Hi – I am repairing a friends JP4 and they are having an intermittent section of keys crop up.
8 keys came to me non-responsive (E 3 major to to B 3 major). The Key assigner on the JP4 has 7 sections (“kbus(s) 0-6 ) that read and register key depressions (completed circuits) for 8 keys each. “Kbus 0-5” read 8 keys each, while ‘kbus 6’ reads one key and that’s C4 on the JP4 or that very right most key (for a total of 49 keys).
I cleaned the male and female pin headers for the ‘kbus’ connection between key bed and key assigner, but it seemed to only temporarily fix the problem. The section of keys was working on my bench for two days, however once they took their JP4 synth to their studio, the intermittent response cropped up.
Is it possible that this issue goes beyond connector issue and is tide to a faulty IC chip on the key assigner? The ‘kbus’ traces, once they make it to the key assigner end up routing to the DATA LATCH – QUAD D FLIPFLOP LS175 IC4-IC5 (page 40/68 in the JP4 service notes) on the key assigner board. Those quad flip flops end up running into the µC 8048 micro controller, however I have a feeling the issue is before this stage. As the issue is intermittency rather than complete failure of ‘kbus5’ connection.
I might try to sever the existing male and female pin header connection, strip the wires, recrimp them, and solder in a replacement male and female pin header.
If you think I might be over looking anything, then any suggestions would be helpful!