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!