Connect with us

HP6034A TMS9981 CPU won't exit reset state.

Discussion in 'Electronic Repair' started by JW, Sep 12, 2007.

Scroll to continue with content
  1. JW

    JW Guest

    Here's one that has me puzzled: I have a HP 6034A power supply that does
    not come out of self-test when powered up. Internally I traced the problem
    to the TI TMS9981 CPU itself

    On power up, the Interrupt code present on it's IC0 - IC2 pins is 000
    (reset) it never exits reset. Tracing back through a few TTL glue chips I
    found that a quad latch has no clock with which to pass D to Q. The clock
    for the latch is derived from the TMS9981's pin 21 "03" clock output -
    which appears to be floating at 2 volts. If I isolate pin 21 of the CPU,
    it goes to a low state. It's not an open collector output AFAICT, so I
    check the CLKIN and OSCOUT pins (33 and 34) where the crystal is located.
    Looks fine at 9.2MHz, levels look good. Check +5 and +12 and GND - looks
    good. So, must be a bad CPU, right? Nope. I place a "known good" CPU in -
    same result, no "03" clock output on pin 21. I'm stumped or missing
    something here... Anyone have any ideas? Even a "That makes no sense to me
    either" would be appreciated.

    Many thanks!
  2. Graham W

    Graham W Guest

    Is something holding up the "o3" o/p?
  3. JW

    JW Guest

    By "holding up" do you mean another device on the trace that is defective?
    If so, I don't think so - as I said, I lifted and isolated the pin 21
    clock output on the CPU chip (it's in a socket) and it's output went to
    zero volts, but that's it. Near as I can tell from the datasheet, all that
    could be required for the clock output would be GND, +5, +12, and an
    oscillator which appears to be functioning. It makes no sense whatsoever.

    Thanks Graham.
  4. Graham W

    Graham W Guest

    So with the IC pin lifted, does the socket trace still show +2V?
  5. JW

    JW Guest

    Yes, but in my experience that would be expected, as TTL inputs would do
    that if nothing was driving them.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day