I'm working on a Zenith 24H20 chassis, and the focus coil is open. I had some similar looking focus coils from other sets, but found that they don't provide enough focus control range.

The reason is pretty straightforward: Unlike most focus coils, which go in series with a current source in the set, this Zenith focus coil is across the B+, so it operates at a much lower current, probably in the 10-20 mA range. Sams doesn't show a part # for this coil, but does mention a resistance of 3300 ohms (compared to 800 ohms for the more common type of coils I have available).

I'd like to check with Moyer to see if they have one, but without a part # that may be difficult. Anyone know a part # for this focus coil? Or have one in your junkbox you can spare?

If I can't find one, I can use one of the other focus coils, but I have to bulid up a special supply for it, which I'd prefer not to do.

I tried running it in series with the B+. This works as far as the focus is concerned (total 130 mA on the B+, and the focus coil needs about 60 mA, which I can get with a 1000 ohm pot across the coil), but it causes too much B+ drop so the horizontal deflection starts to show some problems. So that's not going to be a solution.

I don't want to rewind the coil, since the wire is quite fine, and it looks like thousands of turns.

I tried tapping into the coil at various depths in the windings, but it's as if the coil is damaged in several places. I can't get continuity across any decent fraction of the coil.

I found S-17230 Mechanical Centering & Focus Assy.
but nothing specific to a Focus Coil at these Zenith documents:
http://www.earlytelevision.org/tv_schem ... grams.html
(scroll to bottom at 24H20)

Carl

I found it best to put a resistor there, remove coil, and, put in a mechanical permanent magnet focus assembly in fron another set such as Silvertone, or, RCA.

Not only is that coil high resistance, it has an added permanent magnet as well.
Similar in design to GE, but, GE used much lower resistance.

As far as I know, no replacements were made for these, or, the GE coils. I think that first, they figured not enough would go bad to matter, and, secondly, the sets wouldn't be in service that long anyway.....

Found that with several parts, including the oddball flybacks used in early GE's, and, Bendix tv's.

I used a mechanical focus from a Silvertone on a 19" porthole Zenith once, and, it worked great. Perfect focus.
Bill Cahill

Carl,

That makes sense. The centering assembly is one unit with the focus coil. I drilled a few rivets out so that I could separate the focus coil from the rest and get the focus coil apart to put a new winding in it (albeit a winding designed for higher current than the original).

Bill,

Thanks for the suggestions, and also confirming what the situation with this coil is.

I'll probably go ahead and make the little power supply needed to drive the higher current winding. I don't happen to have a spare magnetic focus unit lying around, and the set will be cosmetically closer to original (no change in focus magnet appearance, and I can have a focus potentiometer in the same back panel position as the original; extra transformer will hide under chassis).

I have a 24h20 chassis that did have a open focus coil. I tried to get one from zenith appox 40 years ago and had no luck. Amazing at that time though was zenith had a headquarter located in cranford nj and had offered me a new set of my pickings in return for the 24h20. Of course I said no. By the way I was a zenith dealer at the time. These focus coils are hard to find. I had my crt rebuilt 16gp4 with a electrostatic gun instead of a 16gp4 with electromagnetic gun. This worked out fine. Then appox 2001 I was rebuilding a old house and one of the framers asked me if I wanted a old tv where he was working at appox 50 miles from me. At first I said no but I knew I had 3 forty yeard dumpsters at my site and said ok. Quess what?A 24h20 combo in really bad shape but the focus coil was good. I then posted on the arf and found a good 16gp4 from analog tv that was original and not modified with a different gun. Keep looking and you will find it. I would have to look at my notes but I can walk you through the process to get you a great picture.

I am going to scrap out another 10" Zenith porthole with a VERY bad cabinet and CRT. I'll look and see if it has a magnetic or electrostatic focus assy. When I am able to get to the storage again.

If you're doing that let me know if you'll part with the flyback or not.

Brian,
I'd prefer to sell the entire chassis.
If you are interested, PM me.

Tom, the parts set I have DOES have a focus coil.
Uses a 10 BP4 CRT.

Hi Frank,

I already put the lower resistance focus coil into the set and built a custom power supply for it. Works well, and the set is back to the customer.

In case someone else needs one, you might measure the resistance of the focus coil. For the 24H20 chassis, you need a focus coil with resistance in the 3000 ohm range. Most focus coils are much lower, including Zeniths.

I'm looking into the parts chassis right now. If I decide to go ahead with it, I'll keep that in mind.