TheMightyMadman
Active member
Hi all,
I have a bit of a weird issue with a Macintosh Classic II that I'm trying to repair.
At first, the analogue board was seemingly completely dead, and the digital board was giving a "stripy" screen.
The digital board now seems to be working OK, following a recap and a CPU replacement (some of the pins fell off due to electrolyte corrosion).
The analogue board still has problems - the system seems to work OK, but only when powered from my isolation transformer.
When powered from direct mains, the system tries to boot - however, when cold the system constantly resets and the high voltage cuts in and out; when warm, it seems to usually boot, but the display is very wobbly and the hard drive doesn't spin up immediately, so the system doesn't boot off it.
The analogue board suffered major electrolyte leakage and subsequent damage.
I've replaced all of the electrolytic capacitors on it (except the line filter cap), and I've replaced the two mains filter capacitors.
I've removed and cleaned most of the larger components including the transformers and heatsinks, and the PCB underneath them all.
I've replaced several of the actives and passives, particularly those in the low-voltage filtering area which was most affected by electrolyte leakage.
Does anyone have any ideas as to why this might be happening?
Is it the current limiting of the isolation transformer, or the decoupling from mains earth?
Could there be a problem on the mains input side of the analogue board?
Many thanks,
Adam




I have a bit of a weird issue with a Macintosh Classic II that I'm trying to repair.
At first, the analogue board was seemingly completely dead, and the digital board was giving a "stripy" screen.
The digital board now seems to be working OK, following a recap and a CPU replacement (some of the pins fell off due to electrolyte corrosion).
The analogue board still has problems - the system seems to work OK, but only when powered from my isolation transformer.
When powered from direct mains, the system tries to boot - however, when cold the system constantly resets and the high voltage cuts in and out; when warm, it seems to usually boot, but the display is very wobbly and the hard drive doesn't spin up immediately, so the system doesn't boot off it.
The analogue board suffered major electrolyte leakage and subsequent damage.
I've replaced all of the electrolytic capacitors on it (except the line filter cap), and I've replaced the two mains filter capacitors.
I've removed and cleaned most of the larger components including the transformers and heatsinks, and the PCB underneath them all.
I've replaced several of the actives and passives, particularly those in the low-voltage filtering area which was most affected by electrolyte leakage.
Does anyone have any ideas as to why this might be happening?
Is it the current limiting of the isolation transformer, or the decoupling from mains earth?
Could there be a problem on the mains input side of the analogue board?
Many thanks,
Adam



