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Macintosh 12” RGB Display (M1299 / M1296) with Vertical Linearity issue?
Posted by: VeryVon on 2022-08-24 17:37:53 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.
Posted by: jajan547 on 2022-08-24 19:51:33 I have information for the M1297 AppleColor RGB Monitor I haven't got around to uploading it yet, do you suppose its similar enough?
Posted by: lobust on 2022-08-25 00:33:26 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.
Posted by: slomacuser on 2022-08-25 02:13:05 Looks like the thread was lost in server crash but I have a backup here
Posted by: VeryVon on 2022-08-25 04:36:21 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.
Posted by: felangga on 2026-06-17 10:08:03 @VeryVon did you finally fixed your monitor?
Posted by: VeryVon on 2026-06-17 11:57:32 Unfortunately, no. I think I moved on to some other projects. Wow, that was 4 years ago? That's an eye opener 😆
Posted by: felangga on 2026-06-17 12:05:35 Haha so sad, I'm kinda having the same problem as yours. But mine the vertical is fully collapsed, only showing the horizontal line. Still investigating what could be wrong. Last time I tried to tapped the input of TA8403K with my finger, the vertical is wobble, seems the power IC is working though.
Posted by: VeryVon on 2026-06-17 18:00:27 Let me know how it goes. Do you have the same board as the one I have pictured? I'm sure you already know, but all the 4XX components are the vertical section, 5XX is horizontal. Thanks for renewing my interest in this project, I pulled it out (I've saved everything) and i'm re-investigating it with the help of AI. AI doesn't solve the problem of course, but often it helps soften the problem up a little bit, and provide new perspectives (even when they're dead wrong haha.) Looking the board over again, I re-capped most of the main board (excluding the big caps near the PS) including the notorious C418. Didn't help. I also remembered there's two 4XX caps around IC 501 I didn't replace because I didn't have the parts, so I might try to get some parts and do those.
Posted by: felangga on 2026-06-17 18:16:45 Let me know how it goes. Do you have the same board as the one I have pictured? I'm sure you already know, but all the 4XX components are the vertical section, 5XX is horizontal. Thanks for renewing my interest in this project, I pulled it out (I've saved everything) and i'm re-investigating it with the help of AI. AI doesn't solve the problem of course, but often it helps soften the problem up a little bit, and provide new perspectives (even when they're wrong haha) Yes I'm having the same board, and I'm just realized about that component numbering LOL 😂 I'm also CRT newbies, so I tried to check the components one by one carefully. Without a schematics it would be slow, but AI also helped me through. Do you still have the monitor? Did you fully recaps your boards?
Posted by: VeryVon on 2026-06-17 19:22:17 Do you still have the monitor? Did you fully recaps your boards? Yes and mostly, yes. I just ordered two more caps to complete the job. BTW, did you check and make sure the solder joints for DY are good? Re-flowing a cracked solder joint could be an easy fix for you, if evident.
Posted by: felangga on 2026-06-18 04:48:46 I see, did you also recaps the neckboard? Yes I reflow the DY and all other "Hot" pins, but currently still stuck on the same issue 😆 How about yours?
Posted by: VeryVon on 2026-06-18 05:39:37 I haven't re-tested mine yet, but I assume it's same state as when I put it away (hopefully it didn't get worse!) Sounds good. I didn't recap the neckboard.
Posted by: felangga on 2026-06-18 09:01:34 I planned to replace the TA8403K even my gut said that it should be normal. But who knows 🤨
Posted by: Retrotechie on 2026-06-18 09:15:52 For a horizontal line I’d suspect the vertical driver. I recently had a different Apple monitor with that symptom. It was a shorted driver chip but that short took out a “fusible resistor” that was there to protect the flyback transformer. Replacing both restored the operation.
Posted by: felangga on 2026-06-18 09:23:38 Can you share which type of monitor of yours? Or better can you share the schematics and point which resistor? I'm CRT newbie, but maybe I can pinpoint mine based on yours
Posted by: Retrotechie on 2026-06-18 12:35:13 Can you share which type of monitor of yours? Or better can you share the schematics and point which resistor? I'm CRT newbie, but maybe I can pinpoint mine based on yours This was in an A2M6020 AppleColor Composite monitor. I can send you the schematic if you PM me your email. The resistor R236 was blown when IC301 failed. It killed the power to IC301 (which is about +24V).
Posted by: felangga on 2026-06-18 18:12:27 Thank you, I found this schematics on google. This one right? I'm wondering, what is the model of IC301? It's not stated on the schematics
Posted by: Retrotechie on 2026-06-19 04:49:06 Yes that is a slightly older version of the schematic I sent you. The vertical amp is a KA2131.
Posted by: felangga on 2026-06-19 11:43:17 Hi this is my output of LA7824 which generate a signal but it seems too weak and noisy compared to the input signal on PIN 14. I already swapped the chip but the result is the same. Wondering it was some passive components around it.
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