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Old 11th Apr 2018, 7:14 pm   #1
Heatercathodeshort
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Default PYE LV20/B18T 1948. Heart transplant.

I found this very dilapidated PYE LV20 in the loft of a large storage shed at the rear of a very long established radio shop in Hanworth Middlesex. The shed had all the appearances of a 1930's scout hut and was crammed with old, partly dismantled receivers including the remains of a number of pre war television receivers. Interesting as the area must have been partly DC mains by the number of DC only receivers that were stored in another part of the shed.

The owner who I believe had passed away in his late 90's was a Cossor dealer and by the look of some of the paperwork discovered, had sold a fair number of pre war television receivers.This was a few decades ago when collecting old receivers did not have the interest it does today.

I managed to find some useful valves and a few semi complete receivers, the LV20 being one of them. At the time I thought it might be of use as spares, having a few of the B18T series in my growing collection back at the shop.
It was stored away in the huge shed at the back of the old shop [It covered the entire what had been a garden] and followed me to my present home, hiding out of sight on the floor at the rear of my 'museum'.

Recently to make better use of the space available I have been parting with mostly run of the mill broadcast receivers. There is nothing of interest for me on AM radio these days and to be honest that goes for FM as well. Time to have a sort out and keep a good representative range and chuck out the junk! This is how this tatty LV20 came to light.


As I dragged it into view the top of the backless cabinet came adrift. The glue had dried out and it simply fell apart. The only advantage is that it is now easy to give the EF50 valves a wiggle without having to delve inside and cop a belt off the EHT..

I'm not going to go into all the details of the repair as this has been well covered in the past. Replacements involving an EF50 valve, loads of decoupling sticky slugs, wax insulated capacitors, well out of tolerance resistors, small electrolytics, slider presets, loads of time but of course all good fun.

If you really want to know all the horror you only have to be cruel to me and ask..Just a note to say that it is well worth taking the time to clean the EF50 pins and spigot. I use a small rotary brass brush powered by a mini drill. They are all now trouble free and can be tapped and rocked in their holders without disturbance. The sockets can be cleaned with a tiny spot of good quality switch cleaner, just a spot to each 'hole'.

The B18T was released in September 1948 and was Britain's first 'transformerless' television. When I say transformerless I mean it did not have a mains transformer for heaters, HT and EHT. It sported flyback EHT and the first television receiver to employ a series heater chain. It would also operate on DC mains providing the supply was not less than 230V but this was not actually intended at the time. It was the PYE agents on DC mains that brought this to PYE's attention.

As I mentioned the EHT was derived from the line flyback and it may have been the first British receiver to do so. It was soon followed about a year later by the Bush TV11 and the Ferguson 941T. It is possible that the KB CV40 may have beaten it to the post but this was a very rare model unlike the Pye that was produced in many tens of thousands.

Back in 1948 the early line output transformers employed in the B18T/LV20 were the height of technology and were operating at the cliff edge. They were very reliable in service and I remember carting home quite a few 9" treasures from jumble sales as a lad and all the transformers were fully functional.

Six decades on unfortunately the insulation is failing as it was on this chassis. The whole assembly is potted in pitch making a rewind a very difficult and messy job. The EHT overwind is constructed in small wave wound sections each very close together and it must be the most difficult task to take on.

Your a brave guy Mike!

As I have a couple of B18T's in working order I decided to have a play with this one and see if I could get a much simpler transformer from my junk box to operate reasonably.[I really must get out more often] It is a fairly simple line output circuit with a PL38 line output valve feeding the primary of the LOPT together with it's multiple section EHT overwind. The isolated secondary feeds the scanning coils with a small well insulated winding supplying the 6.3V required for the EY51 EHT rectifier. A PY31 acting as an early efficiency diode completes the picture.

Removing the original transformer in it's black screening cover is an epic in itself. The smoothing choke has to be removed to gain access to the fixing screws and at first sight the connecting wires vanish into the chassis wiring. It must have been a difficult task to wire up on the assembly line and I take my hat off to the ladies that did it! I wonder how many chassis worked first time on first test?

Extension leads were fitted with crocodile clips to make transformer substitution an easy matter. A number of transformers were tried ranging from an Ekco T161 through to a motley collection of mostly unknowns. Most gave some results with either severe lack of width, low EHT or a combination of the two.

I then struck lucky with a neat transformer manufactured by Allan components. I suspect it had been designed for one of the many home construction kits of the early 50's. It had very similar resistance readings to the original and suitable for low resistance line scanning coils. It had a decent EY51 fitted so I was in with a chance.

It was soon connected up and gave splendid results. I did not wire in the width inductor that was originally wired in series with the scan coils. The EHT measured 6.5KV and what could be seen of the diabolical raster, was of sufficient width.

It was time to get the chassis into shape as mentioned earlier and after a lot of frustrating work managed to get a very good result on the 70 year old PYE. It is exactly the same age as me but I'm not sure who is in the better condition.

It was an interesting project, the sort of thing my school mate Clive and myself would have done way back in 1960. We used frame output transformers for audio output and the reverse, even finding a few that made good heater transformers. All manner of make do and mend to get pictures on those jumble sale delights. We played about with all manner of odd transformers and components the results of which usually ended up in clouds of stinking smoke and a telling off from our poor frustrated parents.

I'm going to tidy it up and fit the replacement transformer in place of the original. There is just about enough space for it fortunately. The line linearity is not perfect, probably due to a miss match between the scanning coils and the transformer or a similar miss match on the primary. I was not aiming for perfection with this chassis, just a reasonable picture. It is only a project.

The pictures show the progress and the end result. I must say the 9" screen is very watchable and does not detract from the entertainment factor.

Way back in 1948 It must have provided enormous pleasure for the family of it's original purchaser. By the look of the chassis, they obtained many hours of pioneer entertainment from their first television receiver.
I hope you enjoyed my excursion back to my school days in 1960. Seems only 10 years ago.
Regards, John.
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Old 11th Apr 2018, 7:19 pm   #2
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Default Re: PYE LV20/B18T 1948. Heart transplant.

The pictures of the final result.
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Old 11th Apr 2018, 9:37 pm   #3
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Default Re: PYE LV20/B18T 1948. Heart transplant.

These are great little sets, I have a total of seven sets that use this chassis.
Great to see another one being restored to working order, I have a spare 1807 LOPT, I was wondering if it could be used as a transplant for a LV20?

It appears there were two different cabinets fitted to the B18T, the difference is the speaker grilles are a square cutout or as in your set, routed out as four apertures.
The later LV20 actually has a wider cabinet than it's predecessor.

Mark
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Old 11th Apr 2018, 10:26 pm   #4
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Great write up John, entertaining reading as always. I have a B18T in the pile, though it's in a very sorry state. You must stop watching that film, you'll be having nightmares about rats on your diner plate!

Cheers,
John Joe.
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Old 12th Apr 2018, 8:04 am   #5
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Hi John,
I enjoy very much your story about this LV20. Interesting story and results.
Good Job. About the age we are almost close. Me a few more: I'm 73.
Best regards to you and everybody there..
Regards,
Roger
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Old 12th Apr 2018, 9:48 pm   #6
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Hi John,
That's a great result or is it? How is it with a test card?? I think we need to know...

Seriously though, I do kind of like these sets. They are let down by their rather plain cabinets together with the poor laquer that PYE used which tends to craze and scratch easily, however from a technical angle they are very good and reliable performers. Luckily lopty failures in these sets are not too common, but when they do happen, like you say, very nasty to rewind.
I once tried to rewind a lopty for a GEC BT2147 (which I'm sure is very similar in construction) using Gerry's wave winder, I soon gave up and had to hold out for a good original to turn up (which it did eventually).

Cheers
Andy
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Old 12th Apr 2018, 10:21 pm   #7
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Brilliant work John and a truly fascinating read! I bet that was a wonderful feeling when it came to life!!! and the linearity looks good enough from here!!!! Really need an interesting old set like this to play with.
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Old 12th Apr 2018, 11:12 pm   #8
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Brilliant write up John. You've done a wonderful job. The line linearity looks pretty good from those pictures.

Thanks,
Peter
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Old 14th Apr 2018, 6:37 pm   #9
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Thanks guys. The line linearity is very bad due to miss match. I have introduced a linearity coil and played about with the line drive but all attempts are to no avail. It's just a bad match but goes unnoticed on the average picture. The ion burn on the straight gun non aluminised tube can easily be seen.

In the 1950's Direct TV Replacements used to manufacture a 'fit all' transformer for these simple line output circuits with a selection of taps. It would be interesting to wind a transformer with tappings brought out from the last 1/3rd of the primary and secondary [scan coil]windings and experiment with various combinations to see if a better match could be achieved. Maybe an experiment I could carry out in the Victorian Home for Faded Television Service men. John.
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Old 14th Apr 2018, 10:18 pm   #10
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Hello John, I think you have done very well indeed with this set. Seeing a b&w picture with the line linearity off brought back memories of the Solus regunned A61-120WR mono tubes we used to fit at Mastercare in the late 70s early 80s. Many suffered from corner shadowing on the top right hand corner (looking from the front), when set up correctly. Even with the scan coils pushed as far forward as they would go. The only way round this would be to off centre the picture to the left, this would just remove the shadow, then adjust the line lin coil or sleeve to stretch the picture on the right just enough to fill the gap. Non of the customers complained about the slight non linear line scan, after all they were just happy to have a picture of decent brightness again.

Alan.
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Old 15th Apr 2018, 9:55 am   #11
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Quote:
Originally Posted by Heatercathodeshort View Post
Thanks guys. The line linearity is very bad due to miss match. I have introduced a linearity coil and played about with the line drive but all attempts are to no avail. It's just a bad match but goes unnoticed on the average picture. The ion burn on the straight gun non aluminised tube can easily be seen.

In the 1950's Direct TV Replacements used to manufacture a 'fit all' transformer for these simple line output circuits with a selection of taps. It would be interesting to wind a transformer with tappings brought out from the last 1/3rd of the primary and secondary [scan coil]windings and experiment with various combinations to see if a better match could be achieved. Maybe an experiment I could carry out in the Victorian Home for Faded Television Service men. John.
Have you seen the line linearity of the little Baird Everyman?! When working 'perfectly' a large circle appears on the screen as an egg shape! But for those who could only afford the minimalist Baird, I suppose it was still magic to watch those moving pictures on a 'silver screen'.

It's nice to think how people sat waiting for the evening TV program (singular!) to begin and would be preparing the room, drawing curtains, extinguishing lights, adjusting chairs, making cups of tea.. Lovely.
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Old 15th Apr 2018, 12:37 pm   #12
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Quote:
It's nice to think how people sat waiting for the evening TV program (singular!) to begin and would be preparing the room, drawing curtains, extinguishing lights, adjusting chairs, making cups of tea.. Lovely.
It's exactly what I do before watching my LV30 !
Regards,
Roger
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Old 15th Apr 2018, 7:19 pm   #13
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Hi John,
Thanks for the test card pics. It is not that bad really, I've seen worse (though not on a PYE). A good result for a fun and interesting experiment.
Btw, I wonder what the pre-war sets were? Anything unusual or just EMI sets (still great of course).

Cheers
Andy
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Old 15th Apr 2018, 7:43 pm   #14
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Default Re: PYE LV20/B18T 1948. Heart transplant.

I played about with some mild variable damping across the line scan coils. It consists of a 5k wire wound pot in series with a .001uf capacitor. With the pot set about mid way there is an improvement that can be seen in the last picture.

You are quite right Andy. I have a number of B18T series and the line linearity is 100% on all of them. My guess is that the primary has too few turns to load the PL38 correctly.

I think this is about as good as it is going to get with this transformer. The EHT is 6kv and remains so all day.

In spite of the non linearity, Fred and Ginger still look pretty cool to me.
Pictures taken under different workshop lighting. The picture is very sharp producing strobing effects.

Thanks again for your comments. John.
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Old 16th Apr 2018, 11:22 am   #15
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Here's a very crudely executed heart transplant which was done almost twenty years ago to my Sobell T121.
Can anyone identify the line output transformer? A Mullard BY182 is used to replace the EY51 EHT rectifier.
Different scan coils were used to match the replacement line output transformer which in turn necessitated replacement of the frame output transformer.
The Sobell performs very well considering the simplicity of the circuit design.
Same chassis was fitted in the Columbia C501.

DFWB.
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Old 16th Apr 2018, 1:14 pm   #16
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Default Re: PYE LV20/B18T 1948. Heart transplant.

I too had a bakelite tele, not sure which one it was without going back in my paperwork etc, but I remember thinking what a big item it was to be made out of bakelite! I mean, Ekco made some pretty large bakelite radios, but a bakelite tele?!
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Old 16th Apr 2018, 1:53 pm   #17
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Default Re: PYE LV20/B18T 1948. Heart transplant.

going slightly off course, the biggest thing I ever saw made of Bakelite was a coffin...
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Old 16th Apr 2018, 2:23 pm   #18
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Default Re: PYE LV20/B18T 1948. Heart transplant.

The attachment shows the very simple circuit of the line output stage in the Sobell T121. Similar line timebase circuits were employed in many other TV receivers made about the same time as the Sobell, 1949/50.

DFWB.
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Old 16th Apr 2018, 2:28 pm   #19
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Default Re: PYE LV20/B18T 1948. Heart transplant.

A beautiful heater dropper I see here..
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Old 16th Apr 2018, 2:51 pm   #20
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Default Re: PYE LV20/B18T 1948. Heart transplant.

Hi Roger,
Here's a picture of the mains dropper in the Sobell T121.
DFWB.
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