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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Posted to sci.electronics.repair,sci.electronics.components
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HP6034A TMS9981 CPU won't exit reset state.
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! |
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