After consulting the service manual, I quickly realised that the keys in question were connected to three row-input lines of the keyboard scanning circuit. This got my hopes up, as it could simply be a matter of a few corroded or short circuited data lines.
I opened up the synth and within seconds came to the conclusion that it had to be something way more serious. Someone had, quite visibly, tried to fix this synth before. Aside from the fact that most of the screws holding the lid were missing, there were a few modifications, repair jobs and even a custom replacement for one of the CEM5580 sample and hold chips. Not a problem in itself, but surely a sign that this synth was in need of some love and care.
|This is probably a factory fix but still looks funny|
|This could also be a factory fix, but the left resistors look horrible|
|Someone has smudged the print on this SaH chip. Maybe we could do a finger print analysis?|
|One of the CEM5510s has been replaced with a cool looking mod.|
The biggest shock came when looking at the keyboard scanner chip. This chip, a Sequential Circuits 68B01, is a rebranded (or cloned) motorola 6801 microcontroller. Take a look at the pictures below:
Someone has cut deep grooves into the packaging and soldered new pins to the exposed internal connectors! I can only guess why - perhaps they cut the legs to get the chip loose from a PCB instead of trying to desolder it, or maybe one or more of the legs broke and they had to dig into the chip to reattach it. Whatever the reason, the person doing this must either have been well informed and highly competent, extremely brave or just desperate. The 68b01 is a very rare chip, and replacing it with a off-the-shelf 6801 will not work as it contains custom firmware. I guess if I owned this synth and the chip died, I could have tried something similar, after all you have little to lose if it does not work in the first place.
The chip even had a small wire connecting the first and second key switches for one of the rows. Now, the first switch detects when the key leaves the top position and the second when it reaches the bottom. The time between is used to calculate velocity. When they are connected, the effect will be that the synth thinks the key instantly hits the bottom position. This explains why 8 of the keys only played at maximum volume.
I tried resoldering the pins and even removing the wire, but this did not help. The chip is defective.
Luckily it seemed that the only defective parts are the row inputs. The chip still sends keypresses for most of the keys, which means it would be possible to decode the protocol. This lead to the only sane conclusion - I had to create a custom drop-in replacement. Challenge accepted!