Hi all,
I'm trying to get a 512k (intl analog board) up and running again, and I think I've tracked the problem down to the power supply. The initial presentation was a dead mac (no video or chime, no HT as far as I could tell) with a rapid clicking/chirping when power was applied. I measured the 12V supply to be peaking around 4V, and 5V to be around 1.2V. I started off by recapping all the electrolytics, with no change. I also followed the suggestions in the Dead Mac Scrolls (rectifier replacement, video cable continuity) to no avail.
After some poking around on the 240V side of the board (with the help of Thomas Lee's excellent notes), I found R53 (1.2R, 1/2W) was an open circuit, so I replaced that with a 1W equivalent, and got what I think is the infamous flup flup flup noise (slower, louder, and lower pitch than the chirping before).
I scoped the 12V and 5V rails, which showed the crowbar was kicking in, even with the voltage turned all the way down.
The feedback circuit seems to be working (output of U1 is going negative as the supply voltage passes 12V), and I replaced the opto-isolator just in case, but the problem persists.
At this point I suspect the SCR Q12. It tests fine to a multimeter, but I understand it's quite a sensitive part (hard to replace, too). Before I go hunting for a replacement for the SCR, I thought I'd see if anyone had any other ideas.
Looking forward to your thoughts!
Tom
I'm trying to get a 512k (intl analog board) up and running again, and I think I've tracked the problem down to the power supply. The initial presentation was a dead mac (no video or chime, no HT as far as I could tell) with a rapid clicking/chirping when power was applied. I measured the 12V supply to be peaking around 4V, and 5V to be around 1.2V. I started off by recapping all the electrolytics, with no change. I also followed the suggestions in the Dead Mac Scrolls (rectifier replacement, video cable continuity) to no avail.
After some poking around on the 240V side of the board (with the help of Thomas Lee's excellent notes), I found R53 (1.2R, 1/2W) was an open circuit, so I replaced that with a 1W equivalent, and got what I think is the infamous flup flup flup noise (slower, louder, and lower pitch than the chirping before).
I scoped the 12V and 5V rails, which showed the crowbar was kicking in, even with the voltage turned all the way down.
The feedback circuit seems to be working (output of U1 is going negative as the supply voltage passes 12V), and I replaced the opto-isolator just in case, but the problem persists.
At this point I suspect the SCR Q12. It tests fine to a multimeter, but I understand it's quite a sensitive part (hard to replace, too). Before I go hunting for a replacement for the SCR, I thought I'd see if anyone had any other ideas.
Looking forward to your thoughts!
Tom
