Another board from my friend Alexander.It’s time for an original Gun. Smoke PCB:
Board booted fine but had a graphical fault where backgrounds were totally wrong (MAME screenshot on the right) :
In absence of schematics, MAME source is always a great help.Looking at it I was able to identify the ROM cointaining the backgrounds data:
ROM_REGION( 0x8000, “gfx4”, 0 ) /* background tilemaps */
ROM_LOAD( “gs14.11c”, 0x00000, 0x8000, CRC(0af4f7eb) SHA1(24a98fdeedeeaf1035b4af52d5a8dd5e47a5e62d) )
I dumped it and it matched the one in the MAME ROM set.So, problem had to lie in the external circuitry.Tracing back connection from this ROM I figured out the its address lines were connected to outputs of some 74LS283 whose inputs were tied to some 74LS273.So, I started to test them in-circuit using my HP10529A and BK Precision 560A.All was fine until my equipment reported a problem on the 74LS273 @8D:
Piggybacking this cleared the issue:
IC failed when tested out of circuit reporting trouble on PIN2:
Fitted a new IC fixed the board completely.
2 Responses to “Gun Smoke repair log”
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Now I want a BK Precision 560A…
Well, BK560A (or the 560) can help in some cases but , as every in-circuit tester, has its limitations.For example when inputs of DUT are tied to GND or VCC, it can’t not perform correctly the truth table.Then, you have to remove the quartz or oscillator so the board under test must be powered but inactive otherwise the BK560 is misled by signals giving false results.
I shoud have somewhere a BK560 (the dark model) upgraded to latest IC library.