When ever I set the appregiator speed to approximatly 8-9 (MM+ version), the LED stays lit, and shows the right speed, but the speed seems to vary significantly. If I hold for example, a C Major Triad, It will play C-E-G like as if the speed was set to about 4-5. The thing that varies is if the envelope stays in sustain, or goes to release quickly, where there will be no sound until it chooses to play the next note. It only happens in that small section. Everywhere else, it functions normally.
A list of other problems that I am getting fixed soon:
- Settings only go up to a value of 096 inastead of 100
- Some patches get erased randomly for no reason
- Pulse and Saw don’t properly mix on any oscillator, any voice - seperatly, or with Triangle, they work fine. Or if Saw is on one osc and pulse is on the other.
- OSC 2 on voice 6 is PITCH HI
- Appregiator problem
- If a note is held when switching out of Monophonic mode, Low C is automatically played for some reason.
Could all of these problems be linked?
Either a PSU problem (bad electrolytic caps), or a failing Z80 CPU.
I’d also check the Lithium battery, as another possible cause of random memory loss.
FWIW, I had a Prophet 5 last year with all sorts of weird behavior, and it turned out to be a defective Z80.
If it is a Z80, how expensive/difficult would it be to find and replace it with a new one?
Not very. Z80 cpu is easily found and inexpensive, and relatively easy to replace (it’s in a socket for easy replacement)
But then again, it might be something else completely, like power supply problems or many other things… Better have a qualified tech take a look at it. It would be much safer. This synth is notoriously fragile, electronically. You wouldn’t want to end up with a completely non-working synth.
Alright… I’ll hope it’s just a Z80, then. I know MC suggested a bad cap in the S&H circuit on a previous thread of mine, but I don’t know if that would effect the appregiator.
BTW - It’s not effecting the LFO or the Sequencer speeds. Those adjust properly. Although, when playing a sequence, the LED next to the Rate selection knob goes very dim, which it’s never done before.
This could end up being a brutal service trip.
These beasts are expensive to acquire, and maintain. And unfortunately, some replacement parts are growing scarce, like those CEM3340 oscillators. Luckily, if it ends-up being only a flaky Z80, or bad caps, or bad PSU all is not lost since these problems aren’t usually that difficult to fix. But either way, it’s going to cost some money. But that could be considered an investment, as the MM will only go up in value especially if it’s fixed. The actual repairs and parts aren’t usually the most expensive things, it’s the diagnostic procedures and the time it takes to perform them, as well as calibration, that is expensive.
Anyway, good luck to you. And don’t let anyone not qualified tinker with it.
Thanks! 
And yeah, I wouldn’t let anyone not qualified work on it. The most I’ve ever done is tune it (and I ended up getting it pretty well tuned), but that’s about as much as I’d do.
Thanks for the replies.