Hi ral-clan,
Wonder if you'd be kind enough to expand on your what you'd said above?
If we look at the photo here:
http://amiga.resource.cx/photos/a520,4
Which bits are we talking about? Within the metal square on the right hand side, the white thing in the middle looks like it has a screwdriver type hole in it. Or is it the two things either side of that?
There are actually four potentiometers in the metal box as shown in that picture. The white plastic thing, the two on either side, and one more roundish one to the south-east of the right potentiometer. They all have slots in the top that you can adjust with a fine screwdriver. You can do this while the Amiga is turned on, but you would normally use a plastic screwdriver. I have done this myself with a metal screwdriver wrapped in electrical tape around the handle. I don't think there are any voltages in there that are dangerous, just the chance of slipping with the metal screwdriver and shorting something on the board. Perhaps you can make yourself a sort of slot type screwdriver from an old plastic knife or chopstick or something if you don't want to risk using a metal screwdriver.
I'm not sure which "pots" do what function, but on other computers and video games they normally adjust the TV channel fine tuning for the RF output, the colour saturation, perhaps luminance and some other related stuff. You would mark their original positions with a fine felt tip marker, and then with the Amiga on and the A520 opened up but plugged into it, you would make gradual adjustments to the potentiometers to see what happens to the picture.
You can also spray contact cleaner into them if you are able, and then wiggle them back and forth with the screwdriver to clean out any oxidization. You'll need something like TV-tuner contact cleaner or other electrical contact cleaner spray that uses one of those fine red straws to focus the spray. This is actually something you could try first with the Amiga off to see if it fixes the problem.
Again, I don't know exactly what these particular potentiometers do, as I haven't seen a service manual, so take my advice with a grain of salt, but they are there to be adjusted by a service person or in the factory.
