Color Classic VGA Woes

[Marconelly]

More things happened today. First, I've replaced the 2nd optoisolator, but as I suspected, the voltages remained the same, and in general nothing changed. Then, I replaced the two 5600uF caps and one 3300uF cap with my preferred Panasonic ones. Again, no changes in voltages, but since the CC was now working fine in VGA anyway, I decided to just call it quits there and just enjoy the fruits of my repair.

And then, as I was playing around, I heard the ramping squeal of high voltage, and seconds later computer shut itself down I restarted it a minute later, and it again worked fine for about five minutes, then the same thing happened.

So I took the board back out, desoldered my caps and put the originals back in. Started it up again, it worked fine for some minutes, but then again the squeal and the shutdown.

Then, as I was taking the board back out again to remove the 2nd optoisolator and put back the original one, I noticed that the CRT molex cable was really stuck and couldn't be removed without some serious pulling. This didn't happen before, so I was puzzled by it. I thought that maybe the connector had some residue in it that got melted due to some arcing and caused this. With some force and lots of wiggling I managed to remove it, and then I sprayed lots of contact cleaner in there. I did also remove the 2nd optoisolator and put the original back in.

With that done, the computer worked fine for the next whole half an hour, so I called it a day there. I'll keep testing tomorrow, but I actually suspect that molex connector was the culprit for this new problem, as I can't imagine the new optoisolator would cause it - especially seeing that replacing the 1st optoisolator actually fixed the original major problem with the board. I just hope that contact cleaner on molex did really fix this long term.

 

[Marconelly]

For a sanity check, today I've put back my preferred Panasonic capacitors back on the board. Then I've thoroughly cleaned the pins on the board where the CRT molex connector goes. Each of the four pins had some darkening on it - some residue or a burnt metal surface due to arcing? Not sure what it was but I've removed it thoroughly, and again cleaned with more contact cleaner. Assembled it all back, and had the computer running for almost two hours. Everything was perfectly stable, no weird sounds or anything. Phew! Lots of little things that can go wrong with these analog boards, other than just recapping.

 

[Marconelly]

I just can't catch a break with this. After working perfectly for whole three days - and I used it a few hours each day - today the computer starting shutting down all on its own again, same way as it did before I thought I fixed it by cleaning the CRT connector. It seems it works fine for about five minutes, but I guess as soon as something in there gets warmed up, it shuts down.
No idea what to do now. Maybe replace those TDA4605 power delivery ICs? Replace some of those large transistors? Try to transplant the Flyback from the LC 575 board and do some adjustments to make it work like TechKnight showed in one of his videos? Or just give up and go back to my regular resolution board, where most things work fine anyway, if I use the fVGA utility :\

 

[JDW]

But the computer is working. If it was a blown component, I suspect it wouldn't ever work. And so, it could be the case of something over-heating, especially now that we are into the warmer months.

What you really need is a thermal camera. Then you can leave the back case off and check temperatures inside.

I have a UTi260B, and here's how the inside of my Mac IIci look when I check its Radius Rocket card:

The downside is they are a bit expensive, even if you get the iPhone attachment kind.

I bought mine from Banggood in 2023, but I see the lowest price on AliExpress today:

UNI-T UTI260B UTI260A Thermal Camera 256X192 Pixels Construction Thermal Imager PCB Circuit Floor Heating Tube Testing - AliExpress 1420

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www.aliexpress.com

Even then it's not a cheap device. Good, but not cheap. And I'm not sure how tariffs would be handled for folks based in the US.

 

[Marconelly]

The things is, it's really haphazard. Today again I've been able to use it for an entire hour, and it still keeps working just fine. Yesterday it wouldn't run for longer than 5 minutes at a time.

I keep suspecting that flyback on this board is dying - it's flat surface looks pretty cracked from the heat - which maybe could explain this weird behavior, if something inside is just slightly shorted, but it manifests only sometimes.

In any case, it feels practically impossible to troubleshoot it when it can at times work for hours at a time. I would never have a real idea if replacing some component actually fixed it, unless I'd use it for days after.