Mac Classic analog board has no voltage

[Bagpuss]

Hello!

I have a Mac Classic that refuses to start up. I received it after being battery bombed, and have purchased a new logic board, but before that can be installed I want to make sure that the AB is behaving. I have changed every electrolytic cap the board, as there were some signs of leakage from the usual cluster, and cleaned the board afterwards.

The analog board is model 820-0395 and shows little signs of life. Measuring the molex connector for the hard disk shows 0 volts. The fan doesn't spin. The only thing that I can see is that pin 6 of the TDA4605 has a fluctuating voltage of between 0 and 8 volts AC, which suggests to me that the board is trying to start up but something is stopping it.

I have checked the fuse, and replaced diodes DP1, 3 and 4. I have also installed a new TDA4605, CNY17G optocoupler and IRFBC40 mosfet. I have reflowed the solder around the larger joints and tested for continuity from the AC input, so the socket where it goes into the board seem to be fine. Nothing seems to help and whatever I do doesn't seem to progress any further.

Of course these boards are notorious for having problems and there are going to be a number of things that could be the issue, but I'm wondering if - given the information I have provided so far - I might have simply missed something else?

Thanks!

 

[Byrd]

Honestly, you've done everything that needs to be done. Is there any damage on the backside of the board - around the mains voltage section? Have you deep cleaned the board in and around larger components where cap goo can leak down (in Classic AB it was extreme - like honey).

 

[s_pupp]

I can understand the hesitancy of risking the new logic board, but testing the Classic’s analog board with no load attached could result in erratic measurements.

 

[Bagpuss]

I can understand the hesitancy of risking the new logic board, but testing the Classic’s analog board with no load attached could result in erratic measurements.

Yes, good one - I have tested it with the logic board both attached and not attached, as well as a hard disk. Same results unfortunately.

 

[Bagpuss]

Honestly, you've done everything that needs to be done. Is there any damage on the backside of the board - around the mains voltage section? Have you deep cleaned the board in and around larger components where cap goo can leak down (in Classic AB it was extreme - like honey).

The board looks pretty good. It came out of a Classic that was already very clean inside, almost no soot on the CRT or board, suggesting to me that it wasn't used much. There is only the familiar 'pitting' of one trace on the back (between DP8 and CP9) but it still measures as connected. The cap juice wasn't too bad under cluster, but admitteldy I cleaned only on that side of the board as I read that you should to take the flyback off before dousing it in soapy water which I didn't fancy doing. The HV side looks like it escaped entirely. I suppose I should just bite the bullet and clean the whole thing entirely in soap and water and see if that helps, but my gut feeling is that there is a component that is not happy and I'm not sure how to go about testing for it.

 

[bibilit]

Hi,

Voltage at pin 6 should be 12 volts (AC) unless you have a faulty DP1 (zener diode 18 volts, and 12 volts after RP11) maybe the Zener