I need to replace the transformer in my Dumont RA-102 TV, which uses an RF-type high voltage supply, similar to the HV supplies used in 7-inch TVs of the late 1940s, only beefier. This transformer has a tall hollow core with stacked coils as seen in the first photo. I believe I could use a Thordarson HVO-50 or Merit TV-231.

Thanks!

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

I have been told that I could also use a Spellman 10-15 KV-1 or Meissner 19-3210. So, here is my revised want list:

Thordarson HVO-50
Merit TV-231
Meissner 19-3210
Spellman 10-15 KV-1

I would love to locate one of these. My original has been damaged. As a stopgap, I'm using an outboard HV supply salvaged from a Westinghouse TV, but the best solution would be to repair and use my Dumont's native circuit.

Thanks!

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Why don’t you repair the existing coil? This type is generally easier to service than the usual wax-impregnated flyback transformer. Perhaps you can post a photograph showing the damage to your coil.

I don’t think that all of these coils were actually manufactured by one firm. There are notable differences of construction, though I can say that the Merit is almost identical to the Miller.

Spellman were, and are, prominent manufacturers of high-voltage power supplies, which often incorporate these Tesla type coils. There were actually a great many different coils of this style made in the 1950s and 1960s. They are still made, but for solid-state driver circuits. Only some of these coils will be compatible with the DuMont set.

There was never a large demand for the Miller #4526 and equivalent coils, so stocks may have been limited. As you suggest, it is likely that many were purchased to construct small vacuum tube Tesla coils, based on plans that appeared in Popular Electronics in 1964 (where it was called Li’l TC) and in some circuit books.

Of the coils cited, the Miller 4526 seems to be the one that most commonly appears on eBay, where the price can be relatively high if the listing is well written.

Your coil (in the set) actually looks quite different from the 4526. Has the 4526 ever been successfully used as a replacement in the RA-102? If so, it may require circuit modifications, if the resonant frequency is different.

If your coil can not be repaired, nor a suitable replacement located, it may be possible to wind a standard solenoid style Tesla coil with the same electrical and frequency characteristics, if there is space above the power supply for the extra height of the form tube.

Thanks for responding. I'm grateful for any advice.







The top connector was torn out and then someone tried to solder back onto the hank of wires hanging off the coil. There was some odd monkey business on this TV, in the HV section and elsewhere. All of the grid cap connectors have been crudely replaced. I don't know if someone cannibalized them to use elsewhere and then later replaced them, or if there was some misguided attempt to restore low/absent HV output. The repairman also eliminated the 500pf/10KV doorknob cap and covered the remaining parts with gobs of electrical tape (allowing replacement of the 12LP4 CRT with one that uses an aquadag HV filter??). There's more, but you get the idea.

I did my best to clean up the mess on that transformer connection and resolder it, but the best output I got was about 1.5KV before corona spots appeared on the coil and the output dived to zero.

As for repairing the original, when I examined the transformer more closely, I found a loose wire hanging out in the breeze on the far side of the top pancake. So there is more than one damaged spot needing repair.

No idea. I'm just repeating whatever people have told me about these various xfmrs. I think much of their info comes from old catalogs.

Yes, but how do you find out those electrical & frequency characteristics? Catalogs & data sheets usually give physical dimensions and an example circuit.

I'd love to be able to send this transformer to someone for repair. But how would they test the repaired transformer? This TV uses two chassis -- very heavy! -- and neither chassis works without the other. The power chassis containing the HV section requires a sync signal from the main chassis to trigger the oscillator and there is also a voltage regulator w/ some feedback. Here are the schematics for anyone who's curious:

http://antiqueradio.org/art/DuMontRA-10 ... atic01.jpg
http://antiqueradio.org/art/DuMontRA-10 ... atic02.jpg
http://antiqueradio.org/art/DuMontRA-10 ... ematic.jpg

Anyway, I appreciate any help & leads in locating a replacement or fixing the original. I have spent considerable time on this RA-102, which was found in ratty condition, and the rest of it is slowly shaping up.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

It was extremely helpful of you to post the link to the power supply schematic diagram. Scrutiny of it confirms that the Miller #4526 R.F. high voltage coil can not replace the coil in the RA-102.

Coils like the 4526 are specifically designed for power oscillator circuits employing Armstrong/Meissner feed-back via a “tickler� coil. They tend to work at 50kc or higher, and have a primary winding tuned by means of a variable condenser.

Your new photos clearly show a coil with a much lower self-resonant frequency; and the diagram shows that the coil is tapped, and has no primary or tickler windings.

The high voltage power supply of the RA-102 works differently than the R.F. types in other television sets. The left half of the 6SN7 is a blocking oscillator (triggered, as you say, by a synchronising signal from the horizontal oscillator circuit, likely so that the high voltage output will be dependent on the existence of sweep). The signal of the blocking oscillator, which is a pulse-type waveform, is amplified by the 807 beam power tetrode, of which the high voltage coil is the load. After each pulse, the magnetic field of the coil suddenly collapses, inducing a very high voltage on the output terminal. Because this circuit is synchronised to the horizontal sweep, the self-resonant frequency of the coil must be just above 15kc.

In the event that a correct replacement coil can not be found, there are few options to retain authenticity other than repairing the original coil, or having a replica or similar coil wound.

Otherwise, since the resonant frequency of the coil is the same as that of a flyback transformer, it may be possible to adapt one of the latter. Flyback transformers were driven by beam power tubes, so it may be possible to find one that will match an 807 without any changes to the load winding.

Still another possibility would be to use a Miller 4526, changing the circuit in the set to its type, as shown in the data sheet, or the Li’l TC diagram. You would have to provide a circuit to make the high voltage output contingent on the presence of sweep, if you desire to keep that function.

I have long thought that the DuMont RA-102 Clifton is the best-looking television set ever made. Even though its appearance is very different, it always puts me in mind of the “interocitor� from the film This Island Earth, 1955.

Thanks for the advice. I'm starting to understand this power supply a little better.

I edited earlier posts to remove references to the Miller 4526.

Perhaps I'll locate the right transformer eventually. It's hard to be patient when the rest of the TV is working well. The only remaining glitch is some vertical instability that I haven't tracked down yet.

It never occurred to me to compare this set to an Interocitor, but yeah . . . it's pretty cosmic looking in its way. We're familiar with This Island Earth -- I'm sure there's a copy somewhere in the house. The Clifton's styling was actually kinda retro for 1947. It resembles pre-war British sets like the Marconi 709 or Cossor 1210 more than American sets of its day.

In other reading, I noticed this clever plug-in HV replacement for a 7-inch Philco TV:

http://www.myvintagetv.com/philco_hv.htm

The earliest incarnation used HV modules capable of 10KV; naturally, those have also become unobtainable (sigh). An interesting idea, but if I have to cheat, I might as well hide the salvaged Westinghouse HV supply inside the Dumont's roomy HV compartment.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Isn't the 6BG6 very similar to the 807 ?

Most old-school photocopiers and laser printers have those HV modules in them...

Some are DC only and usually only around 5 kV but IIRC, some of them have an AC output that make it possible to use a voltage multiplier circuit and get higher voltages. Some have multiple outputs, sometimes one DC and one AC, etc. Depends on the specific module.

It will probably be very difficult to find an example of the existing high voltage coil, short of locating an otherwise irreparable set.

The 6BG6, a standard horizontal sweep tube, is indeed comparable to the 807. In principle, a flyback transformer for a set with a 6BG6 horizontal output stage, and a 10KV or slightly higher accelerating potential, should work in the RA-102 high voltage power supply. The main winding of a simple flyback transformer is tapped, creating an auto-transformer, just like the coil in the RA-102. Any additional windings or taps, except for the link for the high voltage rectifier filament, can be disregarded.

If a test shows that this plan will work, and there is space, the flyback transformer could be mounted above the existing coil.

I suspect that the original coil can be repaired. You mention that the wire on the surface of the top pie is broken and loose. This alone could explain the observations you have made. Surface damage like this can usually be repaired without the necessity of re-winding the pie on a coil winding machine. Do you have a photo that shows this damage on the coil?

Out of curiosity, what sort of power supply is the Westinghouse unit you are using as a temporary repair: R.F. or rectified 60-cycle mains? Either way, installing it would be far preferable to employing a modern high voltage unit; but it would be best to get the original circuit working again.

The close-up photo posted earlier is the best that I have. It looks like several wires were torn loose from that pie winding somehow.

The guy who sold it to me wasn't sure of the model number, but I looked at schematics and found that the Westinghouse H-223 has an HV supply that matches what I have.





The H-233 service manual is at: http://www.earlytelevision.org/pdf/west ... rider3.pdf . This Westinghouse unit supplies 8+ KV, which is sufficient to light up the CRT with good contrast & brightness. At the moment, I'm making temporary connections for plate & filament voltage and ground on the main chassis.

I haven't ruled out trying to repair the old coil. Unfortunately, there seems to be more than one damage spot.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Thanks for posting that schematic diagram. I find these R.F. high voltage power supplies endlessly appealing, and there are many variations.

If you would like to try a flyback transformer, the test would be quite easy to do.

Here is a detail from the horizontal output / high voltage stage of a 1949 Emerson 637 table-top television set:



I cut it from a larger detail posted on these forums by Clark, K90A in the thread:

viewtopic.php?f=3&t=154784

The coil on the left-hand side of the flyback transformer corresponds to your coil. Terminal #1 would go to B+, terminal #2 would go to the plate of the 807, and the outer terminal on the pie, #3, would go to the plate of the high voltage rectifier. A flyback transformer like this would have the plate caps already on the leads, though the leads might have to be extended. The transformer will also have a “link� winding for the rectifier filament.

Even if more than one wire is broken at the surface of the top pie of the original coil, repair may be possible. It might require some unwinding, followed by an attempt to rewind the wire in roughly the original pattern. It looks as if the tap connexion could stand some scrutiny as well.

But you could do worse than incorporate the Westinghouse power supply, if only as a temporary measure.

Hmm. I got a photo of the complete Thordarson HVO-50 data sheet, and to my eyes it does not look like a drop-in replacement for the transformer in my RA-102:

http://antiqueradio.org/art/temp/Thorda ... aSheet.jpg

http://antiqueradio.org/art/temp/Thorda ... heet01.jpg

In the Dumont p-s schematic, the plate of the 807 tube connects to a tap in the T2 transformer secondary. No such tap in the HVO-50 secondary.

Am I correct, or did I just take too many stupid pills today? If the HVO-50 is not a direct sub, that narrows my choices (either try to repair the existing coil, or use the hot-wired outboard HV supply).

Regards,

Phil Nelson
Phil's Old Radios

You are correct. I am sorry I wasn’t clear before, but none of these “high voltage oscillator coils�, of which many types were made from the 1940s to the 1960s, will work in your DuMont RA-102, in the existing high voltage power supply circuit. These coils were intended to be used in power oscillator circuits, with magnetic feed-back, and they worked at much higher frequencies.

The original coil in your set is an “R.F. coil�, in that it is self-resonant at a frequency that is considered to be part of the usable R.F. spectrum. It is resonant at the horizontal oscillator frequency, because the 6SN7 blocking oscillator that feeds the 807 power stage is synchronised to the horizontal sweep. This frequency, of just over 15KC, is lower than employed in “high voltage oscillators�.

Your original coil is a simple self-resonant auto-transformer, consisting of a single, tapped, winding.

Because a flyback transformer of that era is also an auto-transformer resonant at the horizontal scan frequency, it essentially works the same way, in so far as the generation of the high voltage accelerating potential is concerned. Your coil has more turns of wire because it has an air core.

As I described above, I suspect that a flyback transformer for a set of that general era, using approximately the same 10KV accelerating potential, could work in your circuit. A great many flyback transformers of this kind survive as used or “new old stock� items. Since they are so common and inexpensive, I think it would be worth trying one, if you are unwilling or unable to repair the original coil.

OK, it's finally sinking in.

Sure, I may as well try fixing this transformer. If that fails, old flybacks are plentiful and I may find something adaptable.

Thx again for the advice.

Phil Nelson

I'm not entirely sure about this, but I *think* RCA used a simple tapped winding (e.g., autotransformer) air core type flyback in some of their sets for a year or so around 1950. I've seen one, and it had a narrow single pi secondary winding of fairly large diameter (e.g., around 3 inches) wound over a broader primary winding, with just three wires coming out: starting end of primary, junction between primary end and secondary start, and secondary end as a flying lead with clip to attach to HV rectifier tube cap. It also had a 2-turn winding for the HV rectifier filament, and everything was wound on a hollow phenolic tube about 1 inch diameter and 3 inches long. Although it was constructed differently from yours, and apparently would have a larger footprint, I think it was designed to run at the horizontal 15.75kHz frequency, not RF, so I wonder if one of these might work for you. ETF posts on-line a list of parts they offer to television restorers, and they might have one.
R/ John

Thanks, that's interesting. I peeked at a few 1950-ish RCA schematics, looking for something other than the conventional flyback. Are you talking about this sort of transformer?



Regards,

Phil Nelson

I'm pretty sure that's what he's referring too. Here's a picture of one.

Merit HVO-34 and Thordarson FLY-45 are listed subs for the RCA part number. ETF lists 3 HVO-34 available.

http://www.earlytelevision.org/flybacks.html

Thanks, that is very good to know. Maybe I'll ask ETF to measure one of those. Offhand, it looks too big for the available space, so if I installed it I'd need to fashion a bigger HV cage somehow.

I'm also going to contact some transformer rewinders and ask if they would take a crack at fixing the old transformer. I have a general idea how this could be done, but my micro-surgery skills aren't the greatest, and if I'm going to practice, I'd rather experiment with something less unobtanium than this xfmr.

Regards,

Phil Nelson

Yes indeed, that's the transformer. The diameter may be a challenge for you, but I wonder if it otherwise might work. Best of luck.
R/ John