SE FDHD's CRT geometry: a curved bottom edge

[MindWalker]

Hi.

My SE FDHD has a slight geometry problem; the bottom edge of the image is not straight. From about half way horizontally, it's either that the left side curves up or the right side curves down. Looking at the picture attached, it might be that the bottom-right corner is slightly skewed to right and down. 

I've had this machine for years, and I don't think it had this issue when I got it. It might have formed during my disk drive swaps, so it's probably something that I've bumped into during disassembly.

I am familiar with the usual height/width pots and the horizontal/vertical adjustment rings on the neck (and the dangers of CRTs, yes), but I haven't done any detailed geometry adjustments on CRT Macs yet. Any ideas on what to try?

 

[nkeck72]

If you open up the machine and look at the deflection yoke (the coils on the CRT), there will be a bunch of magnets around the yoke that have been siliconed in place from the factory. The purpose of these magnets is to tweak the image geometry around the edges and corners.

To be perfectly clear: I am not talking about the positioning rings (called "convergence rings" on color tubes), but rather a bunch of square magnets arranged around the entire yoke assembly, closer to the front of the tube.

To adjust the edges, simply rotate the magnets on the bottom side of the tube (you'll have to apply a little more force to break the adhesive at first) by twisting them around the stalk they're mounted on. You may have to play with one or two of the magnets to get it perfect.

This is based on my experience with a Mac Plus, the tweaking magnets may be different or not even exist on the SE/30.

 

[MindWalker]

Thanks, tried it and managed to get it somewhat better. Couldn't reach all magnets and didn't have time to disassemble. Need to get back to this on better time.