Poly 61 is "almost" working now

Dead keys
Normally you'd expect an old synthesizer to have problems, for example scratchy pots, dead voices, leaking batteries and dead keys.
Well, the Poly 61 was completely silent when I got it. It lit up and I could edit the presets and store them and everything, but there was just no sound anywhere. Also, I saw the leaking battery.
I was expecting a lot of hopeless searching for errors but I was pleasantly surprised...

I decided to power the thing up again and see if there really was no sound in it, and sure enough, I could hear a note being played from ONE of the keys on the entire keyboard. Wow, 1 out of 61 is quite bad. Anyway, this sort of calmed me down since I was afraid I was dealing with some sort of dead assigner for the voices or something major, but now I was hoping that all the problems were due to the keyboard rubber contacts needing some attention.
I removed the keyboard, disassembled it completely, gave the keys a bath, cleaned the rubber contacts a bit with rubbing alcohol and then rubbed them some more with cotton q-tips or whatever they're called (the things you can clean your ears with). I connected the keyboard before putting all the keys back just to test the rubber contacts and I was actually able to trigger a note from every key! I could also see that the every voice was working because there's a small LED for each voice that lights up when it's in use. Lovely!
So, I put the keys back and attached the whole keybed to the synthesizer and tried it. For some reason almost the entire top octave was not working and a couple of other keys worked only sometimes. Anyway, I now know that the problem was all keyboard related and not electronic.
I will let the keyboard stay like this for now as I would otherwise have to take the whole thing apart again and clean the rubber contacts more.
I DID measure the resistance when the contacts were pressed and I am pretty sure that every contact gave less than 500 ohms resistance. I think that should be enough but I could be wrong.

What about the battery?
Yeah, well. At the same time as I had the unit open I removed the CPU board with the leaking battery on it. I was wondering if I should cut the battery off first and then desolder it but I decided just to desolder it straight away and had no problem at all.
I then used rubbing alcohol, some pieces of cloth from an old pillow cover and a small paintbrush with stiff bristles to try to clean up around the battery area.
I still don't know what to replace the battery with: A new 3.6V NiMH battery or a small 3V Lithium cell that needs a diode as well to prevent charging.
I think I'll get a small NiMH battery pack as it doesn't require any diode and it's the same voltage. However, to make installation easier next time I soldered to wires to where the battery had been so I can install a new battery without having to remove the board next time.

So, this is what it looks like at the moment:

Camera flashes have a tendency to reveal things. I remove some of that dust on the left side of the board after seeing this photo...

I saw some corrosion on some places but since everything was still working I decided to just scrape off the worst visible crumbly stuff and clean as well as I could with rubbing alcohol.

Headphone volume and broken line out connector
I had noticed that the volume in the headphones jumped up and down and sounded bad when I moved some wires around. Bad connection somewhere? So, the last thing I did was to check the connector board. One solder joint was broken on the line-out socket so I fixed that. Also, I noticed that one of the pins on a connetor apparently was a bit loose since I could wiggle it in and out of the connetor a bit, causing the volume in the headphones to drop considerably.
It didn't seem to affect the line out volume though so I just left it. At least I now know what the problem is and I guess I can replace the connector or just attach the wires permanently to the board instead since the other end of the cable has a connector too so I can still separate them.

Oh, there are still problems with the buttons on the panel. I mean some barely respond, some "double click" and some barely respond AND double click. I'm not going to bother with those at the moment because of what I just realized. See below...

What? I knew it had no MIDI, but no CV/Gate either?
OK, I feel really stupid. When I bought this thing I got it for about 100 Euro, convinced by the seller that the problem was something simple. I knew it didn't have MIDI but I just ASSUMED that the Polysix had a CV/Gate interface and that the Poly 61 would have one too. Turns out neither of them have CV/Gate :)
Well... While repairing it I noticed that the ONLY real interface was a trigger input for the arpeggiator. Duh? How useful is that? Not that I have ANY way to currently control a synthesizer using CV/Gate but hey, I could always get a converter or something.
I bet a MIDI retrofit kit for this thing would cost more than twice as much as I paid for it. The Poly 61 is not really good enough to motivate investing in a MIDI retrofit kit. I guess they're difficult to find anyway since the Poly 61M also exists (M for MIDI) so why bother?
I know Korg sold them for the vanilla Poly 61 after they released the 61M but that was long ago :)

But, well... I guess I could always try to make my own MIDI interface for it... In another century or so when I'm done with my other projects :)

Anyway, I decided that I've had enough of this synthesizer for now. It sounds quite nice and a bit fatter than I expected it to, but the absence of any useful interface made me lose interest in it so I'm not going to bother fixing the small annoyances that remain. I know at least that the synthesizer part of it is working and I guess that's the most important thing...

If you have a cheap MIDI-retrofit kit or schematics for one, please let me know! :)