VIC20 refuses to load carts

[rgrocha]

Hi,

I'm in a repairing spree this summer (this is my 3rd this month!) and in a dead end:

This one starts ok, but refuses to load from any cartridge. I've tried with a game one, previously tested on a fully working computer, with no luck.
I've tested with the diagnostic cart + 3K (actually a superexpander cart with the ROM replaced with a EPROM). The RAM gets recognized but again, it boots the the prompt screen.

I've checked everything in the montherboard: all chips works (MOS & ROMs tested in another computer and logic & mem in a logic testes, all ok).
I've also checked main chips (CPU & VIC) traces to other chips, all ok.
I've checked cart slot traces to neares chip, all ok.
I've even checked connectivity between an inserted cart traces to the motherboard slot pins. All ok.

Any idea? Should I tripple check anything in special?

Thank you

[eslapion]

@rgrocha
There isn't a million possibilities here.

I assume you used a simple game which uses only BLK5.

Either the expansion port connector is damaged or dirty or the cart itself has dirt on it's edge connector (that's pretty much eliminated since you tried more than one cart).

If the RAM of a 3K expansion get recognized properly then it means all address lines and data lines are OK, the R/W line is OK and so are the 3 RAM lines (pins 14-16).

The only lines that are used by a game cart but not by the 3K expansion are CA10-CA12 (pins N, P, R) and the BLK5 select (pin 13). AFAIK, the problem can only be with one of those 4 lines or with the 74LS138 responsible for decoding the accessed BLK area.

If you've used a game cart of more than 8k then the problem could also be with the BLK1 or BLK3 select lines (pin 10 & 12).

[rgrocha]

I've re-checked everything twice and three times without lucck

I'll postpone this repair until I get in mood again

[eslapion]

Do you have a memory expansion of 8k or more ?