This is an issue I’ve seen only twice so far. The first time I encountered it, I was stumped. I worked on the board a few times and ended up tearing a pad on one of the big 4 chips – so the board wasn’t fixable anymore. But just this week, I found another board with the same issue. The only colors the console outputs are red and green, no blue.
It actually looks like what is supposed to be blue is red. I was excited to have another go at this puzzle.
I tried a long series of things that did not fix the issue:
- I swapped PPU2 twice. Since PPU2 produces the RGB signals, I figured this was the most likely cause – no blue output! I checked the voltage on pin 97 later on and it did have a healthy output around 3.5V. This is where I wish I had an oscilloscope 😅
- I swapped PPU1
- I swapped the CPU – worth a shot after neither PPU was the cause
- I swapped the video encoder
- I replaced all the SMD capacitors
- I replaced the voltage regulator
After no change still after those swaps and replacements, I removed Q3 and Q5, two small transistors on the back of the board that are the first components for red and green signals (respectively) after those signals are sent from PPU2.
I figured this would isolate the blue channel – so the only color making it to the encoder chip would be blue. But, weirdly, the screen was still only red
WHAT.
The red-only video was the same over both composite video and RF – which meant that the encoder was somehow the issue.
After staring at the schematics, I decided to try replacing C84, a small capacitor that helps stabilize power to the video encoder chip. Bingo. I got an all blue picture.
I put Q3 and Q5 back on the board, and everything was back! Fixed!
Definitely the most obscure fix I’ve encountered so far.
Leave a comment