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Old 10th Oct 2020, 2:05 pm   #21
peter_scott
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Quote:
Originally Posted by beery View Post
Hi Catkins,
Here is the correct switch, a Bulgin S301.
Cheers
Andy
Thanks for posting the photos of the switch Andy. If Catkins' plastic switch panel is like mine I think there is a high probability of damaging it when attempting to remove the blanking plug if so it might be better to hide the switch around the back and use a commonly available 4 pole switch.

Peter
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Old 10th Oct 2020, 4:31 pm   #22
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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Originally Posted by peter_scott View Post
it might be better to hide the switch around the back and use a commonly available 4 pole switch.

Peter
No, I think we should introduce some of those nasty Goblin teasmades to Mr Hammer!
BTW, How is the cover plate attached?

I find it both interesting and a little sad that in both the EMI and Baird sets that an effort was made to expunge the Baird 240 line system from history.
What about the Cossor 137T, were they Baird capable and if so, do the surviving sets have any 240 line circuitry left in them?

Cheers
Andy
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Old 10th Oct 2020, 4:40 pm   #23
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

There is a projection from the blanking plate that fills the switch hole on the under side and I assume the plate is glued but the basic escutcheon is fairly thin plastic. I couldn't see any way to insert a knife blade between the two.

I think all the sets that appeared at the 1936 Radiolympia were dual standard.

Peter

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Old 10th Oct 2020, 7:36 pm   #24
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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Originally Posted by peter_scott View Post
There is a projection from the blanking plate that fills the switch hole on the under side and I assume the plate is glued but the basic escutcheon is fairly thin plastic. I couldn't see any way to insert a knife blade between the two.
The one on this set is different, and the blanking plate is simply held in place with a screw and underside plate, and is easily removed.

See photos.
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Old 10th Oct 2020, 7:40 pm   #25
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Quote:
Originally Posted by beery View Post
Hi Catkins,
Here is the correct switch, a Bulgin S301.
It is not a change over switch as such, but has two N/C and two N/O contacts that are connected as four separate switches, ie there is no common contact between them.
These are quite rare. A 1950's Goblin Teasmade has a slightly lager bodied switch with the same function, but the actuator lacks the pre-war style bobble top.

Cheers
Andy
Hi Andy,

As 240 line operation can be achieved without the V8 valve, I would be very interested in getting the switch !
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Old 11th Oct 2020, 1:11 am   #26
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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Originally Posted by beery View Post
What about the Cossor 137T, were they Baird capable and if so, do the surviving sets have any 240 line circuitry left in them?
Strangely enough I was reading a Radio Times television supplement from 5th March 1937 yesterday, and noticed that it had an advert for the Cossor 137T.

It said "BBC's adoption of single transmission system permits this big reduction in the price of the Cossor combined television and high-fidelity radio receiver" or the 137T.

So it is evident the early 137T sets were Baird capable.

The PDF is here http://www.radiotimesarchive.co.uk/p...-009-72dpi.pdf. Please scroll down to page 11.

Phillip
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Old 11th Oct 2020, 9:05 am   #27
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

The Cossor pricing is interesting. The 137 that had television and radio was priced at 105 guineas and the 237 that added an automatic record changing deck was 120 guineas. The price reduction of the 137 to 70 guineas for removal of the Baird circuitry seems disproportionately large it being the same cost as the record deck! Of course Cossors really hated the Baird system and probably wanted to ensure its departure by emphasising the added cost.

Peter
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Old 11th Oct 2020, 9:52 am   #28
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Correction to the above: Record deck = 15 guineas. Baird cct = 35 guineas.

Peter
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Old 11th Oct 2020, 10:04 am   #29
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Peter Scott wrote: "Of course Cossors really hated the Baird system and probably wanted to ensure its departure by emphasising the added cost."

And I believe Cossor wasn't all that keen on the 405 line system either because they proposed a workable 525 line system for post WWII television.

DFWB.
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Old 12th Oct 2020, 4:28 pm   #30
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Hi Fernseh,
I was wondering if your own 137T had any Baird bits or is the Baird circuitry shown in the manual, if you have one?

Hi Catkins,
I shall send you a PM

Cheers
Andy
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Old 12th Oct 2020, 7:34 pm   #31
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Hi Andy,
The circuit diagram I have doesn't show a standards switch. My Cossor 137T doesn't have a 240/405 line switch and there is no indication that it ever had one. This begs the question did the early Cossor TV sets actually have a standards switch?
The line and frame hold controls have sufficient frequency range for both TV systems. The CRT is an electrostatic scanning and focus type so it is easy to design timebases with a wide frequency range.
6,000 and 25c.p.s. for the Baird system. Marconi-EMI - 10,125 and 50c.p.s.

DFWB.

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Old 12th Oct 2020, 8:42 pm   #32
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

That's an interesting suggestion and looking at the set descriptions in TV&SWW December 1936 the Baird T5 is described as switchable but the Cossor 137 is only described as suitable for reception on both standards.

Peter
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Old 13th Oct 2020, 8:21 am   #33
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

To continue the restoration...

As previously mentioned the state of the television was "upstairs - downstairs", every thing on the bottom of the cabinet was in a much worse state than (largely) anything which had been raised above the bottom.

Looking at the bottom of the cabinet there was clear evidence that rodents had been living in it, and there were pieces of newspaper dragged in for nests, and there was a large quantity of nibbled hazelnut shells and other unidentifiable organic matter deposits. The top of the power supply unit showed corrosion from acidic deposits. In general it was in a rather grimy state.

So far as I was concerned the first thing to do was take everything out, and disinfect the cabinet.

Photo 1 shows the cabinet with the contents removed and vacuumed out prior to disinfecting.

Once done I decided the first thing to tackle was the Power Supply Unit. That is first because it was in a worse condition than anything else, and I think it always best to know as soon as possible if any of the transformers (EHT especially) have failed due to damp (known as green spot due to corrosion of a winding producing a green spot at that point), which would mean they'd have to be re-wound.

Due to the condition of the Power Supply Unit I decided a complete strip down and rebuild, allowing everything to be cleaned treated and repolished, was essential. If there's any place where corrosion and deterioration isn't a good ideal it is where high power is involved.

Before I get onto the restoration proper, it is perhaps worth discussing the unique/unusual or otherwise features of this Power Supply Unit (for those that do not know already).

The Power Supply Unit supplies HT and heaters (filaments) to the TRF and Sync chassis (or the video signal amplification and timebase chassis in more general terminology), it also supplies EHT and heaters to the CRT. It does not supply power to the radio/sound chassis which has its own power supply unit (built into the chassis). This is obviously to allow the radio to run independently without powering the television chassis. Later generation EMI sets (like the HMV 904) have only two transformers, a combined radio/sound/vision transformer and a vision only transformer (which supplies the timebases and EHT). This is because the sound circuitry and video signal circuitry is largely shared. Which produces a substantial component reduction and cost reduction (at the expense of extra circuit switching).

There is nothing unusual electrically speaking about the chassis. One transformer supplies the HT and heater voltages (to TRF, Sync, and CRT, there are separate windings for the TRF and Sync chassis due to the load, there being a total of 14 valves). The HT voltage is full wave rectified and then goes through a reservoir capacitor and a smoothing choke and capacitor. The EHT is supplied via a separate transformer, and is half wave rectified, the output of the half-wave rectifier goes to the CRT chassis for smoothing and the essential bleader chain.

What is unusual (for the 1930s at least) is the attention played to preventing people coming into contact with the lethal EHT. The EHT transformer and rectifier is completely enclosed in a metal box. Additionally, the box comprises a EHT shut-off lever. This lever is normally compressed and placed in the off-position by the presence of the television back cover. If the back cover is removed, the lever will short EHT to earth. Such precautions are obviously because the designers were worried about people coming into contact with the lethal EHT.

Photo 2 is an original (i.e. when I got it) photo which shows the EHT enclosure with the EHT shut-off lever on the right.

The other slightly unusual aspect of this first generation set is it uses a large tag board for the power connections (one tag per connection). Later generation sets considerably simplified connection by using plugs and sockets.

Photo 3 is an original photo which shows the tag board, and its general condition when bought. Note on the left the heat-coil (a type of fuse) has been bypassed by some wire.

Photos 4 and 5 show the rusty state of the EHT enclosure, and of the chassis (with the EHT enclosure and EHT transformer removed and all surface dirt vacuumed and brushed off)
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Old 13th Oct 2020, 8:36 am   #34
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Heat coils are not that common but if you like having one in circuit for original appearance then a possible source (although not the correct value) is the good old GPO telephone system. There are two in the lead in boxes.

Peter
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Old 13th Oct 2020, 10:34 am   #35
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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Originally Posted by peter_scott View Post
Heat coils are not that common but if you like having one in circuit for original appearance then a possible source (although not the correct value) is the good old GPO telephone system. There are two in the lead in boxes.

Peter
They look nice but I always prefer to have the correct value for safety, and so I replaced it with a modern fuse of the correct value. As it happens the EHT circuit short-circuited (due to a short-circuiting EHT capacitor) early on when turning on the set, and the fuse blew rather than burning out the transformer, and so I'm glad I did!

BTW for others I forgot to mention that that heat-coil protects the EHT circuit from overload (as opposed to the HT circuit, which is protected by the conventional looking fuse on the right of the picture).
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Old 13th Oct 2020, 11:09 am   #36
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

This is not to say Peter is wrong (if anyone got that impression), but it is best done after you have discovered any major faults in the circuit!
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Old 13th Oct 2020, 11:18 am   #37
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

Personally I think the shorting bar is a liability and should be permanently disabled. It's better to protect the circuit rather your life! I don't really mean that but you must be able to work on the set with it powered up and if you can do that then you don't need the shorting bar. It's only there to prevent the unwary from suffering an early death.

Peter
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Old 13th Oct 2020, 11:22 am   #38
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

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Originally Posted by peter_scott View Post
Personally I think the shorting bar is a liability and should be permanently disabled. It's better to protect the circuit rather your life! I don't really mean that but you must be able to work on the set with it powered up and if you can do that then you don't need the shorting bar. It's only there to prevent the unwary from suffering an early death.

Peter
I agree, I restored the short-circuit lever, but, it is now disconnected and has no effect.
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Old 14th Oct 2020, 6:25 am   #39
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

To continue the restoration...

Once the PSU chassis was stripped down, I tested the transformers and the choke. This showed that the EHT transformer had an open circuit secondary, which is unfortunately quite common. This transformer was sent off to Mike Barker, who as usual did an excellent rewinding job. The other transformer and choke were fine.

At this point it was simply a question of stripping everything down, cleaning, treating and re-polishing. There is nothing terribly exciting about this work it just takes time. I was pleased to find everything responded extremely well, most of the metal work although rusty or corroded had not yet gone through the plating, and with enough re-polishing effort came up to a very good finish. This is probably a testament to the high quality of manufacture of these sets, all metalwork is thick and heavy and well plated.

Obviously I would have preferred to have had patina, but the level of corrosion meant it had gone beyond mere patina, and it was now an all or nothing approach. The other chassis in the set were not in the same state, and so did not need any stripping down apart from the CRT enclosure.

What follows now is a number of photographs of the work in progress.

Photo 1 is a photo of the EHT transformer before rebuilding. It is covered in pitch which once compromised, seems to do the opposite of its intended function and holds moisture in.

Photo 2 is of the three rebuilt transformers and choke, with the choke on the left, the EHT transformer in the middle, and the mains transformer on the right.

Photo 3 is a photo of the re-polished metal-work from the mains transformer, as can be seen it cleaned-up very well.

Photo 4 is a photo of the stripped down chassis, after cleaning, treating and re-polishing.

Photo 5 is a photo of the cleaned, treated and re-polished EHT enclosure.
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Old 14th Oct 2020, 6:49 am   #40
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Default Re: Restoration of a 1937 first generation pre-war television (HMV 900)

To continue...

Photo 1 is a photo of the cleaned tag board and associated metal-work. This turned out to be quite delicate work, as the legends are very finely printed.

Photo 2 is a photo of the re-assembled tag board in place.

Photo 3 is a photo of the underside of the re-built chassis.

Photo 4 is a photo of the top of the re-built chassis, showing the inside of the EHT enclosure.

Photo 5 is the same photo with the top cover of the EHT enclosure in place. It shows some staining of the plating, but, this is normally invisible (only showing up on flash).
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