Jan 092017

Received this Wild West C.O.W. Boys of Moo Mesa PCB for repair:

PCB was very dirty with flux residual and rust so I first washed it.After dried I powered it up and was greeted by this screen:

The initial self-test reported a bad ROM @T5 and a bad RAM @J4, obviously the board didn’t boot up but kept resetting.Usually Konami testing routines are very reliable so I went to dump the four program ROMs and actually the one @T5 showed at offset 0x3FFB4 a difference  of only one byte (well, one nibble..) compared to the MAME dump:

I reprogrammed the EPROM (a 27C020) and error was cleared but obviously the one regarding the RAM @J4 was still present:

RAM @J4 was one of the three RGB 2K x 8-bit palette RAMs :

Piggybacking it had not effect so, before removing the chip in order to test it out-of-circuit, I checked its connections with a multimeter.All was fine until  I could measure a short between pin 21 (/WE) of the RAM and VCC :

This explained why the RAM was reported as bad as it was never be written (write enable signal is active low).The /WE signals of the other two RAMs are generated by the near ‘054338’ custom :

so I went to inspect it with a digital microscope and found two bridged pins:

I removed the bridge and board successfully booted but sound was totally missing:

This is a common issue on these Konami board that use the ‘infamous’ hybrid audio module.This board has the ‘054986A’ one :

Usually you can fix this issue by replacing the capacitors but I opted for a drastic solution removing the module and installing two rounded machine-tooled strips with 1.778mm of pitch:

Lastly I used one of my ready module:

Job done.

 Posted by at 10:27 pm

  2 Responses to “Wild West C.O.W. Boys of Moo Mesa repair log #1”

  1. Great repair as always, also I was just wondering if you know if bad caps on the 054986A module can cause the ‘Sound System Bad’ error at start up?

    It’s a bucky board in my case but with the same 054986A module, I’m going to recap it anyway but I have a feeling that isn’t the cause of the problem.

  2. I can exlcude that capacitors can cause the “SOUND SYSTEM BAD” error, this is related to digital sound circuit while capacitors are used in analog one.
    So the error is due a problem in the digital sound circuit, I suggest you to check all the involved ICs : YM2151, RAM, ROM and obviously the ‘054986A’ hybrid module as well check traces.

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