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Macintosh Plus Analog Board with Power problems
Posted by: nvdeynde on 2012-06-22 05:58:53 I have something weird going on with a Mac Plus Analog board that I recapped. I regrettably didn't power it on before I recapped the board but last time, a few weeks ago, it was still working though. After replacing the capacitors, it didn't want to power on anymore. I triple checked the polarity and the value of all the capacitors, ESR tested them to make sure, but everything was fine. There were no lifted traces during soldering: I already re-verified all traces from the capacitor's pins to the next component: all OK. It's not a faulty switch ( I bridged it for testing ) and the fuse is also good. However, when playing with the power switch a few times, I found that the board suddenly powers on. It's very random, sometimes after 3 or 4 attemps, otherwise much more. When it's on, I tested the voltage on all reference points from a schematic in Larry Pina's book: everything is spot on. I left it on for 3 hours: nothing smoked/overheated, stable as a rock but the power on problem remains. Has anyone seen a problem like this ? It's not a dead set but there's definately a problem somewhere. Unless someone has an idea, I will have to look for the schematics and verify the whole power supply section. :'(
Posted by: trag on 2012-06-22 13:40:09 With the machine on and the bonnet removed, oh-so-carefully, flex the analog board by gripping it at the top rear corner and push or pull a little bit. If the machine stops working, then you've got a cracked solder joint somewhere, most likely. I have one like that. One of these days I'll track down the fault -- or just desolder and resolder the whole board....
Posted by: techknight on 2012-06-22 18:09:40 sounds like a bad solder joint. However, Also check the tolerance of all your resistors as well! carbon and carbon composition resistors drift over time, and thats a discrete transistor power supply so things are more critical there before itll oscillate. They also become heat sensitive as well. So you could have a resistor in the feedback and/or soft-start circuit drifted just enough so the circuit wont fire. turning it off and on sends spikes/ripples and that can trigger the oscillation. Just a thought.
Posted by: nvdeynde on 2012-06-23 04:59:41 sounds like a bad solder joint. Definately not a bad solder joint. You can wiggle and tap the Analog board as much as you want, once it's ON no problem. This will be a very time consuming job to get it repaired as I could not find the Schematics for the board ( it's the Internation, 240 Volts version, not the USA model ). I have one of a USA model but the layout is completely different. Resistors and capacitors have other numbers, even other values.... I'm afraid I have no other choice than using my spare board as reference to look for the problem. I also think it's a faulty transistor or resistor somewhere. It's something I'm going to keep for next winter as it's too hot in my small workspace now. :-/
Posted by: nvdeynde on 2012-06-24 06:37:59 It's repaired. It was an open R55 resistor 🙂
Posted by: techknight on 2012-06-24 13:34:34 I figured.
Posted by: BEU on 2018-05-31 15:35:24 However, Also check the tolerance of all your resistors as well! carbon and carbon composition resistors drift over time, and thats a discrete transistor power supply so things are more critical there before itll oscillate. They also become heat sensitive as well. So you could have a resistor in the feedback and/or soft-start circuit drifted just enough so the circuit wont fire. It's repaired. It was an open R55 resistor 🙂 Hi nvdeynde and techknight. I got a defect Mac Plus (240V) that was dead. No power on the analog board so I started to check all components. It was exactly the same fault, R55 had no connection, completly open. It was 33k but I replaced it with a 20k resistor ( according to Larry Pina, Macintosh Repair & Upgrade Secrets) and its working perfect. Maybee should be 33k https://68kmla.org/forums/index.php?/topic/29763-macintosh-plus-analog-board-q12-replacement/&tab=comments#comment-575154 but its working anyway.
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