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saturizer

Macintosh Classic chessboard screen

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I've bought a Macintosh Classic computer recently, it was in a very nice condition with original keyboard and mouse, but with an issue of a chessboard-like pattern displayed on the screen all the time. At first I thought that it might be caused by capacitors leak on the logic board, but after I've cleaned the logic board in a dishwasher and than with pure alcohol, nothing happened — the pattern on display was the same. So I decided that caps are dead and need to be replaced. I've replaced all the capacitors on the logic board, but again with no result. After that I've found the information about this issue might been caused by broken capacitors on the analog board, so I've replaced a group of caps mentioned here (http://www.maccaps.com/MacCaps/Capacitor_Reference/Entries/1990/10/15_Macintosh_Classic_Analog_Board.html) total 8 caps, although they aren't all the same as in the guide, cause I have a later model (1992) I suppose. Still no result, the pattern on the screen is absolutely the same.

 

What else could I try? Any help or ideas are much appreciated!

post-6389-0-22898800-1459983539_thumb.jpg

post-6389-0-67656600-1459983567_thumb.jpg

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How can I measure voltages? I have a multimeter, but don't know where to measure voltages in this computer.

 

At the Hard drive Molex connector, you should have 12 and 5 volts.

 

 

there is a lot of corrosion on this board.

Edited by bibilit

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At the Hard drive Molex connector, you should have 12 and 5 volts.

 

 

there is a lot of corrosion on this board.

 

I have 4 and 11 Volts there.

 

The most of the board is just darkened, the only place really corroded is at the small chip signed as VR1 near the four caps, two of its legs were pretty corroded and I'm not sure if they are properly connected to the board. I've scratched off the oxides with a needle but I can't check the connection between the legs and the board because the probes of my multimeter are too thick.

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This is Analog Board related, double check your work, but also can be a problem related with a cracked solder joint somewhere.

 

Try to check voltage on the Analog board side first to confirm readings, and maybe clean the solder on the AB for connector going to LB.

 

There is a potentiometer also named PP1 (IIRC) that can be adjusted, but should be only used as last solution.

Edited by bibilit

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This is Analog Board related, double check your work, but also can be a problem related with a cracked solder joint somewhere.

 

Try to check voltage on the Analog board side first to confirm readings, and maybe clean the solder on the AB for connector going to LB.

 

There is a potentiometer also named PP1 (IIRC) that can be adjusted, but should be only used as last solution.

 

For the caps I've replaced I'm 100% sure everything is okay cause I checked every capacitor, its placement and +/- position before replacing it and after. The analog board is in very good condition, with no leaking caps, as I removed the dust, it looks like new. Maybe some other capacitors are broken on the analog board? Or other components?

 

Where can I check voltage on the analog board safely (without a risk of damaging the board or multimeter)?

 

I also found a story pretty like mine, but with no solution:

http://appletothecore.me/files/leaky_caps.php

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http://old.pinouts.ru/Power/macclassicpower_pinout.shtml

 

 

This is the pinout for the J12 LB connector, should be colour coded and easy to locate on the AB side.

 

 

 

I also found a story pretty like mine, but with no solution

 

No solution because if i remember clearly the Ab was swapped with another one without troubleshooting it first (and second hand)

 

I have repaired many Classic ABs, and very few were beyond repair.

Edited by bibilit

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http://old.pinouts.ru/Power/macclassicpower_pinout.shtml

 

 

This is the pinout for the J12 LB connector, should be colour coded and easy to locate on the AB side.

 

 

No solution because if i remember clearly the Ab was swapped with another one without troubleshooting it first (and second hand)

 

I have repaired many Classic ABs, and very few were beyond repair.

 

Thanks, I'll check this tonight!

But what might cause lower voltage?

 

Yes, it seems strange for me that he didn't tested both analog boards and compared the result...

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Saturizer, next to the line of capacitors you replaced are 3 chips that probably handle data/video signals and that might be causing your problem.

If you check the output of the analog board and the voltages are within spec, theres a encoder and a buffer to the left (the 16 pin maxim chips) and a omp amp below (8 pin).

Considering that the mac classic isnt that old, I'd suggest retinning all of the connections near the capacitors, after cleaning off the bad corrosion.

I'd also recommend reflowing all the solder joints on the analog board, because they crack and can disconnect over time. I'm pretty sure the reason that the link you posted didn't

fix his problem was because the analog board wasn't the problem in the first place, because the problem came back even after he adjusted it the first time.

 

Anyway hope you get things sorted out.  :) 

Edited by K55

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Saturizer, next to the line of capacitors you replaced are 3 chips that probably handle data/video signals and that might be causing your problem.

If you check the output of the analog board and the voltages are within spec, theres a encoder and a buffer to the left (the 16 pin maxim chips) and a omp amp below (8 pin).

Considering that the mac classic isnt that old, I'd suggest retinning all of the connections near the capacitors, after cleaning off the bad corrosion.

I'd also recommend reflowing all the solder joints on the analog board, because they crack and can disconnect over time. I'm pretty sure the reason that the link you posted didn't

fix his problem was because the analog board wasn't the problem in the first place, because the problem came back even after he adjusted it the first time.

 

Anyway hope you get things sorted out.  :) 

 

Yep, it looks like the problem is in the logic board, but in that Mac Classic story the logic board and those microchips near the capacitors were undamaged with no significant corrosion. That story seems important to me cause it's the only case where the pattern on the screen is exactly the same as the one I have. In all the other cases I saw, when there were leaking or dead caps, the patterns were more chaotic, while I have perfect b/w rectangles.

 

I'm not quite sure I got you right about the microchips you've mentioned, could you please name them by their signs on the board (like VR1, UE8 and so on)? I was trying to find some kind of a scheme of the logic board with microchips' names and specifications, but haven't find anything so far.

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Yeah in order the UH6, UG6 and VR1, in particular the VR1/LM751 looks pretty bad. I don't believe the classic has a public schematic.

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Yeah in order the UH6, UG6 and VR1, in particular the VR1/LM751 looks pretty bad. I don't believe the classic has a public schematic.

 

Just checked the voltage from the analog board.

+12V and -12V — 22,5V

+12V and ground — 11V

+5V and ground — 4V

 

So, what components on the analog board could cause lower voltage? I can replace all the capacitors left, but it would be much better to know the reason.

I would like to know what this VR1 thing is, cause if it's somehow related to video, than probably it's the reason of my problem, although that guy with the same problem got all those microchips in perfect condition with no corrosion at all...

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TTL min vcc is around 4.75 v, so with a 4v line and a 11v, you're probably causing a few logic circuits to not get enough voltage to operate correctly. I'm not good at analog board debugging, but most of the time it's old capacitors and cracked resistors.

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the opto oscillator is an issue with those 230v AB, for some reason.

 

How does it look like or is it somehow signed on the analog board? Is there a way to test it to know it's corrupted for sure?

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