The "Cursed VIC20" - A repair job I've hit the wall with

[GeekNathn]

Hey everyone,

Long time reader, first time poster.

I've been hitting my head on the wall with the repair of a VIC20 for a few weeks now. I'm fairly experienced with these things but this one is befuddling me a bit.

It initially presented with a black screen. Replaced the CPU and it kicked into BASIC like a normal machine. Tried to run a diag cart on it and the screen loaded full of characters. I tried a game cart (Radar Rat Race) and it wouldn't load and went straight to BASIC. I tried running some simple basic programs like a 3d maze and flashing border and it returned a munged screen. (I tested those on VICE and they worked fine).

I then opened it up and started looking around. No evidence of prior repairs. Nothing obviously at issue. I checked voltages using a multimeter, and started pulling chips to test in a known good machine. I also tested the RAM and logic in a Retro Chip Tester Pro. All them were fine. Just to be sure, I replaced quite a number of logic chips with brand new ones. I also checked the cartridge port and reflowed the joints and continuity tested. The problem continued.

I then stepped up to probing around with my Rigol scope. I first focused on the cartridge port and connected logic first. I have a good VIC20 and started to compare signals/waveforms and noticed something odd. There was a repeating anomalous signal on RAM3 at the cartridge port.

This image is comparing two different VIC20 (CR). Cursed VIC20 on top and good VIC20 on bottom.


I probed around a bit more, started using a second probe to try and find the cause. I took measurements of the signal and interval appeared to be 8ms of normal signal, and 12ms of abnormal signal. I am not sure what turns on and off for those periods in the system.

In trying to identify the correlation a friend on Noel's discord suggested I look at the video signals. When zoomed right in I do see some correlation between the composite signal and the interference but I'm still not sure what the cause is. I did play around with the screen colours by poking in white screen/border which dampens the amplitude in luminance but it made no effect. So I don't think the video signal is the cause, but rather is effected.



Here's am image of the transition between a good period (8ms) and a bad period (12ms):


I also ran around the board probing and looking at the "loudness" of the signal to try and triangulate the source. The signal seemed "loudest" around C47 ceramic on the south side of the board. I replaced it, but no change. I also noticed up near the VIC the interference was dampened by ferrite bead 6. So I checked C6/7 and then replaced C8. No joy there either.



I'm kinda at a loss now. I've rarely had to venture this far for a repair so I am a little in new territory.

Anyone got any ideas?

[DarwinNE]

Interesting repair! I have a stupid question: have you checked that the noise on the power supply rails is not too large?

[GeekNathn]

Interesting repair! I have a stupid question: have you checked that the noise on the power supply rails is not too large?

No stupid questions here, thanks for asking. I knew there was a good chance after the weeks I've spent on this I would miss mentioning many things I've tried.

But yes I have checked. There is noise that seems to be "loudest" near C47. I've seen that on good boards too, but no where near the same amplitude.

I am using a good quality aftermarket power supply from Electroware and I used that on all my Vic20CRs and C64s (and I use an adaptor for my C128s). There is negligible noise/ripple on the 5v rail on the din connector. As you probe inwards towards the rail that runs along the bottom of the board the noise starts to appear.

I'm going to go a check all of the ceramics sitting on the main power bus. If one of them is shorted it may be the cause of the noise. I don't have any spare 0.1uf @ 50v ceramics, but I think I could get away with 35v if I needed to replace a couple.

[MysticSword]

Welcome aboard (and I'm still somewhat new around here myself).

I think the GIF you posted of them trying to get rid of the evil-spirit from the 'Cursed Vic20' motherboard is hilarious.
Good luck with your repairs of the 'Cursed Vic20'.

[DarwinNE]

I am using a good quality aftermarket power supply from Electroware and I used that on all my Vic20CRs and C64s (and I use an adaptor for my C128s). There is negligible noise/ripple on the 5v rail on the din connector. As you probe inwards towards the rail that runs along the bottom of the board the noise starts to appear.

I'm happy my suggestion have helped, though I'm not an expert about the VIC20 repair in particular.
I've not seen often a ceramics to fail short, yet (it is common on tantalums, but then you have other symptoms). Noise on power supply rails is a typical sign of a failing electrolytic capacitor. I would try to see if they are still working properly. In some cases, you can also temporarily solder a known good capacitor in parallel with the power supply rails, to see if the situation improves. Then change the ones that have failed.

[GeekNathn]

I'm happy my suggestion have helped, though I'm not an expert about the VIC20 repair in particular.
I've not seen often a ceramics to fail short, yet (it is common on tantalums, but then you have other symptoms). Noise on power supply rails is a typical sign of a failing electrolytic capacitor. I would try to see if they are still working properly. In some cases, you can also temporarily solder a known good capacitor in parallel with the power supply rails, to see if the situation improves. Then change the ones that have failed.

Another one I failed to mention as it was so long ago...... I have replaced all the electrolytic capacitors with new ones.

This morning I was probing around on the board. I had my scope keeping an eye on (CH1) the interreference on RAM3, another probe (CH2) on the output of C47 and was using CH3 to poke around the 5v inputs around the board.......

I noticed the interference pattern was generally very quiet around the outside of the board but was very loud in the middle, particularly on UE8/UD8. I've replaced those with brand new chips. I tried starting the board with each one taking a turn not being in there and of course the board did not start, but without them the power lines around the system were very quiet.

[DarwinNE]

If capacitors are ok, maybe a bus contention is present somewhere? In any case, 2V of noise on a power supply rail is definitely not normal

[GeekNathn]

Update: started looking at it from a power filtering hypothesis which lead me to check all decoupling caps and ferrite beads to see if any filtering was shorting on one of those components.

I replaced all the decoupling caps. The board looks nicer . But it did not fix the issue.

I thought maybe it my be some kind of parasitic oscillation, but the amplitude seems to 'clean' and there was no issue with either the caps or the beads.

So I'm back to looking for a short somewhere (despite all the visual checks I've done with an eye glass)......

[GeekNathn]

Update:

This has been a tricky one. I finally gave it and decided to remove the cartridge port to see if there was something about it in my testing that I missed.

I also tried injecting some power on the RAM3 line with all the logic ICs in that area removed to see if I could find a short that way. I did see a lot of 100-500mV on adjacent traces, but it was pointed out to me that's probably normal. I did not see any change in ground levels so it did not appear anything was shorting to ground.

So removing the expansion port was a little difficult. The de-soldering was easy with my hakko, but the port itself was glued to the PCB with really powerful double sided tape. It took a fair bit of heat to loosen it to remove the connector without damaging the surface of the PCB. But it left a very sticky mess in that area. There were several traces and vias under that connector too including the RAM3 line and I did start thinking to myself if the adhesive itself may have become conductive over time and was causing the issues.



I gave the area several sprays of contact cleaner and IPA and a good scrub. I try to methodical in my troubleshooting and do tests every time i change something. So even though I could not run cartridge, I previously had tried running BASIC programs that were easy to type in and they failed reliably. So I tried again and lo and behold.......



.....It is running a program for the first time in probably ~30 years and weeks after I started repairing it which has involved socketing just about every chip (except the 555 timer) and every capacitor (including the ceramics).

Here it is running the stack display program correctly.......



Next steps will be to remove all the logic IC sockets near the port (at this point they are pretty worn out) and giving the whole area a thorough scrub again, replace the sockets with better ones, cover the traces near the expansion port (probably kapton tape) and solder in the expansion port again and retest everything.

What's the root cause? Was it the adhesive? just a dirty board? a short somewhere I managed to clear some how? I'll probably never know, but it feels likely to be one of these. It might have been the expansion port connector itself but I have tested it pin by pin and it seems ok.

I'll post again when I have it fully reassembled and tested in a day or so

[Lechuck]

Congrats!!. and thanks for sharing. This was a tricky one, but that makes fixing it much more rewarding.

Cheers

[DarwinNE]

Congrats!!! Nice to see a VIC20 coming back to life!

[mingle]

You might also want to check the continuity on the tracks under the expansion connector.

I had issues on several Roland synths with the keybed PCB having intermittent crack and splits in the very thin tracks.

Try flexing the PCB slightly while testing the continuity...

Cheers,

Mike.

[GeekNathn]

Sorry for the delayed update.

In order to verify the issue I soldered the port back on and the issue returned. While the connector was off, I did test it thoroughly and I can't detect any particular issue with it. I don't have a spare connecter to put on the board, but I have a replacement on order from Digikey. If the new connector goes on and the issue returns it has to be the board. But it would be a bit of mystery that a perfectly working connector causes problems on a board. I can't think of any specific conditions on a board that would cause problems like this when a good connector is on the system.

I guess we'll see what happens when the new connector arrives.