After using the set for a couple of days, the left and right sides of the picture became jagged, along with an intermittent sizzle from the line output transformer and the smell of scorched insulation. No smoke or sparks visible, but the sizzle seems to be coming from the driven coil, not the EHT or the yoke driving coil. This lost its pitch insulation long ago, and the paper covering is fragile and loose, perhaps from heat. Should I remove the loose paper and have a look?

Have a look with lights tuned off.
You may be able to see if it is arcing between windings.
A whiff of ozone is usually the giveaway.

Hmmm, I have just replaced its predecessor transformer in a T18. Exactly the same symptoms.

You can verify its failure (primary winding heating) using a laser thermometer, you know the ones, common during Covid.

That transformer is made from Unobtainium. The one in the T23 is basically the same electrically as the earlier type that was used in the T18, T19, T20 and T21 so it had a long run. The only difference with the T23 is the 1S2 anode connector is part of the EHT winding.

I grafted a Philips NT3101A transformer into my Pye. They are easier to find.
It works perfectly.
The later version, the NT3102, would also work.

Drop me an email. I'm writing it up at the moment, I may post it here.
This T18 had many issues. The line transformer was only a small part of the story!

Thanks. I've got no infra red thermometer to hand, but I've removed the loose insulation on the driven winding, and there's some sign of melted pitch there. Looking at my T21 data, it seems the main electrical difference in the LOPT is the T23 version has taps for both 110 and 114 degree deflection, and the EHT is 20kV rather than 16kV. The Pedigree has a 23HP4 picture tube so it's wired for 110 degrees. I'll email you shortly.

My T18 also has a 23HP4. I know the T23 chassis reasonably well too. The higher EHT increases the brightness of what is probably a Thomas CRT like mine.

The last of the Pedigrees used a 24" square corner CRT and consequently had a different cabinet shape. Later models used the T26 all solid state chassis,

Yes, it's a Thomas picture tube.

Can't see either of those Philips transformers for sale. I've seen a few 1960s LOPTs for sale in the US, but who knows if any of them would match? The Pye is a kind of split autotransformer, isn't it? Then there's the possibility I could rewind the driving coils, assuming I could get the pitch off, open the ferrite frame, and not lose my patience counting the wires.

Your Philips transformer would have to come from a scrapped TV chassis, as mine did.

I am currently working on a method of determining the suitability of any line transformer for replacement purposes. My method involves injecting 15kHz at 10 volts P-P from a signal generator and measuring the voltage on the taps. My current reference transformer is the Philips NT3101A because it can replace any transformer that drives parallel-connected yoke windings, like the Pye.

The "dead" transformer that I pulled out of the T18 is still a candidate for this test because it still works OK when dead cold and my test voltage is insufficient to cause a breakdown.

The fact that it is a split winding (Pye seemed to like that circuit!) doesn't matter, you simply wire the replacement up the way it was used in the donor chassis. The Philips transformers have the yoke connected to either side of a centre-tapped section of the winding with the B+Boost on the centre tap. This balanced connection reduces crosstalk between the horizontal and vertical windings of the yoke.

If not sure about the polarity of any tertiary winding, follow the waveforms (hopefully) shown in the schematic. You may still get it wrong, my T18 with the Philips transformer fired up with a left-to-right flipped picture. Easy to fix.

To work out if a transformer is a candidate for a transplant, look at the original circuit to see if the yoke is parallel or series connected - everything else can be worked around. The original circuit for the donor is a good guide to how to install the transplant!

The ETF in Ohio seem to have a large range of "flybacks" (US term) but most US designs use a series yoke connection, unfortunately.

If you wind 10 turns of wire around the core and (accurately!) measure the voltage obtained, you can derive a turns ration for any or all of the transformer. Scale that to get the number of turns of wire, you don't have to count them. But you do need a good working transformer.

Are you seriously considering rewinding your transformer? I've been following a UK forum where many have tried this. Sometimes they even worked for a short time!

The most common failure in the two most common Philips transformers we see here (NT3101 and NT3102) is shorted turns in the EHT overwind. You cannot pull the cores of these transformers apart, they are glued with epoxy into which is mixed glass beads of an accurately-defined diameter. It you do pull the cores of another transformer apart, it is vital that the material on the core ends is not disturbed because it sets the core gap which greatly affects the performance of the transformer.

One guy I helped who had one of these transformers with a failed overwind cut it away with a hacksaw and used a tripler from an old colour TV driven from the 6CM5 anode with good results.

The Philips transformers or a clone of them were used in the largest percentage of B&W TVs in Australia so they are usually not too hard to find. That incredibly bad Thorn R2 chassis used in the last B&W AWAs was an NT3102 clone made by Rola.

Now, assuming you can only get a transformer designed to drive a series-connected yoke, it should be possible to re-wire the yoke to work this way. The yoke in your Pye T23 was actually made by Astor and they ran it series connected in their TVs.

A rewind attempt would be a nothing-to-lose last resort.

Last night I tried running the set in the dark to look for flashes, but it refused to malfunction! Perhaps something changed when I removed the loose and melted pitch from the driven winding. If it's overheating, the layer of pitch wouldn't help. Like your T18, there's no obvious faults with the LOPT when it's cold.

Did the Pye T23 LOPT have a close Astor equivalent, then?

(PS. I'm busy the next couple of days, I'll be back afterwards.)

The one in my T18 did that too!

Maybe you should pull it out, boil it in Turps (I'm serious!) then soak it for a few days in thinned polyester lacquer.

Put it in a warm oven to bake the lacquer and repeat a few times, with the last coat not thinned.

If that doesn't save it, nothing will.

No the Astor line transformers were quite different. They drove the yoke with the two windings in series rather than parallel. Astor line transformers were made by Rola. Pye used the same yoke at the time, but with the windings connected in parallel.

The Pye transformers were coated in "Berry Wiggins Compound" which was a mix of bitumen and wax I'm told. They used a plant that had been built in WW2 to make tropic-proofed aero engine ignition coils. It would have made a great horror movie location!


I'll be in Albury over the weekend visiting another collector (he has a T21 Pedigree), I could drop in if you like...

OK, I'll try the turps boil if it starts acting up again. I'm also going to put in a used 85 percent emission 6CM5 in the hope of reducing the DC current slightly.

I think I have an Astor yoke that looks just like the Pye one, unless I passed it on to Andrew thinking I'd never need it. I didn't realise the windings were connected differently.

You're welcome to visit us, and the countryside around Daylesford is quite attractive, but that's about 300km from Albury.

It occurred to me afterwards that you might have had one of those lead glass 6CM5s that Philips made in the early 70's.

The lead would migrate onto the mica and cause internal arcing when the valve heated up.

You can recognise these by the large patches of dark staining on the inside of the valve around the micas after it's been in service for a while.

Why did they add lead? It was an attempt to stop the glass from cracking around the base after the plant moved from coal gas to natural gas. It worked, but it caused other problems....

My transformer runs for about 15 minutes then the raster starts to shrink in from the sides as the inner winding heats up. I soaked it in WD for a month but it made no difference which is why the chassis now sports an NT3101!

300km? Tah! Albury is 660 km from Belrose!

No, it was using an AWV Radiotron, and now I'm testing it with a Matsushita 6CM5. I did see a flash of a spark on the last test, but I don't think it was the LOPT. With the chassis down it should have lit the floor, not the inside of the case. Possibly a wire too close to metal, or the CRT spark gap?

Well, I got lucky and managed to watch an hour of monochrome TV today without serious problems. I'm guessing that the LOPT had absorbed a bit of damp during winter while the set was parked with its back to the dining room window, and this has now dried out with use. I've taped a sachet of silica gel to the inside of the LOPT cover, and I'll use the set regularly to keep the damp at bay. I'm also wondering if a thin coating of silicone oil might help.

I think a strip and soak with polyester while it's still OK might be a good idea, before it breaks down forever.

You know the wire is cotton insulated? Cotton is hygroscopic so it's not surprising that once the outer seal is broken it can absorb water.

The UK vintage TV forum reports success with this technique.

I need to ask, where can I get polyester varnish? My local hardware certainly doesn't have it. Insulating varnish I've seen online is either polyurethane, or unspecified. I've got polyester resin, and I guess that can be painted on, but would that do? It's too thick to fully soak into anything but open weave materials and coarse paper.