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

not necessarily, it does not seem to be that high voltage, and it's going to a pin that is not there, so it is not likely to hurt anything.

No I need to restore the set first. I just wanted to secure a CRT before i even tried becuase there would be no point. the original CRT was an open cathode. I tried to weld it but i was tapping too hard on the CRT and necked it.

Here are some pics of the set I am working on.

We're these pictures before you necked it?... because besides the bakelite base being off (a fairly trivial fix I can explain in a paragraph) it looks okay.

Yep. I took the base off on purpose because it was loose, I did this to determine if it was a broken wire causing no emissions.

But it was not. I let it cook for an hour, with no emissions whatsoever, I tried to go through the H-K Short removal/cathode welding process and tap on the neck. I got nothing. I tried different angles of the CRT and eventually i cracked the neck and that was that.

After looking, and looking... and looking...

This could be the tube that is in that box.
https://frank.pocnet.net/sheets/127/1/17BP4.pdf

This is just a guess on sight alone, no way to know for sure, also it's doubtful that it is NOS at this point, could be used or rebuilt, how does it read on a emissions test?

Well it is definitely the same brand tube that is in there. The electron gun shows no signs of wear either. No I have not tested it yet but that's actually a good idea...

The 17BP4 & 17HP4 are identical mechanically and size wise, but differ as is the biasing, as in the 17BP4 is an older version, other than that, no dif, but that SHOULD have been stated on the box that it is a 17B not a 17H...

But it's pretty certain you have a 17BP4 there and it will be a pain to rig a circuit for focus for it. :yes:

Are you by chance confusing biasing with basing? Both share the same standard 12 pin basing with the exception a 17HP4 normally should have a focus pin in location 6. :scratch2:

I think every CRT from the 40s through the mid-50s used the same pinout. I might have a spare PM focus assembly if you do end up needing one.

yeah, i got the words intermixed, :/

key being that 17HP4 has 12L and 17BP4 (which that tube seems to be) has 12D basing.

and if bandersen has a PM assembly that will work, then that's the best way to go, cause as mentioned, the one I have from the broken 21EP4B is too strong for this tube. :(