Greyscale Video Output

[jibberjabber]

Hey All,

After 28 or so years I finally got my first VIC-20 last week. My best friend at school had one, but I only had a ZX Spectrum!

Anyway, it's been working great. Playing Matrix over and over in the office all week. Today I brought it home and was unable to get a colour signal when tuning it into my both my HD TV and a small CRT portable. Thinking it was the TV modulator, I dug out the soldering iron and made a composite video lead but got the same results - a greyscale image.

Also, when turning the machine on - even with the composite cable - it can take a few (up to 5 from cold) seconds for the picture to become stable and appear. Although from then on it's rock solid.

Does anyone have an suggestions as to what the problem may be? I suspect from reading troubleshooting guides that it's an issue with the VIC chip.

It's a PAL machine, made in W. Germany with a paper rainbow sticker and a square two-prong power socket. I know nothing of this machine's history.

Thanks,

Jonny

[matsondawson]

Open the cage the vic chip lives in and give the red variable resistor a twist, see if that fixes it. It's the red circle thing up the top right in the image.

Edit: Actually I think it may be a variable capacitor.

[jibberjabber]

Thanks. But I've got something a bit different inside my VIC20. Looks like it's the European ceramic version of the VIC chip for starters, and the other components are different. My guess is that the variable capacitor with the blob of yellow on it is the equivalent, but I rather know for sure before twiddling away! Any thoughts?

Edit: Ah - there are two components with blobs of yellow! I meant the right most one. Looks like it's labeled C35.



Thanks again.

[jibberjabber]

Being an impatient soul, I went ahead and twiddled.

The left hand variable resistor (?) changed the brightness. The middle one didn't seem to do anything (sound perhaps?), and the right most one (variable capacitor?) made the screen go blank/black when turned clockwise. Turning anticlockwise made no difference.

I'm still all grey

Edit: Actually - the screen going black was my screwdriver touching the side of the trimmer. Turning the right most one in either direction has no effect. Also - the middle trimmer does change the screen in some way. I'm just not sure how. There was a definite brightness flicker when adjust it.

[matsondawson]

Yeh, the right most one was the one I was thinking of.
It can make my screen go grey if it's in the wrong position.
Once you've adjusted it you'll need to turn your computer off and on as it tends to get stuck in grey scale. Possibly you have a faulty variable capacitor, or as you said it could be the VIC chip.

I havn't seen that extra variable resistor before.
Is it a revision e board?
Here's the schematic to compare to...
http://www.mainbyte.com/vic20/e_schematic.pdf

You'll find the colour signal comes out of pin 2 of the VIC if you're keen on trying to track what parts might be not working

[jibberjabber]

Thanks. I tried twiddling again, this time cycling the power between turns and still it's all grey. I guess the last thing to do before considering replacing the capacitor is to take it back to work and try it out on the only TV I've seen it display colour!

I'm not sure about the revision. I can only see one potentiometer in that schematic, but it's been a while since my electronics degree so I may be a bit rusty at reading these things. It does, however, look to be the same as the VIC shown on this page:

http://www.nightfallcrew.com/?p=1403&lang=en

It has the same ASSY number (Edit: 324003), although the code underneath the "COMMODORE INTL" text is different on mine (Edit: TP-S08HB).

Thanks for the help on this. I'll post an update if I manage to resolve.

Cheers,