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Macintosh Classic Checkerboard screen
Posted by: theorime on 2020-05-21 03:34:15
Hi everyone,

I just bought a Macintosh Classic from 1991. When I turn it on, I have to wait for a few seconds/minutes to make it startup (probably due to the analog board capacitors), but then I have the checkerboard screen issue with a wobbly image.

I washed the motherboard with solder flux cleaner, and then changed the capacitors, but it had no effects. So I have a few questions :

- Can the lack of battery cause this issue ? I couldn't find a suitable one for the moment

- I did not change the analog card capacitors, could it be the reason ?

- What are the expected voltages on the analog board to motherboard cables ?

- Do you think solder flux cleaner is not enough to wash the board ? Should I use water and soap ?

Thank you very much !
Posted by: Papichulo on 2020-05-21 17:48:43
Could be a ram problem
Posted by: OsvaL on 2020-05-21 21:47:21
I'm also in the process of restoring a Macintosh Classic, I will try to answer your questions according to the research I did so far. I may be wrong.

1) You don't need a battery to boot this model of Mac but every time you restart the computer you will need to adjust the time.

2) Most of the time, if not 100% of the time, you will always find bad capacitors on the analog board. Not only the capacitors but some other bad components like the optocoupler and diodes).

3) The expected voltages are 5V and 12V depending of the rail, you can use a voltmeter and a paperclip to test the voltages without opening the case through the external floppy port on the back of the Mac.

4) Using regular warm water and soap is a good option but you should be very careful giving the logic board enough time to let it dry completely, use Q-tips with isopropyl alcohol to clean the most stubborn parts.

There is a very good troubleshooting book you can check to learn more about all these issues, please DM me if you want me to share it with you.
Posted by: rplacd on 2020-05-21 23:06:54
Could be a ram problem
Agreed – you might also try removing, cleaning, and reseating the ROM (watch out for the extra column of unused pins in the socket!)
Posted by: theorime on 2020-05-22 00:23:28
Thank you all for your answers !

I washed the board with soap and water, but still no results... On the motherboard, I have 3.6V or 9V, but never 5V or 12V. Maybe  the analog board doesn't deliver enough voltage to make the motherboard works properly. I think i'll try to recap it.

Could be a ram problem
Agreed – you might also try removing, cleaning, and reseating the ROM (watch out for the extra column of unused pins in the socket!)
What could be the problem on the RAM, and how could I check it ? About the ROM, I have to admit that I'm scared to remove it. Is there a special technique to do so ? I'm trying to pull it raw but it doesn't move even a little bit
Posted by: bibilit on 2020-05-22 00:43:56
Capacitors on the A board. 

Common issue, won’t work without stable voltages. 

4,85 and 11,9 volts are the minimum requirements to get things going and capacitors on the Logic board should been replaced as well. 
Posted by: theorime on 2020-05-29 19:26:03
I finally found the problem !

After cleaning the logic board, replacing the capacitors on both boards and two diodes on the analog board (as suggested by @bibilit on another thread), still nothing worked. My voltage was a little bit better, but still not enough.

So I changed the CNY75GB optocoupler, and now everything works perfectly fine !

Thank you all for your help
Posted by: bibilit on 2020-05-29 19:35:07
Yes, common issue as well.

Well done.
Posted by: Roninfourseven on 2020-07-29 16:57:57
I currently have the same problem, Mac Classic with a wobbly checkerboard screen on boot. It had worked previously, and in fact was fine about 6 months ago when I got it out to test and see if it was working, which it was, then the next time, wobbly checkerboard.

Voltages were poor, with 4.5v and 11.6v so I swapped out the caps on the analog board and now get 5v exactly and 12.2v. I also recapped the logic board, but I still had the same problem, even after cleaning the board around the caps and chips with IPA. I would say at this point there wasn't any evidence of leaky caps on the logic board, and neither had the battery leaked. The analog board caps were wet around the base on removal and clearly leaking, so needed to be replaced. The horrible fishy smell was present too!

I washed the board last night, so it's currently in a warm cupboard drying out, but the talk about the CNY75GB is new to me, I will look into that next, thanks. I'll post up here when I solve my problems.
Posted by: DomDom on 2022-07-27 13:41:41
Update I got it booting everything's working fine now I reseeded the chip u10 and that did nothing but after receded chip u3 because it looked super corroded and then it booted perfectly
Posted by: imactheknife on 2022-07-27 15:58:06
Well, even if caps dont “look” like they are leaking they are. I bought a mac classic on ebay with original box, and was mint. Once I received it, first thing i did was open it up and check battery etc. board was clean and looked really good. I thought, easy quick recap. She worked before recap. After, just horizontal lines, no sound. I found two eaten traces between the 4 47 caps by power plug and the little regulator thats in the same area had to legs corroded (found out once i started to clean board). Had to tske sound chip off and clean. Still horizontal lines.. did a warm vinegar bath for an hour, good rinse in sink, then an hour in isopropyl. Let air dry. Worked fine after that.
Posted by: DomDom on 2022-07-27 16:03:27
Well sound is working I recapped him myself and I will continue checking for any problems floppy drive works hard drive works thing couldn't run better. But I will run it through speedometer and compare it to a normal one.
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