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Mormislaw

macrumors newbie
Original poster
Aug 23, 2018
27
5
Poland
Hello
I've bought a 350mhz iMac G3 Blueberry and after some time the image started to get weird, I'm no CRT expert, but I've read that it's called pincushion distortion. In my case it looks like this:
Screenshot_20200322-151100~2.png

At first, it got normal after about 15 seconds from startup or if I hit the side of the Mac, but now it gets normal only after 10 minutes of hitting the side, and it's not a good fix, also, I think hitting a 20 year old computer might not be the best idea.
Does anyone know any fix for this, or a better workaround?
Thanks in advance for any help.

Sorry for my poor English
 

philgxxd

macrumors 6502
Feb 11, 2017
303
246
Malaga, Spain
Your english seems perfect to me but what do I know, I'm just another non-native english speaker.

Luckily I think I can help you there.
I already repaired 3 iMac's with the same issue.
Best case scenario is you only have to replace a little potentiometer.
[automerge]1584892879[/automerge]
Let me prepare a summary for you and others.
 
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philgxxd

macrumors 6502
Feb 11, 2017
303
246
Malaga, Spain
I wrote about it already some time ago in the Collectors section of the forum. And here also.

Disclaimer: I want to thanks a german engineer on macuser.de called schnaggenhals who guided me through the repair.
I think he won't mind if i attach a photo he sent to me once.


You have to replace a potentiometer called VR901 on the PAV board. That alone will probably get rid of your problem and is
dirt cheap.
If that doesn't help try replacing the flyback transformer (expensive) and the big cap C905 (tricky to find a replacement).

Good luck!

I also recommend searching for help in the Collector section. @MacTech68 is a very knowledgeable soul over there, a big thanks goes out to him also!
 

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Mormislaw

macrumors newbie
Original poster
Aug 23, 2018
27
5
Poland
I wrote about it already some time ago in the Collectors section of the forum. And here also.

Disclaimer: I want to thanks a german engineer on macuser.de called schnaggenhals who guided me through the repair.
I think he won't mind if i attach a photo he sent to me once.


You have to replace a potentiometer called VR901 on the PAV board. That alone will probably get rid of your problem and is
dirt cheap.
If that doesn't help try replacing the flyback transformer (expensive) and the big cap C905 (tricky to find a replacement).

Good luck!

I also recommend searching for help in the Collector section. @MacTech68 is a very knowledgeable soul over there, a big thanks goes out to him also!
Thank you so much, that is big help!
Also, I've never attempted a CRT repair, and people say it's dangerous and I can get electrocuted. Should I try to discharge the CRT or is it not necessary?
Thanks in advance
 
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philgxxd

macrumors 6502
Feb 11, 2017
303
246
Malaga, Spain
I would always be very cautious as the 15" CRT screens seems to have voltages as high as 20.000!
I was very aware of that and asked my wife to be near of me when I did the discharge just in case.
But whenever I put the screwdriver with an earth connection under the suction cup I never got an actual discharge. It seems as long as you use common sense and wait a little bit after you turned of the machine you are safe.
My iMac G3 400Mhz and newers seem to have a good implementation of a bleeding resistor so that residual voltages from the CRT get burned fast.
 
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Mormislaw

macrumors newbie
Original poster
Aug 23, 2018
27
5
Poland
I would always be very cautious as the 15" CRT screens seems to have voltages as high as 20.000!
I was very aware of that and asked my wife to be near of me when I did the discharge just in case.
But whenever I put the screwdriver with an earth connection under the suction cup I never got an actual discharge. It seems as long as you use common sense and wait a little bit after you turned of the machine you are safe.
My iMac G3 400Mhz and newers seem to have a good implementation of a bleeding resistor so that residual voltages from the CRT get burned fast.
I can't thank you enough! I thought I'd have to replace the whole display or the whole board, but it seems that even I am able to repair it :) I'll try to do it in a few weeks, and will send updates
 
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