This is a cheap and easy arcade pcb repair I did without much knowledge.
I got this Mortal Kombat bootleg pcb with some problems:
- Sound was horrible
- When turning the volume up the pcb resets
- Both BLOCK buttons where constantly pressed down
- Some graphic glitches
So first thing I did was to clean the entire pcb, focusing on the contacts that join together the two PCB´s. This solved the graphical glitch problem! Let’s go for the other issues.
The sound was fine upon starting the machine, but then after a couple of minutes it transformed into a noise, like ‘TV white noise mixed with the Mortal Kombat sound effects’. I’ve replaced all capacitors and this fixed the problem. It fixed the resetting issue too.
I’ve also tried removing the amplifier chip and connect the board straight to an external, better quality amplifier. Sound was greatly improved, but finally I decided to keep the pcb with it’s original sound chip for practical reasons.
The BLOCK buttons problem
These chips on the photo below shows the input buffers for the buttons and joystick. Some testing with a cheap logic probe showed there were a couple of them bad. I’ve tested them out of circuit on my cheap EPROM programmer and indeed, they were faulty. So, after a couple new 74ls254 IC, the board was fully working!