CRT turns on vertically squished, picture corrects over time

[Michael_b]

I have a weird problem with a 12" 640x480 display. When I first turn it on, the picture is extremely squished vertically - otherwise fine. Over about 10-15 minutes, the display gradually stretches vertically until the picture looks normal. This happens so slowly that it really is imperceptible.

I'm guessing this is a capacitor issue, though would be good to hear if there is another potential fault before I decide to invest any time/money into repairing this.

Thoughts?

 

[Byrd]

Yes, I'd suspect a PSU/caps issue - do you hear any cracking/noise as the tube warms up?  If so, the flyback might be going out

 

[mdeverhart]

My 13” (M1212) does the same thing, although mine has a visible overlap of the scan lines, so it’s both visibly distorted and squished. I suspect caps in the vertical drive circuit, but I haven’t had a chance to crack it open and investigate. I don’t hear anything when mine is warming up.

 

[techknight]

definitely capacitor related issues in the vertical deflection circuitry. you have a bad one, or multiple bad ones. 

If you dont take care of it soon, you risk damaging the vertical output IC. 

 

[mdeverhart]

Good to know, thanks @techknight. Fortunately mine doesn’t see much use, but I’ll see if I can crack it open soon.

 

[Michael_b]

definitely capacitor related issues in the vertical deflection circuitry. you have a bad one, or multiple bad ones. 

If you dont take care of it soon, you risk damaging the vertical output IC. 

Well, I replaced every single electrolytic capacitor in the monitor. Annoyingly, the display is still letterboxed, though now it doesn’t get any better over time. 

What else could it possibly be? I don’t imagine I’ll put any more time into this monitor, but I’m curious as to what could cause this problem.

 

[Byrd]

I assume you’ve adjusted the trim pots for horizontal width? If so I’d be looking at the flyback next.

 

[Michael_b]

I assume you’ve adjusted the trim pots for horizontal width? If so I’d be looking at the flyback next.

I maxed out the vertical width with little effect. If it is the flyback, I likely won’t bother to repair it... 

though it does seem like the electron gun is shooting just fine, it’s just not being deflected up and down enough to reach the top and bottom of the tube. In that case, would the flyback still be the primary suspect?

 

[techknight]

Well if it was improving with time before replacement, and its not now, after replacement, you should go back and double-check your work. Making sure you used the correct values in the correct spots.