Classic II analog board voltage problem

[Cam]

Hello, I'm hoping someone can help me with a Classic AB voltage issue I'm having.

I got a dead (not even the fan spun up) machine cheap figuring a recap and a cleaning would bring it back to life as it has on other machines I've worked on. I replaced the capacitors on both the AB and LB, as they looked to be "original equipment" from the factory. The battery died, but did not explode (thanks to some miracle).

Now I have the fan spinning up, and the AB is ticking softly so definitely forward movement. The problem now seems to be that the 12v power rail swings between 8v and 12v continually. Reading on the forum I thought replacing the CNY17G might settle things down; nope! Next up were the diodes people mention DP3, DP4 as I had a bunch on the shelf; nope, same problem.

Anyone have any ideas where to go next? I've restored several old Macs but this one seems to be possessed by some kind of evil spirit.

Thanks.

C

 

[Kouzui]

I have a very similar issue with my Macintosh Classic, and have a thread going here documenting my process of trying to fix it. I'm waiting for the AB capacitors to come from Mouser at the moment, and will be posting the results there. Hopefully we can get our Macs fixed!

 

[chiptripper]

Check the backside of the analog board for cracked solder joints, particularly at plug connectors / wires and the transformers. Not as common as on SEs and Pluses, where it happens almost 100% of the time, but still find it shows up often enough on Classics.

We’re certain the LB is good?

 

[Cam]

I have a very similar issue with my Macintosh Classic, and have a thread going here documenting my process of trying to fix it. I'm waiting for the AB capacitors to come from Mouser at the moment, and will be posting the results there. Hopefully we can get our Macs fixed!

I read your other thread and you've done a lot of work on your classic. Battery bomb damage to the LB is always fun to repair!
Check over your AB carefully - I found that the Classic AB is VERY cheaply made and to much heat causes damage to the copper traces.

 

[Cam]

Check the backside of the analog board for cracked solder joints, particularly at plug connectors / wires and the transformers. Not as common as on SEs and Pluses, where it happens almost 100% of the time, but still find it shows up often enough on Classics.

We’re certain the LB is good?

Yep checked for cracked solder joints, and the only ones I found held the back bracket in place; I resoldered them anyways.

The LB? I recapped it and didn't see the usual cap leakage 'sheen' on the PCB I usually associate leakage. Once the AB is working I'll know more about the LB, in the meantime I'm using the hard drive as a load for the power supply.

 

[dochilli]

Most often: DP3 and DP4 (1N4148), TDA4605, CNY17G, IRFBC40.

 

[Cam]

Most often: DP3 and DP4 (1N4148), TDA4605, CNY17G, IRFBC40.

So I've read. I replaced them all, but to no avail.
One odd thing; the forward bias on DP3 & 4 look normal (0.5v), but the reverse bias does not (2.7v). These are the only 2 diodes that test 'oddly' on the AB, but I'm putting it down to in circuit testing (?).

 

[chiptripper]

Yeah I wouldn’t trust a diode reading in circuit, probably OK if you just replaced it.

I’ve occasionally had problems with PP1, it’s close enough to the big cap cluster so seems to take a hit.

 

[Cam]

A quick update . . .

Finally had time to sit down with the AB and a soldering iron.
- reflowed any connections that even looked questionable - didn't fix anything
- replaced the 1N4148 diodes, and used aluminum alligator heat sinks when soldering new ones in - no fix
- replaced the IRFBC40 - Winner, winner, chicken dinner! Ticking sound stopped, and the voltages all came up and stabilized.

So the next bit is to adjust the voltage to get 12v and 5v up to the right levels and to do that I need to find a tool small enough to adjust the PP1 potentiometer.

Baby steps, but still steps in the right direction.

 

[Kouzui]

Glad to hear the progress! I've done just about everything I can think of for my classic at this point, and can't shake the Sad Mac.

 

[Cam]

Glad to hear the progress! I've done just about everything I can think of for my classic at this point, and can't shake the Sad Mac.

Do you get any error codes, or just the sad Mac icon?

 

[Cam]

So today I found a screw driver bit small enough to fit PP1, and guess what? Adjusting it does NOTHING. The voltage on the 12v rail never changes from 9.8v no matter what position the potentiometer is in.

I said many bad words,

 

[Kouzui]

Do you get any error codes, or just the sad Mac icon?

0000 000F
0000 000A

Basically an unknown error given the context.

 

[dochilli]

Sometimes the pp1 needs some cleaning with ipa and sometimes it needs to be replaced. You can measure the resistance when you turn it. May be the resistance will not change.

 

[Cam]

0000 000F
0000 000A

Basically an unknown error given the context.

the first line is some kind of compatibility error - I got it on another machine. I think it was bad RAM? the second line; no idea.

 

[Cam]

Sometimes the pp1 needs some cleaning with ipa and sometimes it needs to be replaced. You can measure the resistance when you turn it. May be the resistance will not change.

Sometimes the pp1 needs some cleaning with ipa and sometimes it needs to be replaced. You can measure the resistance when you turn it. May be the resistance will not change.

Yea, it seems OK (varies from 0.5 to 180 ohm), but my meter shows the resistance creeping a little, even without touching it. I think for the sake of $1 part, I'm going to replace it.

 

[Cam]

Just to close this off . . .

I ordered new low ESR caps based on the list from Amega of Rochester (https://amigaofrochester.com/recap-services/capacitor-list/) which I didn't see before I did the cap work.

Replaced all the regular caps with the low ESR type, and the damn thing chimed and came right up!
I wish I had tested the AB after individual cap replacement to get better info on which one was the culprit, but maybe this info will help someone else out there.

C

 

[chiptripper]

Nice job!

They do respond best with low ESR caps, though it still should’ve still operated for a while. Maybe one of the first round of new caps was a dud, or the flaws in the circuit (which you’ve now addressed) burned one out.