Macintosh Plus analog board issue

[ijnfuso]

Hello everyone.
I recently bought a Macintosh Plus 1MB with an analog board problem.
The computer initially had a strange issue where it would boot-chime loop for some minutes (sometimes multiple restarts per second), then when it warmed up it worked normally. Initially I simply thought it was some soldering problem, and in fact there was a couple of cracked joints, that I fixed and it worked for some hours. Then the issue reappeared; except this time it rebooted just a few times, and, total silence and nothing... completely dead. I checked all the capacitors and found some that had some measurements off... including one that had a corroded leg (C18). I carefully replaced them (C18, C27, C28, C24), and, unsurprisingly the computer still wouldn't do any kind of noise or sign of life. So I spent some hours checking every connection and voltage; at first I noticed that the logic board was not getting any voltage, apart from a pin that was very low (0,3V or so)... Left the computer off for some minutes, came back to test again voltages, and when I flipped the switch the fuse and my main power popped... and now I'm here writing this post.

Maybe this is an incredibly stupid thing, but it's my first time operating such an old computer... the oldest thing I've ever put hands on is an iMac G3...

I checked the Dead Mac Scrolls for help, and looking through there I noticed that CR21 has all blackened legs with some corrosion on it, almost like it was burnt. I also checked CR5 and noticed that the Ohms for it were much lower the expected value.... which points at a flyback failure... (still unsure)...

Something else I noticed, was that transformer T3 looked kind of damaged (?), almost like some pins of it were burnt, and the coil on top has some oxidation... I don't know if it's bad, but it is worth mentioning.

Another thing, the seller told me that his father had repaired it back in the day, 20 years or so, I don't know what kind of repairs he did but I did notice a weird junction on Q6... maybe it's normal, maybe not, I don't know.

This is an european Macintosh Plus (model M0001AP), 220V - 240V rated, the fuse was a 1.6A 250V...
Any kind of help is welcome... this has been frying me for the past weeks....

 

[Builder68]

Hi there,

Since the AC fuse is now popping, you likely have a direct short on the primary (AC) side of the power supply.

You mentioned CR21 looks blackened. According to The Dead Mac Scrolls, CR21 is a Schottky Barrier rectifier. If it’s reading low resistance, it needs to be replaced. CR5 is part of the feedback/protection loop; if it's reading low, it can prevent the board from starting or cause the 'chirping' sound of a power supply attempting to start into a short.

If the fuse is blowing instantly, check the Bridge Rectifier (CR22, CR23, CR24, CR25) and the Switching Transistor (Q11). If those are shorted, they will blow the fuse every time.

Check for RIFA capacitors. On European models, these often crack and short out. You can temporarily remove all RIFA caps and operate the board without them, provided you are taking the AC from a UPS or a surge protector.

T3 is the power supply chopper transformer; these rarely fail.

That 0.3V you saw earlier suggests the power supply was shutting itself down to protect the logic board or that the startup circuit was failing.

 

[cheesestraws]

If the fuse is blowing instantly, check the Bridge Rectifier and the Switching Transistor (Q11). If those are shorted, they will blow the fuse every time.

There is precedent here. I had a Plus where a couple of the diodes in the bridge rectifier suddenly decided to become resistors quite sudenly one day. Certainly check the diodes.

 

[ijnfuso]

If the fuse is blowing instantly, check the Bridge Rectifier (CR22, CR23, CR24, CR25) and the Switching Transistor (Q11). If those are shorted, they will blow the fuse every time.

I checked the bridge, and it seems that CR25 reads 0,001V in both directions... certainly have to change that.

Check for RIFA capacitors. On European models, these often crack and short out.

Good to know this, because the two smaller 4700 pico farad ones seemed really cracked up. I will also change the 0,47uF one just in case.

CR5 is part of the feedback/protection loop; if it's reading low, it can prevent the board from starting or cause the 'chirping' sound of a power supply attempting to start into a short.

I have re-checked it, and now it reads around 0,6MΩ in the normal direction and 3,3MΩ in the other, I don't want to assume wrong, but, could this mean that the flyback transformer is bad?

On a side note, I checked other many diodes and some either read a lower voltage than expected or they are shorted and read both ways...
Should I try to repair this board or should I just get another board/sell the Macintosh as non-working...?

 

[Builder68]

I checked the bridge, and it seems that CR25 reads 0,001V in both directions... certainly have to change that.

Highly recommended to change all the bridge rectifier diodes at once!

Good to know this, because the two smaller 4700 pico farad ones seemed really cracked up. I will also change the 0,47uF one just in case.

I have re-checked it, and now it reads around 0,6MΩ in the normal direction and 3,3MΩ in the other, I don't want to assume wrong, but, could this mean that the flyback transformer is bad?

Flyback failure in compact Macs usually involves shorted windings, so those resistance readings might not tell the whole story. I would focus on fixing the AC section first before worrying about the flyback. I would also change Q11, the optocoupler, and CR5. There is a very good chance the flyback may not be the issue.

On a side note, I checked other many diodes and some either read a lower voltage than expected or they are shorted and read both ways...
Should I try to repair this board or should I just get another board/sell the Macintosh as non-working...?

In my opinion, selling it is like trading one 40-year-old car for another—you'll likely run into the same old parts failing anyway. If you aren't comfortable doing the work, it's worth finding someone nearby to help. Since these boards are so well-documented now, there is a very high success rate for bringing them back to life.

 

[ijnfuso]

I would focus on fixing the AC section first before worrying about the flyback.

Thank you for all the advice. By fixing the AC section which parts do you suggest me to concentrate more?

 

[Builder68]

Thank you for all the advice. By fixing the AC section which parts do you suggest me to concentrate more?

Check this thread.

The AC section are these ones:




I had to replace those four big caps (C34, C35, C38, and C39) in my INTERNATIONAL AB because they were bulged. I recommend checking everything (resistors included) and replacing the four diodes of the bridge rectifier without hesitation; I would also check if CR31 is okay.

With a little patience and luck, you should find one or more faulty components in this section.