Help with an ailing VIC

[Gabriel Angelos]

I pulled my VIC-20 out of the closet to do some testing. The last time I used it was a 4-5 months ago and all was well. This time however, when I flipped the power switch, nothing comes up on the TV.

After messing with it a bit I discovered powering it on and off a few times and finally on would give me a few seconds of normal screen. Then the screen starts to jitter until it loses horizontal hold. The colors flash to black and white and then the signal is gone. Restarting it will repeat this.

I'm at a loss as it worked fine when packed away. I've tested the power supply and the A/V cable and those are working perfectly.

Can anyone offer advice on how to diagnose / repair the system? I'm afraid I'm not too familiar with the innards of a VIC-20 or how to go about diagnosing. It appears similar to the C64 in basic layout and I recognize some of the chips as predecessors of some in the C64, but beyond that I could use some pointers on where to begin.

[English Invader]

If I had to hazard a guess, I'd say it was the 6560/6561 VIC chip that's playing up. I believe this is the most common fault with the VIC and it definitely fits the symptoms you describe. The only remedy is to replace the chip with another one from a known-working VIC 20.

Don't quote me on this, though. There are plenty of people on this forum who are better qualified than me to give advice.

[channelmaniac]

Swapping the VIC chip is the easiest way to test since it's socketed. A bad crystal or 7402 IC that drives the crystal can cause this too.

[Gabriel Angelos]

Thanks for the replies so far. Looks like the VIC is up first as a suspect. I'm all for trying the socketed chips first

Is the VIC inside the metal shield? My VIC-20 is one of the later cost reduced models. My older C64 has a similar shield over the VIC-II, though it has a flat side that's easy to pry up. This one seems to have clips all around. How does one go about removing it without destroying it? Is there a simple trick to removing it?

[Wilson]

Gabriel Angelos,
The VIC is inside the shield. The shield comes off really easily though. No special tricks or tools needed (assuming it is like my cost reduced model, which indeed seems to be what you are describing). So, yeah just pull it off . I've heard it's a good idea to keep the shield off once you remove it to let more heat out.

[a1bert]

I've tested the power supply and the A/V cable and those are working perfectly.

It could also be the voltage regulation inside the machine, for example dried-up electrolytic condensators.