I'me close to pulling my hair out on this Sys9 board and am hoping you could lend some assistance to me:
Problem: Board on bench, booting to LED 7.
Probable Issue: Pin 4 on 6808 (IRQ) measures a steady 4.7 VDC.
Things I've attempted:
1. Replaced both 6808s with new / tested CPUs
2. Reseated all socketed ICs. Replaced some sockets (ROM at U20, RAM, 6810 RAM as someone did a crappy job before me)
3. Burned new ROMs, tested working in another board.
4. Replaced RAM with tested working RAM.
5. Did some testing of the logic gates. Replaced U39 which had a short at pin 4 or 5.
6. Dropped in Leon's Test ROM. Program boots. RAM checks successful. IRQ still stuck at 4.7 VDC.
7. I've read all other threads based around Sys9 LED 7 error boards. Haven't found anything too interesting.
8. Pin 39 (Crystal), Pin 5 (VMA) measurig between 2-3 VDC on DMM.
Questions: What components in the IRQ am I missing? I've never had to fix that circuit before and was wondering if there is something very stupid I'm not checking?