A lot of boards are awaiting on the “to be repaired” stash, today the random pick is one of my favourite arcade system ever … an SNK MVS board! So i moved it from the stash to my bench for a repair. A clean MV2 dual slot board :
The usual visual inspection, reveals the BIOS IC (on the red square) is missing, the battery seems to be in good condition and no damage has been done due to its acid leakage.
The overall condition is good enough to perform a smoke test, so i plugged in the JAMMA harness and powered up my supergun, finger crossed and i’m been greeted with this error:
A video RAM error is a common issue on this hardware, the error address is not zero boundary, so the culprit should be a damaged video ram chip. The self test program is expecting a 5555 hex value (101010101010101 binary pattern) but the lower half memory chip probably reply with wrong and garbage data, as you can see on the READ lower half value (hex FF instead of 55) and the random characters on the screen.
So i’m going to replace it. On my spare parts i found a 32Kb SHARP LH52B256 a little bit faster than the original TOSHIBA ic, 70ns instead of 85ns, but it should do the job in the same way, it’s the one in the yellow box below:
At this time i decided to give the board a try and i powered it up again, no more garbage on the screen, but there are still strange video errors (picture missing sorry) tipically due to a problem with NeoGeo FAST video RAM. The Sony CXK5814 ICs, a 6116 2Kb standard compatible static RAM, over the ages are very prone to failure, so i replaced both and fitting the proper ic socket, you can see them in the light blue box below:
I replaced the FAST video RAM chips and powered up the board again … yeah board fixed!
Now it’s time to do some deeper testings … 🙂
The board runs fine, the I/O controls, the video and audio sections now are fully working. After the “testings”, i made a minor fix to prevent further damage due to battery leakage, so i removed the original Ni-Mh battery and the 470ohm resistor to deactivate the charging circuit:
… and fitted a CR2032 battery socket on the rear side for easy swapping and maintenance, once the board will be closed with its metallic case.
Board fixed. Another case closed! 🙂