Started by doing a deep cleaning with deoxid agent, baking soda and IPA to remove the rust as much as possible. There were some ICs that had pins very rusted, particularly those on the left bottom corner. So I went and decided to replace all the sockets and socketed many ICs to be able to test them. Unfortunately during this process, the two ICs on the bottom left got some of the pins loose because they were heavily rusted.
I ended up with a fairly clean and derusted motherboard:
All socketed ICs were tested on a working VIC 20. Fortunately none of them seem to have failed.
However, after all this work, I am now getting a black screen once the computer is powered on.
I have purchased a diagnostics cart on ebay... not sure if these carts work similarly to the C64 dead test card which will allow the computer to boot with fewer ICs (No ROMs) or other non-essential ICs. However, with the cart the behavior is the same.
There are scope captures from some of the PINs on the CPU and the VIC chips:
CPU:
- 6502 PIN 2:
- 6502 PIN 3:
- 6502 PIN 4:
- 6502 PIN 6:
- 6502 PIN 7:
- 6502 PIN 34:
- 6502 PIN 37:
- 6502 PIN 39:
- 6560 PIN 2 (Chroma):
- 6560 PIN 3 (Luma/Sync):
- 6560 PIN 4 (R/W):
- 6560 PIN 35:
- 6560 PIN 36:
- 6560 PIN 37:
- 6560 PIN 38:
- 6560 PIN 39:
- PIN 1:
5.16 VDC
- PIN 2:
GND
- PIN 4:
- PIN 5:
