Vintage Synth Repair

>>Duncan, 
I tried your suggestion, swapping out the U13 chip with one from another working mm+ but the problem remained. Any other suggestions?<<

U57, which is a 40174 latch on the demux board. it's the only other place where those signals & ONLY those signals are one one chip.

what threw me off its scent in the first place is that I assumed (NEVER assume with an MM!) that the same demux & latch would light the panel indicators. looks like it doesn't. 

there'll be a good reason for that, no doubt, but I haven't time to read the rest of the schemo now..... :-(

let me know how you get on.

d.