iMac DV display raster rotated wierdly...

[Schmoburger]

As the title suggest, the display on the iMac DV SE i just built is rotated noticably clockwise. It wasnt too oddly positioned when I booted it the first time prior to installing a hard drive, however the second time after I had unplugged it and put the hard drive in, it came out rotated like this...

ANybody got any ideas on what might be an easy fix? Ive tried the control panel, there just isnt the adjustment to set it straight.

 

[CelGen]

It's as if your deflection yoke slipped.

 

[Schmoburger]

Having done some other generalised research on the workings of CRT's in my absense it seems as tho you may be right. It'd stand to reason as the geometry is basically as it should be, and there are no anomolous hues or focus issues i can see... just a tilted display. I shall try some adjustment and see if I can get it playing ball.

 

[mcdermd]

Take off the outer shell, loosen the yoke clamp from the CRT neck and turn it.

 

[Schmoburger]

So I now have the shell off again, and there would appear to be two different clamps in close proximity to each other on the neck of the tube... one has the screw mounted horizontally, the other has the screw vertical, and theyre one behind the other... which is the one i need to undo or adjust? Also, if the tube hasnt been discharged, are these clamps to be considered live or are they outside of the realms of stored CRT voltage?

 

[Schmoburger]

Problem solved. Did some careful, cross-referenced research on CRT servicing procedures and precautions, ascertained that the yoke is safeish, at least with the machine powered down and allowed to auto-discharge some. Just to make sure tho, I used a hollow-plastic handled screwdriver with a shaft that only goes as far as the base of the handle, wrapped from top to bottom in electrical insulating tape just to make sure, and stood in a cardboard box with rubber soled shoes, and one hand behind my back to minimize any risks.

I carefully loosened off both clamps but it appears the rear is the one i am most concerned with for the purposes of yoke tilt adjustment... turned it the oppposite direction to the way the screen display was rotated, and gently tightened both clamps up just tight enough to secure the yoke in position without damaging anything. Put the shell back on it, booted it up and it was far better. The allignment was still slightly off... with it sitting a bit down and to the left, and ever ever so noticably tilted, but well within the viewable area now and easily able to be straightened out to where i want it with the monitor geometry panel with plenty of adjustment to spare. I could get it better if I fiddled with the yoke further but i think illl leave that until the plastics absolutely need to come off again.

 

[Byrd]

To the uninitiated it's a scary thing, adjusting CRTs, but once you get past that it's pretty easy I recently enjoyed fixing up a 14" 15khz CRT housed in a cocktail arcade cabinet, the glue holding the yoke and rings had either perished or been moved in transit, took a lot of work but finally got it as good as I could get it. Happy that a CRT still lives instead of an LCD put in its place.

 

[Schmoburger]

Yeh I have worked around them before and given them the room and respect deserving of something that can store enough charge and impart a high enough current to cause death, but never had any desire to explore them any further even tho i have a sound understanding of electrical principals and the basic ins and outs of a CRT. For something where there consequences of an error are painful and destructive at best, a i didnt consider a basic understanding to be sufficient to go into it with boots on lol.... CRTs are scary!

 

[techknight]

you have to be VERY careful with CRT yokes for color CRTs. any slight mis-alignment or slippage of the yoke will throw off the purity and the convergence.

I doubt the yoke slipped, because if it did youll notice a change in purity, and at the very least convergence.

I think a cap failed in the pincussion or keystone correction circuit, you just corrected for the symptom, and not the cause. The one thing that is sticking out at me for this reasoning of thought is the fact that the bottom is at a slant much larger than the top. usually the slant will be equal if the yoke slips.

 

[Schmoburger]

Mmm that's a fair point too, and there could be something to it... There did apear to be evidence of movement in the yoke (computer was a mess and had endured some very poor handling, so it'd not surprise me), tho the color convergence before and after adjustment seemed to be pretty much spot on. This said, on closer inspection it does appear however that the geometry isnt quite what it should be, as with everything basically squared up there is a slight "droop" downwards at the top left edge. Will have to see how it fairs and if it deteriorates again it might be an analogue recap time.