I wasn't planning to log this one but as the fault turned out to be something I haven't come across before I thought it was worth mentioning. That's also why the screen shots are from mame.
playtronic Spy Hunter pcb would either start with no background
graphics or run fine for an hour or two and then the background would
faults are always time consuming and a lot of guess work, it looked
like a failing chip that was failing as it heated up but often from cold
there was no background and spraying chips in the background circuit
with freeze spray made no difference.
schematics came in very handy, the playtronic board doesn't have
screened chip locations but the schematics include a picture of the pcb
with locations. After ruling out all the single wipe sockets for the ram
(2b) and eproms (3a - 6a) used in the background I was left scratching
I had previously replaced all the ribbon cables and also reflowed the connectors with no change to the fault.
I took a short break and when
I came back to the board the background had returned, this makes
finding the fault impossible so I had to leave it on for around an hour
for it to vanish again.
Once the background vanished I checked all the outputs of the TTL connected to eproms (3a - 6a).
outputs on the 74LS174 @ 13e were all low, the truth table for the 174
and schematics confirmed that pin 1 should be high as it's connected to
+5v through a 1K resistor but it was reading low.
Best of all out of circuit the multi meter says that the resistor is fine and exactly 1K.