Apple 12" RGB (LC Monitor) - Focus Issue

[Scott Baret]

I've got a 12" RGB, commonly known as an LC Monitor (the kind a shadow mask tube in place of a Trinitron), and am having some focus issues with it which seem to go beyond the internal focus knob (which I have not yet tried adjusting).

When I turn the monitor on, the focus is fine. As I leave it on, it goes out of focus. This became worse over time.

I get a feeling this has something to do with a component in the power supply. It seems to be fairly common on this model of monitor. If anyone has a guess as to what this could be and what I would need to fix it, please let me know.

 

[MidnightCommando]

I, too, have this issue with my LC Monitor, and would like to know if it can be repaired.

 

[Strimkind]

I'm not very knowledgeable when it comes to CRTs but I am thinking a bad capacitor myself on the power board.

 

[techknight]

bad flyback.

For potential power supply issues, you can monitor the B+ line feeding the Horizontal Output Transistor with a multimeter, as time goes on you can monitor this.

But ive seen this in the past with other monitors, and its always been a failed flyback, because the Focus/G2 block starts to go bad on the flyback itself. Eventually it shorts and will blow the neck on the CRT (intense arc/cracks neck).

the HV has a built in regulator/x-ray protect circuit. if the HV starts drifting out of spec, the focus/brightness will as well. But the X-ray circuit will trip shutting down the monitor. Bad caps in the HV regulation circuit could cause this, but as i said it would effect overall brightness. You could grab a high voltage probe and stick it up under the anode cap. Watch the reading as the focus drifts. if it stays solid, replace the flyback. If you cannot find a replacement/NOS flyback, you could go the hacker route and disconnect the focus lead off the CRT along with the G2/Screen Drive lead.

Trim those back and HV corona dope/isolate them away. Use an in-line focus/G2 block off the high voltage anode lead (like how projection TVs did it). And problem solved.

 

[techknight]

This site explains exactly as I am talking about. Funny that i actually found something like this. lol.

http://www.jestineyong.com/?p=562

And this is where you purchase the new Focus/G2 block:

http://www.noahtec.com/mbb.htm

And here is the site to perform that hack i was talking about:

http://www.electronicrepairguide.com/flyback-divider.html

In case you are wondering, Ive done this before several times. But of course back when i fooled around with CRT monitors, this stuff wasnt documented on the internet. Now it is, so its easier to look at/learn/do.