Vic 20 brightness fluctuating

[loftcat]

Since purchasing my first Vic 20 in February I've been on a bit of a journey of discovery. Since the early days of learning about memory to recapping the machine and cleaning the cassette deck it's now running really fantastic. Well, almost! Whilst using the Vic, particularly with light backgrounds, I notice distinctive brightness fluctuations in the picture, sometimes going slightly brighter than after switching on, then often several seconds later reverting to normal brightness. I'm using a new DIN to RCA cable which otherwise produces a good stable picture. I'm interested to know if anyone else has experienced this? I'm wondering if it might be something on the main board that needs looking at, or perhaps just because I'm using a modern TV?

[JonBrawn]

I'd start by blaming the power supply. Are you using the original power supply from way back or a modern replacement?

[loftcat]

Thanks for replying. I'm using an original power supply, I've not had any obvious problems with it so far, but it does sound similar to one or two problems in threads I've found on this forum. I noticed they power supplies are very expensive to replace on eBay Is there any ways to tell if the power supply is working without getting a replacement one? I suppose I could try monitoring the voltage output?

[JonBrawn]

Thanks for replying. I'm using an original power supply, I've not had any obvious problems with it so far, but it does sound similar to one or two problems in threads I've found on this forum. I noticed they power supplies are very expensive to replace on eBay Is there any ways to tell if the power supply is working without getting a replacement one? I suppose I could try monitoring the voltage output?

You could try measuring the 5V level while the VIC is running and see if it's wobbling up and down as the screen changes brightness. Something else you could try is popping the top off the metal box with the video circuit in it and tweaking the potentiometer with a small screwdriver, just a little and back again (that's the one near the VIC chip, not the one near the crystal), and see if disturbing any oxidation by doing that improves things.

I wouldn't worry too much if it's a 2-prong 9V AC power supply, but if it's one with the 7-pin DIN plug, then I'd be considerably more worried - the old capacitors inside may have degraded over time. There are tales of Commodore 64 power supplies failing and frying the '64; I don't know if the same can happen with the DC VIC power supplies. The VIC DC power supplies I've seen have all had "potting compound" used to seal all of the components into the supply (probably to stop 16-year-old me from sticking my screwdriver into the live terminal), so they're impossible to repair. If you buy something from eBay, this is one occasion where you don't want "original commodore" equipment, you want something newly built.

Traditionally at this point, one starts talking about Ray Carlsen. However, he's based in America, and if you aren't, then that could be problematic. If you look at the power supply pages on his website, one of the products he offers that you could reproduce relatively cheaply is to use a 9V AC wall wart and a 5V DC wall wart and wire the outputs into a 7-pin DIN plug. You'd want wall warts capable of delivering 2A. If you're prepared to make the sacrifice, you can get a 7-pin DIN by cutting the cable off your current power supply and stripping the ends and wiring to that - if you've recapped the motherboard, this is easy by comparison. Heat shrink sleeving will make it look more respectable. Remember to work out which pins are 0V and 5V and which two are 9V AC before you cut the cable.

[loftcat]

You could try measuring the 5V level while the VIC is running and see if it's wobbling up and down as the screen changes brightness. Something else you could try is popping the top off the metal box with the video circuit in it and tweaking the potentiometer with a small screwdriver, just a little and back again (that's the one near the VIC chip, not the one near the crystal), and see if disturbing any oxidation by doing that improves things.

I wouldn't worry too much if it's a 2-prong 9V AC power supply, but if it's one with the 7-pin DIN plug, then I'd be considerably more worried - the old capacitors inside may have degraded over time. There are tales of Commodore 64 power supplies failing and frying the '64; I don't know if the same can happen with the DC VIC power supplies. The VIC DC power supplies I've seen have all had "potting compound" used to seal all of the components into the supply (probably to stop 16-year-old me from sticking my screwdriver into the live terminal), so they're impossible to repair. If you buy something from eBay, this is one occasion where you don't want "original commodore" equipment, you want something newly built.

Interesting , I fear I may own the potentially problematic power supply - it's a 7 pin round DIN socket, although only 4 pins on the plug itself. I'll look into how to measure the voltage and try tweaking the potentiometer and post back how I get on.

[loftcat]

Hey, just wanted to say first thing I tried was tweaking the potentiometer. I did notice a small amount of brown residue on one side of the potentiometer which I thought could be concerning. In any case I careful adjusted it, only about 30 degrees or so. So far so good! It's early days and I'm monitoring it closely, but having run the machine for about 40 minutes and loaded two games the picture is looking steady. Thank you for your help!

[6502dude]

More likely the problem is in video section, rather than power supply related.

If 5vdc supply was significantly unstable to cause video degradation, there would be other issues - such as Vic-20 freezing or crashing.

Try spraying contact cleaner / Deoxit on pot you adjusted in video section, while rotating it fully clockwise & counter clockwise a few times. Do this with power off, reposition pot to old setting and power back on.