Another WonderBoy bootleg board was laying around on my stash, so it’s time to pick it up and check out what’s wrong with it …
The usual visual inspection didn’t reveal anything strange, but once powered up the board seemed to suffer several graphics issues and also the audio part isn’t working as it should.
The sprites were totally missing, and there are some jailbars over some graphics tiles:
The sprite generation circuit on SEGA system 2 is almost handled by a couple of custom ICs and on the bootlegs board they were replaced by the big doughter board in the middle of the PCB
Probing with the scope its connections with the mainboard i found at least one floating input pin and tracing the signal backwards in its path i was lead to a Fujitsu 74LS157 with floating outputs. It’s a common issue for these TTL IC chips, some of them are prone to fail over the ages, they probably loose it’s internal connections between the pins and the die.
Piggybacking it with a good working chip restore the sprite graphics. The piggybacking technique it’s always a good thing if the damaged chip has lost its internal connections, while is totally useless in case of internally shorted pins.
We will take care later about this chip because as you can see, jailbars over the graphic tiles are still there …
The circuit which handles the tiles is close to the EPROMs which stores them, you can find it in this section of the PCB:
Working on this layout of PCBs stack can be quite annoying because you cannot check and diagnose easily the ICs which are laying underneath the doughterboard, but i own another Wonderboy bootleg board, so i tried to swap the small doughterboard hoping it could help me to diagnose the culprit faster … This daughterboard in the bootleg boards replaces three custom Sega ICs in the original boards.
Swapping it resolved the jailbars issue, so i can easily check the TTLs involved, because they should be located in the daughterboard. It ends up with 2 TTL replaced : a 74ls157 with floating outputs and a damaged 74LS299 with internally shorted pins:
I replaced all the three TTL ICs damaged, two in the tiles handling doughterboard and one in the sprite generator circuit
Now the board is playing graphically fine:
The only issue left is in the audio section: sometimes it was totally silent and sometimes it played a some kind of motorboating ramp sound … the culprit can be easily spotted, it’s due to a leaked bad capacitor near the audio power amp:
Once replaced everything was OK in this board! Case Closed 🙂