• Updated 2023-07-12: Hello, Guest! Welcome back, and be sure to check out this follow-up post about our outage a week or so ago.

Mac Classic II randomly restarts

brayne

Well-known member
Hi all, this is a duplicate of a post I published on a Mac group on Facebook, so my apologies if you're reading this for the second time.

I was hoping to get some help from one of the tech gurus out there regarding a Mac Classic II analog board (820-0525-B 240V). 

The board wasn't working, so I started by replacing all of the leaky caps (specifically CP2, CP6, CP7, CP8, CP9, CP10, CP12 & CP36) and giving the board a good clean. When I powered it up, it immediately blew the fuse, so I replaced the QP2 MOSFET and the IP1 voltage regulator which solved that problem.

Now it fires up fine, I'm getting a nice, steady 5v and the Mac boots and operates normally. The only problem is that it occasionally restarts all by itself. There doesn't seem to be any consistency to it. Sometimes it restarts after just a few seconds, and other times it will work perfectly for an hour before restarting. It's a known-good logic board, so the problem is definitely on the analog board somewhere.

Larry Pina's book on Classic & SE repair and upgrade secrets suggests that this symptom is being caused by a cracked solder joint on the high voltage transformer ZP1. So I removed ZP1, cleaned all the pins and soldered it back on, but it doesn't seem to have made a difference.

Has anyone else encountered this issue before? Or can someone offer any suggestions on what might be causing these intermittent restarts? Apart from this, everything else about it seems to work perfectly.

Thanks in advance.
 
Last edited by a moderator:

techknight

Well-known member
You need to get a multimeter and keep an eye on your voltages. See where they are hovering at. The fact that you started with a shorted MOSFET makes me highly suspicious there is more damage in the power supply than meets the eye. Start there. 

 

brayne

Well-known member
So far I've only monitored 5V, but there's no noticeable change at all when it restarts. If it's dipping or spiking, it's happening so fast that I can't see it on the multimeter. I'll check +12 and -12 too.

 

Paralel

Well-known member
I'm thinking along the same lines as TK. My Classic II had a similar issue, and it was in the power supply.

Also, is your meter digital or analog? If it's digital, depending on the model, the sample rate may be low enough that it can't see a true flicker type dip, which would be enough to trigger a restart, but not long enough to show up on a digital meter that has a low rate.

 

brayne

Well-known member
I'm thinking along the same lines as TK. My Classic II had a similar issue, and it was in the power supply.

Also, is your meter digital or analog? If it's digital, depending on the model, the sample rate may be low enough that it can't see a true flicker type dip, which would be enough to trigger a restart, but not long enough to show up on a digital meter that has a low rate.
I have a DMM, and I don't think it's responsive enough to show any sort of flickering. I haven't been able to observe any real voltage fluctuation when it restarts.

 
Last edited by a moderator:

techknight

Well-known member
Then use a scope with a set trigger just below 5V and do a one shot capture. when it dips, itll trigger the scope. 

 

MacintoshMan1999

Well-known member
I know this thread is kinda dead but,

My Macintosh Plus does the same thing when I move it. I think it’s the connector between the Analog and Logic Board. Not sure if it still does it, but it's been a while since I’ve moved it so I don’t think so.

 

maceffects

Well-known member
Probably a bad solder joint, the Mac 128k-Plus had that issue, often detected if hitting the side by the analog board fixed or worsened the issue.  Doesn’t seem to be the same issue as the OP. 

 
Top