Author Topic: iMac G3 CRT shrinks/stretches picture?  (Read 1704 times)

Offline Tobibrocki

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iMac G3 CRT shrinks/stretches picture?
« on: April 25, 2015, 12:50:12 AM »
Hi everyone,

I seem to have a little issue with my iMac G3's Internal Monitor, but before I get to that I should mention a few things about the iMac.

I got this Computer (350Mhz iMac G3, Summer 2000 Model without Airport and Firewire built-in) a little over a week ago and have replaced the PRAM Battery, swapped the Harddrive for a 80GB one, did a Reset of the PVRAM Settings on bootup just in case by holding down Command+Option+P+R and installed MacOS9.2.2 on it.
The Battery seems to work fine since the time and date doesn't reset itself to 1904 anymore.

But here is the issue, the Screen seems to not keep the picture where it should be based on my settings, I have used external CRT's in the past on Computers like my Amiga 1200 and everytime I turned that on the picture was as it should be in the spot I set it to and at the exact height and width that I adjusted, not on this iMac's internal CRT though, I set it to a certain size, turn it off, come back to it maybe an hour later, turn it back on and suddenly I'm greeted with a weird aspect ratio.
I tend to have it set to 1024x768 and I like having the picture take up as much screen as possible, so like I said above I turn it off after I set everything in the OS9 Monitor Settings and let the System cool down, turn it back on and the left and right side is shrinked to the factory settings almost, and the top and bottom go outside of the viewable area a little, especially the bottom portion, as I leave it on the picture slowly (about 5-10 Minutes) goes back into a state that resembles my settings, but not quite, actually. I have tried keeping it at the factory settings but moving the picture a bit more upwards to have a even border at each side. Turn it off, leave it for a while, turn it back on and the bottom oddly enough didn't leave the viewable area, but it's very close to it, as time passes it shrinks back into a position that, again, looks more like what I set it to, but never quite, as the monitor heats up the picture on the sides stretches itself out a few millimeters, so even at near factory settings it doesn't quite stick to it.
So far, everytime I turn the thing on, my first stop is usually the monitor settings panel to adjust it and it is quite tedious as I even have to back into it after the thing was on for a while to "unshrink" the image a little again.

Considering this is my first iMac G3 (or Mac with a CRT in general) I don't know if that is normal behavious for these or if it means the CRT is on it's last legs?
To be honest that would explain a lot but would also be a bit bitter considering I took this thing apart and cleaned it (there were also broken plastic bits flying around so I had to open it regardless).

Does any of you guys know a little more on the subject and what else I could do?

Offline Philgood

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Re: iMac G3 CRT shrinks/stretches picture?
« Reply #1 on: May 24, 2019, 10:14:52 AM »
Hi all.

I just came across some posts from a german mac forum called macuser.de where a guy called schnaggenhals posted about the repair of his iMac g3.

https://www.macuser.de/threads/imac-g3-bildgeometrie-veraendert-sich-laufend.135160/

Just run it through the google translator but he's saying that the fastest and cheapest way to repair this behaviour is changing a potentiometer on the PAV board.

*G4 MDD 1.25GHz (Single 2003)* with 2x 80Gb harddrives, 1Gb RAM, Tascam US-428 and Edirol FA-101 USB/Firewire soundcards-*iMac G3 DV 400MHz* with installs from OS 8.6-OSX Tiger on different harddrives-*Powerbook G4 1.67Ghz* with new SSD ! Love it.