Ct-100

[Tom Albrecht]

Thanks for the tips on the degaussing coil. I checked eBay a few days ago, and only saw one for $39, which I thought was perhaps a little steep. Buyers for degaussing coils these days are probably few and far between! Anyway, there is certainly a nice selection of degaussing coils today, so I decided to order one.

I've parted out a few color sets over the years, and have always pulled the degaussing coils as a very useful source for a lot of nice magnet and coil wire in a very accessible form. I may have one of those fully intact somewhere, but I guess it will be nice to get one of the commercial degaussing coils instead of trying to make use of an old coil from a junked color set. Certainly sounds workable enough, though.

I've been making some measurements of what's happening when the focus goes out on bright scenes. I'm becoming convinced that the problem is focus voltage instability, not a HV regulation problem. The HV hardly budges, while the focus voltage changes by several hundred volts out of about 2.5 kV. Does anyone know of a solid state replacement for the 1X2B? I may also try a string of a bunch of 1N4007s just to see if a lower impedance rectifier would make much difference. I suppose I should check whether the voltage at the output of the 1X2B (as opposed to at the focus output to the CRT) is stable before worrying about the rectifier itself. It's possible that the voltage dropping potentiometer and resistors are the problem, driven by a change in current on the focus grid. The circuit looks like it was designed with the assumption that the focus grid would have little or no current flow, but I certainly seem to have enough to cause a problem.

I've been hearing from some of you that defocusing on bright scenes is par for the course on these sets, so this experimentation with focus stability is perhaps really more about understanding the design limitations of the set, and not true restoration, I suppose.

I'm also wondering if there is just a touch of gas in my CRT, which may be having some subtle effects.

Still working on the vertical dynamic convergence -- that remains one trouble spot with the set. It's watchable, but would be nicer at the bottom of the screen with better dynamic convergence.