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 of 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.

See also:

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

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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.

30 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. :-|)


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