Thank you Mr. thesteve. I messed around with it a little more this morning before I saw your reply. I soldered in a spare card slot from a TG (I don't have any spare Duo slots laying around) just to see if there was a change if I try and boot a card. The only change is intermittently (about every 5th boot) I do get a purple or blue screen instead of white.
I also took the liberty of checking readings from HuC6270 and HuC6280 with the logic probe (with no card in).
HuC6270:
- High 4, 5, 6, 7, 8, 9, 10, 12, 15, 16, 17, 18, 19, 21, 22, 24, 28, 32, 39, 40, 47, 70, 72, 77, 78
- Low 11, 27, 29, 30, 31, 33, 34, 35, 36, 37, 38, 55, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 73, 74, 75, 76
- Pulse 1, 2, 3, 13, 14, 20, 23, 25, 26, 79, 80
- Nothing 41, 42, 43, 44, 45, 46, 48, 49, 50, 51, 52, 53, 54, 56, 57, 58
HuC6280:
- High 1, 2, 8, 11, 12, 14, 15, 19, 22, 24, 25, 26, 27, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 47, 57, 59, 60, 76
- Low 7, 16, 21, 30, 48, 58, 75
- Pulse 3, 4, 5, 6, 13, 46, 49, 50, 51, 52, 53, 54, 55, 56, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 77, 78, 79, 80
- Nothing 9, 10, 17, 18, 20, 23