26 Apr 2017
Sega Megadrive 2 (repair)
After a quick test I obtained only a black screen, no sound. The first obvious reason can be a dirty cartridge port or some bent pins on the port.
So I cleaned it carefully and checked the pins with no success. Well, I didn't expect much since both previous owners must have tried this before too.
From my experience Megadrives are rock solid unless you use the wrong AC adapter.
So I inspected the board carefully and found a trace going to the video encoder has already been repaired but nothing more.
It was time to go deeper. I dug out my logic probe and quickly found address line A17 was floating on the main CPU. Normally all address lines are tied to VCC via pull-up resistors so there was 3 possibilities :
- either a trace is cut somewhere
- or the pull-up resistor is bad
- or the CPU is faulty and drags the signal somewhere between Vcc and ground and my logic probe detects it as "floating" (not a valid state)
It was option 1. I was about to patch it with a piece of kynar wire when I saw a very small but clean cut in the concerned trace near the cartridge port. Some solder was enough to fix it:
Time for a test and... Bingo!
I really wonder what has caused this cut???