Macintosh Plus Analog Board with Power problems

[nvdeynde]

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. :'(

 

[trag]

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....

 

[techknight]

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.

 

[nvdeynde]

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. :-/

 

[nvdeynde]

It's repaired. It was an open R55 resistor

 

[techknight]

I figured.

 

[BEU]

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.