Baird T5 restoration project.

[FERNSEH]

"check for woodworm, it gets the legs first"
Hi Kevin,
at least the rebuilt cabinet shows no trace of any woodworm.
Actually, there is almost enough spare cabinet parts to build another Baird T5 or T23 look alike.
But let's get this CRT installed. We're all looking forward to seeing pictures on that 15" CRT.

DFWB.

[peter_scott]

Originally Posted by FERNSEH

Legs are still a bit wobbly so the CRT remains in it's cradle until I'm better.
In the meantime a 10mm ex-valve top cap connector has been fitted to the CRT base. The CRT will be positioned so that the EHT connector is facing the front of the cabinet.

DFWB.

David, Your connector looks "the business". Good stuff!

Peter

[FERNSEH]

Preparing the T5 to accept its CRT.

To avoid glass to wood contact three rings made from felt material are attached to the aperture in the CRT support shelf.

DFWB.

[stevehertz]

Obviously three layers of felt is better, but would glass to wood contact be so bad?

[FERNSEH]

Better play safe than sorry. There is sight surface imperfections on the glass bulb and the same goes for the hole cut out in the shelf.
We're dealing with a fragile eighty-seven years old CRT.

DFWB.

[FERNSEH]

That 15" CRT truly is a big scary thing.

DFWB.

[FERNSEH]

Last Saturday I attempted to re-install the Baird Cathodevisor CRT.
The tube in it's cradle was brought along side the cabinet, everything was prepared to drop the tube into the receiver.
Well, like everything about this set nothing is easy. It proved impossible to insert the CRT neck through the focus coil. I'm taking no chances with this tube so it has been returned to its storage cradle. We know that the 15MW4 CRT is not the original tube and was fitted just after WW2. Because the tube bulb is 20mm shorter than the 15MW1 an adaptor block was fitted to raise up the focus coil. Even though every care was taken to ensure the tube neck would pass through the coil and that rolled metal tube the tube would just not go in. Can't use force with this one.
A solution to this problem will be to raise up the focus coil support by 20mm. This was applied to the Baird T23 which has the much shorter 15" Cossor CRT, except the focus coil support is raised up more than 100mm.

DFWB.

[Station X]

I don't understand how raising the focus coil support will help. Surely the diameter of the hole is too small?

[Panrock]

I don't understand either.

David is doing sterling work and is really quite brave. I well remember how much force was required when pulling up and out the CRT in the T5* I acquired from BBC Wood Norton. In my case, I didn't need to put it back in later.... the heater was o/c and a shorter post-war tube went back in its place.

*Perhaps this had originally seen service as an off-air monitor at Alexandra Palace?
'Cathovisor' or 'Cathodevisor'? I always thought it was the former. Please explain and put me right on this.

Steve

[peter_scott]

Originally Posted by Station X

I don't understand how raising the focus coil support will help. Surely the diameter of the hole is too small?

+1

Peter

[FERNSEH]

It is the focus coil assembly and some blistering on the CRT base that is preventing the CRT neck to pass through. A paxolin disc on the focus coil was found to be very slightly off-set from centre, <0.5mm. That can be easily corrected.
The blistering on the CRT base around the filament contacts must have been caused by high contact resistance, the filament current of this CRT is 2.5amps.
I'm sure after these faults have been corrected the CRT will drop in. Just have make sure there are no issues with that rolled metal tube.

Unlike most magnetic deflection CRTs the Baird Cathovisor 15MW4 has a 42mm diameter neck, the base is 44mm.

The 15" Cossor CRT in the Baird T23 has the 35mm neck necessitating different line scanning coils and focus coil. It is the focus coil in the T5 which is of concern, it has a resistance of 19,000ohms and the current through the coil is controlled by a 50Kohm wire-wound variable resistor.
The focus coil in the T23 has a resistance of 2,000ohms.

DFWB.

[FERNSEH]

Originally Posted by Station X

I don't understand how raising the focus coil support will help. Surely the diameter of the hole is too small?

I was considering removing that crudely made spacer block and raising up the focus coil support panel 20mm.

DFWB.

[FERNSEH]

NEWS FLASH!
This afternoon the CRT was re-installed. No drama, the neck of the tube passed through the focus coil without any difficulty.

DFWB.

[Panrock]

Congratulations!

Steve

[Jac]

That's quite a relief David!

A major hurdle taken.

Jac

[FERNSEH]

Hi Steve, Hi Jac,
well that's the difficult hurdle overcome so now the next task is connect the CRT to the power supplies and feed video to the CRT grid, or to give it's old name, the Wehnelt electrode.
We'll soon know if the tube is still usable.

DFWB.

[beery]

That's great news. That tube is frightening.
Can't wait to see if it works.

[peter_scott]

Great news!

Peter

[FERNSEH]

Today. the CRT was connected up. All the connections for this tube are on the base, including the EHT.
Grid voltage variation is correct, 0 to -125V. This proves the EHT bleeder chain is OK and 5KV is available for the CRT anode.
Timebases are working correctly. CRT filament voltage present.
Even with the workshop lights switched off there is no illumination on the screen.
CRT faulty?

DFWB.

[high_vacuum_house]

Just a few thoughts,

What is the cathode voltage in respect of the grid? I presume the CRT cathode circuitry is ok.

If possible, could an ammeter be placed in between the CRT cathode and the rest of the circuitry, This would prove there is some current draw through the CRT?

Also, could the deflection be temporarily disconnected (with the brightness turned down) to get a spot on the screen? This would also prove if one of the deflection circuits is being turned hard on and the beam being deflected hard in one direction onto the wall of the CRT.

Could the focus be too strong or weak to cause the beam to scatter or not emerge from the electron gun?

Christopher Capener