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HP 8640B repair headaches

Discussion in 'Electronic Repair' started by Ancient_Hacker, Jul 16, 2007.

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  1. Now don't get me wrong, I think $16,000 signal generators are worthy
    of respect. The 8640B, not so much. I had the bright idea of buying
    a few of these in as-is condition and fixing them up, reselling them
    for many $. Profit !! Woo Hoo.!!

    here's how it's gone so far;

    These things are heavy. There seems to be some unbreakable rule, if
    you want to generate signals down to -130dbm, you need fifty pounds of
    thick aluminum castings to contain all the stray waves. The weight
    means you're $50 in the hole right away just for shipping.

    They're wonderfully complex. What you'd do today with a fly-speck of
    PLL synthesizer, HP did with a half-bazillion discrete parts. Even if
    the parts only fail one every X hours, there's so many parts the
    effective Mean Time To Failure is not much more than a day. The number
    of gold-plated SMA connectors, switch contacts, pc board edges is
    large. Each is a potential point of failure.

    They're both sturdy and fragile, which is a bad combination. The case
    is sturdy but the plastic knobs and nylon gears are crumbly. Most of
    them arrive with broken bandswitch and FM deviation knobs. The power
    transformer has its fragile nickel-tin pins soldered to the PC board.
    Guess what breaks if the unit is set down a bit harshly? The slide
    switches and some of the rotary switches are custom gold-plated jobs
    that are unacceptably fragile and of course unobtainable.

    They have unacceptable amounts of unobtanium. Luckily, not a bit of
    software or EROMS in these. But a half-dozen gold-plated analog and
    ECL IC's with the doomed HP 1820-xxxx part numbers on them. If the
    ECL ones go, and they will go, the counter and phase-locking breaks.
    If the jumbo TO-3 amplifiers go south, and they will, as the chips
    were not glued down into the carrier properly,you're SOL, there's no
    microwave 25dB amplifier chips in Jumbo TO3 that run off 44.6 volts!
    The fan is only slightly less complex than a Pratt and Whitney R4360.
    And the aforesaid gold-plated switches.

    They're very hard to work on. Many modules are hidden under 30
    screws. I've opened and closed the divider and filter boxes so often
    the threads are wearing out. And the filter module still cuts out if
    i tighten all the screws! Arggghhh!

    The militarized -323 version is a teensy bit better-- at least the
    knobs are sturdier.

    So lesson learned::: even if HP got these to work well just
    marvelously as they left the factory in 1975, they're kinda a DOG
    these days.


    So far I've been able to make two mostly working units out of five
    basket cases.
  2. AH-

    I talked to one person at a Hamfest who claimed to have done what you
    are doing. He said that he had some custom metal gears made to replace
    the crumbly nylon gears in the units he refurbished. If you have a
    sizable investment, you might consider doing that as well. It should
    completely eliminate that as a failure mode.

    I haven't opened-up mine, but have one with a broken meter needle. I
    suppose it is floating around inside the meter case, and it might be
    possible to glue it back if I could figure out how to get to it!

  3. They sure were great units back in 1982!

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"©

    "Treason doth never prosper: what's the reason?
    For if it prosper, none dare call it treason."

    "Follow The Money" ;-P
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