CC trouble Red colour dropping when warm

[ArjenCNX]

Hi there Guy's/Gals, im Arjen, im fixing up a CC and i have a real headscratcher on my hands, I have here a colour classic, and it has the red gun deactivate about 10 minutes after turn on, i did do some digging, and have a second analog board here, and i swapped the neck board, the video amplifier, LM2418 the Video processor XC1186, swapped out the capacitors on the RGB lines, and the resistors, the CRT, and the mother board, and this problem keeps showing up.

has any of you ever seen this before? and if so, any hints, tips ? i would welcome them at this point!

 

[cheesestraws]

This may be an annoying suggestion but have you thoroughly cleaned the edge connectors, both on the boards and on the wiring harness? When I've had a single colour drop out on these it's often actually been the connector playing up.

 

[ArjenCNX]

Good point, they look good, those connectors, but ill look at them under a microscope. also, tapping the PCB doesn't do anything at all, so i though a PCB trace broken, or a dodgy connector is a less likely scenario

 

[Byrd]

RGB wires going to the CRT and yoke, unclip the housings around and reflow/resolder anything going in

 

[ArjenCNX]

RGB wires going to the CRT and yoke, unclip the housings around and reflow/resolder anything going in

Thanks for the advice, i did that when replacing the neck board i have also replaced all electrolytic caps with nichicon or chemicon caps. no change this issue

 

[ArjenCNX]

I suspect it is a small SMD component at this point, somewhere under the video shield, that disconnects when warm

 

[ArjenCNX]

ive also noticed a difference between the RGB DC decoupling caps on both boards, on the 820-0360C one we have film caps, 47nF/250V the other 820-0360B is 1206 SMD 100nF unknown voltage. so the later revision changed to film caps, must be a reason for that as they tend to be more expensive
im ordering new ones just to see what will happen

 

[ArjenCNX]

Okay update time, a very strange failure mode, it was either the DC blocking caps from the LM1825 Video signal amplifier IC to the neck board, or the diodes on the RG and B lines. my suspicion is that the diode for the red channel disconnects when hot. after replacing the diodes and the caps it now runs well. those 5AA caps by the way are 100nF, high voltage (I think 250V) so I replaced them with 100nF 400V X7R 1206 caps, this should be safe. after adjusting convergence and colours it looks pretty sharp now. Parts replaced are: DV3/DV4/DV5 for the diodes, and CV1/CV2/CV3