YAMAHA DX7 - Too much harmonics and artifacts in most of presets and there are a few keys that plays with low velocity. Is there any way to fix that problems? My unit has Jahn spx 1.6 installed. I would be grateful for aby help.
Hey, welcome to the forum!
I’d check out the power supply with a voltmeter. Have a look at the power supply circuit in the schematic. It outputs +15 V, -15 V and +5 V. You’ll want to verify those are all there and within about 10% of the expected value. Bonus points if you can check them for ripple with an oscilloscope. Also look closely at the capacitors and make sure none are bulging or have a shrunken plastic wrapper (evidence of prolonged heat stress).
I’ve worked on a handful of synths from this era that had high frequency artifacts that went away after getting the power supply back to as good as new.
The key problems are probably from the contact strips or key contact boards getting a bit dirty over the years. We have a blog post going into detail about that.