Hello all,
I finally pulled out my Concertmate that has been sitting in storage since I used it in a band back in the early 80’s. I replaced all the sliders and switches and everything works good except 10 keys don’t work when in KEYED mode. When I trace the keys back to the board they go to three different MM5823s. Could all three MM5823s really be bad or is this something else? The keys in order from the low end of the keyboard are:
G#, A, B, D, D#, E, F, G#, A, B
Any help would be appreciated.
Thanks,
Harry
I would check the IC sockets, if there are any. They may be corroded, and if so, it is best to replace them. I would use NTE replacements. Clean the pins of the chips, as well. Use 90% isopropyl alcohol and a Q tip. Did you remove the foam of death? 
Thanks a bunch for your input. The black foam/goop was a fun cleanup
There was only one IC in a socket and it is not on the keyboard board. It checked out fine and was cleaned and reseated. Got all the foam of death out and cleaned everything. I did some more tests and it doesn’t matter what mode the keyboard is in or what tone source is used, the keys mentioned in my previous post don’t trigger anything. Also when using the polyphonic tone source only, an additional two keys just make an electronic tick when pressed. They are the first two keys from the low end of the keyboard.
Any other suggestions anyone?
Harry