Focus coil does not have sufficient range

[LukeSimon]

What are possible issues to investigate when focus continues to improve as I move the slug further to the “out” extreme, but it isn’t ideal focus yet? It is like the focus coil doesn’t have sufficient range. Focus gets more blurry in the other direction, but lacks sufficient range in the good direction.

[Electronic M]

This post has WAY too little context. Make chassis are important to know what era focusing system and circuit.
Rectangular screen tube TV could be anything from a 1952 rectangular monochrome with a electromagnetic focus coil to a mid 70s color set with a focus adjustment coil on the flyback.

If it's one of those later sets sometimes the tube or SS focus rectifier is bad and sometimes ceramic peaking caps are going open, and or other issues.

[Yamamaya42]

I shall assume you mean with the Maggie T933 (or like) chassis, most likely reason is a worn R150, (66M 6kv) , these are always getting pounded and drift up in value, R149 can also get worn (2.2m), and to a much lesser degree, C122, (130pf), sometimes gets worn, but very rarely.

[zeno]

If its got a focus tube start there. Dont trust a checker.
If its a all tube Japan set they usually barely change the focus.Most
dont focus well even 40 yrs ago. You do get what you pay for !

Zeno

[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?

[Yamamaya42]

No idea why they do what they did, that's most likely lost in the RCA engineering archives someplace, but they must have had some reason for it, but I've had very good results replacing those 2 resistors mentioned, especially the 66m one, I have seen it drift as high as 74-75m, yes I DO have one multimeter that can measure up to 100 meg ohms, I got it just to test these resistors. :O

[LukeSimon]

Originally Posted by Yamamaya42

No idea why they do what they did, that's most likely lost in the RCA engineering archives someplace, but they must have had some reason for it, but I've had very good results replacing those 2 resistors mentioned, especially the 66m one, I have seen it drift as high as 74-75m, yes I DO have one multimeter that can measure up to 100 meg ohms, I got it just to test these resistors. :O

Thanks. I will see if I can source resistors rated for 6kv. On a related topic, my T933 also has limited contrast range and limited sharpness range. The contrast gets bright enough, but there is very little difference between min and max contrast. Sharpness is similar. There is some difference in sharpness between min and max, but not nearly as much as my T931 where the highest sharpness setting makes the video very sharp to the point where pressing a button on a wired game controller causes enough electromagnetic noise to show up in the video. Were you able to fix the limited contrast range on your T933? Have you seen limited sharpness range?

I am hoping it doesn’t need an RF alignment, though I do own an oscope and a B&K 415 Mark & Sweep Generator. I know how to use my oscope, but haven’t spent the time to read the 100 page manual for the B&K 415.

[Yamamaya42]

Originally Posted by LukeSimon

Thanks. I will see if I can source resistors rated for 6kv. On a related topic, my T933 also has limited contrast range and limited sharpness range. The contrast gets bright enough, but there is very little difference between min and max contrast. Sharpness is similar. There is some difference in sharpness between min and max, but not nearly as much as my T931 where the highest sharpness setting makes the video very sharp to the point where pressing a button on a wired game controller causes enough electromagnetic noise to show up in the video. Were you able to fix the limited contrast range on your T933? Have you seen limited sharpness range?

I am hoping it doesn’t need an RF alignment, though I do own an oscope and a B&K 415 Mark & Sweep Generator. I know how to use my oscope, but haven’t spent the time to read the 100 page manual for the B&K 415.

With mine anyway, the issue with weak contrast was that it was VERY VERY picky about the tubes put into it, even though NOS tubes were used, it just did not like them, had to try 2-3 to get best resuts for video ouput, video amp, etc.

[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...

[JohnCT]

The RCA TVs of the '60s were the same way. The tunable buck transformer was mostly reliable (even if the 2AV2 wasn't), but I remember cranking the slug in it a dozen turns in both directions and trying to see which way it looked better.

The Zenith focus method was less reliable but I loved the 300 degree pot. So much easier to focus sharply. I changed a ton of those brown rectifier sticks and focus pots.

John

[zeno]

For those hi ohm specialty resistors check an old Workman catalogue.
They repacked tons of OEM odd ball parts. Then check with the old time
parts dists. You can also use those ##'s to watch e-bay

73 Zeno
LFOD !