68kMLA Classic Interface

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Mac Classic II - Smoking Resistor

Posted by: Nev on 2022-01-08 02:21:02 I've now had two Mac Classic analogue board (updated version - 240v) where the 220 ohm resistor RL1 which is right next to the flyback transformer has started smoking and burning out. I haven't read about anyone else having this problem, but it's happened to me twice. Has anyone had this issue before and problem solved it ?

Posted by: ironborn65 on 2022-01-08 03:14:59 Hi @Nev, Have you recapped the board? This is something new, at least to me. I'd follow the traces looking for shorted capacitors or faulty parts. Do you have the issue with the joke disconnected as well? I can not help much, but there are smart guys here who have more skills ...

Posted by: Nev on 2022-01-08 03:28:29 Hi Iron, I have recapped the board and gone overboard and replaced all the caps, not just the usual suspects. I figure the problem is elsewhere on the board and not with the resistor itself.

Posted by: ironborn65 on 2022-01-08 15:35:34 Is it smoking with the connected anode? it might be the flyback I - again - would follow the traces and check other passive parts, e.g .the diodes The attached schematics might help PF

Posted by: Nev on 2022-01-08 16:25:52 Thanks for the schematic. I had the Classic schematic, but not the Classic 2. I pulled out the flyback and inspected it and there is no sign of any heat damage. I'll work my way through the board and see if I can find any shorts or other issues.

Posted by: PPZ on 2023-05-22 18:26:17 Hello, I am a new member. Sorry to dig this thread, but I have exactly the same problem with my Mac Classic. My analog board is 630-0525. I received it with leaky capacitors, which I have replaced. Also, voltage regulator LM339 had cold solders, so I fixed that. After powering on, with the yoke connected, K get a bright horizontal line on the screen, and general power drop. Resistor RL1 (220ohm) goes up in smoke. I temporarily fitted a 270ohm replacement, which goes red hot when powered on. When yoke is disconnected, I get normal voltages and computer boots normally (I can hear the chime, hdd spins). I replaced the vertical deflection IC TEA2037A, with no chanhe to the behavior. It gets the power, I can measure around 15V on the IC. I have no other yoke to test, but it looks ok (horizontal coil 0.3ohm, vertical 8ohm). Took it out and checked, no signs of burning. Flyback seems to be ok, I reckon there would be no high voltage if it would be shorted. I am really puzzled about this one. Analog board is very clean, there are no obvious shorts, or any sign of damage. Did anyone solve this problem before?

Posted by: PPZ on 2023-05-25 15:24:23 No one? I have so far replaced: - LM317T - IRF740 HOT - BYV26B diode - all of the BC558/548 transistors - 2k2/1W resistor which supplies the TEA2037A IC Unfortunately no change, and I am at the dead end. The only thing more I am willing to do is to change the yoke if I find one in Europe, and then next step would be to change whole analog board. However yoke measures well, and it's hard to believe it's faulty, but there's nothing else left except the flyback itself, which again is generating high voltage normally. I would really like to fix the original analog board, since I put a lot of effort in i cleaning and recapping.

Posted by: chiptripper on 2023-05-27 01:12:30 Check for cracked or cold solder joints on the back of the yoke / neck board, and for each neck board wire where it connects to the analog board. And check the back of the analog board where the four pin yoke power plug connects (BD1). Re-flow anything that looks questionable.

Posted by: PPZ on 2023-06-05 12:58:05 Thank you for answer, but finally I resolved the problem (although I still have issues with logic board). Well, it seems someone was "cooking" something in my Classic, and swapped either analog board, either CRT with yoke. After extensive research, I realized that I have rev.B analog board, and on my CRT is YK1000 yoke, which os NOT COMPATIBLE with rev.B analog board. There should be a YK1001 yoke. So I ordered a new part (the one with smaller connector is YK1001) and after a replacement everything worked.

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