Baird T5 restoration project.

[FERNSEH]

Originally Posted by Brigham

Is the Dual-Standard switch still fitted?

Hi Colin, There is no evidence of any components in the timebase chassis ever been fitted for the 240/25 Baird system.
In 1947 the receiver was returned to the works for a complete overhaul. New electrolytic capacitors fitted and a number of modifications carried out to improve reliability. For example, 50 ohm resistors fitted between the anodes of the IW4-350 HT rectifiers and the secondary of the mains transformer. The replacement electrolytic capacitors are of TMC make, dated 1947.

The 2.5volt CRT filament is suppled from a separate winding on the mains transformer. In series with the CRT filament is a variable resistor. This component has gone high value so the CRT filament was underrun.

The attachment shows the timebase and receiver units have been successfully removed from the cabinet. The wooden shelf is close to disintegration, another part to replicate.

DFWB.

[FERNSEH]

Testing the HT and heater supply power supply unit. The smoothing capacitors are useless and will not reform. Two 16microfarad 450volt capacitors and one 24microfarad 450volt electrolytic capacitor are required. The 24mfd capacitor will be replaced by an 8 + 16mfd component.

With the two Mullard IW4-350 HT rectifier valves removed 360 VAC can be measured at the anode connections of the rectifier valves.
The mains transformer runs cool, so it's likely there will be no problems with that component.

The 5KV EHT unit will receive attention next.

DFWB.

[FERNSEH]

Results of last night's work. The EHT is delivering 5KV which I believe is the correct figure. A bleeder resistor chain consisting of five 2Megohm resistors is connected across the EHT supply. All the resistors have gone high in value and will have to be replaced.

The timebase and receiver unit baseboard is beginning to disintegrate, see third picture.

DFWB.

[mark pirate]

Good news that the transformer looks OK, I guess you have tested the CRT filament?
You certainly have a task to rebuild the cabinet, looks like the only thing keeping it together is the woodworm holding hands!

Mark

[Heatercathodeshort]

The 'works' looks fine David. I would imagine that with some hours of dedicated work, this should revive well.

That cabinet is really bad. Those wood pests love the animal glue used in the manufacture of plywood.

The suspense is just too much... Can you please connect that EHT supply to the CRT and see if you get a 'blob'. Is it an original Cathovisor tube or a post war replacement. Time to put on your bomb disposal work gear. John.

[line sync]

Hi David
Well you have certainly taken on a big job here but we all know that your the man to rebuild it , I will also be following this.
I have a question , I notice your using your trusty rank EHT meter to measure the EHT , well I was told you couldn't use these type of meters for testing mains EHT sets and if I remember correctly the correct one is an electrostatic meter?

Robin

[FERNSEH]

Hi Robin,
I've been using the RBM EHT meter getting on for forty-five years.

I'm sure the load that the meter imposes on a mains derived EHT system is negligible. The source impedance of EHT from a mains transformer is far less than any flyback EHT system, be it colour or black and white TVs.

Electrostatic meter explained:
https://en.wikipedia.org/wiki/Electrostatic_voltmeter

DFWB.

[line sync]

Thanks for that david , I now know I can use my rank EHT meter on mains eht sets.
Getting back to the Baird , can you still buy the right kind of plywood ?
I know this sounds a silly question when you can walk into any DIY shop and buy ply off the shelf but its not the same stuff.
Have you got a stash of 1940/50`s wardrobe doors and side panels ?

Robin

[Graham G3ZVT]

I can imagine Dave walking in to his local charity shop, calipers in hand.

I suspect the advice not to attempt to measure mains derived EHT by one means or another, is given purely on personal safety grounds.

[Heatercathodeshort]

Just a word of warning here.

For those not familiar with mains derived EHT systems. These are lethal if stringent precautions are not observed. Great care has to be taken when servicing including the stability of the chassis that can fall and trap the unwary with very dangerous results.

O/C bleeder networks and poor insulation are very real hazards.

Just this week I had a very odd incident involving a PYE D16T and a glass mirror. I won't bore you with the details but it just went to prove how very careful you have to be.

The D16T particularly can be a very dangerous receiver in the wrong hands but any 50c/s mains derived high voltage power supply has to be treated with the utmost of caution.

Play safe! John.

[Ian B]

Regarding plywood: what you need is Birch Ply. It's available in thicknesses from 3mm upwards, and a good timber merchant should be able to order it for you. There are different grades according to whether you need a totally un-blemished surface or not; obviously when veneering over it, a few filled knot holes won't matter.

Unlike the cheap Asian ply, the individual plies are all the same thickness, so it's much stronger. Naturally, it's quite a bit more expensive!

Ian Blackbourn

[FERNSEH]

Hi Ian,
Good quality birch plywood is available from at least two timber merchants in Gateshead. On Monday I'll make more enquiries. The plywood is usually sold in 1200 X 2400mm or if you prefer 8 X 4feet sheets.

Rambo1152 wrote: "I can imagine Dave walking in to his local charity shop, calipers in hand."
I've found some useful pieces of veneered wood, a bed headboard and another piece removed from a table. Sadly neither is any good for the restoration of the Baird T5 TV.

A picture of the underside of the timebase unit.

DFWB.

[peter_scott]

Looks like a horrible thing to work on.

Peter

[Heatercathodeshort]

Hello David,
That under chassis view looks remarkably clean. I think the majority of the work will be from your cabinet maker experience.

Oh, by the way. I'm sending up some timber that may be of use.

[Huge Oak tree that was felled last week in the lane. 380+ rings counted and the trunk alone weighed around 18 tons. Massive bracket fungus around the base, the biggest the guy had seen and a rotten core. Terrible shame and the end of a bit of history.]

Is there room to park an artic at the back of your shop?
Regards, John.

[FERNSEH]

Originally Posted by peter_scott

Looks like a horrible thing to work on.

Hi Peter,
The can type electrolytic capacitors are post-war replacements for the card box types.
The line output transformer is subject to a high voltage flyback pulse which could result in the possibility of insulation breakdown. It will be a good idea to warm it up to drive out any moisture.

DFWB.

[beery]

Hi David,
Here is a photo of how the timebase would have looked with the original caps.

Cheers
Andy

[FERNSEH]

Hi Andy,
that's how the timebase chassis looked like in my Baird T23. I still have the old capacitors and intend to rebuild them so to return the chassis to it's original appearance.

DFWB.

[Ted Kendall]

Is there not some virtue in restoring the set to its 1947 factory overhaul state?

[FERNSEH]

Hi Ted,
That's what I intend to do with the Baird T5 because the work carried out in 1947 was done to a very high standard.
However, My T23 did have all the original card box electrolytic capacitors which were of course useless. It might make sense restuffing those.

DFWB.

[Ted Kendall]

With you. Should have paid more attention...