[Coco] Coco1 3003B color fix - finally

CoCo Demus retrocanada76 at gmail.com
Tue May 25 14:25:53 EDT 2021

Hey all,

After a long time I finally found why all coco1 64k white with melted keys
(3003B) have broken artifact colors, they show as cyan/pink instead of the
famous blue/red.

Following the original coco1 I found out the clock line coming from the SAM
chip is directly connected to the VDG and then between the VDG and MC1372
there is a 10K resistor (R37). This is the resistor that holds that ugly
hack inside the PVC tube, which seems a desperate last minute fix for the

Well the original Coco1 has a 470ohm between SAM and VDG and between the
VDG and the MC1372 there is nothing (justa 56pf cap).

My color fix works for composite video (I drive it from mc1372 in the
classic video driver from datasheet, so I don't know if the fix will work
for RF) but the fix is basically:

Lift the left side of ferrite bead FB3 (close to SAM) and put a 470Ohm in
series. Replace the 10k (R37) to a jumper and put a 56pf cap between the
jumper and the ground (next to it) and voila, beautiful artifact colors on

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