Received from USA this Wonder Boy PCB for repair, actually the board was a factory conversion on Sega System 1A hardware:

When powered it up I got most of times a steady black screen, rarely an ‘ERROR ROM 4’ message:

I fired up my Fluke 9010A to perform a ROM test.On some devices sometimes a valid signature was returned but more often I got error and a different signature :

There are six program ROMs:

When I dumped them my programmer complained about some bad contact:

It turned out legs of some EPROMs were dirty and oxidized.After polished them the board booted up but  clearly a color was missing (the RED one) and GFX faults were present :

Doing a visual inspection I found a couple of broken 1K bussed resistor networks :

After checked connections I figured out they were used as resistor ladder in the palette circuit.I replaced them, this restored the three RGB colors and showed better the grahics faults:

Backgrounds had wrong colors and some sprites (like main character) had missing parts and vertical lines through.As for first issue I went back to the color circuit where @IC151 lies a TMM2016 (8K x 8-bit) static RAM (whose data bits are routed to some 74LS175 flip-flop and finally converted to analog by the above mentioned resistor ladders)

Probing it revealed a unhealthy signal on an address line (A10 ,  pin 19), here’s a comparison with a good signal on the left:

Sure enough I removed the RAM :

Device failed the out-of-circuit testing:

Now the sprites issue.Relevant data is stored in four 27128 EPROMs installed on a small piggyback board:

Devices were dumped as good.After a quick check with my multimeter I found a missing connection between a data line of a ROM and the header which goes into the sockets :

I restored the connection and this fixed board completely.Another repair accomplished.

Received from Germany some faulty boards to repair.There was this Prehistoric Isle in 1930 PCB, an horizontal  shoot ’em up game developed by SNK and released in 1989.

Game boots up but some foreground graphics had vertical lines through them:

Relevant GFX data are stored in a 2Mbit MASK ROM located on video board:

I dumped the device and it was good.The data of the MASK ROM are read by the near custom silkscreened ‘SNKCG’.A visual inspection in this area reveleaed some corrosion on a trace :

Trace was connected to pin 17 of the custom, it  was silent when I put on it my logic probe so either it was a dead output or an input not receiving any signal

I was not able to find where this pin was connected to so the trace was really broken due corrosion.Looking at the other near pins, they were all connected to data lines of the MASK ROM, I found corrispondance for all of them except pin 26 (D6).I run a jumper wire and foreground graphics were restored:

Happy with result I was about to close this repair when, during testing, I noticed that voice samples were missing, I could hear only some crackling noise instead of them:

Looking at sound section on CPU board I quickly found the culprit:

The 640MHz ceramic resonator (which provides clock the uPD7759C speech synthesizer IC) was missing, this a common issue on all boards that use it, it’s very easy that it comes off

I took the spare from a dead Sega System 16B motherboard:

Repair finally completed.