Symptom: scrolling playfield okay but alphanumeric garbage in four evenly spaced columns the length of the screen, sprites and text wrapped (top of screen at bottom), not playable. Self test reports RAM error at 3P on power up.
Fix: RAM was replaced (no change). Investigated associated circuitry, found a nasty Fujitsu 74LS153 @ 3F on video board with high impedance outputs.
Symptom: Game crashed during demo mode then would only boot to white or black
screen. In test mode powers up to garbage and looks like it's
constantly resetting.
Fix: Confirmed reset line was pulsing on
T11 main CPU. Disabled watchdog self test now reports 'interrupt
failure'. Checked interrupt circuitry and vblank as suggested by ops
manual. All fine. Eventually traced to the area labelled as 'address
decoders' on schematics and the problem was with 74LS138 @ 3M on CPU pcb
which had all outputs stuck low.
Symptom: random resets.
Fix: Voltage variation between +5v line on CPU and video pcbs (separate power supplies for each). Adjustment kept drifting and was traced to nasty connections in the harness between the multicore wire and the plastic connectors (the header pins on the pcbs had been reflowed and cleaned previously). Moving the harness caused variations in excess or 1volt. From past experience I would say these connectors are getting pretty flaky with age so I soldered the wires into the plug. Not pretty but reliable.
Symptom: missing sounds.
Fix: self test revealed silence from the right pokey. Replaced pokey.
One other thing of note is that the pcb also wouldn't boot to game mode without the 6502 sound processor but it would go into self test and try the 6502 comms test which obviously failed.
And no, I haven't been saving these up, they all occurred on the same game in the same week....
karlcdoe2013-05-29 03:55:33
Fix: RAM was replaced (no change). Investigated associated circuitry, found a nasty Fujitsu 74LS153 @ 3F on video board with high impedance outputs.
Symptom: Game crashed during demo mode then would only boot to white or black
screen. In test mode powers up to garbage and looks like it's
constantly resetting.
Fix: Confirmed reset line was pulsing on
T11 main CPU. Disabled watchdog self test now reports 'interrupt
failure'. Checked interrupt circuitry and vblank as suggested by ops
manual. All fine. Eventually traced to the area labelled as 'address
decoders' on schematics and the problem was with 74LS138 @ 3M on CPU pcb
which had all outputs stuck low.
Symptom: random resets.
Fix: Voltage variation between +5v line on CPU and video pcbs (separate power supplies for each). Adjustment kept drifting and was traced to nasty connections in the harness between the multicore wire and the plastic connectors (the header pins on the pcbs had been reflowed and cleaned previously). Moving the harness caused variations in excess or 1volt. From past experience I would say these connectors are getting pretty flaky with age so I soldered the wires into the plug. Not pretty but reliable.
Symptom: missing sounds.
Fix: self test revealed silence from the right pokey. Replaced pokey.
One other thing of note is that the pcb also wouldn't boot to game mode without the 6502 sound processor but it would go into self test and try the 6502 comms test which obviously failed.
And no, I haven't been saving these up, they all occurred on the same game in the same week....
karlcdoe2013-05-29 03:55:33
