Color Classic VGA Woes

I spotted a "for repair" CC analog board on Ebay a few weeks ago. Seller said that the board was VGA modded and worked fine for a while. Seller then attempted to undo the VGA mod, but something got messed up in the process, and it stopped working. I got this board thinking I could try to repair it, and if not, I could at least keep it for parts.

It arrived the other day, and after careful inspection, I spotted a problem that I figured was the culprit. The tiny J78 jumper was not re-soldered properly. In any case, I wanted to try a VGA mod on this board, so I did all the steps very carefully and properly (including the removal of J78). Then I tried it - and it worked! I got a nice 640x480 picture, and after some picture adjustments, I got to try a few games, everything ran perfectly. I was very careful not to make any adjustments on the CRT itself - all the picture adjustments were only done on the analog board pots in the back.

But one thing bothered me - even though I turned the green sub-brightness pot all the way down, the green sub brightness was still too high. I could see a green background all the way around visible picture, and there was never a proper black color displayed on the screen, it was always green-tinged and visibly bright. Watching the @JDW 's picture adjustment video and seeing the CC service manual, I saw that part of the adjustment process is to tweak the larger PL2 pot which controls the "VG2", whatever that is. So I turned that pot a bit - and green sub brightness became perfect! I was really happy with the result, turned the computer off and left it for the day. The next day I tried it again, and that's when the problems started: The picture was really flickery and unstable, kind of scarily so, considering that there were buzzing sounds coming from somewhere on the board each time the picture became unstable. I turned the PL2 back up a bit to where it was originally, and the problem seemingly stopped. Then I turned it back down to make the picture look the way I wanted, powered the computer off - and the next time it would not power on. The front LED just blinked quickly. So I turned the PL2 back up - and then computer powered on.

I figured that maybe the PL2 not being turned in decades was a but rusty and losing contact, so I sprayed it with some contact cleaner (while the computer was off). Let it dry and tried it again. It seemed to make things better. I could adjust the PL2 and the picture remained stable. So here I am a day later, and the computer no longer powers on, no matter what
The light on the front just blinks quickly, turns off, and that's it. The high voltage never seems to spool up when I flick the switch on the back. I tried more contact cleaner on the PL2, some more on the AB main connector, turned PL2 back and forth - but no luck.

Did I mess something up by spraying the PL2? Am I just experiencing the same issue that the seller was? I guess I will remove the PL2 and test it somehow first. Maybe transplant the PL2 temporarily from another board to see if that's really where the problem is. I hope someone can help, if you've experienced the same kind of problem.

*edit* I've checked the resistance of PL2 pot while it's still on the board, and it seems to work OK - I measure the range between 0 ohm and 1.1 Mohm while I rotate it (if I measure the other two pins it's the opposite range).

JDW said:

I truly wish I had the answer for you, but I unfortunately do not. I can only guess it must be something more than just a questionable POT. Did you examine the Analog Board for burned parts? There are a couple diodes that often leave black spots. Some parts on the analog board get so frightfully hot it calls into question the sanity of Apple engineers who created it.

You might want to have a look at the following thread where we talk about burned parts and beefier replacements:

https://tinkerdifferent.com/threads/color-classic-analog-board-trouble.2367/

My CC mentor (先輩) is @Kay K.M.Mods . He taught me a lot about the Color Classic. He might not know what's going on either, but maybe he will share his thoughts if he does.

Click to expand...

Thank you, I will look through that thread carefully. I was thinking of replacing the optocoupler and those diodes, even though nothing looks obviously burned.

I’m worried it’s the flyback though. I can hear a brief buzz from it while the LED is on, and it all shuts off a moment later. Maybe that’s what caused my picture instability in the first place.

I wish there was some way to test the flyback without having to transplant a new one onto the board.

I've made a potentially significant discovery on the nature of the problem. Once I removed the LC575 board, and placed the original CC logic board, the computer boots fine. It of course cannot display the VGA resolution correctly, so the image is distorted in terms of width and barrel distortion, but it shows the picture with the bootup computer and everything - meaning that I will hopefully always at least be able to salvage the flyback and other parts from this board.

But also importantly, having a stable bootup has let me measure the computer voltages - and they're not looking good. I measure 4.6V instead of 5, and 11V instead of 12. So it's really the low voltage problem in the end, it seems. The board is nearly fully recapped from what I can tell though, maybe just those small 1uF / 50V caps are not replaced, which I don't think would cause this. However, I do notice what looks probably like some green corrosion on the low voltage side optoisolator IC legs. Can that be a cause of this? I'm going to be replacing these ICs in any case.

I've been reading a bit how optoisolator works, and it does seem that a decaying dLED inside of it could cause a voltage regulation issues, and specifically lower voltage issue. Not much point posting a photo of it now as I"ve mostly scraped the green corrosion from the IC's legs, but some of the legs were fairly covered in green. Wonder if I should also replace the TDA4605-2 ICs?

On my other CC, with the mystic board I was getting issues when the voltage dropped to around 4.6V, I think. However, the issue looked completely different - the computer would still power on, but I was getting a death chime. This one won't even power on with the mystic board in place.

Good news! After replacing the corroded optoisolator, the board is working again I'm back in the VGA club!
Now, the voltages I measure are 4.8V and 11.5V. Better than before, but still not quite right. Tomorrow I will replace the other optoisolator, although that other one sits next to the high voltage transformer and it doesn't look the least bit corroded. I will also replace two big capacitors on the low voltage transformer side - the ones that are there now come from the newer Console5 recap kit, and are 5600uF / 16V, rather than 5600uF / 10V. They are also kind of smaller than the Panasonic 5600uF / 10V ones I have, and have used before. I know this voltage and size difference shouldn't matter, but since the voltages are still not quite right, I've got nothing to lose trying this.

JDW said:

Note that modern electrolytic capacitors tend to be physically smaller than those from the 80's and 90's, even though you have the same capacitance and voltage specifications.

Click to expand...

I'm aware of this, but the 5600uF / 10V Panasonic capacitors I have are also new. They just happen to be similar in shape to the ones that were originally on the board. The capacitors from the Console5 kit are thinner but longer, so maybe their overall volume is similar. I'll see what happens, no harm in trying.

Yes, some components on CC analog board stay really hot unless the switch on the back is set to Off. For example, the black square component right in front of the mains power plug is too hot to touch. I also think the whole high voltage section, including those two diodes and the 3300uF / 16V cap are permanently powered and heated up unless the switch is set to Off.