Korg Polysix additional storage board? And resonance issue

[acobberson]

Recently I picked up an old Polysix. The previous owner was very open and honest about the condition of the synth and it's issues.
The synth has had the old battery removed, PCB "repairs" and traces "fixed." A Panasonic lithium battery was inserted, however the synth will not save patches.

I knew I'd need to give the synth a bit of TLC to get it working well, but when I popped the hood I noticed an additional circuit board, probably a DIY job, had been added. At first glance it looks like possibly an additional storage circuit, but I can't be sure.
Anybody know what this is?

My second issue is with the resonance control. It does not effect any of the voices. No matter where the controls are set it makes no difference to the sound. The pot seems to work (I measured the resistance as I did a slow sweep) so I expect there is a problem elsewhere. I've read that ic18 can cause issues with the resonance, but all the other controls that this IC works with work perfectly.
I'm pretty handy with a multimeter, but I unfortunately don't have an oscilloscope. Will I need more than my multimeter to diagnose the problem?
Anyone else have a problem like this and can point me in the right direction?

One thing I found odd was that pins 5, 12 and 13 of IC1 on the 370 board measured continuity between them. Looking at the schematic these should not be connected as they feed pin 2 of the sustain, resonance and effect Int respectively. Am I misunderstanding that?

The 367 board looks to be in pretty good shape, and I've tested all the suspect traces for continuity, and many of the chips on the board habe already been replaced, so it would be a shame to drop 200€ on a replacement if that won't solve the problem.

Here's a pic of the added circuit

[acobberson]

Update:
Something must be up with the CPU board. I loaded factory presets and most of them sound helicopter-ish and after holding the keys long enough EVERY preset eventually morphs into a modulated mess.

On the upside, the Res control miraculously started worrying. Sooooo, there's that. I'm leaning toward replacing the CPU board, but I'm not sure if I want to go the Kiwisix route or just a replacement board and separate MIDI control board.

Any Kiwisix fans want to talk me into it?

[micahjonhughes]

I did the KiwiSix mod, both the power supply and digital board. It is fairly easy to do, fixes all of the issues from a battery leak and adds functionality without changing the character of the synth.

I liked that it was a complete package kit that is reasonably easy to install. I'd recommend it.

[acobberson]

I did the KiwiSix mod, both the power supply and digital board. It is fairly easy to do, fixes all of the issues from a battery leak and adds functionality without changing the character of the synth.

I liked that it was a complete package kit that is reasonably easy to install. I'd recommend it.

Thanks for the response. Do you use the extra features other than MIDI? I like the Arp clock feature, but I'm not sure I'll need most of the other functions. Also, living in Germany means I'll have to pay import tax on it which bumps the price up considerably.

Back to my original post, does anyone know what the additional circuit is for?

[acobberson]

Any ideas on what that extra circuit is for?

[acobberson]

So for anyone who may care, I was able to track someone down who was kind enough to offer insight into the mysterious board. Turns out it is "most likely" a polarity circuit allowing the P6 to be used with more modern triggering devices.

I received my replacement CPU board and will swap that out this week. After that I may give the calibration procedure a go, otherwise it'll be sent off to a friendly Chap in Berlin.