exxos's Atari, Amiga & retro forum

I thought I'd open a little topic here to record the problems I'm seeing on both the VIC-20 and Atari 600XL, plus anything I do to fix.

Symptoms are:

600XL - shimmering/waving screen. Picture is bright and clear, but a sort of wave runs up it causing shimmering. I have tested with an old linear PSU in case it was the modern SMPSU, but no change. I'm using a VBXL that was fine in my 800XL.

VIC-20 - The colours are very muddy and there is some interference, though the BASIC screen is perfect. I can get some improvement by tweaking the pots in the video circuit, but it's still not great, so might be a marginal 6561. Need to delve in and see why I'm getting interference and muddy colours on games. I think maybe a small basic test program to change screen and border colours might be useful.

Ok, so cleaning up the legs of the VIC 6561 and spraying the socket with contact cleaner made no difference, which is kind of what I thought would happen.


I tried tweaking the 2 pots in the video circuit (The two next to each other and just above the VIC itself) and was able to see a minor improvement, but not enough to warrant saying it's fixed.

I'm not touching the pot next to the crystal as I think that could easily upset the applecart since I don't know what it's for. Interestingly, it has no metal box around the VIC 6561 and associated circuitry like later models, and no evidence it was ever there from what I can see. Probably normal though since it is an early model going by the 2 prong plug for power.

So, I found a place in the Netherlands that has NoS PAL VIC 6561 ICs and have ordered one. There is obvious evidence that there has been some recapping, but I'll pull the mainboard and look underneath to be doubly sure.

Time to get the schematics too.

The circuit board looks really nice and clean.

stephen_usher wrote: 25 Mar 2022 10:32 The circuit board looks really nice and clean.

It is really nice and clean, but all the ICs look really really old :)

Date codes in the middle of '82 is reasonably late for a VIC=20, surely?

stephen_usher wrote: 25 Mar 2022 10:43 Date codes in the middle of '82 is reasonably late for a VIC=20, surely?

Well, from what I can find it was released in Japan in 1980, but not until May '81 for the US & Europe, and wasn't discontinued until '85, so could still be an early model even in '82.

There are at least 3 ICs that are marked 81, a couple of logic ICs and one ROM so I suspect probably an early '82 machine.

It's made in Germany, serial is 84331. Wonder if there is a serial number DB anywhere?

Some days I feel like I'm an archeologist...

Mainboard is dated 1981, so probably assembled in '82.

That 6561 has a date code of the 24th week of 1982, which is half way through the year (2nd week of June). The machine was probably assembled in July/August if that's the latest chip.

stephen_usher wrote: 25 Mar 2022 12:06 That 6561 has a date code of the 24th week of 1982, which is half way through the year (2nd week of June). The machine was probably assembled in July/August if that's the latest chip.

It's the latest of the '82 chips, everything is earlier, so you're likely right. The oldest, one of the ROMs has a date code of 5281 suggesting it has either been replaced at some point or Commodore had a surplus they were working through, either is possible.

So, with the VIC 6561 on order, I turned my attention to the 600XL shimmering video. It looks as below:


https://youtu.be/jRSg1IW6AP0


It's a side to side shimmering (Difficult to pick up on the vid, but it is there), and since the sync signal for the 1088XEL style 13 pin socket used by the VBXL is derived from a CD4050BCN (U19) in the original video circuit, which I know is already good as it's new, I decided that as there are a few caps up in that corner of the board, plus some directly connected to U19, I figured I may as well replace them.

I then discovered that there are three 4.7uF capacitors that are bipolar, and of those I have none, and also no 22uF axials that are in the same area.

So they are on order, we'll see if all works as expected once they are all replaced. I doubt it but it makes me feel better replacing them.

If as I suspect the problem remains, then I need to figure out a way of finding out what is causing the problem.