Korg Poly 61 pitch issue consistently up 5 semitones.

[Midphase]

Working on a Korg Poly 61 (non MIDI version) which has one of the strangest issues I've come across -- up and down the 61 keys, everything appears to be transposed up a perfect 4th. There are no tuning discrepancies across the octaves, but obviously since hitting a C yields an F, this is not a desirable thing.

Rest of the synth appears spot on, working 100%. Voltage out of the power supply also appears to be exactly where it should be, looks good under load on both my multimeter and my scope.

Before some of you chime in with the dreaded exploding battery issue, this has been addressed by a previous tech and there doesn't appear to be any damage worth noting.

I already fiddled with the various tuning knob and trim, and looking at the pitch bend assembly everything looks fine there. The client is telling me that the synth was working perfectly until this issue started about a week ago out of the blue.

Since this is affecting all of the voices across the board, and all of the presets, I suspect some type of IC or other voltage controlling component that affects the entire tuning voltage of the synth globally.

The service manual that Korg made is a mess honestly, and there aren't a whole lot of instructions for dealing with something like this.

Any thoughts? Has anyone run into a similar issue whether on this or other synths that might give me some inkling as to where I should be looking next?

Many thanks in advance!

 

[radardoug]

Have you done a reset?

 

[Midphase]

Have you done a reset?

The Poly 61 doesn't have a reset function unfortunately.

 

[capacitorless]

Is it a perfect 4th, and does the pitch bend seem to work normally (meaning it goes up to max bend the same amount as down)? If not, the pitch bend circuitry might have an undesired offset of approximately 4 5 semitones.

If you pull the battery, that may reset the digital side of things. The usual caveat about either not caring about the internal presets, or making a backup applies.

Service manual in case you didn't have it already: https://www.florian-anwander.de/korg_poly61/Korg_Poly-61_ServiceManual.pdf

 

[Midphase]

Is it a perfect 4th, and does the pitch bend seem to work normally (meaning it goes up to max bend the same amount as down)? If not, the pitch bend circuitry might have an undesired offset of approximately 4 semitones.

If you pull the battery, that may reset the digital side of things. The usual caveat about either not caring about the internal presets, or making a backup applies.

Service manual in case you didn't have it already: https://www.florian-anwander.de/korg_poly61/Korg_Poly-61_ServiceManual.pdf

I tested the pitch bend range and it seems consistent both up and down (about a 5th each way). The Poly 61 has a pitch bend range dial that effectively defeats the effect of the pitch bend when set to 0, and this doesn't have any effect on the keyboard transposition issue.

I'll try a hard reset on the entire synth by removing the battery next, was hoping not to lose all the presets but I might not have any other choice. However I am not terribly optimistic that a reset will fix things.

 

[capacitorless]

One other thought: if the master oscillator is running high, everything should be faster. One easy way to tell would be testing the rate of the LFO (verbiage is in the service manual - it says something like if it is set to 5, the rate is around 5-6 Hz, if I read that correctly). 40% more should be noticeable.

It's possible the pitch bend circuit is offset 40%, but still providing the full range in either direction.

Those schematics are hard on the eyes, haha! Way past my bedtime but please post back what you find. I used to wrench on this era of synths and have a soft spot for them

 

[Voyager10]

The master clock running fast seems the most likely explanation, if it's not a software problem. AFAICT (the circuit is hard to read on a phone screen) the master oscillator is a multivibrator whose supply voltage is varied to do vibrato/pitch bend.

If you have a scope, can you check around the oscillator? I'd expect a clean, symmetrical square wave at a few volts pk-pk. Maybe one of the capacitors is faulty?

The oscillator is Q5 and Q6 in the pic below, if I read it right.