Yamaha EX-5
At first, it looked like an easy job, but once I opened it up, things got much more complicated and definitely very time-consuming. The problems: most of the keys didn’t work (only about a dozen out of 76 produced sound) and none of the MIDI ports worked (the EX-5 has two full MIDI sets, including IN/OUT and one with THRU).
Keyboard issues were caused by several things:
- Oxidized metal contacts under many keys (the old DX-7 style system), which required both mechanical and chemical cleaning (Kontakt Chemie).
- A damaged ribbon cable connecting the IDC sockets on the key contact boards (MK1 and MK2) to the PLS sub-processor board that handles the keyboard.
- Corroded and broken traces on the PLS board, which had to be repaired and reinforced.
MIDI problems:
The EX-5 uses a fairly complex switching circuit for its MIDI ports, based on a few simple 74HC series ICs. One of them must have failed. These chips were in hard-to-find SO-20 200 mil packages. Instead of wasting time probing with an oscilloscope and logic analyzer, I just replaced all three (they’re very cheap) after first checking the entire signal path from the sockets through the optocouplers up to the logic chips. The keyboard repair took the most time, but in the end the synth is now fully working again.
