Mac classic checkerboard

[arthegor]

Hello 

I bought a mac classic with checkerboard problem. After many search i have determined a low voltage on analog board. i have changed 8 capacitor and the opto-oscillator. i have checked the qp2 transistor and diode bridge and is good. i have mesurated with oscilloscope a voltage and the ripple is hudge. in your opinion of where can it come from ?

cordially

alim 12V mac classic.bmp

 

[apm]

What's the DC value of the 5V line? I just recapped a Classic analog board; all the hi-temp caps in the power supply section had leaked and the voltages were very unstable. A bad 12V rail will affect the screen and the drives, but won't be the source of the checkerboard.

You'll also need to recap the logic board, which is the most common cause of this problem.

 

[arthegor]

hello 

I have already reccaped the logic board. the voltage on 5V line is good only if the analog board is alone. If I connect logic board, screen and drive the voltage is only 4V and 11V. 

 

[apm]

What about just the logic board, or just the drive? You'll want to make sure there isn't a fault in one or the other that is dragging the voltages down.

The next step would be to replace the rest of the electrolytic caps on the analog board (especially the PSU part). You say you did 8 of them, but there are quite a few more.

 

[bibilit]

I agree, a bad Hard drive can be responsible (happened to me once)

Plug the Logic board alone, won't work without the Logic board anyway and see what's going on.

 

[arthegor]

when i plug the logic board alone, the problem is the same. I have check the hard drive with external atx supply and is good. 

In this case i go reccap all analog board and hope that it will work. I keep you informed.