(Topic ID: 285977)

Black Knight player 3 and 4 commas stuck on

By solarvalue

10 months ago

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  • 12 posts
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  • Latest reply 10 months ago by Schwaggs
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#2 10 months ago

The caps on the MPU board are filter caps and rarely fail. To test if the problem is on the MPU or Master Display board, unplug J8 from the MPU board and see if the commas turn off. If they do, I would suspect the MPU. If they don't, I would suspect the Master Display board.

#4 10 months ago

That points to IC36 6821 PIA being the culprit.

Is it and another PIA on the MPU or Driver board socketed by chance? If so, you could swap them around to see if the problem follows the chip.

#6 10 months ago
Quoted from solarvalue:

Not socketed unfortunately, but I will put a socket in if I need to replace it. Is there a way to test the chip with a logic probe?

You can test pins 16 (comma 3-4) and 17 (comma 1-2) on IC36 to see if they are HI when the commas should be on and LOW when the commas should be off.

Pin 16 is probably stuck HI

#8 10 months ago

You can connect the power connections to the test points on the MPU board (see the circles on the picture I attached). Set the switch on the probe (if it has one) to TTL and touch the probe to the leg of the pin you want to test. Try not to bridge 2 pins together.

0D7E2612-5949-4754-AED6-ABDCE2FD1AC3 (resized).jpeg
#10 10 months ago
Quoted from solarvalue:

Thanks, 16 is stuck HI, 17 is pulsing HI and LO as it should in attract mode.

Looks like you found the smoking gun!

#12 10 months ago

Awesome! Congrats on the fix!

You are welcome, I’m happy to help!

And in this case, the problem could easily have been one of the AND gates or the 7180 on the master display board. As you said, nice to know for sure before replacing perfectly good parts. It will be especially important as these parts are used up.

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