Jul 292018
 

Bought a Super Cobra PCB that was meant to be fully working, but of course wasn’t. It was too much hassle to return to the seller, and I wanted the PCB so I decided to fix it.

First issue was corrupt graphics in-game and the board could not run a Super Cobra test rom correctly

Test Rom Video

The test rom was failing at the screen ram test, so I checked the rams on the main board and found that a 2114 @ 3K had failed so replaced that and got all the GFX back

Next was the audio. To help with this I built Phil Murrs audio probe here which was very useful!

I was getting a low pitch, really loud noise, when I powered up the PCB. It is quite horrendous! Adjusting the volume pot just seemed to make things worse

I checked the z80 on the top board, and the ram. I took the checksums on the roms which I need to check later. I popped out the AY chips, and the 8255 chips and the sound was still the same.

Clearly the amp was working because I am getting some noise!

Next I checked the outputs from the AY chips CH A, B & C – pins 3,4,38 with the audio probe and got nothing.

I checked over the z80 CPU which seemed ok.

Turned my attention to the 8255 chips. Popped them both out again and noticed a bent leg on one of them. Straightened the leg and the game sounds now returned when testing with the audio probe. I then tested the board properly on a cab and everything was working fine

Thanks to PhilMurr for his help

Now to do the multi-game board 🙂