VIC 20 ASSY 324003 Black Screen

[beamrider]

Going back to the weird signal on pin 4, if you remove the Vic chip what do you see on pin 4 on the scope? And if still present can you not trace it back with the scope to see where it originates from? If it's not present you could try making a socket with certain pins unconnected and see if you can affect it by isolation.

Did you say you have tried the CPU in your working Vic also?

[SmakyTakyOne]

Yes, all the ICs I could have brought into the working VIC were tested there... particularly the VIC, ROM (only one ROM is socketed on the working one), and CPU. However, when I remove the VIC the computer does nothing I think the clock that feeds the CPU comes from the VIC, right?

This is what happens when I turn on the VIC-20 switch.



Now on the measures... without the VIC, as I suspected pretty much all activity stops.. as the CPU is no longer getting the clock provided by the VIC. However these are the scope images from some of the pins that do show some activity (the input clock coming from the oscilator circuit) and the outpit of the pins 1 to 4 of the 6560:

Pin 2:

Pin 3:

Pin 4:

Pin 40:

Pin 39:

Pin 38:

[beamrider]

Did you get any further with this?

[SmakyTakyOne]

Unfortunately, no... I guess I need to continue watching youtube and reading to get more ideas. Something I did found out is that the VIC on this motherboard at some point failed and now is dead. I had to look for some (expensive) replacements to get back to where I was. However, I am a bit weary to continue troubleshooting with the replacement IC in place as the fault in the MB may be very well the one causing the VIC to go dead. So I want to prevent the the replacement IC to go bad.

[beamrider]

Seems like it's causing you a lot of trouble. It's annoying when you can't get to the bottom of things - I have the same situation with a C64.

I mainly watch Adrian's Digital Basement and Noel's retro labs. Both those have a lot of C64 and Vic20 fixes.

Perhaps you just get a Vicky Twenty board and use that...

[SmakyTakyOne]

Do you have posted on the C64 issues... I have fixed quite a few of these.

[beamrider]

Thanks...

No I haven't posted on it.

I need to get another working C64 for IC testing. I have only one other but don't want to mess about with it like yourself.

I'm facing a blank screen issue but it works (did work) with dead-test and radar rat-race. I swapped out the 3 rom chips, PLA and U15 so far, but I have only made the problem worse since swapping out U15 as de-soldering messed up two pads - but now it works only with corrupted graphic characters