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Classic Analog Board low power

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Hi, I have a Classic and Classic II. I have a problem with the Classic II analog board that I can't fix. It was leaking caps which I have replaced. The Molex connector on the HDD was reading 4.52 and 11.32 volts on the classic II analog board. On the Classic analog board they are reading 4.98 and 11.92 and both the Classic and Classic II logic boards are working fine with the classic analog board. So I know both logic boards are working. With the classic II analog board nothing appears on screen with the classic II logic board and with the classic logic board I get a checkerboard and the HDD doesn’t startup. So I gradually turned up PP1 watching the voltage (turn off, turn up a bit, turn on, check voltage etc), when it reached the maximum on PP1 it read 5.2 and 11.81. I turned it back a bit it to bring back to 5v, the HDD now starts up but nothing changes with the screen so obviously the 12v circuit isn’t enough to drive the logic boards.

What might be not allowing the 12 v circuit to get past 11.8v ? I have changed every Electrolytic Cap on the analog board except CP1 which I can’t find a replacement for, I have taken it off and it doesn’t look like it has leaked.

I don’t get a bong start on the classic II logic board on either Analog board but I am assuming that there is a bad sound circuit on the logic board given all caps had leaked on the classic II Logic board. It still boots up ok and reads the HDD on the classic Analog board.

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Maybe, just maybe, since both the +5V and +12V rails are lower than expected, there might be a voltage pot you can adjust on the AB? I don't know what it is off the top off my head.


Strange, but I've also had an uncapped Classic II LB (rev B) that had no sound – don't know what it was, and I guess I won't know for a while since I lifted a few pads while trying to recap it.

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Read through cheaper 9 of this book. You'll find most of your answers there. It's likely you need to replace the opto coupler.


What version of AB do you have? CP1 should be available from any major electronics dealer.


For sound my CII originally had scratchy sound then lost it all together. After recapping the AB and I think desoldering the sound chip and cleaning and putting it back on it stared working fine. One of the caps on the logic board directly affects the sound. If you pull that trace and don't repair it will have issues as well.


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  • 2 weeks later...

Hi all happy to report that I have fixed the issue,  I replaced the Optoisolator (QP1) and DP3 and DP4 as per the advice from bibilit.  I also noticed that the two RIFA capacitors (CP19 & CP38) were severely cracked on the top of their cases so I also replaced those.

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Hi Cinan, everyone,


I'm going trough the same process,I have just bought a Mac Classic and a Performa 200, both have a faulty analog board.

I have replaced the capacitors as per https://recapamac.com.au/macintosh-classic-analog/

If was not enough, no voltage at all, so I replaced the TDA4605 and the RIFA capacitors (CP19 & CP38) because they were cracked for me too.

Now I have 5v and 12v, but the voltage soon drops, so now is the turn of CNY17G, IRFBC40, DP3 and DP4 (1N4148as dochilli is recommending.


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