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The long HP1980B repair story...

Discussion in 'Electronic Repair' started by Robert Obermayer, Aug 3, 2006.

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  1. Hi,

    Just finished repairing it.Quite interesting problem, and real easy to
    fix once one knows which of several hundred ICs is faulty...

    First of all, what is a "HP 1980B"?
    Its one of the most advanced analog scopes made, being fully digitally
    controlled either via HPIB or locally by piezo buttons and a rotary
    encoder.It has a well-working autoset function and with a option (which
    i have installed) can also provide digital waveform storage.
    It looks like that:

    So, first of all, what was broken at all?
    The text display didnt work, being compressed to vertical lines where a
    whole line of text should be.
    This happened one day after 2-3mins of running and stayed since then.
    All other functions were ok, indicating that the controller should be okay.

    The first step was looking for a service manual, which proved to be near
    impossible.None of the usual manual dealers had one, just with one
    having a printed edition for about 4 times the price i paied for the
    whole instrument.Oh well.
    All i could find was the operating manual, containing zero repair
    Looking around inside this really complex instrument showed no obvious
    problems like blown tantalum capacitors or what else.Randomly probing
    around the horizontal output stage didnt really show useful results.
    Probing around at the z-axis amplifier showed a lot of pulses when text
    display was on, so whatever generates the text display at least produces
    Y and Z signals, with the X signal being dead.
    More random probing and testing didnt help, and led to the [false]
    conclusion that the hybrid DAC probably produces the ramp signal and is
    faulty, which would be a problem.

    Since the scope was more or less useable i didnt do anymore and started
    looking for a replacement.


    While searching around for possible info on how the waveform storage
    works (turned out it uses progressive sampling and a rather slow ADC) i
    came across a document listing a lot of test eq. manuals, and a email
    adress where to order.It included the 1980B service manual (with all 612
    Quickly i asked for the price (15euro shipped!) and bought it.

    Now i had 612 pages full of schematics, descriptions and other information.
    Much time was spent gluing together the printed schematic pages to
    something that is readable.

    Finally, it turned out that the text display X and Y ramps are generated
    by simple integrators with a reset and enable signal coming from the
    video controller.
    A good time probing around showed the enable signals were okay, as were
    the enable circiuts.
    Some time later, i was working on the X ramp generator, having looked
    for every component with no results, scoping the enable signal at the
    base of the transistor, which looked like a square wave.I didnt pay
    attention to the amplitude.
    A good time later, i came back to this part again, this time looking at
    the amplitude.
    It measured 0.3v on the transistor base.Not much.
    I unsoldered the transistor to see if its shorted but tested okay.
    Since i had no 2N4904 i substituted a BC547 for testing, no sucess.
    Measuring around the base transistor showed a 0.3v signal both sides, so
    now it was clear the transistor couldnt turn on!
    Soldered it back in and looked where the signal came from, which
    happened to be the video controller, oops no, a 74LS373 octal latch
    after the controller.
    Scoping at the video controller shows a 5v signal, perfect.
    After the latch, it was reduced to 0.3v.
    Measuring (with power off!) from the latch output to GND it had 5Ohms of
    resistance.With the chip removed, it was much higher.

    So what caused the latch output to resistively short to GND?

    Bridging the input and output with a 1k resistor (so chances are lower
    to fry the video controller) brought a perfect display!

    Of course the latch had to be replaced since whatever caused it to short
    would now damage the hp-custom video controller if it came back.

    3 Days later, i had some LS373 in the mail.
    Quickly put one in, works perfectly!
    Having it running now for a longer time, it still works so i think i can
    announce sucess finally!

    What happened?
    What causes a LSTTL Output to resistively short to GND?
    Can the condition reappear?
    Can this happen to other parts too?

  2. Ohmster

    Ohmster Guest

    I have no idea of what caused the problem. Faulty IC I would imagine, but I
    did indeed enjoy reading the story. You did a good job, Bob.
  3. Might be, but what causes a IC to work flawlessly for 20 years and then
    suddenly fail?

    I think (but cant really remember exactly) that the problem appeared
    while adjusting focus and that i heared some click (probably one of the
    many relays in there).

    I already checked if the signal goes anywhere close to a high voltage
    wire (that could spark to it if moisture was present) but didnt turn out
    to be so.
    I found the +18kV-wire being rather close to unrelated wires however and
    tied it away from them just to be safe.

    Btw, thats how it looks now:
  4. Ohmster

    Ohmster Guest

    Uh parts just fail, dude. That's just life. Nothing is expected to last
    forever, although low voltage and low current ICs sure can last a very
    long time. Who knows what made it fail. Maybe the mixture was not as good
    as it could have been at the factory the day the part was made and it did
    not last as long as other ICs did. Maybe the case had small flaws in it
    near the leg openings and allowed moisture to enter the package. Maybe
    the etching was a bit too close when it was made and it finally gave out.
    Who knows, don't take it personally.
    Might be totally unrelated. Maybe you messing around in there caused a
    small arc to jump. Hard to say, doesn't really matter anyway.
    It is always good to be safe.
    Looking good there Bob!
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