Type: Digital Programmable Algorithm Synthesizer
Serial: 78489
Purchase date: march 2012
Provenance: Kijiji
Purchase price: 50 $
Previous owner: John, Verdun
Status: 100% working
Bought non working. When started-up, displayed some weird chars on the LCD and a 4 or a 8 on the LED display.
The DX7 has 2 CPUs: a 6805 (keyboard and pannel scanning, analog I/O) and a 63B03 (memory management, sound generator management). I quickly noticed that reseting the 6805 by grounding the pin 40 (/RESET) leads the synth to boot up. I measured the time /RESET takes to go high: 40 ms, while the datasheet expects 100 ms. As I have another working DX7, I swapped the mother board; the issue follows the board. Good: everything is working fine, and the problem is located in the initial clear circuit.
I replaced the 10/16 capacitor in the initial clear circuit; no change. I replaced the 2 transistors; no change. Hmmm...
As a side work, I replaced the battery and put a battery holder; cleaner and easier to service. It's then I noticed the second 10/16, close to the battery, was a tantalum; having serviced an ARP Omni full of those, I know how badly time affects them. I measured it; it was open! The problem was no the initial clear circuit switching to fast, but the battery circuit feeding initial clear with +5V too quickly.
I replaced the tantalum by an electrolytic; I know the leaking is different, and I will probably reduce the battery life expectation. But CR2032 are common, cheap, and with the holder, it's really a 2 min service time.