Focus Coils

[LukeSimon]

I am having trouble finding cross reference information for 1960s color TV focus coils. Is there a JW Miller catalog that covers 1960s color TV focus coils? I can only find their older 1950s catalogs. How interchangeable are different models focus coils? How many different focus coil models did the aftermarket brands make (such as Thordarson and JW Miller)?

Is it possible to use a voltage divider, like used in solid state TVs, instead of a focus coil?

[Electronic M]

Dividers can be used. You may need to find a 5-10M pot too since the set probably doesn't have one.

Focus coils can be found, but I don't have the cross reference info. If you're dealing with an RCA clone you could probably look up the RCA chassis it clones then look up newer and older RCA chassis and see if they kept using the same part number and if Sam's crosses it to other generic replacement numbers in other sets folders.

[old_tv_nut]

??? Please explain what is a "focus coil" for sets that used electrostatic focus?

[LukeSimon]

This evening, I did some patent searching, and I found the RCA patents that explain the design and operational theory behind how focus coils work. There are two main patents corresponding to the focus coil design in RCA's early color TVs and the coil design used in RCA CTC-15 and later chassis designs. The second patent describes a refinement to the coil design that allows it to adjust for a larger range of voltages than the earlier coil design.

Vintage TV focus coils look like an early form of a buck boost transformer. Just like early television gave rise to the first flyback power supply (the first switching power supply), it looks like early television also gave rise to the first buck boost transformer. Someone needs to make a list of everything in modern electronic devices today that was born in early television. I bet it would be surprising how much of vintage TV tech is still alive today in electronic devices manufactured in 2022.

The patents mention the advantage of a focus coil versus a potentiometer is a greater power efficiency, which is important for non-solid state color TVs, which pushed the limits of power output for a horizontal output stage with no transistors. So while a pot could be used in place of a focus coil, it would place a greater power drain on the horizontal output tube and the flyback transformer. Both of these components are pretty much running at their limit in non-solid state color TVs, so I think it is best to stick to a focus coil after reading the patents.

My TV uses the second (improved) type of focus coil, and I think an exact match focus coil isn't required. I just need to replace with a coil that can be adjusted to a voltage that is close to the optimal point in the OEM coil.

[Electronic M]

Originally Posted by old_tv_nut

??? Please explain what is a "focus coil" for sets that used electrostatic focus?

I had the same brain fart on the first read...He doesn't mean a coil on the neck of the tube to create magnetic focus of the guns like a 40s monochrome set, but an adjustable transformer coil in the HV cage that controls focus voltage is to the focus pin of the tube.

[Yamamaya42]

It's more of a tunable transformer than a coil in the way it works, is it not?
and is there not normally a list of sub parts in the SAMS?

[LukeSimon]

Yes they are a buck boost transformer. I didn’t study the patents until after my post. Strange that some chassis designs placed these focus control transformers in the HV cage because, as RCA’s patents explain, they are not high voltage and do not require any special insulation protection. They run around -700 to 700 volts AC.

The Photofacts for the Mangavox T931 that I am working on does not provide any aftermarket parts. JW Miller has a dozen models, and their focus control transformers come with a datasheet, but since nobody has scanned and uploaded the datasheets, I don’t know which one to buy. I see Early Television Museum has many different JW Miller models in stock. Any chance they could scan the datasheets and upload them on their website?

[zeno]

Use the OEM p## first. Format is ######-# for old Maggy.
Cross reference that to Workman & JW Miller etc numbers.
Then watch E-bay.
Also do a WTB here & the AR site.

73 Zeno
LFOD !

[old_tv_nut]

Originally Posted by Electronic M

I had the same brain fart on the first read...He doesn't mean a coil on the neck of the tube to create magnetic focus of the guns like a 40s monochrome set, but an adjustable transformer coil in the HV cage that controls focus voltage is to the focus pin of the tube.

Aha - thanks

[LukeSimon]

Originally Posted by zeno

Use the OEM p## first. Format is ######-# for old Maggy.
Cross reference that to Workman & JW Miller etc numbers.
Then watch E-bay.
Also do a WTB here & the AR site.

73 Zeno
LFOD !

I have the Photofacts and the Magnavox part number, but there are no Workman or Miller catalogs available online that cover the mid to late 1960s transformers and coils. I can only find catalogs for Workman and Miller that cover years up to around 1959. I bet one of one of the Miller focus coil models on eBay or Early Television Museum is a match, but without either datasheet info or cross reference info, I am at a loss. I will email the Early Television Museum to see if they can share pictures/scans of the datasheets in the little coil boxes from Miller for the several focus coils they have in stock.

[damen]

The Sams Photofact for Magnavox chassis T931 shows a Magnavox part number for the focus coil as 361306-1. The Sams for a T974 chassis shows the same 361306-1 focus coil part number, but also lists a Miller number as H-214.

[LukeSimon]

Originally Posted by damen

The Sams Photofact for Magnavox chassis T931 shows a Magnavox part number for the focus coil as 361306-1. The Sams for a T974 chassis shows the same 361306-1 focus coil part number, but also lists a Miller number as H-214.

Thanks! I should have realized later Mangavox models would likely re-use the same parts and may have a more complete cross-reference. I found a partial picture of a datasheet for the Miller H-214 and the datasheet lists an alias model number “Miller H-1033”. See the lower left corner of the datasheet for the aliases.

[nasadowsk]

Odd that Zenith used focus pots for a while . Though it might be a case of patents, or NIH. Or maybe Zenith knew something…

[Electronic M]

Originally Posted by nasadowsk

Odd that Zenith used focus pots for a while . Though it might be a case of patents, or NIH. Or maybe Zenith knew something…

On color sets Zenith used the adjustible focus coil through 1970, then switched to an HV divider in tube sets after that and all SS sets. The advantage of the resistor divider is it's fairly reliable, and if resistance drifts you can add 2-8M of resistance to either side of the pot to bring the control back into range...As I had to do on my 1971 the first couple of years of daily service (out of something like a decade) until it stopped drifting.

Motorola was doing the resistor divider method before they came out with the SS WID sets.