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Macintosh Classic II Analog board randomly restart

N

N3rd

Member
Joined
Jun 21, 2020
Messages
34
Location
France
Hi everybody,
I am curently trying to revive a Macintosh Classic II. I have trouble with the analog board : it randomly restart. Most of the time, on a cold boot, the computer first enter in a restart loop. It restart every 5 or 10 seconds and after some time it finally boots and works perfectly.
The logic board is recapped and works perfectly with an other analog board.
The analog board is also recapped (I have replaced all the electrolytic capacitors). In first time, it presented a low voltage issue. I solved this by replacing both DP3 and DP4 diode and the QP1 optocoupler. I now have a stable 5.08v on the 5v line and 12.2v on the 12v line. I have also no display issue.
The only issue is these restarts.
I have tried to replaced the TDA4605 ic. It has changed nothing. I have mesured at multimeter all the diode on the board. They all seems correct to me.
Do you have any clue about this issue ?
Many thanks for your help.
 
Check for bad solder joints. The heat inside compact macs from the CRT will degrade the solder over time and cause the solder joints to crack/separate from the PCB and/or legs they're attached to. I would recommend redoing all solder joints on the analog board.

A test for bad solder joints can be done by tapping the insulated end of a screwdriver on the analog board and see if anything happens while the Mac is running. If you get screen glitching, machine crashes or other odd behavior, you likely have bad solder joints.

When the joints get bad enough, you'll start to be able to see the cracks, they'll usually look like rings inside the solder.
 
Thank for your reply. I have try to reflow most of the solder. Unfortunately, it has changed nothing.
 
If there was a problem with the CRT itself, it wouldn't go away, it would always be present.

There's likely still a problem with the analog board.

I would recommend to start from one end of the supply and check backwards or forwards every component. Resistors, diodes, etc. and make sure nothing else is open/short/way out of tolerance.
 
I had a similar Analog board issue, and ended up washing the board several more times to clear it up, there was still left over liquid from leaking caps over the years.
 
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