Vic 20 video issues. Help troubleshooting

[gmc]

Just got myself a Vic 20 off eBay and thought I'd try my hand at restoring it.

Powered up and can see horizontal lines on the screen almost like there is a sync issue.
I used the same screen/cable with a Commodore 64 and it works fine (Built a AV to SCART cable)

eBay seller also said there were video issues.


Tried:

Code: Select all

POKE 36878,15:POKE 36876,150

And I can hear the sound so CPU is all good. Looks like just a video issue.

From looking at the pic can anyone point me in the right direction before I pull out my scope and start probing around.

I've reseated all the chips I can.



Any pointers would be appreciated.

[eslapion]

Just got myself a Vic 20 off eBay and thought I'd try my hand at restoring it.

What type ?

From looking at the pic can anyone point me in the right direction before I pull out my scope and start probing around.

It looks to me the video amplitude is wrong.

If your scope has video trigger, adjust the amplitude when displaying a white background so the delta from the lowest part of the horizontal sync to the peak of the white signal is exactly 1v p-p.

See:
https://ez.analog.com/cfs-file/__key/co ... e0822c.jpg

[norm8332]

Unfortunately the prime suspect in these cases is the VIC chip. They are hard to come by and expensive. Not saying for sure it is, but it's a good possibility. The fact that you can hear sound is a good sign since the VIC chip also does the sound.

[gmc]

Thanks, I'll try that with the scope.

It's a Pal, REV D. Assy 250403

Failing that looks like I'll have to try my luck with another VIC chip.

[gmc]

Quick update, I bought another VIC chip from aliexpress which has fixed the issue.

[topcat]

I also need a VIC chip, but can't find any sellers on AliExpress, could you provide a link to your source please gmc?

[srowe]

AliExpress search is rather poor, you need to enter "6561-101" to find VIC chips.

Looks like a single unit will cost you £15 at the moment, I bought a number a while ago for a repair and could probably let you have one for less than that. PM me if interested.