Sunday, 8 October 2017

Spy Hunter pcb repair

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.

This playtronic Spy Hunter pcb would either start with no background graphics or run fine for an hour or two and then the background would vanish.

Intermittent 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.

The midway 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 my head.

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).

The 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.

Checking the resistor and 5v was going in one side but nothing coming out the other, I changed the resistor and sure enough 5v back at pin 1. The resistor was obviously going open but not just from heat as from cold it was often open too. The resistor has no signs of damage or corrosion.

Best of all out of circuit the multi meter says that the resistor is fine and exactly 1K.

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