Thundercade repair log

Got for repair an original Thundercade PCB, also known as Twin Formation and 特殊部隊UAG, a vertical shooter developed by SETA and released in 1987.Board was in very good condition :

When I powered it up I got a black screen as the power supply of my test rig had gone into protection so I turned off the power.Using a multimeter I found there was a dead short across GROUND and +12V, the measurement was taken with the probes on the protection zener diode just near the egde connector :

I also noticed there was some burn marks over the sound amplifier heatsink, this was a clear sign of some heavy event (overcurrent or overvoltage on the +12V line)

Anyway, after I removed the zener diode, the short was cleared so I could power the board up again.Game was playable with sound too but sprites were glitched :

Sprites data are stored in eight 2Mbit MASK ROMs :

I dumped all of them (as 27C400 and then I splitted the files in two halves).Doing a MAME ROMs compare all the files were matching the existing set except the one marked 'UA0-10 W51'.To have  confirm I replaced the MAME dump with mine, I launched the emulator and could reproduce exactly the same issue :

 

Since there are no direct replacement of 40 pin 2Mbit MASK ROM I programmed a 27C400 EPROM doubling the MAME ROM file into the progrmmer buffer :

This fixed the issue and board completely.Repair accomplished.