Macintosh Color Classic - Intermittent Red/Other Colors

Hi All!

First post here, so please let me know if this would be better suited elsewhere on the forum! Somehow after many years of PC/Mac repairs, I have never made a post!

I have a Color Classic with some unusual display issues - essentially, sometimes it boots without the red channel present that then kicks in after a certain amount of time, but sometimes I can leave it on for hours and it remains missing. I have confirmed the red gun is working within the CRT tube itself, it seems to be a missing color channel. I have upgraded this with an LC575 (no resolution mods), however the issue was present on the stock logic board as well. I'll do my best to note down everthing I've tested and done so far:

• Recapped the entire analog/logic board and thoroughly cleaned them. All components that I am able to test have been tested and are working within spec
• Checked and reflowed any solder joints multiple times
• Reflowed edge connectors on both the analog and logic boards and thoroughly cleaned the contacts, including inside the chassis
• Measured key voltages (5v - 5.01) (12v - 12.09) (B+ - 64.5v)
• Checked for any broken traces or lifted pads multiple times (of course it's always possible something was missed)
• Replaced a couple of resistors and doides that were measuring slightly out of spec or burning the PCB
• Brushed against a high voltage component and got a nasty shock (ouch!)
• Adjusted all pots relating to color and convergence
• A firm hit on the front of the chassis (notably only below the CRT, not above) will sometimes cause the raster/beam to collapse or colors to change rapidly, but this does not happen when manipulating the neckboard or yoke

Thanks all, it may be something obvious I've missed but I'd appreciate any pointers Fairly new to this particular model.

EXDosis said:

A firm hit on the front of the chassis (notably only below the CRT, not above) will sometimes cause the raster/beam to collapse or colors to change rapidly, but this does not happen when manipulating the neckboard or yoke

Click to expand...

Then you've still got cold solder joints or a loose connection somewhere. Look again...

My bet is that it's still the one of the edge connectors. I've had issues like this where the only thing that fixed it was taking it apart and putting it back together several times until everything was just in the right place. Bloody awful machines, but weirdly charming when they work...

luRaichu said:

Then you've still got cold solder joints or a loose connection somewhere. Look again...

Click to expand...

You could be right. But even pushing down on the analog board relatively hard in different locations has absolutely no impact whatsoever. Locating it is becoming a bit of a nightmare.

Reflow (with new solder) all the wiring going to the tube and behind the plastic housings to it, same goes with the yoke PCB

Byrd said:

Reflow (with new solder) all the wiring going to the tube and behind the plastic housings to it, same goes with the yoke PCB

Click to expand...

Thank you - I’ll give it a try tonight.

the issue was present on the stock logic board as well.

Click to expand...

This means you know it isn't the logic board

A firm hit on the front of the chassis (notably only below the CRT, not above) will sometimes cause the raster/beam to collapse or colors to change rapidly, but this does not happen when manipulating the neckboard or yoke

Click to expand...

I had a Mac Plus with the cold solder joint issue; a whack on the side just behind the faceplate would often fix the issue for a while. On my Color Classic, whacking the front below the CRT was almost always related to the edge connector -- or more specifically, to the board contacts where the edge connector connects. I used to sometimes get dust or flux in there between the case contacts and the board contacts, causing an issue like what you describe. To fix it, I cleaned off the contacts with IPA and bent the case contacts a bit more so they pressed more firmly against the board. Worked fine until I sold my CC -- the shipping wasn't nice to it and the buyer had to fix it all over again. If shipping a CC again, I'd remove the board and pack it separately.

I would clean the grounding points for the logic board in the chassis and make sure all grounding clips are installed (and cleaned) on the logic boards. The CC is oddly sensitive to this for some reason and what you are describing is a symptom.

Also make sure your internal cage isn't bent so the clips are making good contact.

See this post:

https://68kmla.org/bb/index.php?threads/color-classic-screen-fritz.39468/

Thanks for everyone’s advice so far, I have a few updates. In addition to my previous list, I have:

• Reflowed everything again, on both the analog and logic boards, paying particular attention to the edge connectors and testing for continuity
• Carefully checked each grounding point and tested for continuity
• Replaced a dodgy looking Rifa capacitor and any other components prone to failure or that are looking like they run hot
• Checked grounding on the logic board tabs and ensured a good connection to the metal cage
• Replaced every component on the neckboard despite testing fine
• Tested the CRT tube itself in a friends machine (works perfectly fine)
• Cleaned the edge connectors and ports within the chassis
• Replaced the video chip under the metal shielding

Annoyingly, the issue persists. I cannot figure out what’s causing it. The only thing slightly out of the ordinary is that my B+ measures a little lower than some others I’ve see on the forum, but to my understanding, that wouldn’t cause issues like this. Frustrating…

Is it likely that there is an issue within the connectors in the chassis (perhaps the ribbon cables) between the logic and analog board?

Well, you've already addressed all the common issues, including possible gaussing effects, so having eliminated those... yeah, I'd check all the ribbon cables next.