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Classic repair issue

Svenska

Veteran Member
Joined
Mar 19, 2007
Messages
732
Location
Sweden
Hi,

I have recently acquired a Macintosh Classic, my first Apple device ever. Which means I have zero experience.

When turned on, the machine shows a checkerboard pattern, likely indicating that it wants its logic board capacitors replaced. There was no battery leakage yet (although the Lithium battery measured at negative 0.5V), but small signs of corrosion around the electrolytics on the logic board (plus a fair amount of dust). I have cleaned the board with ethanol ("rödsprit"), removed the leaky electrolytics and replaced them with tantalum capacitors which seems to be the correct approach.

Unfortunately, the machine only produces a little noise on the speaker when turned on and only shows a checkerboard pattern (some pixels are flickering). I did some poking around with a multimeter on the logic board, but did not find any broken traces. The corrosion was also very minor and cleaned up nicely. To my eyes, the board looks fine - but does not work. I haven't done any live tests because the cables are very short and I did not feel particularly brave near a running CRT yet. :)

I have not removed or looked at the analog board yet. The voltages on the hard drive power connector measure fine (4.9V and 11.9V) and since the screen shows an image, it should be fine for now. According to some information I found, it should also need a recap, but that can wait until the system works... right?

In any case, I don't really know where to start troubleshooting these systems, so I've attached pictures of the board (scanned at 300 dpi) in the hope that you see something that I don't.

Best Regards,
Svenska
 

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I would highly recommend doing the analog board as well. The caps there have leaked, I can assure you. Even when the voltages seem fine, the ripple is probably way beyond specs. Not sure if that can cause the checkerboard pattern, too. But since the PSU is the first part in the chain, that part must work 100%.
 
According to my handheld oscilloscope, there are about 280 mV ripple on the 12 V rail, and 500 mV ripple on the 5 V rail.
I'll report back when the analog board is recapped.
 
I've completely recapped the analog board, except for the two big 220 µF / 400 V caps. Not sure whether they are prone to leaking, but all other caps in the whole system have been replaced with fresh ones now.

Unfortunately, the system still displays a checkerboard pattern and I don't know much about these systems. What would the next steps be?

At least now I understand what people mean when they talk about the fishy smell... :)
 
500 mV ripple on the 5V rail is too much. TTL devices are designed for 4.75 to 5.25 Volts operation (at the most). 500 mV of ripple is 0.5 Volts (pk-pk) which is +/- 0.25 V; which is only just on the limit.

What is the ripple now you have done some recapping?

Dave
 
Both my Classic and Classic II produced very similar symptoms (one a checkerboard, the other vertical stripes, neither with any audio). Both had significant, though barely noticeable cap leakage.

I was advised that an ultra-sonic cleaner would resolve this where isopropyl probably wouldn't (and didn't, despite several attempts). Both systems are now working, including sound. The Classic AB was not cleaned (it still needs it).
 
What is the ripple now you have done some recapping?
About 40 mV on 5V and 60 mA on 12 V, measured on the hard drive power connector (without floppy/hard drive connected). Looks good to me.

I was advised that an ultra-sonic cleaner would resolve this where isopropyl probably wouldn't (and didn't, despite several attempts). Both systems are now working, including sound. The Classic AB was not cleaned (it still needs it).
Hmm, I don't have access to an ultrasonic cleaner... used ethanol and a careful application of toothbrush, nothing else.
So you're saying that a deep cleaning was all your systems needed to work again?
 
Many people put the board in the dish washer. When the caps did leak, a very good cleaning is needed, as the stuff can also be under the ICs and cause leaking current there. That is one cause of the checkerboard pattern.

I did understand your first post so that you did gave the board a good cleaning. But if not, you need to do that.
 
Hmm, I don't have access to an ultrasonic cleaner... used ethanol and a careful application of toothbrush, nothing else.
So you're saying that a deep cleaning was all your systems needed to work again?
Actually, yes in the case of my two systems, I think so. I sent my boards away for recapping, and was told when they were returned that they had actually needed only a good ultrasonic cleaning. Once refitted, both computers booted normally.

I since checked the photos I had sent off, and compared them with the boards that came back - there's noticeable difference. On the photos, there was quite a lot of slightly textured surfaces on the boards, and on the cleaned boards that came back, none at all. I think this would probably be true under components too, where you can't really get with a brush or a suitable fluid to clean.

I suspect that it might also depend on the degree to which the 'cap goo' might have damaged the board - I was lucky that my Classic II had been stored in dry but cold conditions for some years, so hadn't deteriorated as much as some might have done.
 
I did understand your first post so that you did gave the board a good cleaning. But if not, you need to do that.
I had cleaned the area under and around the caps with alcohol and a toothbrush, but not the rest of the board.

Many people put the board in the dish washer. When the caps did leak, a very good cleaning is needed, as the stuff can also be under the ICs and cause leaking current there. That is one cause of the checkerboard pattern.
Did some (a lot) more scrubbing on the board, which changed the checkerboard pattern to a garbage screen with a fully corrupted Sad Mac.
Then I did even more scrubbing, which turned it into a regular Sad Mac (error code 0000000E / 0000FFFF).
Then I went the dishwasher route, and ... it is back to a checkerboard pattern. :-(

Actually, yes in the case of my two systems, I think so. I sent my boards away for recapping, and was told when they were returned that they had actually needed only a good ultrasonic cleaning. Once refitted, both computers booted normally.
Thanks for the story. Unfortunately, ultrasonic cleaning is something I can't do, but I am quite certain that my board is now clean.

It also doesn't work, so now comes the actual debugging. Sigh.
 
Then I went the dishwasher route, and ... it is back to a checkerboard pattern. :-(
You only used water (no detergent) and gave it a couple of days to dry, right?

Because when the checkerboard pattern appears because of leakage under the chips, it's because that stuff is electrically conducting. And any remaining water left from the dish washer is as well.
 
We eventually replaced the mainboard with a working one (after recapping). The donor machine had a broken analog board, floppy drive and hard drive.
 
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