Problem with voltage on Classic (analog board)

#1 Thu, 12 Mar 2015 - 09:42 68kMLA

Hi, i have a old Classic. But the problem is in low voltage. In most of time it working fine, but sometimes the hard disk is slowing or is totally stops (commonly if heads is running) and screen is a little bit unstable on up corner. Then mac is freeze. I have a 220V version (no jumper JP1).

#2 Thu, 12 Mar 2015 - 11:02 68kMLA

Caps are done.

Here is ground zero for leaking caps

http://www.maccaps.com/MacCaps/Capacitor_Reference/Entries/1990/10/15_Macintosh_Classic_Analog_Board.html

#3 Fri, 13 Mar 2015 - 02:54 68kMLA

OK, I replace all caps (i look your link), but it not resolve the problem. Screen is still unstable.

#4 Fri, 13 Mar 2015 - 07:03 68kMLA

Are the caps from the Logic Board OK ?

#5 Fri, 13 Mar 2015 - 09:09 68kMLA

All caps on logic board (not analog) i have replaced. And i replace 8 caps on analog board (i used instructions from uniserver's link. I tested voltages and if computer is idle, the disk have 11.59 and 4.86 volts. But if using (eg load os) then voltage is unstable

#6 Fri, 13 Mar 2015 - 14:05 68kMLA

Opto oscillator ?

#7 Sat, 14 Mar 2015 - 15:28 68kMLA

I've replaced it, but noting happen. Screen is still unstable on up corners. But hard disk is working fine for 15 minutes after system boot.

#8 Sat, 14 Mar 2015 - 19:20 68kMLA

Check your smoothing caps again, There is one cap that if you look at the polarity is confusing on the pcb.

also keep in mind cap goo could still be under something so squirting the smoothing area down with IPA 91%

i use these bottles from ebay all the time to apply my Flux / IPA / Acetone love em!

http://www.ebay.com/itm/Two-1-OZ-bottles-with-stainless-steel-needle-tip-dispenser-for-Flux-/111004315495?pt=LH_DefaultDomain_0&hash=item19d85f6f67

#9 Sun, 15 Mar 2015 - 17:41 68kMLA

Now I replaced all caps on analog board. Voltage on disk connector is 4.85V and 11.51V (i think, this is quite bit low. On my mac pro is 12.04V and 4.95V), but mac is working fine (no errors or hangings). Now screen is unstable only.

#10 Wed, 18 Mar 2015 - 14:11 68kMLA

How can i do with this screen?

#11 Thu, 19 Mar 2015 - 09:03 68kMLA

Ok. I adjusted voltage on 12V adjust resistor then problem with unstable screen is gone. Great! I think all is done. Now i have 12.04V and 5.06V on disk. But 20min after that classic is freeze and the fuse on analog board is blowup. Something goes wrong?