MATRIXSYNTH: Sequential Circuits Prophet 600 SN 5371

Tuesday, March 23, 2021

Sequential Circuits Prophet 600 SN 5371

via this auction

"So yes, it seems to work great with the original Z80 CPU, but it behaves a little strangely with the mod installed. Basically, with GliGli, oscillator pitch seems slightly erratic and janky. Sounds a little like random/noise modulation. Sometimes you can hear it on the first note you play, sometimes you can't hear it until you have played all six notes. Even weirder, this issue also goes away a bit when pitch bend is not perfectly centered. I think a multiplexer IC on the front panel PCB might be causing these problems, but since It doesn't have this issue when it's stock, I'm not so sure. I haven't messed with it. I'm including a couple replacements just in case.

I bought my GliGli mod from a pro builder on Reverb, so I don't think it was assembled/soldered together wrong. I'm pretty sure I installed it correctly as well! I'm stumped.

This synth also had a slight whine in the signal path when I first got it, but after I installed GliGli it went away and never came back, even after I reinstalled the original CPU this evening. Even when it was present, it wasn't too noticeable. (Hopefully it won't return!)

Cosmetics and other info:
This Prophet, serial number 5371, was built some time in mid-1984 and is in really beautiful cosmetic condition. A couple signs of use here and there but nothing very noticeable. Some of the lower black keys are a little dull-looking compared to the rest. Two front rubber feet were missing so I replaced them with adhesive ones. A GliGli "cheat sheet" from Synthgraphics is also now on the front panel.

All keys work!! Battery seems to be holding patches fine as well.
If you haven't had a Prophet 600 before, be aware that (when it's stock) it takes several seconds to boot up and tune each voice. This is normal!"

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