A scary Baird T23.

[FERNSEH]

Keeping up with the scary TV theme here's some pictures of the interior of the Baird T23. About the Baird T23. The set differs from other Baird TVs because the 15" Cathodevisor 15MW1 has been substituted by a Cossor type 65K. Same screen diameter but the latter is about 4 inches shorter. Other T23 sets have the same type of Cossor CRT so we know this is a factory conversion and the execution of the work was not done very well, at least that's how it was in my set.

The Baird T23 in the ETF collection: http://www.earlytelevision.org/baird_t-23_pics.html

My set does work in a fashion but the CRT has gone bad, very bad. It's a lights out job the see the very dim picture. A suitable replacement is to hand, another Cossor 65K, however, it is a later type and has an ion trap gun assembly and is thus designated as the 65K/2.
It's possible that Cossor invented the ion trap gun?

The set was acquired in 1993 and was found to be in a terrible state. None of the modules were properly screwed into position and fact the set was stuffed full of newspapers to prevent everything inside the set moving about.
An easy set to get going but some of the practices of those times such as the fitment of those awful his-vis capacitors and PVC wiring will have to be addressed. Quite a lot of tidying up to be done.

The third attachment shows the compact vision TRF receiver. The sound unit is similar to the one installed in the T5 except it is modified for 41.5mc/s TRF operation. The 1936 T5 was a superhet receiver.

DFWB.

[Cobaltblue]

I am not making any claims here as I don't have any original evidence to hand but the Cossor 902 from April 1947 claims to be the first TV to use an Ion Trap.

I have a 902 in poor condition minus tube and scan coils.

So its not close to the top of the heap to be restored

Cheers

Mike T

[Jac]

Good morning David,

Yes, the 15" CRT certainly is quite scary.
I think the 12" 6/6 and the mains derived 6 kV was scary enough for me...

Good luck with the restoration, but that will present no problem for you, if the CRT is in good order.

Jac

[FERNSEH]

Good afternoon Jac,
Well I summoned up the courage today and switched the set on. Signals supplied from an Aurora.
Plenty sound from the speaker so no problems in that department. As for the vision all I can say there is something apparent on the screen, all can one describe it as an extremely dull defocused mess, no hint of any modulation whatsoever. Safe to say the CRT is useless. Also, a smell of burning resistors was traced to the small unit situated between the two cabinet base mounted power units.
There's obviously a fault in the TRF receiver unit.

DFWB.

[peter_scott]

Hi Fernseh,

Is that with your original CRT or the replacement one with the ion trap?
I've got a poor quality photo of your set from 8 years ago. It was a bit dim but I hope your replacement is not worse.

Peter

[FERNSEH]

Hi Peter,
The original Cossor CRT was replaced sometime in the early noughties, maybe 2001, so I'd guess the picture you took eight years ago would certainly be from the screen of the replacement CRT that is presently fitted in the set.
But before condemning the CRT the heater volts should be checked. The Cossor 65K has a four volt heater whereas the Cathodevisor 15MW1 had a 2.5volt heater which was supplied via an adjustable resistance from a winding on the timebase and receiver sections mains transformer.

DFWB.

[FERNSEH]

The heater voltage at the connector plugs of the CRT read spot on 4 volts.
The reason for the burning resistor in the power supply is because there is a low resistance reading between HT and chassis in the TRF receiver unit.

DFWB.

[Panrock]

Originally Posted by Jac

Yes, the 15" CRT certainly is quite scary.

One of my least favourite vintage TV moments was when I wanted to remove the tube on the Baird T5 discovered in the cellar at BBC Wood Norton in the early 1980s. This had an o/c heater.

The tube neck was jammed tight inside whatever went round it. I had no choice but to grab the huge bulb bodily and H-E-A-V-E it upwards as hard as I could, hopefully not straining it where it joined the neck.

Fortunately, it moved! (Yes I wore goggles just in case.)

Steve

[FERNSEH]

The smell of burning resistors was traced to the small power unit which is situated between the much bigger EHT and timebase power supply units. The small unit supplies HT to the TRF vision receiver unit. Actually, it is nothing more than an plug adaptor because the later T18 TRF unit has a six pin connector plug whereas the original superhet vision receiver had a B5 supply plug. Also, a degree of extra HT smoothing is provided by two resistors and a can type wet electrolytic capacitor which has long since dried up. A low resistance reading was measured between the TRF high tension was finally traced to failure of the insulation of the sleeving of one of the wires inside the unit. That's been put right and the resistance reading between HT and chassis is about 60Kohms.
The TRF vision unit will be refitted in the T23 and I will report back my findings.
The attachment shows the video amplifier components. The large valve is the Mullard TSE4 secondary emission video amplifier. This valve also provides the useful function as a phase splitter. Positive going video from the anode is supplied to the grid of the CRT and negative going video from the secondary emission cathode (K2 for +ve syncs) goes to the sync separators.
From the Radiomuseum: http://www.radiomuseum.org/tubes/tube_tse4.html

DFWB.

[FERNSEH]

The TRF unit has been refitted and is working very well indeed, shame about the CRT though, it's really dim.
The attached picture has been enhanced because what illumination there is on the screen has to viewed in almost total darkness, that tube will have to be replaced. But before doing that there's nothing to loose by giving it a tickle from a tube reactivator.
Note there is a problem with the vertical shift. The Baird timebase resembles a high power blocking oscillator, only one triode valve is employed in the frame timebase, a Cossor 41MP.
The oscillator transformer has pole pieces which extend out to the neck of the CRT.
The DC current though the primary of the transformer is cancelled out by an addition winding. The set under discussion has a mechanical picture shift assembly which can adjust the position of the focus coil, no amount of adjustment of this device can centre the picture so the picture displacement fault must be in the timebase.
The second attachment shows the picture shift mechanism.

DFWB.

[beery]

Hi David,
I remember visiting in the late 90's and watching Wimbledon on it.

Btw. I don't know what the life was of the 15MW1/15MW2, but I've seen a good one first hand in a T5 and it is even better than 6/6. However we do know that the 6/6 seems to have a very long operational life.

I'm not sure how the Cossor CRT compares, but I've yet to see a good one. So was the original fitting of the Cossor tube less to do with reliability and more to do with the Cossor tubes being easier to obtain?

Cheers
Andy

[FERNSEH]

Hi Andy,
the date codes on many of the components in the T23 indicate it was made in 1938. All the other T23 sets still in existence are also fitted with the Cossor type 65K CRT. All evidence of the conversion of the set to accommodate the shorter tube shows it was done in the factory. However, the execution of the conversion work appears to be have done in a very sloppy manner.
It's all very strange because Baird Television Limited owned the Cathodevisor CRT works. Most likely Cossor offered the tubes at lower price than BTL tubes works could offer them. The Cossor 65K must have been cheap because you have to factor in the cost of the additional components required to install the Cossor tube.
The 65K CRT was also installed in the well known Cossor 1210 TV set which was made at the same time as the T23.
The timebase unit in the T23 is similar to the 1936 model T5 save for the addition of an interlace diode. Some sets use a diode connected Mazda AC/HL which can be replaced with a V914 double-diode. The best power triodes for the timebase oscillators has been proven to be the Cossor 41MP.
The Mullard TT4 and Osram ML4 will work OK as replacements.

DFWB.

[Heatercathodeshort]

Looks good David. Fit that 65K/2 just to see how good the picture is. J.

[FERNSEH]

More scariness, the CRT donated by HKS will be fitted in the T23 later today.

Check out those "horns" it looks like a WW2 mine.

DFWB.

[yesnaby]

Just hope no earthquakes are detected in the north-east area!

[Heatercathodeshort]

Caution! The evacuating horn on that tube David is very delicate. I do hope it has a bit more emission compared to your example. Being side pumped they usually stand a better chance. [Less cathode poisoning due to the 'dirty air' being extracted from the bulb rather than being dragged over the cathode via a pinch pump tube.]
Only a test will tell. Fingers crossed...John.

[FERNSEH]

Hi John,
Caution is an apt word for this tube replacement task.
The attached pictures show the problems I'm going to experience changing the tube. Those "horns" are precariously close to the wooden board and also the metal adaptor plate which was made to mount the bowl of the shorter Cossor CRT. The final anode cap is comfortably clear of the board but as the second picture shows the evacuation pip is dangerously close to the woodwork, in fact a notch has been crudely nibbled out to clear the evacuation pip.
Modifications to the CRT mounting arrangement will have to be considered.

DFWB.

[line sync]

Hi David
I'm following this thread with interest.
You could really do with someone to help you with changing that CRT as it looks fraught with danger.
I'm looking forward to seeing a much brighter test card.

Robin

[FERNSEH]

Hi Robin,
Good advice. In fact removal of the tube is the easy bit, it's the reinstallation of the replacement tube that's gonna be kinda scary.

DFWB.

[unitaudio]

Most people only think of a television as scary when it's showing a horror film. We know better, don't we? They're only truly scary when they're like this one and showing nothing at all!

Regards,
Paul