Korg Poly 61 - Some Keys Produce Out of Tune Notes.

[scottie_uk]

Preamble and Introduction:
So, about a month ago I was at a flea market near and when I spotted a Korg Poly 61 on the grass by a seller. I was kicking myself for not getting there earlier. I asked the man "How much did you sell it for?". "Sell it?" he replied, nah it's f***ed I got it in a job lot I'm going to cut the plug of it and thow it in the dumpster.

It did have one broken key, a dent beside the appegiator and a dent in the particle board underneath. My first thought was to part it out as many old synths contain unobtainium and that this would be a sorry waste. I plugged it in and it powered up. It even made noises, but one of the six voices was shot making random sounds. Anyhow I asked if I could have it and ended up paying $5.

So, I took it home and opened it up. Turns out that shot voice was the dent in the particle board causing the foil grounding sheet to touch the PCB. I fixed that and while at it replaced the broken key and the battery. The battery had corroded, but careful inspection of the PCB showed little if any significant corrosion on the board (a bit of the green PCB coating by the battery, not on a trace, but nothing else other than moisture from the battery which I cleaned.

Problem:
So the keyboard was put back together and it sounded ok. However, some keys are out of tune. I know it's not the voice board, as I watch that cycle and it works fine (i.e. each voice makes the same sounds as the others).The pattern of out of tune keys is like this

Image here of pattern:
http://i.imgur.com/vjejbkM.png

That is, if you count the notes in groups of 16, then 9 and 10 are out of tune. However, sometimes this pattern shifts when powered down and then on again. I have shown is the most common combination which appears 95% of the time. Despite occasionally shifting, it always with the same spacing. If you hold these notes down after about 2-3 second they do a glitchy portamento to the right pitch. Then if you press them down again the pitch is correct, that is until you press one of the non-glitchy notes, the faulty keys then go out again.

I know it is not the keyboard because pressing A2 (an ok note) with the appreciator in 2 octave mode will fire A1 (a wonky note) gitches and all.

Work so far:

• Replaced the battery on the CPU board,
• Replaced C8 C6 C1, C25, C37, C52 (electrolytic capacitors that I had matches for) on the CPU board
• Replaced IC44 (an operational amplifier 4558DV on the lower right that seemed to be running hot and no longer does) on the bottom right of the CPU board.
• Replaced 4023BP (IC27 ) below the battery because two of it's legs had signs of corrosion and put a short between pin 13 and 5 because the board had this but I may have damaged the link when un-soldering the previous IC.

It now sounds more sharper and in focus. However, the problem remains. Does anyone have any further ideas on what it could be. I'd like to know what parts of board KLM 509-A are responsible for tuning perticular keys.

Thanks for reading, I'd love to get this synth going again. I'm almost there.

[scottie_uk]

It's been a few days since I asked this. Is there anything I can do that may make getting help with this more likely?

[scottie_uk]

Further investigation shows that when the octave of DCO1 is changed to 4, 8 or 16, the position of the two out of tune notes moves. I've listened carefully and hear that both DC01 and DC02 go out of tune.

[scottie_uk]

Is there anyone that can help here?

[crafter57]

Hi, are you still battling with this demon or did you find the problem?

[scottie_uk]

No, I am still battling with this error. Or well not battling now, but got fed up and left the synth in my basement gathering dust. If anyone does know of a probable cause I would be happy to hear them out.

[synthchaser]

Sounds like a stuck data bit or bits going to the DAC (which in this case is just fixed resistors). I'd check the buffers (inverters) on the data bus ahead of it with a logic probe or scope. IC18 & IC21.