Focus coil does not have sufficient range

[LukeSimon]

Yeah, Magnavox T933, but to a lesser extent also my Magnavox T931 has similar symptoms but not as extreme. The T931 reaches optimal focus just before maxing the focus control whereas the T933 is still far from ideal focus when at max. R150 is part of a voltage divider that is inbetween the focus coil, which is an adjustable buck boost transformer and the kinescope focus anode, and so I had suspicions it was a problem. I will test it to see. I will also try swapping in a new focus tube, HV rectifier, and HOT.

Side thought: if the chassis is going to use a voltage divider on the focus rail, why even bother with the buck boost transformer? Why not just use a 50meg pot as the focus control?

[Electronic M]

Originally Posted by LukeSimon

Side thought: if the chassis is going to use a voltage divider on the focus rail, why even bother with the buck boost transformer? Why not just use a 50meg pot as the focus control?

It was probably a matter of cost. The 15M focus and convergence pots rated rated for that voltage in the CT-100 are expensive unobtanium. A slug tuned buck boost transformer is cheap.
The 70s Zenith's that used a resistive divider off the HV for the focus used IIRC a 5M pot and kept it on the ground side of things to make a conventional pot viable, and prevent the need for something expensive and exotic.

Tube based focus I usually bypass with a modern diode...It almost always results in a significant improvement even if the original tube was doing well enough not to complain.

[zeno]

As far as alignment goes DONT do it unless you have to or really want to.
If you must flag your diddle stick & write down the CW or CCW so you can go
back. Without signal you need to get a heavy clean snow without
streaking, ghosts etc. If you dont start easy & safe with tubes.
After that inject at video det diode & trace things through if IF seems good.

Zeno

[Alex KL-1]

These HV resistors sometimes used to be hygroscopic... in one TV I have in my small collection, I needed to replace all focus resistors due to this. Value changed with temperature and especially with humidity...