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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ToughDev

ToughDev

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.

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

  • February 3, 2014 at 2:20 am
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    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.

    Reply
  • February 3, 2014 at 10:52 am
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    Hi PaulFW,

    You can read my other article that has detailed information on the Option 14 board herehttp://www.toughdev.com/2014/01/experimenting-tektronix-tds-340-100mhz.html 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.

    Reply
  • February 3, 2014 at 2:00 pm
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    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! :)

    Reply
  • February 3, 2014 at 2:30 pm
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    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.

    Reply
  • March 10, 2017 at 11:20 am
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    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.

    Reply
      • May 24, 2017 at 2:51 pm
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        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.

        Reply
        • ToughDev
          May 25, 2017 at 10:57 am
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          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.

          Reply
        • October 9, 2017 at 10:14 am
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          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?

          Reply
          • October 9, 2017 at 10:35 am
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            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
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            Hi,

            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
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            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
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            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
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    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. https://www.youtube.com/watch?v=jZjRVAb3Ykc

    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!

    Reply
  • October 10, 2017 at 7:22 am
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    Typo on the above comment… I meant some of the capacitors need to be a higher resistance.

    Reply
    • ToughDev
      October 11, 2017 at 2:34 pm
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      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.

      Reply

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