Help with a Macintosh Color Classic analog board

[retroguy2019]

Hi everyone! My first post here (I think...)! My name is Marco, a vintage computer enthusiast. I have a collection of old Macs, Amigas, Ataris, MSX and such... simply love them! I spend a lot of time restoring them to bring them back to their glory days... now, I am strugling to bring back a Mac CC...

Yes, yes... sorry about that! But, yet again in this forum, we have an issue with a Mac CC :-(

I have looked all over for info about this, couldn't find it anywhere. I am about to throw the towel here, so I thought about asking for help before doing it

Little background first... I bought this Mac CC a while ago, for a decent price at eBay. The Logic board was totally damaged by battery corrosion, but I was able to salvage it after recaping and cleaning it up. The CC booted up but image was distorted. I then proceeded to recap the analog board. I replaced almost all the caps, along with the DL21 & 22 diodes and RL62 (47 Ohms) and RF11 (390 Ohms) resistors (the ones that normally looks burnt). CF8 cap was part of the recap work, of course (replaced with a 3300uF 25V 105C). After the work, the CC turned on and image was looking fine. It lasted for a few boots and then I started seen varying brightness and flickering, up to a point where the screen went totally dark. CC was still on and working, only the screen went dead.

O opened it up once again only to realize the resistor RL62 was scorched again. I, once again replaced it and also double checked for solder cracks and other issues. Reflew some solder joints that looked sad and give it another shot. Same thing... The CC stays on for a bit, then image gets unstable and, then, blackness. Again, RL62 scorched. 

It seems there is a problem somewhere in the high voltage path. I am no electronic engineer... can't do complex troubleshooting here (I don't have an osciloscope and, even if I did, I have no clue how to use one)... 

Any ideas what to check? I am ordering a replacement CF8, just in case... but I don't think that's where the problem is. Maybe the diodes? 

NOTE: You will notice, on the back of the board (2nd picture), there is a pretty bad burnt cluster where the diodes and the affected resistor connect. Even though it looks bad, the traces and connectivity is fine. I double checked that with a multimeter and it tested fine. I also resolder all those components in place, so the solder is fresh. The burning was already there before I started the restoration. Those analog boards don't look well designed... but it might be an indication of an issue somewhere else... not sure.

Follow some pictures...

Thank you everyone!!

 

[retroguy2019]

This is how the picture looks right now... it is kind of stable (no brightness or flickering issues), after a few more interventions... but it is distorted as it can be seen... I don't think this is just a matter of correcting the geometry... it seems to be an analog board issue, still. I left it on for a while to see if the dark screen returns, and it did. After only 5 minutes on. Screen shut down, but everything else looked OK. I could use the keyboard to shutdown the Mac, even without the screen. Sound also is fine. Only thing affected seems to be the video. 

 

[bibilit]

Try correcting the geometry first, screen is too wide and tall anyway.

There is a lot adjustements on the back once the rear casing has been removed.

 

[retroguy2019]

Try correcting the geometry first, screen is too wide and tall anyway.

There is a lot adjustements on the back once the rear casing has been removed.

Thanks for getting back to me, bibilit! The geometry adjustments are fine. Picture was perfect before the flickering and brightness oscilations. The geometry was affected after I resoldered some components in the board. So, I am pretty sure the adjustment is unrelated. More like being a defective component or trace... and there is the issue with the screen shutting down on its own after 5 minutes. That wouldn't give me enough time to adjust anything, even if I tried ;-)

The picture below was taken yesterday... see that it looks OK. No distortion. 

 

[techknight]

I never encountered this issue so its going to take some extensive electronics knowledge and troubleshooting skills to figure it out. 

You make sure you ordered the correct replacement diodes? 

 

[SuperSVGA]

The pincushion distortion makes me think something might be wrong in the pincushion circuit, though I don't know much about it on this board. You could check the various transistors for bad joints (QL1, QL2, QL13, and QP11, not sure which one is for the circuit in question though).

That resistor burning up makes me wonder if there's a short or something, but could also be a bad joint I suppose.

 

[retroguy2019]

The pincushion distortion makes me think something might be wrong in the pincushion circuit, though I don't know much about it on this board. You could check the various transistors for bad joints (QL1, QL2, QL13, and QP11, not sure which one is for the circuit in question though).

That resistor burning up makes me wonder if there's a short or something, but could also be a bad joint I suppose.

Tks for the help SuperSVGA! I will look carefully through all the transistors! Let's see ;-)