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HP6034A TMS9981 CPU won't exit reset state.

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

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  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
    (Datasheet http://www.datasheetarchive.com/pdf/3434384.pdf)

    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.
     
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