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Tek TDS420 help?

Discussion in 'Electrical Engineering' started by DaveC, Jan 31, 2010.

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  1. DaveC

    DaveC Guest

    This scope has the famous "FAIL ++ Acq" error which points to the infamous
    failed (and leaking) SMD caps on the acquisition PCB.

    I'm moving on to troubleshoot another (unrelated, I think -- and hope) issue.

    During the 25 second "on" period, relays click on & off, the monitor goes
    through it's boot display (all bright, then dark, then the clock icon), then
    the graticule is drawn, which is quickly replaced with the "Tek Running
    Autoing" display listing the "FAIL ++ Acq" error.

    During the power-up period, the front panel LEDs all turn on for about 25
    seconds, then off for 5 seconds, then only the CH 1 LED lights.

    The service manual says (in the Front Panel troubleshooting chart):

    Do all
    of the front panel
    LEDs turn on and then
    turn off a short
    time later?


    Does one
    front panel LED
    turn on and then turn off,
    followed by the next LED until
    all LEDs have turned
    on and then
    turned off?

    The LEDs do not go through this one-at-a-time sequence. The manual's flow
    chart points to a failed CPU PCB if the LEDs don't blink this sequence.

    Can someone with a TDS4xx confirm power-on LED behavior?

    I'm trying to determine whether, because the LEDs aren't blinking "properly"
    and most of the front panel buttons are not working (the buttons bordering
    the monitor, plus all the CH input buttons), does this mean a replacement CPU
    PCB is in my future, or if it's possibly related to cap failure (ie, other
    cap failures beyond the acquisition PCB)?

    Has anyone else here have a similar problem (non-functional buttons) in the
    past? I'd appreciate hearing your experiences.

  2. DaveC

    DaveC Guest

    All my experience is with the 540, but I think you're wasting your time
    OK, I suspected that, but non-functional buttons seemed a symptom not related
    to caps issue. But I'll take your advice to heart and do *all* the caps.
    Yeah, I plan to do that. Making a complete list right now.

    What techniques and chemicals did you use?

    Thanks for your reply.
  3. DaveC

    DaveC Guest

    A little more info on my non-functioning button problem:

    I disassembled the front of the scope -- bezel, buttons, front panel

    The elastomer button strip (the "rubber' buttons) have a conductive backing
    which presses down onto the menu flex circuit (flex pcb).

    The back of the button looks good (very light wear indications). The flex
    circuit traces are not metal, which surprises me. It's made from pretty
    fragile carbon-like traces.

    When I measure the continuity of the closed contacts of a button that works,
    I see approximately 200 ohms. Closed contacts of a button that doesn't work,
    I see 400 ohms (sometimes more).

    What resistance should I see at the contacts when the button is pressed?

    Can I clean these contacts with alcohol? Or...?

    Can the traces on the flex circuit be renewed (ie, silver-bearing paint)?

  4. Nico Coesel

    Nico Coesel Guest

    Tektronix has an excellent user forum covering these sort of repairs.
  5. DaveC

    DaveC Guest

    The method I use for cleaning those conductive pads on button switches
    Thanks for that input Bill. I'll see if scrubbing lightly helps.

    The flex pcb switch contacts are more what I suspect than the button. And
    those are seriously fragile. It looks like a tiny bead of carbon-filled
    rubber cement. I dented a trace (actually chipped off a *tiny* piece) with a
    DMM test probe. Yikes!

    Thanks again,
  6. DaveC

    DaveC Guest

    Tektronix has an excellent user forum covering these sort of repairs.

    Thanks Nico. I'll check it out.

  7. DaveC

    DaveC Guest

    Tektronix has an excellent user forum covering these sort of repairs.

    Nothing I could find regarding the button switches.

    Can you point me to a Tek forum topic about button contact repairs?

  8. Nico Coesel

    Nico Coesel Guest

    I'd register and post a question there.
  9. James Sweet

    James Sweet Guest

    I've fixed a lot of similar buttons in TV remotes. What I do is wash the
    rubber membrane with warm soapy water, liquid hand soap works fine.
    Scrub it gently with your fingers, rinse, and lay it out on a cloth to
    dry. The PCB can be cleaned with a rag dipped in the same soapy water,
    or alcohol would probably work too.
  10. James Sweet

    James Sweet Guest

    Don't scuff them! You don't want to remove material, the problem is skin
    oils or oils from the rubber that leach out and reduce the conductivity.
    All you have to do is clean it thoroughly and it will work like new and
    be fine for years.
  11. James Sweet

    James Sweet Guest

    I suppose the surface could wear over time, I've just never seen it
    myself. Certainly try just cleaning before doing anything more drastic
    though, liquid dish soap and warm water works as well as anything I've
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