From Kenny Balys Sent Sat, Mar 10th 2018, 02:58
From Kenny Balys Sent Sat, Mar 10th 2018, 02:58
Thank you for taking the time to describe this. On 10.03.18 02:50 , Michael E Caloroso wrote: > I studied microprocessors during college and I know how they operate > inside. CPUs can steer data into multiple registers for processing. > Besides data, there are registers for program counters, interrupt > vectors, relative addressing, etc. > > Two things clued me in: 1) it would crash on occasion and wipe out the > patches, 2) when I ran the C7 routine to monitor autotune it would > report rational numbers then as it warmed up they would be all zeros. > So I had an intermittent temperature related malfunction - the worst > to troubleshoot. > > After being lead down a few wrong paths, it dawned on me that there > may be a register on the Z-80 that was going bad. I happened to have > a surplus MOSTEK Z-80 in my spare stash - swapping it out solved the > problem. There was indeed a register on the CPU substrate that was > going bad, apparently one that didn't affect the operation of the rest > of the synth. A very misleading symptom and the one IC I least > expected to be bad. I've done this long enough to NEVER rule anything > out, no matter how remote... > > MOSTEK made Z-80s under license for Zilog before they owned their own > fab labs, so for all practical purposes the MOSTEK Z-80 is an > authentic Zilog part. > > I've pulled completely dead CPUs before, but that's the first one I've > pulled that was fractionally dead. Shortly after that a friend asked > me to look into a malfunctioning guitar preamp and the reason the > front panel buttons didn't work was a missing clock signal which was > generated directly from the 8031 port pin. Yet the rest of the preamp > worked. Sadly, that 40-pin 8031 was not socketed. > > Sent from my iCPUWrench, > MC > > On 3/9/18, Brian Willoughby <xxxxxx@xxxxxxxxxxxx.xxx> wrote: >> Thanks for the report, Michael. Should be great information for the >> archives. >> >> Meanwhile, I had intended to respond to your earlier message about the “bad” >> Z-80 with a hypothesis. I recently repaired a Roland CR-78 where one bit of >> a latch had partially burned out. The whole thing was working except for the >> Bass drum, and I tracked it down to that one bit. My thinking on the Z-80 is >> that it must have been mostly functional if it was able to run the synth >> apart from tuning, but perhaps there is a Port IO pin that is exclusive to >> tuning and which had gone bad. Not that you can repair an individual pin >> without replacing the whole chip, but I was curious. Typically, a CPU is >> either completely dead or totally functional. It’s actually quite rare to >> have some features fail while most are still working (at least not unless >> there is a bug in the firmware - in which case the same problem would appear >> on all units). >> >> Again, thanks for detailing your work and discoveries. >> >> Brian Willoughby >> >> >> On Mar 9, 2018, at 5:32 PM, Michael E Caloroso <xxx.xxxxxxxxxxx@xxxxx.xxx> >> wrote: >>> The last mile to the finish line was the toughest... autotune >>> wouldn't. Even though the display reads "6 TUNED" does NOT mean that >>> it tuned the VCOs! After isolating many of the support ICs and >>> confirming the audio path between the VCOs and autotune circuit the >>> root problem turned out to be.... the Z80. EVEN THOUGH THE REST OF >>> THE SYNTH FUNCTIONED. >>