Received from Austria this Heavy Barrel PCB, a run and gun arcade game released by Data East in 1987 :
Board was in really good shape and game was fully playable too but lacked of sound :
Using an audio probe revealed no sound came out from analog circuit hence the fault was of digital nature.Audio system is ruled by a 6502 CPU located on top board :
Its pinout :
Checking it with a logic probe revealed that the /NMI line (pin 6) was asserted as well as two address lines were stuck low (A12 and A14)
A non-maskable interrupt triggered is usually due to some hardware failure hence for first I dumped the ROM containing the audio code (located on bottom board), it turned out to be good.Then I focused on the RAM accessed by the 6502, a 8K x 8-bit device (6116 compatible)
As soon as I piggybacked it with a good chip I got some random sounds played.The scope revealed weak signals on some data lines of the RAM (a good signal on the left of below picture for comparison)
I removed the IC :
And sure enough, it failed the out-of-circuit testing :
New chip fitted :
Sound was thus restored and another board fixed.
2 Responses to “Heavy Barrel repair log #2”
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I love your blog and have learned a lot. Can you say what led you to the RAM chip instead of thinking something was wrong with the 6502?
A bad CPU is less likely to encounter.RAMs are the first to be suspected.In this repair CPU was alive while RAM was clearly abnormal in its operation when analyzed with a scope.