CRT gone wonky! - Page 1
Forum section: electronics / repair

author: cylindricpanda
author_rank: Newbie
timestamp: January 03, 2020, 03:28:52 am
content: Hello there, I've got a powermac G3 all-in-one (the one used mostly in schools) that is having a little trouble. When I first got it the display worked fine, but after letting it sit for a while when I turned it on again, I can only describe it as completely shifted...In this pic I've got the room as dark as possible so you can see the screen, but in normal light it's almost impossible to see this much.In #2 you can see I got the system to boot and turned the brightness all the way up and that's what it looks like. I've got more pics on flickr, if you want to see them I'll post the link instead of spamming on my first post. As far as my research has shown, this is possibly an issue with the flyback transformer. My next step is to try the official apple display adjustment tool, after that I'll get into the guts and see if I can see anything blown or misaligned, but I wanted to see if anyone had any ideas what this could be from and I figured this place would be one of the best to find out!Sorry for such a big post for the first one but what can you do...

author: james_s
author_rank: Super Contributor
timestamp: January 03, 2020, 04:52:43 am
content: Don't try adjusting anything, something has obviously failed. I wish I had a dollar for every monitor, TV, etc that somebody has rendered uneconomical to repair by twiddling every adjustment they could find when something failed.First thing I would check is the power rails, especially the B+ that is feeding the video amplifier. Also take a look at the CRT heaters and see if they're glowing the normal yellowish orange. The video amplifier is clipping, that's what makes the streaks to the right, and the picture is very dim, suggesting some of the voltages may be low. Electrolytic capacitors are a common fault.

author: cylindricpanda
author_rank: Newbie
timestamp: January 03, 2020, 04:38:40 pm
content: yeah I should have thought to check the caps, hopefully it's only a couple few that are bad...

author: cylindricpanda
author_rank: Newbie
timestamp: January 07, 2020, 05:18:47 am
content: Did some preliminary poking around inside and aside from evacuating some dust lions, the caps on the video board and the CRT board look ok, I'll have a better look and get better pics when I take them off and outside to remove the dust. I'll look in the power supply itself too; if it's not the caps, what else could be cramping the voltage?

author: james_s
author_rank: Super Contributor
timestamp: January 07, 2020, 05:46:41 am
content: There usually won't be any visible signs of trouble, you have to test the capacitors with an ESR meter. Have you measured the voltages? Are they labeled anywhere on the board?

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 07, 2020, 01:02:45 pm
content: Remove the shielding on CRT board, first check for dry solders there.If you are not used to testing capacitors (other than looking at them) this is the first place to start a shotgun approach.The first ones I'd be after are the video amp. PS decoupling.

author: andy2000
author_rank: Frequent Contributor
timestamp: January 07, 2020, 03:39:59 pm
content: Start by suspecting any small electrolytics in hot areas (near big heat sinks, or dark areas of the board). Pay particular attention to caps in the video processing circuits, often mostly on the CRT socket board. There should also be a roughly 10-33uF 160-300v cap that filters the power supply to the video output circuit. It's usually near the flyback, or on the CRT board.Since this is an all in one, don't overlook the possibility that the computer's video card has a problem. Older Macs were notorious for having leaking SMD caps on the motherboard. I think this is too new for that, but it's worth checking.