Macintosh 12” RGB Display (M1299 / M1296) with Vertical Linearity issue?

[VeryVon]

I have this monitor that looks pretty good but the top of the display stretches out and then squeezes towards the top (see picture below.) Must be pretty low hours on it as well because I can find no sign of electrolyte leakage, the caps test out good with BlueESR, even the ones near the big hot resistors and heatsinks are fine. I've gone over the board a few times and replaced two of the transistors that others have reported going bad (Q401 & Q402) but no change. I've also moved around all the pot adjustments on the board (especially V-LIN which expands / contracts the image a bit, but the loss of linearity is relatively constant.)

Does anyone have an idea of which PCB components could lead to a loss of linearity in the current near the top of the scan? Without schematics I'm having a tough time figuring that out.

 

[jajan547]

I have information for the M1297 AppleColor RGB Monitor I haven't got around to uploading it yet, do you suppose its similar enough?

 

[lobust]

For that sort of fault, my money is still on capacitors. Note that ESR testing is not 100% reliable!

I work with CNC machines, some of them old, bad caps are fact of life for me. My typical approach when I suspect bad caps and have no schematic is just to brute force replace them all, but I understand that is a pretty serious undertaking on a typical CRT!

I'd start by following the traces from the V-LIN pot and replacing caps as you find them.

 

[slomacuser]

Looks like the thread was lost in server crash but I have a backup here

 

[VeryVon]

I have information for the M1297 AppleColor RGB Monitor I haven't got around to uploading it yet, do you suppose its similar enough?

@jajan547 I don't know but if you can upload the information I'll compare it and let you know.

For that sort of fault, my money is still on capacitors. Note that ESR testing is not 100% reliable!

I work with CNC machines, some of them old, bad caps are fact of life for me. My typical approach when I suspect bad caps and have no schematic is just to brute force replace them all, but I understand that is a pretty serious undertaking on a typical CRT!

I'd start by following the traces from the V-LIN pot and replacing caps as you find them.

@lobust totally agree. At first I take a shot at understanding theory of operation before brute force, but If it comes down to it I will definitely just replace all caps.

Looks like the thread was lost in server crash but I have a backup here

@slomacuser thank you! I have watched all @techknight 's LC RGB M1296 troubleshooting video's which were extremely helpful, especially in realizing the bad design of how they located capacitors near those two big resistors and along side a heat sink.