Macintosh Color Classic occasional screen instability/brightness jump

[jibsaramnim]

Hi all! It's MARCHintosh, and so I really wanted to give myself a push to see if I could finally fix the display instability my Color Classic has been having for as long as I've had it. This has been a project I've been working on off and on for a bit over a year now, hopefully with your help I'll finally be able to bring it across the finish line.

The issue is a fairly common one I've seen described here many a time, though a recap and reflow (two, actually) both have not yet solved the issue. The display jumps in brightness where it occasionally becomes too bright where its retrace lines (I believe that's what they're called) are visible. This does not happen all the time, sometimes it runs perfectly fine for hours on end, other times it does this song and dance soon after booting or after being on for some time. I can't exactly find a pattern as to when it does or does not happen.

I recapped the analog board myself with components from the replacement kit from Console5, and have gone through and re-flowed pretty much every solder point on the analog board twice. Technically the first time I did that I was a bit more targeted, but the second time I just went for reflowing pretty much every single solder joint on the analog board. I have also re-flowed the video board once, it honestly looked totally fine but I did it anyway just to be safe. I think I did this well enough that the behavior is likely not due to a bad solder joint, but I fully admit that it's still possible that I missed a particular wonky connection. The flyback transformer appears to be in good condition without visible cracks or so, at least that I could find.

I'm thinking that there might be a diode or other non-capacitor component on the analog board that is struggling to keep chugging along. The board does not show signs of burn marks like I've seen in some photos shared here, but that of course doesn't really mean they're not potentially struggling after all these years.

Onto my main question; I've seen the DL21 and DL22 mentioned somewhat frequently. Could these potentially explain the display behavior I described above? Are there any (other) diodes or other non-capacitor components that I should look at replacing? I would love to have an idea which ones I should look into more so I can order replacement parts in one go. As I'm dealing with international shipping costs on most of these things, it's better for me if I can do as much as I can in one go.

I'd appreciate any help you can give. Thanks so very much in advance!

 

[bibilit]

Hi, your best option is to monitor the voltage rails at the Molex connector while booting.

Those diodes are always cooked, but not bad most of the time.

 

[Byrd]

Possibly the flyback going south, but hope not. I would also resolder the wiring that runs to the CRT yoke and the neck board. I’m yet to come across a CC that doesn’t have some sort of dry joint around here that doesn’t have some sort of missing colour/jumpiness/flickering issue.

 

[Callan]

Sounds like your b+ is varying/starting to go bad. Technight (TheTechknight) did a youtube on this recently. It was on a Mac TV, but same principle. Check it out and see if it matches your problem. It's near the end of the video, but I'm sure he'd appreciate it if you watched it all and gave him a thumbs up!

 

[jibsaramnim]

Hi, your best option is to monitor the voltage rails at the Molex connector while booting.

Hi! I did measure voltages last year, but at the time I could find nothing wrong with the voltages as measured from the molex connector. I could try measuring them again, but I don't think this is it, sadly.

Possibly the flyback going south, but hope not.

Yeah I sure hope that's not the case either.. I am not sure how I could check if this is the case though, it's not like I have a spare one of those...

I would also resolder the wiring that runs to the CRT yoke and the neck board. I’m yet to come across a CC that doesn’t have some sort of dry joint around here that doesn’t have some sort of missing colour/jumpiness/flickering issue.

This was recommended to me before too, which is why I focused on them and that general area the first time I went in to re-flow joints. Similar to how Techknight (gently) tapped some components to see if that would trigger the issue, I did that too back then, including wiggling those wires coming off of the analog board, but to no avail. They seem ok, but at the same time if its a hairline fracture somewhere, it's entirely possible things appear alright even when they're not of course.

Sounds like your b+ is varying/starting to go bad. Technight (TheTechknight) did a youtube on this recently. It was on a Mac TV, but same principle. Check it out and see if it matches your problem. It's near the end of the video, but I'm sure he'd appreciate it if you watched it all and gave him a thumbs up!

I did try tapping components around there too last year, but I suppose just like as it did with him it doesn't always trigger it that way. I wonder, if it is one of these components.. is there any way to find replacements without going the "donor board" route?

Thanks so much everyone!