I was sent a a 288V for repairs. The 80 channel was distorted and channels 120, 140, and 160 were dead.

I was a bit hesitant to work on this module as there are no schematics and there are a lot of mod wires on this board. However this version follows the 288 schematics very closely with the exception of the storage array. I had to trace the signals from the output jacks through the Molex 8 pin connector to PCB2. There is a daughter PCB on the front for the sliders.

Instead of shift registers this design uses 4164 DRAM ICs. The rest of the circuitry on this board follows the original schematics except for the DRAM control in the lower left corner. There are a lot of wires on this board. I felt that the last three channels being dead might be related to a memory failure. I probed the Dout pins and found where they stopped. Rearranging ICs would move the dead channel around. Since there was also a problem with channel 5 and memory is inexpensive I chose to replace it all which restored the last three channels.

Channel 5 was still distorted. Working on sandwiched SMT boards is always slow. The 9 channel D/A's are along the right side of the board but not in order. Channel 5 is the top circuitry. The fault wasn't obvious but there were some loose pads and sloppy soldering so I touched up a lot and added jumper wires to the loose pads. I replaced the P-channel JFET and a cracked ceramic SMT capacitor which restored full operation. It would have been fun to play with this module more but I choose not to tempt fate and returned it to the owner. 

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