Colour Classic Display Issues After 84v Mod + Recap

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Doctorkillbydeath87

New Tinkerer
Sep 30, 2026
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0
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Hi Everyone, I recently attempted the 84v mod including the "beefy" upgrade as outlined by @JDW in his excellent youtube tutorials (thank you so much!). I also recapped a good portion of the analog board (the logic board was done by the previous owner). My CC was running flawlessly before my tinkering; it still boots fine, but I'm experiencing some major wonk on the display:

CCdisplaywoes.jpg

So far I have:
-Tested voltage @ J66 (84v)
-Replaced the new zener @ DL21/22 with the originals as I was worried installing heatsinks to the new one may have damaged it (no change).
-Tested for contact between new solder points and adjacent pads
-Reflowed a good portion of the board
-Checked for shorts caused by new soldering work

I am also looking for some advice on a small component that's detached on an LC 575 board. It is labeled L1 and need a replacement recommendation if possible:

mysticboardcomponentl1.jpg

Hopefully I can get some direction from someone more knowledgeable than I, thanks in advance.
 

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Mark_W

Tinkerer
Feb 2, 2026
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The screen geometry and color are both way off which is a good hint that something's wrong with secondary voltage outputs from CRT transformer.Start with checking all voltages on CRT side, make sure 24V, 8.4V from transformer output are normal, as well as regulated 18V, 8.2V from zener diodes.

You can get the schematic here:

@jonschultz also has an excellent reverse engineering project on the XC1186B jungle chip and you can get more info here:
github.com

GitHub - jonschultz/colorclassic_video_processor: Project for the reverse-engineering of the Macintosh Color Classic's CRT video processor

Project for the reverse-engineering of the Macintosh Color Classic's CRT video processor - jonschultz/colorclassic_video_processor
github.com github.com

As for the LC575 board, L1 is a 100uH filter inductor for sound chip's 5V voltage supply. You can get replacements from any scrap board that uses DFAC-II chip, or just bridge it

1790825717793.png
 
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jonschultz

New Tinkerer
May 16, 2026
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github.com
Excellent suggestions regarding the voltages to check. I was also going to mention that checking a flyback-generated voltage would be a good verification, as well. Most are pretty large, but one is reasonably low for safely measuring with a DMM. That's the one on pin 11 of the LM2418T (the cathode amplifier), which should be about 75V.
Mark_W said:
The screen geometry and color are both way off which is a good hint that something's wrong with secondary voltage outputs from CRT transformer.Start with checking all voltages on CRT side, make sure 24V, 8.4V from transformer output are normal, as well as regulated 18V, 8.2V from zener diodes.

You can get the schematic here:

@jonschultz also has an excellent reverse engineering project on the XC1186B jungle chip and you can get more info here:
github.com

GitHub - jonschultz/colorclassic_video_processor: Project for the reverse-engineering of the Macintosh Color Classic's CRT video processor

Project for the reverse-engineering of the Macintosh Color Classic's CRT video processor - jonschultz/colorclassic_video_processor
github.com github.com

As for the LC575 board, L1 is a 100uH filter inductor for sound chip's 5V voltage supply. You can get replacements from any scrap board that uses DFAC-II chip, or just bridge it

View attachment 30627
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Doctorkillbydeath87

New Tinkerer
Sep 30, 2026
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Thank you so much, I was able to spend a little bit of time tonight trying out these suggestions. I'm not overly familiar with reading electrical schematics so bear with me please. My results as best I can describe them:

On the LB:
DL21/DL22(zeners) = 18v
DF2 (zener) = 11v

On the CRT neck board:
Not sure what to label these as so, on the connector that attaches to the tube itself, moving clockwise starting from bottom left I tested each pad of the connector as follows:
0v, 40v, 39v, 0v, 89v, 88v, 88v, 0v, 260v, 0v

The four solder points at the bottom of the neck board with wires leading to the shielded cage on the LB, left to right:
88v, 89v, 89v, 0v

I will add some pictures with labels tomorrow to aid I'm my description.

I also measured the leads coming off the LB leading directly to the CRT but realized after I must have wrote the wrong IDs down for them after referencing a high res picture of the board. I will remeasure and post tomorrow, but here is what I noted:
HL 2: 183v
HL 3: 0v
HL 4/HL 5: 40v

If any of this is useful off hand to anyone in coming up with a diagnoses please let me know, and thanks again! I will keep trying in the meantime.
 
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