the even numbers are testable where odd number pins are on the bottom row and I am not sure how to test at this time. Maybe row of pins are same values also as with J5.

    J2 (V+ unless noted): Pin2 = 0, 4 = 9.94, 6=0, 8=4.94, 10=0, 12=-5.07, 14=5.06, 16=0.2, 18=5.06, 20=4.49, 22=3.99, 24=0, 26=2.3

    J26 (V+ unless noted): Pin 2=0, 4=0.97, 6=0, 8=-15.10, 10=-15.10, 12=14.98, 14=0, 16=0, 18=-5.07, 20=-5.07, 22=0, 24=0, 26=-5.07, 28=-5.07, 30=-5.07,32=0, 34=0, 36=4.94, 38=4.94, 40=4.94

    J27 (V+ unless noted): Pin 2=24.4, 4=0, 6=5.0, 8=0, 10=5.0, 12=0, 14=5.0, 16=0, 18=5.0, 20=0, 22=4.9, 24=0, 26=5.0, 28=0, 30=5.0,32=0, 34=5.0, 36=0, 38=5.0, 40=0

    J5, J26 and J27 do not seem out of specification on the A11. Therefore, those I am thinking have traces that are OK or suitable for now.

    J2 per Figure 6-30 6th Step down Pins 2, 4 are 0V and should be +15V and -15V respectively and Pins 16 should be +5.1V and is 0.02V.

    I am also wondering what the specification of 19 through 26 are as they seem lower in value. I do not see a specification for those, however they may be attenuators values relative to settings for feedback to the A11 DRAM Processor Display board or something else.

    Looks like more testing components and traces where J2 is missing power and the values are not correct or questionable where I don’t have a spec value. I have to see what the vias and components values are going backwards from the J2 connector pins next.

  • ToughDev
    November 25, 2017 at 11:48 am


    Thanks for the detailed info.

    Can you closely examine the front panel module and the processor/display module with a flashlight or a magnifying glass to see if there are any hairline cracks? Usually one of the failure modes is from someone pulling hard on the front panel buttons, causing tiny cracks on the boards. Since you seem to have most of the voltages correct, except for those on J2 which read ~0V instead of the correct values, I suspect one or more broken traces somewhere.

    This link may give you some hints:

  • November 26, 2017 at 2:30 am


    Not sure if you celebrate Thanksgiving, though Happy Thanksgiving. :-|)

    I noticed the A12 Front Panel Module attenuators looked corroded or something not clean.

    Beside the fibers/lint or particles… the attenuator corrosion or bad solder work really stands out. I’ll do more inspection of the traces to see if I notice anything on both the A11 and A12 modules.

    Here is the video I made after a few hours trying to get the microscope to work since I lost the driver CD and reticle as well as method for recording (no large stand and ContaCam settings).

  • ToughDev
    November 26, 2017 at 12:22 pm


    Happy Thanksgiving to you too!

    Thanks for the video and the reference to my blog. You can try cleaning the corrosion from the boards with something like diluted white vinegar and see if there’s any difference.

    Look forward to seeing the TDS520 working again. It’s one of my favorite scopes!

  • December 12, 2017 at 3:28 pm


    Snow has fallen and back inside working again.

    I wound up cleaning the A12 front panel module board with a brush, tested the pots in circuit, created a better thermal sight mount ( ), added a ground cable to the A12 front panel board to the chassis and found out the connector board between the A11 DRAM processor/display board and A10 Acquisition board was upside down.

    Now there is a different effect on the CRT, progress in startup with LED’s going off and I need to go through the troubleshooting steps again to test voltages and what we’ve discussed already.

    I’m waiting on a stereo microscope I found on eBay for a great price so I have a manual way to inspect as you noted plus not a bad idea since the electronic devices are so small with SMD’s and I still need to pick up the desktop mount arm for the USB microscope I made last winter in storage. I have some old ring stands from QC jobs over the years I was given, so thinking I can modify to mount the old stereo microscope on one of those with a better basemount.

    Here is the latest startup video:

  • December 12, 2017 at 3:33 pm

    I forgot to note that I tested continuity in the contacts that go around the CRT Screen to the A12 front panel board plug.

    I also tested the J2 cable to A12 Front Panel for continuity.

    Both were fine. As noted in the thermal image, the pots readings seemed strange in circuit… though values are different in circuit. The R48 variable resistor (I called pots for potentiometers above) isn’t like the component list or boards I’ve found online. Not sure if that would cause an issue also.

  • December 12, 2017 at 4:01 pm

    I also forgot to note I used IPA to clean the A12 front panel board with a Q-tip and brush noting to inspect for cotton left by the Q-tip which I had to remove two spots with tweezers. Seemed to help the look at least. Now like I said above… need to check for corrosion. Man, labor work seems to effect the brain like some diets. :-)

  • ToughDev
    December 13, 2017 at 10:29 am


    Thanks for the update. Glad to see that you have made some progress :)

    I think whatever causes the system to hang (maybe the acquisition board connected upside down) has been at least partially resolved. And the CRT display that you’ve got now looks promising. If you take a look at the display interface of a working TDS 520, that line is actually part of a scale that shows the sampling rate. The next thing the oscilloscope should have been doing after plotting that line would be plotting some text above it (e.g. ‘Sample’ for a default boot-up). See this:

    So I guess something causes the scope to hang when trying to draw the text. Most likely there’s an issue reading the video BIOS ROM (a 32-pin PDIP EEPROM with the Tektronix label pasted on top of it, at the corner of the board, next to the DALLAS RTC chip) to retrieve the character set for drawing text. Unfortunately the label obscures the part number so we won’t know exactly the pinout, but my guess is that it should look similar to below (most 32-pin EEPROMs are similar)

    Can you use your other scope and probe the OE (Output Enable) and CE (Chip Enable) lines to see if this chip gets activated during startup? From there we’ll see how to troubleshoot further. :)

  • December 14, 2017 at 5:44 am


    Thank you very much and your very welcome. :-)

    Yes, I will try to use my scope and probe the OE and CE lines on the U1331 chip. Do you know roughly what I am looking for range, i.e. 2V, 5V or will be 25V range? Are the signals like the J18 display pins?

    Below is an update of what I’ve worked on:

    I’ve been trying to read into the details of the U1331 EPROM chip and there isn’t much in the documents detailing specifications for the pins. I did find from the “TDS-520B Digitizing Oscilloscope Component Manual” that U1331 is a Tektronix part #160-9335-00, IC, MEMORY CMOS, EPROM, 128K, X8, 200NS, 10%VCC, PRGM 156-4025-00, 27C010, DIP32.6.

    The electrical components information isn’t in any of the TDS-520 or TDS-520A manuals I have.

    My TDS-520 U1331 chip reads on the label: Tektronix (1991), 160-8505-00, A11111331 VER 1.5

    I thought I can also re-do the troubleshooting steps from the TDS-520 Service Manual again just to be safe as I was curious the effect of replacing the A10 to A11 connector in the correct orientation.

    Here is what I’ve done so far:

    Performed Figure 6-22 Primary Troubleshooting Procedure up to step where “Is the Display Readable and Stable” is “No” perform the “Display Troubleshooting Procedure” and also since I wasn’t sure the definition of “readable”, I did the “Yes” perform the “Processor/Front Panel Troubleshooting Procedure.” Per the Figure 6-30 Processor/Front Panel Troubleshooting Procedure, I need to replace the A12 Front Panel module. Per the Figure 6-26 Display Troubleshooting Procedure, I need to test J18 pins to determine to replace A20 Display Assembly or A11 DRAM Processor/Display module.

    Basically, this tells me like you are noting where to trace on which board for the error more. I thought this would save some time in inspection of the board and better narrow down where to look more for repair.

    J2 unplugged now allows the system to power on and startup as last found (other than when unplugged the system when first powered on DS1 displays “.8″ and the CRT doesn’t do anything and the LED’s stay on, though after powering down and on again… this goes away until unplugged from 120V main).

    J2 pin 25 does have a 3.125MHz clock as read with VICTOR VC3165 COUNTER.

    Below pins measure with VIOT M7 DIGITAL MULTIMETER (V+ unless noted)
    J2 – pin 2: 15.12, pin 6: 15.00, pin 7: 4.95, pin 11: -5.07, pin 16: 5.07

    J5 – pin 1: 24.3, pin 2: 25.7, pin 3: 5.0, pin 4: 5.0

    J26 – pin 7: -15.0, pin 11: 14.9, pin 17: -5.0, pin 35: 4.8

    J27 – pin 1: 24.4, pin 17: 5

    All of the pin Voltage values are in specification now.

    I still am going to read the J18 pin’s 1 through 3 (where per the TDS-520A service man. notes as J62 pin 1 to test instead of pin 3 as with TDS-520 service man.) signals and U1331 OE and CE values with the oscilloscope.

    Do you know what am I supposed to observe with the U1331 OE and CE values? I am guessing that chip is 5V though not sure if higher potential or not.

    Thanks again for the excellent feedback, motivation and support. :-|)

  • ToughDev
    December 14, 2017 at 11:00 am


    U1331 is a standard 128K EPROM. The OE (Output Enable) and CE (Chip Enable) lines are active low, which mean they normally stay at 5V for ‘disabled’ (logic 0) or 0V for ‘enabled’ (logic 1). The lines from A0 to A18 are the address lines and O0 to O7 are for data output. A typical EPRROM usage flow is below:

    1. Oscilloscope sets CE to 0V to enable the EPROM and sets OE to 5V to disable the output pins.
    2. Oscilloscope sets A0 to A18 to the address of the byte to read from and then sets OE to 0V to enable data output
    3. The EPROM will then output the value of the byte at the above address to the output lines (O0 to O7) to be read by the scope
    4. Scope will repeat (1) with different address to read another byte.

    From this you can see that you should expect CE to stay at 0V at least for the duration of the startup, and OE to switch between 0V and 5V. The address lines and data lines should also switch between 0V and 5V as well. Look for any pins that seem to be stuck, e.g voltage does not change. This could indicate a faulty connection (for the address lines) or a stuck bit (for the output lines), which is an issue in old EPROMs.

    It is also worth checking the VCC line of the EPROM with a scope to make sure that it doesn’t appear too noisy, which may cause issues reading the ROM.

    Also as mentioned earlier, the Dallas RTC chip with a dead built-in battery next to the EPROM could also be the issue. Try to remove it and see if anything changes.

  • December 14, 2017 at 11:23 pm


    OK, makes sense. I tend to like to know exact details so I’ve been scared to remove the EPROM and NVRAM until I have a reader/writer and practice soldering and de-soldering more (I’m confident with non-SMD soldering, de-soldering I am still not so good yet). Also, I really went Holiday investment spending and I ordered a GQ GQ-4X V4 (GQ-4X4) EPROM chip Burner USB Universal Programmer 29F400 PRG-055.

    The Universal Programmer will help with backing up and updating the chips and other chips i come across in the future. I was wanting that also and an FPGA system too, which I have to read up more if the above will work with FPGA’s also. I’d guess not, though not sure yet as I still need to read into and will have to wait until I get into learning to program those later.

    I also made a best offer and won a TDS8000 with the HD and OS not always working. That looks like will need power supply work and I’ll find out what else. Kind of risky investment, though for the $200 and shipping seems like worth the risk investment.

    Now, back to the TDS-520. I have a Tektronix 1740 Waveform Vector Monitor (I got for $8 from the GVSU Surplus Store) that is smaller and I was thinking of setting that up to work on the TDS-520 and other projects unless I need to use the TDS-520B. Looks like from some quick reading… I need to read more still… that the waveform monitor will work as the oscilloscope though is a more basic in regards to information on the screen I’m thinking oscilloscope. I still need to read more into what the specifications are like bandwidth, time and voltage divisions, etc.

    Therefore, give me some time to read into the 1740 for use as electronics test equipment if feasible. Feedback would be great if you are familiar with. I’ll see what I can find out about reading. Not thinking other than video and audio testing that the vectorscope or vector monitor will be required… though is interesting when reading how to interpret the images to test the video qualities, chroma, hue, etc. of video signals. Analog video that way in phase and vector interpretations is definitely different than I am used to.

    I’m excited about investing in the EPROM programmer finally. That will be neat to work with and have as a tool. Then the TDS8000 deal and man… upgrading into more than the FCC Technician license I earned sooner than I expected. I think is definitely worth the student investment however when compared to books and tuition. With edX classes and the amateur radio and electronics community that is mainly helping also. To think I started with cheap Baofeng SDR and RTL systems and now using old superheterodyne radios, signal generators with modulators and oscilloscopes for radios and spectrum analysis that is expanding in frequency range. Just in two years of learning mostly online and the library. Really neat. Many thanks again!

  • ToughDev
    December 15, 2017 at 8:43 pm

    Wow, that’s a lot of toys to play with. You’ll for sure have a lot of fun exploring the equipment and learning new knowledge!

    I do have an old basic CRT waveform analyzer. Very basic with only brightness/contrast and some scale adjustments. The only time I played with it was two years ago, right after purchasing it from eBay. I found it to be much less useful than my 20MHz analog oscilloscope. :) The machine has been in my store room ever since!

    I also have three spectrum analyzers and one 8 1/2 digit high precision multimeter. Again, only played with the devices briefly after purchase and then once in a while for my curiosity. I have few needs for these devices except to have some fun playing with them :)

    On the other hand, my EEPROM programmer (also from eBay) turns out to be very useful. I use it to dump old BIOS chips from old motherboard and graphics cards. Sometimes the data stored on those can be very interesting …

    The TDS8000 is fun to play with. I used to own one but sold it a few years ago. You might wish to dump the contents of the hard disk drive into an image file and clone it onto another hard disk drive (or a CompactFlash card) as the hard disk inside those units will probably die soon. I think the hard disk uses a standard 2.5″ IDE interface. Not sure about the floppy – the ones that I have seen use a proprietary interface and not the standard 34-pin found in PC.

    On the TDS520, let me know how it goes after you’ve observed the waveforms at OE and CE with the EPROM connected. I really hope to see the machine working again. As mentioned, it’s one of my favorite scopes :)

  • December 23, 2017 at 4:36 am

    Hi again,

    I didn’t check signals details on the A11 Board quite yet since I am still wondering if I need to replace capacitors (caps) or repair traces. Trying to save the more morbid tasks of desoldering the chips and replacing with sockets until I have to, at least I hope that is correct logic to proceed. I know…, I am trying to avoid the inevitable. :-)

    After re-reading again the posts you noted and some others I found and since I have a handheld DMM, ESR capacitance meter and brush… I figure I better brush off the board and check the caps in circuit for continuity and values just to see if anything appears different since whomever replaced the caps before used all the same 33uF 16V 85’C Nichicon’s everywhere I find. I still haven’t made room for the larger test equipment where I have the TDS-520 apart (kitchen table), so like above noted to factored into using the handheld devices for testing. :-)

    First, I figured I’d better try continuity testing of the caps positive and negative leads/pads with the VIOT M7 DMM with the common probe clipped to the chassis. I found three caps where there was no beeping sound on the positive lead/pad and all the caps negative lead/pad beeped. Of the three with no positive lead/pad beep; two (C1256 and C1257 ) had a ~782 value (I guess a diode value) and one (C1220) had a ~886 value.

    I decided to use the ESR next to measure capacitance in circuit on all the A11 module caps tested above. All tested when ~ stable above 21uF with the exception of those three caps noted above. C1256 and C1257 tested 7.2uF and C1220 tested 19uF.

    I then did as you noted above to test the EPROM with the DMM and found the following:
    VCC when not powered on was 0.08V and powered on was 5.08V
    CE & OE stayed steady 4.5V and zero when not powered on.
    O0 to O7 stayed steady 1.71 to 1.73V depending on pin and zero when not powered on.

    I don’t have a replaceable electronics parts list in any of the manuals for the TDS-520, only the TDS-520B. That list notes C1256 as 10uF 35V and C1220 33uF 10V, C1257 is not listed.

    I’m thinking I should de-solder the C1256, C1257 and C1220 and replace with new tested capacitors before and after in circuit. Clean those pads and area and startup the scope again to see any difference.

    I’ll also try to look at the traces from those caps in more detail to see if I notice anything.

    What are your thoughts?

  • December 23, 2017 at 9:22 am

    Looks like I’m going to get another update in for today.

    I pulled the Dallas DS1245Y NVSRAM chip out and looks like someone had already soldered in a socket.

    I installed the USB driver for the GQ-4×4 and installed the latest version of the GQ Software.

    I removed the DS1245Y chip to see the contents via the Buffer tab in the GQ-4×4. I am not sure what I am supposed to see however. Do I set the file offset to 0x4E with this chip also?

    I am thinking I want to install the NI PCI-GPIB card to see what that can demonstrate during startup as I do have the GPIB connection option installed though no Centronic, RS-232 or VGA connection.

    Do you have a copy of the NVRAM for a TDS-520 so I can see what looks like?

    I do have the GPIB connection though no others on the back of the scope. I haven’t installed the NI PCI-GPIB card yet in a desktop and am thinking I will do that next to see if I can read any more data to determine what is going on. I do have a Windows 7 Ultimate desktop PC that I was planning on installing the NI PCI-GPIB card in. Not sure if software I have (tektool.c, stackframe_tektool, NI4882_1500f0 driver-software, and WaveStar Software for Tek OSc.) and not installed yet will aid in this process of troubleshooting.

    Thanks in advance for your time and support!

  • December 23, 2017 at 10:44 am

    OK, I promise this will be the last post for today. Embarrassed, I just realized when reviewing the traces (I haven’t found a bad trace yet) that there are resistors missing. There is a suspicious solder blob though and the capacitors I noted above. Thinking the EPROM function may have to do with missing resistors, though didn’t trace out in detail yet or de-solder and read the EPROM chip.

    I made a generic video that documented the locations of the missing resistors.

    I noted in the video description the details of what I found and since I do not have a TDS-520 electronics components parts list, I am not sure exactly what the resistors specifications are supposed to be.

    Based on the TDS-520B, I have some information. However, R1998, R52, R13 and R15 are not listed.

    Thinking this will make a significant difference in performance.

    I also noticed that the memory chips (I guess are 32K) have pads for larger memory chips that I guess can go to 128K. Similar to the NVRAM which looks like 128K… there are larger 512K chips that I was reading may work.

  • ToughDev
    December 25, 2017 at 11:29 am


    Thanks for the detailed updates.

    From your information, I can say that the EPROM is not activated after power on since CE/OE are always high (4.5V) and O0-O7 are floating. Of course it’s possible that CE/OE were pulled low (0V) for a short while and not displayed on your multimeter but you can only tell for sure by looking at the waveforms for these pins on your other oscilloscope.

    In a typical EPROM configuration, CE/OE is connected to VCC via a pull-up resistor (4.7k-10k), and also to dedicated pins on the micro-controller/micro-processor. When it’s time to use the EPROM, these pins will be set to 0V to get the EPROM activated. You can trace where OE/CE are connected to, and see if there’s anything broken.

    For the suspected missing resistors, you can perhaps tell from whether these resistors were not populated from factory or somebody removed them during an attempted repair. Are the solder holes for these resistors clean? However, my take is that these missing resistors may simply be intended for larger memory chips, since the board may be for different EPROM/NVRAM sizes, like what you noted.

    The NVRAM contents are typically uninteresting – just the date/time, oscilloscope calibration constants, and user waveforms. If I recall correctly, the TDS520 allows users to store waveform to internal memory, which is the DS1245Y NVRAM. Most oscilloscopes can recover from a corrupted NVRAM simply by performing a quick calibration after the machine has successfully powered up. The DS1245Y is still manufactured by Maxim so you can always purchase a new one, replace it, and see if anything changes. Also refer to the datasheet here (, check the CE/OE/WE/VCC and DQ0-DQ7 pins (in a similar manner as the EPROM) to see if you can find anything.

    To answer your question, I think the file offset simply specifies the location from which to read data, so you will definitely want to put 0x0 as the file offset to simply read everything from the NVRAM.

    Unfortunately your U1331 EPROM is not in a socket. I do not recommend desoldering it except as a last resort since the heat may damage the chip, unless you have a temperature controlled soldering iron, suitable heat sinks, and have done similar stuff before. If you do desolder it and put in your programmer, make sure the programmer is in read-only mode. Do not do any testing of the chip in the programmer as this will perform simultaneous reading/writing cycles on the chip and destroy existing data. Unlike the NVRAM, the oscilloscope will not power on if the EPROM is corrupted. I do not have the EPROM data for the TDS520.

    Regarding the capacitors, I think the best course of actions is to replace all SMD/through-hole electrolytic capacitors for which you’re sure of the current values. Your in-circuit capacitance measurements using your DMM, although encouraging, do not say much because the DMM is not designed to measure capacitors in circuit. The replacement capacitors should be rated for 105C (and not 85C).

    If you still want to measure, you will need an ESR meter, something like this or this. A good electrolytic capacitor will have ESR < 1 Ohm while a bad one will have very big ESR value, and you’ll know exactly whether a capacitor is good. The resistors, however, can be left untouched since they seldom fail. Keep me updated on the progress :)

  • January 3, 2018 at 5:45 am


    Figure I’d post this update for the record regarding the missing (or not) resistors.

    “Siggi” posted from that:

    “R2 and R4 are 0 Ohm resistors tha pair with R1 and R21 to select
    alternative clocks for CPUCLK and TSCLK.

    R6-R17 are 0 Ohm strapping resistors that tie straight into an IO register,
    maybe HW config/identification. Alternate number designations tie high or

    He motivated me to get a copy of the service manual from Artek and now I have a copy of the TDS-520 Component Level Service Manual volume 2 (070-8313-00) where I can now find out what the missing resistor and capacitor stock values are.

    I’m working on making a bench so I can get all the mechanical, computer, electronics and optical systems located in one place and work more effectively. :)

  • ToughDev
    January 3, 2018 at 11:43 am


    Good to hear that you’ve finally identified the suspected missing resistors. Now with the service manual, hope you can get the TDS520 up and running in no time. :)

    Is the service manual in PDF format? if so, can you share it with me?

  • January 3, 2018 at 1:32 pm


    Yes, is nice to move forward again. :)

    I just reviewed the manual and found that all the resistors required that are missing are 0Ohm.

    The only resistor I can’t find a value for is R1998 and R1999. R1999 happens to be in my board and is a 0Ohm (green label with “000”). I see others that look the same and are found on the schematic and are 0Ohm.

    Do you happen to be able to see what R1998 is? Is just next to J2 (Front Panel connector side closest to CRT).

    Different to me having so many 0Ohm resistors. I assume from reading that for certain EPROM or Firmware settings, or maybe later boards NVRAM since Firmware Boards are not present in later models, from what I’ve read during startup/booting the model can be determined.

    I read that also on this blog:

    Would be interesting to know what options can be made to make the TDS-520 more bandwidth or sampling rate. I am thinking I’d have to add more memory so no or less memory samples lost/dropped. Do you have any ideas? Would be neat to enable FFT as well as increase the bandwidth to 1Ghz and sampling rate to 4GS/s.

    Sure, you can find info on this link:

    Also, on a side note… do you happen to have a copy of the HP8640B OPT323 AN-USM323 mil version. Air Force #: T.O.33A1-8-684-1 / TM-07570A-15?

  • January 5, 2018 at 9:57 am


    Wound up buying the download copy of HP8640B OPT323 AN-USM323 mil version. Air Force #: T.O.33A1-8-684-1 / TM-07570A-15 and TDS-520 service manuals from Artek Manuals website. The T.O.33A1-8-684-1 and 4 are the only manuals for that system I can’t find online or from the library.

    Found out I already have copies from Tektronix of the same manuals or slightly different for the TDS-520. I haven’t went through the files to determine what the exact differences are. Cool thing is Artek is going to refund me for the TDS-520 manuals since I already have from Tektronix, buying my scopes or online hunting around.

    Here is a summary of where I am and some more notes found related on the schematics for the missing resistors:


    – I’m still not finding the board ID number on my board yet and haven’t read in the manual where found. Sounds like that is missing factory info potentially.

    – U4 isn’t present on my board. R7, R9 and R11 are placed though. I’m guessing since U4 isn’t present I have an older version board. I’m guessing R7, R8 and R11 can be removed and placed elsewhere and will do that plan for now.

    – There is a typo on the manual and the R1999 and R1998 resistors are found at 11-12, not 11-14.

    – There is a note there also noting if 32K RAMS installed, do not place R1998. I’ll assume for now that 32K is what is installed, though will verify as I see discrepancies where other resistors are placed and chip is not, i.e. U4.

    – I noticed the board has room for larger chips at U1007, U1014, U1026-U1028 and U1031-U1033 with open pins labeled also as U1008, U1009, and U1034-U1039 on above the open pin section where on 11-14 the chips are memory and note 128k. Is this for option 1M? I wonder if these can be increased? I’m guessing will have to study why, how and what the circuit for the memory will require, and existing board circuit capabilities.

    I found this video and am thinking so:

    – For the missing resistors, I almost wanted to just jumper with wire or maybe pins and jumpers, though ordered 0.125W 0 Ohm resistors in the 0805 size since a reel I found on ebay will arrive sooner than the 1206’s from Digikey or Mouser that are on back order for 20 days. $15 more for 3,318 and I only need ~19 minus what is required for the version of my board as noted above. Looks like I am going to be selling resistors soon. :-)

  • January 5, 2018 at 12:11 pm

    For the record, I’ll note there are a few other locations on the A11 not populated. Maybe there is upgrade/improvement potential.

    U200 through U207 (74F1940, D41264V-12 maybe) are not populated. U76 is also not populated. These look like shift registers and ZIP RAM maybe? U76 (74F157AD maybe) looks like an Encoder/Multiplexer.

    Not sure if this is for color option or something else for a different model.

  • ToughDev
    January 6, 2018 at 10:56 am


    Sorry for the late reply.

    Wow, that’s a lot of detailed technical knowledge that you’ve learned about the TDS 520. Thanks a lot for taking the time to share it with me :)

    I have also uploaded the files you provided to a permanent location on my server and added a link to the end of this article for other readers who might be interested.

    For the missing resistors, I really doubt that anyone who previously attempted to repair the scope really bothered with removing them, unless that person was also experimenting with upgrading the RAM configuration. Can you use a multimeter and check the continuity between the pins where the missing resistors are supposed to be to see if they’re not already connected somehow? I am suggesting this because your board might not exactly match the schematics, and attempting to construct an exact match by soldering resistors without knowing the specifics might cause additional issues.

    Also a word of warning on the zero ohm resistors. They might have been included for impedance matching purposes, especially around the acquisition board where high frequency signals can be expected. Replacing them with normal wires/jumpers might disturb the characteristics causing additional noises and/or signal attenuation, although the oscilloscope might still appear to work fine at first.

    The note on the “version ID resistor placement” looks a bit sketchy, but what I understand from it is that, only on boards whose ID number is greater than 11110000 (binary), R7, R9, R11 will be missing, and pins 5-9 of U3 will be high (~5V). If this is the case, U4 and R12-R17 will be present, and the last 3 binary digits of the board ID (e.g. XXX in the note) can be measured via pins 2, 3, 4 of U4, e.g. ~0V means 0 and ~5V means 1. You can easily verify this with a multimeter. I don’t think messing with R7, R9, R11 is advisable as it might cause the scope to fail to boot if it detects an firmware incompatible with the board revision ID.

    Based on the component service manual (page 156 / 370), U1008 and U1009 is populated with HM628128LFP (128K 100ns CMOS SRAM), totalling 256K of SRAM memory. Adding 1M of SRAM memory requires 8 pcs of 128K SRAM, matching nicely with your observations (U1007, U1014, U1026-U1028 and U1031-U1033 are empty). So I’d hazard a guess that you are correct that these are for Option 1M. You can verify this by looking at the datasheet here and check if the voltage at VCC is correct, if CS1, CS2, OE and WE are pulled high (connected via a resistor to 5V) and if the data pins and the address pins are also connected to the same set of pins on the existing 2 SRAM chips.

    32K of RAM most likely refers to 32 pcs of the AS7C164 8K x 8-bit CMOS SRAM chip (U301-U316, U401-U416). Each chip provides 8Kbit (1Kbyte). Count how many AS7C164 there are on your board and you’ll know whether the configuration resistor for 32K of RAM (R1998) should indeed be missing.

    D41264V-12 is a 4-bit, 12ns, 64K DRAM chip, sometimes found in old video cards. From this, I am guessing there are some potentials for upgrading the video memory of this scope, although I am not sure if the extra memory will be recognized and utilized by the firmware.

    Beyond this, I seriously doubt if there is any feasible way (for hobbyists like us) to increase the bandwidth or sampling rate. The limitation is in hardware (for example, missing memory required for higher sampling rate) as well as in the firmware for oscilloscopes of this era. For modern oscilloscopes, I know that some can be easily upgraded. I have a Rigol DS1052E which has been upgraded from 50MHz to 100MHz simply by switching the firmware. Although some people say doing this might cause noises or attenuation at high frequency since some components on the board were never rated for the higher frequency, I personally have not experienced any such issues.

    By the way, have you replaced the electrolytic capacitors?

    As always, thanks for the hard work in repairing the scope and keep me updated :)

  • January 7, 2018 at 5:50 pm


    Thank you for all your support. No hurry or need to be sorry. My apologies for all the information that may be confusing since my background is Chemistry and C.I.S. and I’ve not worked on electronics since the early-mid 90’s other than an instrumental analysis course for one lab. :)

    In regards to the memory on the market still (though I did find some HM628128LFP on eBay):
    An SRAM chip for the A11 proc board is (IS62C1024AL SOP-32 and another that may fit better (SOIC-32) and may even perform more efficiently (lower power requirements and higher op. temperature range).

    Any thoughts for replacements to perform the 1M upgrade other than making sure firmware is newest to support and setting 1M option?

    Is this a faster substitute (10ns) for the A10 acq board than AS7C164, though will be a SOJ-28 pain in the pins still (will have to rollout carefully):
    I noticed the I/O capacitance is 8 and 10 versus 6 and 8 for what you used. Thoughts on effects of difference in capacitance?

    I have to verify the above chips match access times.

    In regards to replacing the electrolytic capacitors; I see there are solid though really low ESR tantalum capacitors that are the Kermet-Cap. :

    The other that looks like a higher ESR is the Jameco:

    Are you familiar with these? Would be nice to not have electrolytics and have a longer capacitor maintenance cycle that won’t cause board damage. I wonder how the newer non-electrolytics are designed into the newer or higher end boards?

    Looking forward to having a bench to work on so I can have all my equipment together as I am working in a house that I am restoring so kind of a scattered with not enough room in one spot. I also have the stereo microscope and 5/8″ stainless steel rod to make a boom mount so easier to use with larger boards. Next up is completing the bench and clean room. :)

  • ToughDev
    January 8, 2018 at 10:50 am


    The I/O capacitance does not matter. These parameters are highly speculative and usually just estimates.

    A rule of thumb is that you can usually replace electrolytic capacitors with tantalum equivalents in most circuits except audio circuits. Just make sure the capacitance values are the same (or very close) and that the temperature ratings of the replacements are the same or better. For example, if the old capacitors are rated for 85 degrees C, you can replace them with capacitors rated for 85C or 105C. If the old ones are rated for 105C, you should only replace them with capacitors rated for 105C. As for ESR, replacements should have the same or lower ESR values – capacitors marked as ‘low ESR’ like the one you listed should suffice.

    Audio circuits are a different story. Those circuits make use of special characteristics of electrolytic capacitors (which are not present in tantalum capacitors) to deliver good, high-fidelity sound playback. Replacing with tantalum and the audio will never sound the same again :)

    As for the RAM access time, although you can replace the chips with modern equivalents that have faster access time, the RAM will still function at the original (slower) speed which is already programmed by the firmware. In other word, there’s no point changing to faster RAM unless you can also change the firmware to utilize the faster speed :)

    For Option 1M, besides checking if OE, CE, WE, CS1, CS2 are pulled high like what I suggested, you can perhaps use another scope to check if those lines are ever pulled low (set to 0V) during startup. If this happens, you can be confident that the firmware already checks for the presence of these chips and will hopefully make use of them if it detects extra memory installed. Other than that, I guess we can only try and hope for the best :)

  • January 9, 2018 at 7:47 am


    Thanks for detailing the capacitors info as I am finding impedance and ESR values as noted in the article referenced to be missing in datasheets for the components also:

    There is a link to a calculator that is interesting to see more details of the capacitors performance over a broader set of variables. The KEMET (T583D336K016AHE060) polymer for space applications looks most impressive like you noted for the T582_583.

    I wasn’t sure of the Nichicon ESR and Impedance specs. over a larger frequency range.

    The Nichicon datasheets show Impedance values.

    The Panasonic datasheets show ESR values.

    I requested a TDS-520, or at least Mfr.Part No. Replaceable electrical Parts List for the TDS-520, equivalent to the TDS-520B Mod CM Digitizing Oscilloscope Component Service Manual 070-9710-03 and am waiting a reply back. I also asked for schematics as I’d like the most current version directly from Tektronix.

    The unit I have has all 33uF 16V 85’C Nichicon electrolytics that are not SMD. I’m not sure what came stock.

    I’ve read that there are other values on the board stock (10uF and 33uF) and I notice the TDS-520B has others listed also, i.e. 33uF 10V and 10uF 35V (both 20%).

    10uF 35V 20% is listed for the C1256 and that is one of the suspect capacitors along with C1256 and C1220.

    I still need to test the voltages of the capacitors in circuit (though with the unit powered on) and record those voltage values to further narrow down potential trace or other component issues. :)

    I also have to read more into J40 and see what the details are for connecting a RS232 cable directly and in regards to software. I’m thinking Putty will be easiest (though I just downloaded RealTerm) since I just noticed the desktop PC here has a serial connector. I didn’t realize there was a computer working here with one until now. I’m thinking I am going to try to read the RS232 from the pcb edge connector to debug before I receive the components for the USB Debug Cable that I am about to build. :)

    I’m way overthinking this… though better to repair for longer value and less future maintenance. Or at least, that’s the plan. :)

  • January 10, 2018 at 6:45 pm


    I finally tested all the voltages of the capacitors in circuit on the A11 board with the power on with the scale line appearing on screen before testing and found all capacitors are 5.06V with the exception of the three suspect with different power off capacitance and continuity readings as noted before; C1256 and C1257 @ 11.85V and C1220 @ 3.13V.

    Those are supposed to be ~15V and ~5V respectively (though S1002 notes 12V which is down from U12 Vout so maybe that regulated Vin of 15V down to 12V, with C1256 & C1257 in between that still can be closer in spec.).

    I also tested the resistance powered off and found all were around 80 Ohm with C1256 and C1257 being 1.4K Ohm and C1220 not reading a value (way high). :)

    Hoping that is all. :) Not noticing trace or other component issues yet. Board seems clean.

    Waiting for the capacitors to replace and test more from there. I ordered the T491D336K020ZT and those are a 33uF 16/20V (depends where I read mouser notes as 20V though Kemet notes 16V), 800 mOhm ESR noted, and rated to 125’C.

    I have some 33uF 50V Jwco cheapo brand that have ESR values between .53 and 1K. Figured I could test with these… though I will wait for the new ones to arrive.

  • ToughDev
    January 10, 2018 at 9:17 pm


    Capacitor voltage ratings are not critical. You can replace with capacitors that have the same or higher voltage ratings and the circuit will still work.

    In general you need an ESR meter to measure capacitors in-circuit. That said, C1220 seems suspicious as most capacitors should show something on your meter’s resistance scale. Try to swap the meter’s probes; if it still reads infinite maybe that capacitor should be replaced.

    The capacitors where 5V is measured across their legs are most likely decoupling capacitors used for noise filtering. Most of these capacitors are not critical and any replacement with similar specs should suffice.

    If 11.85V is found where 15V should be expected, maybe your power supply is having issues. One thing you can do is to measure the voltage of various DC power rails. Look where the power supply interfaces with the circuit board and measure the voltages at the connector.

    For J40, you should be able to locate the GND, TX (Transmit), and possibly RX (Receive) pins using your other scope. Connect the GND and TX pins to another computer’s serial port and you should be able to view the debug output (if any) by using Putty/TeraTerm in 9600bps, 8 data bit, 1 stop bit, no parity (9600 8N1) configuration.

  • January 11, 2018 at 8:05 pm


    Haven’t finished the work bench yet.

    I wasn’t sure the format of the 20 pin edge connector of the J40. Figured I’d need a format converter chip at least before RS232/DB9 readable.

    Here is a link with a TDS debug cable design with J40 pin information.

    J37 looks like the same data streams though with six more pins (2 GND, DS, DSACKO, A, 5.1V)

    Will the R/W act as a TX, TX and RX or TX or RX alone or is IRQ and CS information required? I do not know.

    Thanks in advance.

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