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HP 8640B

Discussion in 'Electronic Repair' started by beerbarrel, Jun 26, 2005.

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  1. beerbarrel

    beerbarrel Guest

    Picked up a HP 8640B for a song. When I powered it up, the counter
    display was inop. After removing the cover, I noticed that the LED for
    the 5V regulator card was not lit. I removed the card and found that
    the fuse was blown. Well, needless to say, the new fuse blew
    instantly. My question is this: Has anyone ever torn into one of these
    that might have had this problem? I thought that I might ask before I
    dug any deeper.

  2. nanu

    nanu Guest

    First thing is to check the 5V bus for a short. If that is OK then
    check the regulator card and external pass transistor for problems.
    A little patience and I'm sure you'll find the problem. Good luck.

  3. beerbarrel

    beerbarrel Guest

    Thanks for the heads up Nanu. The bus looks good as does the card
    components. Tomorrow I'll check the pass transistor which is a 3055. I
    have a sneaking suspicion that is my problem.

  4. beerbarrel

    beerbarrel Guest

    Found out that emitter to collector was shorted in the pass
    transistor. They are very easy to replace. I noticed that it had been
    replaced before at some point. They used a mica insulator between the
    transistor and ground. The insulator was broken. I think the collector
    might have been grounded. I had a 3055 to replace it. Now I just have
    to get some of those hard to find little fuses that solder onto the
    regulator card. I only have 35 dollars in the generator so if it never
    works I'm not out too much.

  5. beerbarrel

    beerbarrel Guest

    Dumb question do ypu ID the card that has the 1 ghz option.
    As of yet, I have not powered up the unit for more than a few seconds.

    Thanks for the tips. I also found info on the problems with the delrin

  6. beerbarrel

    beerbarrel Guest

    It does have the 1024 range on the switch. Maybe that's a good sign

    I won't know until I get a fuse for sure.

  7. Jamie

    Jamie Guest

    the power transistor on the botton side comes loose! tighten the
    screws, also check for bad caps. other than that if you got the
    doubler option for 1 Ghz, you have something there.
    its an easy thing to repair/
  8. Jamie

    Jamie Guest

    i have 2 of those units with the 1 Ghz option.
    old but very nice with the hard line feeds in side ect. :))
    make sure you lubricate the range selector universal because the
    original grease (safe for plastics) tends to harden up. also check for
    alignment and make sure there isn't any slack between the joints.
  9. beerbarrel

    beerbarrel Guest

    I replaced the pass transistor. I installed a 5 amp fuse. It calls for
    a 3 amp. I did not have one so that will have to wait a few days. The
    transistor runs mighty hot. Is that normal for this particular
    generator? It seems to work fine though. I noticed whomever replaced
    the last transistor did not use any conductive paste. I'm thinking
    that may be some of the heat issue.

  10. Jamie

    Jamie Guest

    Hmm, i not sure about that, i also got these 2 units from ebay, the
    selector dial has the 1024 position on it, when you snap the switch over
    to the mode your freq counter should display it.
    as far as i know its simply a doubler module put in there.
  11. Tracy-

    Apparently the doubler can be added to one that doesn't have it. As I
    recall, the internal counter displays frequencies in the 1024 range, even
    if the doubler is not there. I have the impression that the counter is
    not connected to the output, but to an earlier stage, and displays a
    calculated value.

    Look at the HP label that has "OPT" and see if any are listed. I think
    the doubler is option 3.

    If you need a manual, do a search for TM 9-4935-601-14-7&P.pdf. It is
    about 10 Megabytes.

  12. Jamie

    Jamie Guest

    mine does not run hot, you mite want to check for leaky caps.
  13. beerbarrel

    beerbarrel Guest

    Thanks Fred...

    There were no options listed on the decal so I', inclined to believe
    it is the base model.

  14. beerbarrel

    beerbarrel Guest

    I did not have any paste handy. I ordered some. I also noticed that
    one of the screws that holds the transistor down will not tighten. It
    seems to be stripped out. Once the paste gets here, I'll repair the
    screw problem and see if that helps. If it does not, I'll move on to
    other things to see where my problem might lie.

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