Symptoms
The PCB simply displayed a solid blue screen with no activity of any kind.
Troubleshooting
The ROMs have previously been verified so these were ignored. Activating TEST had no effect. Clock was fine, however upon checking pin 40 (RESET) on the 6502 CPU I could see that the watchdog was barking (flipping Hi/Lo constantly).
The CPU socket seemed to be in very poor condition, so this was swapped out for a fresh turned pin variant. No change.
Resolution
After some poking around with a logic probe, I noticed that the 3rd Color Bit Selector circuit was not firing correctly.
After some checking, I noticed that the Triple 3-Input and Gate logic at N3 (74LS11) had all three outputs floating.
This was swapped out for a NOS LS11 and the PCB came back to life.
Equites2014-02-05 13:34:07
The PCB simply displayed a solid blue screen with no activity of any kind.
Troubleshooting
The ROMs have previously been verified so these were ignored. Activating TEST had no effect. Clock was fine, however upon checking pin 40 (RESET) on the 6502 CPU I could see that the watchdog was barking (flipping Hi/Lo constantly).
The CPU socket seemed to be in very poor condition, so this was swapped out for a fresh turned pin variant. No change.
Resolution
After some poking around with a logic probe, I noticed that the 3rd Color Bit Selector circuit was not firing correctly.
After some checking, I noticed that the Triple 3-Input and Gate logic at N3 (74LS11) had all three outputs floating.
This was swapped out for a NOS LS11 and the PCB came back to life.
