[sdiy] Interesting CPU failure mode

Michael E Caloroso mec.forumreader at gmail.com
Sun Mar 4 08:02:38 CET 2018

I had restored a Memorymoog Plus for a client and the last mile was a challenge.

Autotune wouldn't.  It would report "6 TUNED" but none of the voices
were tuned.  When the machine was cold, the C7 routine would report
back legit numbers, then they would be all zeros.

Other symptom - random crash with loss of patch memory.

Great, a temperature failure that is intermittent - the most fun of
any debug jobs.

That initially pointed to a bad RAM but that turned out not to be the
case.  After confirming the path from voicecard to autotune circuit,
supporting TTL/CMOS, CTC, cabling and interconnect, I finally found a
surplus MOSTEK Z80 I had forgotten about... and autotune is WORKING.

The original Z80 must have a bad register on the substrate.  That is a
failure mode that none of my bench equipment could had picked up.

Had it running C7 for three hours and can't get autotune to fail now.

Lesson learned: just because the MM reports "6 TUNED" does NOT mean
that autotune works.


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