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Restoration of a possibly unique pre-war TV

 
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Restoration of a possibly unique pre-war TV

Post by Briancuff » Thu Feb 06, 2014 12:10 am

Hi all.
I have been asked to restore a Pye 12C console TV - what an honor!
The 12C was the first set which used the EF50 valve but due to the imminence of WWII, the government ordered Pye to recall any sets dispatched and to scrap all examples. The 12C table model, I think, was the 915 which was shown at the 1939 Radio show. However, one 12C seems to have escaped!
I can't find any service data on the set but I have obtained a paper on the design of a TV receiver intended for presentation to a professional institution in 1941 but the presentation was cancelled.
Fortunately, the paper contains a lot of very useful info including a circuit of a sound section which, with comparison of the existing set, proves that it is very, very close to the 12C - the differences are in the tone control circuits in the AF section. The vision and timebase info in the paper are a bit more sketchy but nevertheless useful.
The set comprises two separate chassis, each with its own PSU. The CRT is carried on the vision chassis, RF, Video and TBs and the sound chassis is bolted to the LS baffle board.
Unfortunately, as can be seen in the pictures, there is a lot of rust on both chassis which will need to be treated and the chassis painted which means, of course, all the electronics will need to be removed.
Here's a picture of the sound Chassis

Sond-Chassis-web.jpg
Complete TRF sound chassis (separate aerial)

Whilst dismantling the chassis, I came across this date printed on the loudspeaker:

Date-on-Loudspeaker-web.jpg
Pretty convincing proof (would they save loudspeakers for the duration?)


Meanwhile, The electronics have been removed by drilling out some copper rivets. I have ordered some similar rivets and will re-rivet the assemblies back in place.

RF-Skeleton-1-web.jpg
RF section (2 stage TRF)

AF-Skeleton-1-web.jpg
AF section - the modern cap was installed for testing purposes

Mains-Transformer-Skeleton-web.jpg
Rectifier valve base still attached

Sound-OP-Transformer-Skeleton-web.jpg
Sound O/P transformer


I have cleaned and primed the metalwork so tomorrow, I will be spraying them and assembling the transformers. The chassis will be left to harder for a couple of days.

 
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Re: Restoration of a possibly unique pre-war TV

Post by marc » Thu Feb 06, 2014 12:52 am

Hi Brian,

What a job you've taken on there, fabulous !
I for one will be monitoring this one with great interest.

Marc.

 
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Re: Restoration of a possibly unique pre-war TV

Post by IJK2008 » Thu Feb 06, 2014 3:48 pm

Brian

What a brilliant project to take on.

I too look forward to following this one.

Cheers

Ian

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Thu Feb 06, 2014 5:54 pm

Hi Brian,
I'm stimmt I've got somewhere in the shop service data for the 915 chassis. It is essentially a development of the model 815. As soon as I find that info I'll let you know.
The line output valve is the then all new Mullard/Philips EL50, it replaces the Mazda AC2PEN used in the earlier set. The frame oscillator-output valve was specially developed for Pye TV receivers, the Mullard 6153T triode-hexode. The valve is is for all intents purposes a modifed ECH2 frequency changer. The oscillator injection grid is omitted or returned to the cathode. The Mullard Maintenance Manual suggests replacing the 6153T with the ECH21. the oscillator injection grid returned to the cathode.
The ealiest version of the EF50 had those hooked pins similar to those on the EE50 secondary emission valve.

Till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Thu Feb 06, 2014 11:33 pm

There must be a finite limit on the stuff waiting to be found, Trev! A pre-war Austin 10 eh? My first car was a 1939 Austin 8 and I have fond memories of it - I used to drive up and down from Birmingham and Bracknell, Berks when I first started working for the BBC in Brum.
I know the 815 well, Till, having restored two of them. The Edwards' paper suggests that the type of TB oscillators are blocking oscillators which indeed the 815 uses but of course, it doesn't use the EF50. The first thing I checked when I got the 12C was whether the EF50s were the very early hooked pin type but no, they have the standard straight pins. It will be interesting to see whether the 915 uses the same sloping chassis of the 12C as that will partially confirm that the 915 is the table version of the 12C.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Fri Feb 07, 2014 9:37 am

I must admit that I've never seen a hooked version although I've been "around" EF50s for a very long time. Can you post pictures of it as I have only seen a drawing of one and they're not even mentioned on the Valve Museum website. Do you have a base for it? I bet they're even rarer!

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Fri Feb 07, 2014 12:55 pm

Hi Brian,
This might be the circuit diagram you are looking for. Drawn July 1939 this is the 915 chassis. The other doc is part of the the C.E.Edwards description of the design of a television receiver.

Till Eulenspiegel.
Attachments
915_1.jpg
915_2.jpg

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Fri Feb 07, 2014 1:14 pm

The sound channel of the 915 chassis.

Till Eulenspiegel.
Attachments
915_3.jpg

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Fri Feb 07, 2014 1:25 pm

I have the complete Edwards IEE paper and have cleaned it up with Photoshop - that includes the sound chassis circuit which I have redrawn, adding the PSU, using ACAD (not the best system for schematics!). If you look carefully at the pictures, the redrawn circuit is on the bench under the bits. However, the 915 circuit could be most useful as I have only the information from the paper and it is much more general when it comes to Vision RX and timebases. Could you send me a hi-res scan pretty please??

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Fri Feb 07, 2014 1:52 pm

Hi Brian,
The circuit diagram is a huge thing, >A3. I could either post it to you or have it copied at the local library.
I'm certain it's the doc you need for your set.

Till Eulanspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Fri Feb 07, 2014 2:06 pm

If you could post it to me, I'll take great care and scan it on my A3 scanner. The resulting files can then be shared! I'll pay for all expenses, of course.
Thanks for the offer, Till.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Fri Feb 07, 2014 5:33 pm

Hi Brian,
I've just posted off the 915 vision unit circuit diagram sheet.

Till Eulenspiegel.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Fri Feb 07, 2014 5:49 pm

You're a star, Till. :thumb :thumb

 
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Re: Restoration of a possibly unique pre-war TV

Post by Wolfie » Fri Feb 07, 2014 8:09 pm

This looks like a great thread Brian, will be watching this from the wings..

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Mon Feb 24, 2014 12:19 am

Till posted the Pye 915 main chassis circuit to me (thanks Till) and I have scanned it (8 A3 files) and processed it in Photoshop so that I now have a single file covering the whole circuit and 3 files, each covering a section (RF, TB and PSU/CRT) which are much more convenient for workshop use! I will be uploading them to Dropbox so if anyone wants copies, let me know. Strangely, the original had a 12” x 6” hole in the centre but fortunately, the missing info was confined to interconnects so was easily restored by using the circuit in the Edwards paper.
Back to the sound unit: I painted the metalwork, after a coat of primer, using matt chrome appliance paint which gives a good, hard finish. I didn’t paint the underside of the chassis as it has been zinc plated, unlike the top and there was some discolouration but absolutely no rust. The pictures show the chassis before “treatment” and all the metalwork (including mains transformer clamps and chassis rails) afterwards.

Sound-Bare-Chassis-2.jpg

Prepared-Metalwork-web.jpg

The screening can fixing plates were re-riveted in position using the copper rivets I found on the web. I used them to fit the valve bases as well. All other components were fitted using 4 & 6BA machine screws and nuts. The screening cans are to be nickel plated and I need to paint the loudspeaker which will be difficult as it is mains energised and has an open voice coil!
Unfortunately, during the transplant of the components two things happened: a tag broke off a valve base and one connection broke off two RF coils – I presume because of metal crystallisation – say! Fortunately, I had a couple of NOS ceramic bases and I fitted those (one ceramic and one Paxolin would not look good). The coils were easily rewound (only 9 turns) as the formers were grooved to hold the turns in position. The electrolytic and paper caps were re-stuffed with the main electrolytic printing produced in Photoshop and applied using decal paper.

Pye-12C-partial-restore-CU-side-view-2.jpg

Pye-12C-partial-restore-under-view.jpg
The original was this messy, honestly!

The screened cables were in pretty bad shape so I stripped some PSF1/3 broadcast grade double screened coaxial cable to obtain the very closely woven braided screens. When silicone rubber wire is threaded through this braiding, the resultant cable is a very good reproduction of the original.
I've now got to do the loudspeaker!

 
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Re: Restoration of a possibly unique pre-war TV

Post by IJK2008 » Mon Feb 24, 2014 10:22 am

Hi Brian

How do you do your riveting? Is it by hand or do you have a tool?

I would like to have a go at replacing valve sockets into a rebuilt chassis using rivets to keep the original appearance.

Cheers

Ian

 
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Re: Restoration of a possibly unique pre-war TV

Post by freya » Mon Feb 24, 2014 11:59 am

I meant to ask the same question last week when considering the same thing about the rivets, a search revealed nothing online about the original type of rivets or any that may be available to buy. obviously Brian's pre war is in a different calibre to a `49 Ekco thou. :bba

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Mon Feb 24, 2014 1:50 pm

There's nothing wrong with a just post war Ekco! I have several myself. :qq1
I found them using Google. Just enter "copper rivets" or "solid rivets". Just entering "rivets" brings up loads of sites selling pop rivets although you could try those - I don't know what the smallest size is but it could be 0.125" which is a bit larger than 6BA clear but most valve bases have large enough holes.
The actual riveting is done with rivet snaps. These are like pin punches but have a rounded recess in the end. These are used to form the metal into a round head or a ball pien hammer can do the same job.
Mount one snap vertically in a heavy vice and place the rivet head (the rivet and socket being in place in the chassis) in the depression and either using another snap or a ball pien hammer, round over the extended shaft of the rivet. The general rule is that the rivet should extend by 1.5 times the rivet diameter beyond the parts to be riveted together. The holes should not be a tight fit on the rivet or it will jam when being formed and perhaps separate the parts to be fixed together. Use a rigidly mounted vice as the blows have to be resisted as much as possible to ensure good riveting. Pincers can be used to cut the rivets to length.
The other method is to use brass or copper eyelets which are available in many small sizes. In fact eyelets were used in many sets - they're the hollow rivets that are easy to drill out!!

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Tue Feb 25, 2014 3:33 pm

I have had a quick look at the main chassis of this set. All non-sound components (except for the volume control!) are mounted on this chassis including timebases, RF and PSU (the sound chassis has its own PSU).
My first job was to remove the CRT - a 12" Mullard MW31-6. This proved a trifle difficult as the scan coils were stuck fast. However, about 15 minutes of 1A DC through the line coils freed it up and enabled their removal. One thing to remember is that very often the CRT base moulding is larger than the neck so be very careful not to damage the tube by placing undue stress on that moulding.
I mentioned earlier that this set, although it was a TRF, had been tuned to Ch2 (Holme Moss) which did not exist when this TV was made.

RF-closeup-bad-joints-web.jpg
The quality of the coil connections are definitely questionable

The picture says all. It is pretty obvious that the connections to the coils were post manufacture and, in my opinion, not very well done! So it definitely has been modified (phew!). After talking with the owner, we have decided to put it back to as originally made i.e. CH1 (AP). So I need to find a WWII PYE radar IF strip in order to copy the coils. Does anyone know of such a beast that can be bought or borrowed for the purpose - the best would be a scrapper so that the coils could be re-used in the 12C. I am going to post a request in the "Wanted" section.
In general, the set is in a dreadful state with dangling cardboard electrolytics to replace a 4 section bolt-down cap. Here are pictures of the underside and top of the chassis.

Under-Chassis-1-web.jpg
Not exactly professional

Top-of-Chassis-web.jpg
A trifle "barn" like!

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Wed Mar 05, 2014 3:39 pm

A bit more progress on the main chassis. All the components have been removed together with as much of the original wiring as possible, especially in the RF section. The B9G valve bases are terrible. They have gone very brittle (crystalline) and snap off at the merest provocation and will have to be replaced.
The workmanship in this TV is pretty bad - nothing like the quality of the contemporary EMI sets. This does suggest that this could be a laboratory model which was used for testing mods and could explain why it was modded to CH2 - it happened to be still around in the Lab after hostilities ceased (say).
The chassis was originally zinc plated so the all metalwork will go off to the platers on Monday for re-finishing. Meanwhile, I can get on with cleaning up the chokes, transformers and small tagboards and rebuilding/making the block and can type capacitors - There are 2 cardboard blocks and two can types. One of the can types is ot the continental single screw variety and contains 4 caps., 3 with common neg. and the other stand alone which means that there are 6 wires passing through the central spigot. I have re-stuffed this one but not yet applied the decal. These piccies show how it's going:

Bare-Chassis-Top-Web.jpg
The original finish is coming off in flakes

Bare-Chassis-Under-Web.jpg
The underside is in quite good condition

Metalwork-Web.jpg
Ready for the platers

RF-Components-Web.jpg
Now. Where did that bit go?

It can be seen that there are broken contacts on the valve bases - only one came off unscathed.
One tool that was absolutely necessary was my 100W temperature controlled iron. It is really good for soldering/desoldering connections to steel chassis and of course, soldering up tinplate cap boxes.
I have ordered 10 colours of braided silicone rubber wire for interconnection and I hope that I can remove the braiding and insert 22swg tinned copper wire in silicone rubber sleeving into it as some of the wiring needs to keep its form when routed! We shall see!

 
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Re: Restoration of a possibly unique pre-war TV

Post by Briancuff » Fri Mar 21, 2014 3:26 pm

Slowly, slowly, catchee monkey - I have at last got the metalwork back from the platers. The pictures show them as received. The finish is a bit blotchy but so much cleaner than the very rusty and pitted state it was in. The smaller components such as the screening boxes, choke clamps and other bracketry has come up very well.
I'm waiting for some fluid for my ultrasonic cleaner bath so that I can get most of the grime off the non-wound components. The front panel with its range of slider pots will benefit from a bath too! I just wonder whether a mist coat of acryllic "appliance" paint would help with the appearance of the main chassis!

12C-Metalwork-after-Plating.jpg
A bit blotchy!

 
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Re: Restoration of a possibly unique pre-war TV

Post by freya » Fri Mar 21, 2014 4:01 pm

I had always assumed that it would come back looking like new, is it the metals rough surface that causes the blotchy appearance ?
Looks much better though.

 
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Re: Restoration of a possibly unique pre-war TV

Post by marc » Fri Mar 21, 2014 4:09 pm

Hi Brian,

For what it's worth, my point of view would be how happy would I be each time I saw the chassis ?
Even though no one else will see it I bet it would still nag a perfectionist like your self :aaq
So maybe a waft of appliance paint would be in order.

Marc

 
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Re: Restoration of a possibly unique pre-war TV

Post by mark pirate » Fri Mar 21, 2014 9:34 pm

I agree a bit of paint would make it look more uniform, I too thought it would have looked better after plating.

 
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Re: Restoration of a possibly unique pre-war TV

Post by Till Eulenspiegel » Fri Mar 21, 2014 9:45 pm

When the chassis is populated with components the metal finish defects will not be as noticeable. The corrosion of the metal obviously had some effect on the plating process.

Till Eulenspiegel.

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