Focus Coil circuits?
 

Does anyone have any examples of Electromagnetic focus coil circuits theyd be willing to share?

I have an electrostatically focused set that I am attempting to adapt to electromagnetic focus because i got my hands on a CRT, and its NOS.

I found a Merit MF-4 coil listed on ebay, but I do not know which sets it was associated with or how to build a focus coil circuit outside of whats built-in to a normal set.

Thoughts? Thanks.

Dig around here some.
http://earlytelevision.org/postwar_t..._diagrams.html
Probably better than %90 of the 1950 and older sets in their postwar sets at the museum gallery are electromagnetic focus...Search those models in the schematics page I linked and you'll find various schematics with focus circuits.

Thanks! I will take a look.

You could also consider a permanent magnet focusing mechanism and need no external circuitry. Early RCAs, such as the KCS 49 chassis, had such an arrangement. I also think some of the early Philcos had PM focusing units.

Yeah i know, like the focalizer. Problem is. How do I find/locate one? I have not been successful so far.

I think the General Electric 10T1 locomotive set had a very straightforward electromagnetic focus. Pretty much a high impedence coil, a pot, and filtered DC power if I recall.

the only I worked with was on my FADA (rca 630ts clone) and had a 12 inch screen 12LP4, which needed a bit stronger magnetic field vs the 10 inch 110ma vs 100.
otherwise , the circuit was almost the same as below, except for having a PM ion trap, and having that circuit omitted.

http://www.qrp.gr/technology/vttv/rca630ts.pdf


i still have the PM focus magnet of of a necked 21ep4b, but at almost 120ma equivalent,it's no doubt way too strong to use on the CRT you have,
you never DID say what that was...

Thought I did say? Sorry.. its a 17FP4. it does not have the focusing anode. the CRT that was in the set originally was a 17HP4. I picked up the CRT NOS from a guy who has tubes listed on the ETF Spreadsheet. he drove down locally and I met up with him to pick it up and do the exchange.

I picked up a cheap 100mA focusing coil from ebay that I need to build a mount bracket and a circuit around. But i dont know if that is enough current or not. I may make a pitstop back down to the tv shop warehouse and have another look around to see if i can find a junker set i can steal a coil from if not.

Something does not add up here...

according to frank.pocnet.net datasheets, BOTH the 17FP4 and the 17HP4 have electrostatic focus and the same bias of 12L

https://frank.pocnet.net/sheets/127/1/17HP4.pdf
https://frank.pocnet.net/sheets/127/1/17FP4.pdf
and thus are pretty much the same.
Meaning focus is p6 and magnetic focus wont work.
Is the " 17FP4" above an error? cause it does show it as ES focus.

That's what I thought it was, but it is not an electrostatic focus tube. the Tube literally has no markings though, and the box says 17FP4. See pics:

https://media.discordapp.net/attachm...512&height=683
https://media.discordapp.net/attachm...512&height=683
https://drive.google.com/uc?export=d...eed6oZdOxr7eUB

then you are in a very tricky situation, you really don't know WHAT THE HELL that tube is, or what it may do if you try to hook it up and use it. :scratch2:

Welp, that was the tube that was in the box. and I opened that box. so that is how it came. But who knows what Moyer did 100 years ago.

As far as a tricky situation? Story of my life. I am used to it.

Have you tried the tube? Quite possibly it will focus just fine with no focus magnet or focus voltage. If you read the typical operational conditions there's a note C that specifies the preferred condition for best focus. :scratch2:

...Won't you also have to disable the existing electrostatic focus circuitry?

to be on the safe side, I would check it with A VOM ans see that the biasing matches, hell if you were blind and nearsighted like I am you could almost peek inside ans see what goes to what, and one can normally spot if it's an ion trap type tube or not, like this one no doubt WAS.
https://i.imgur.com/yy8qIUO.jpg