Thread
Macintosh Plus Analog Board with Power problems
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. :'(
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. :'(
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....
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....
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.
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.
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. :-/
It's repaired. It was an open R55 resistor 
I figured.