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
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!