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Old 14th May 2021, 6:39 pm   #361
FERNSEH
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Default Re: Baird T5 restoration project.

Hello Jac, Hi Steve,
after the initial warm up time the picture is dim. Allow five minutes and the picture becomes quite acceptable, room lights dimmed of course, but the pictures are of entertainment value.
We're hoping the 15MW4 CRT in the T5 will the same or even better perhaps?
The type number seems to indicate that after WW2 Cinema Television was making a replacement for the pre-war 15MW1 and 15MW2 tubes.
I'll be returning to the workshop later this evening to try out the T5 EHT unit in the T23.

DFWB.
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Old 15th May 2021, 9:27 am   #362
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Default Re: Baird T5 restoration project.

Tested the T5 EHT unit yesterday. There was no problem with the CRT heater because the main power unit provides the heater supply. 2.5V or 4V depending on the type of CRT in use.
The 5KV EHT is OK so it's time to move on to the restoration of the timebase unit.
As mentioned earlier there a few differences between the T5 and T23 EHT units which means no direct interchangeability without doing certain modifications.
The T5 power unit does not have the video DC restorer in the brightness control circuit and the brightness control is connected to the unit by a long three core cable.

DFWB.
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Old 15th May 2021, 10:00 am   #363
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Default Re: Baird T5 restoration project.

Hi David,
That's great news about the EHT units and even better news about your CRT.
I wonder if the Cossor CRT that you swapped out a while ago would still work now you have fixed the bleeder chain issue? Did you actually check the emission on a CRT tester?

My Invicta T102 had a seemingly good MW22-14 CRT, but last time I powered it up the emission seemed to have fallen off a cliff. I meant to in find out why over Christmas, but covid restrictions prevented that. It is a mystery similar to the Cossor tube you had in the T23, let's hope it is not the tube.

Keep up the good work.

Cheers
Andy
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Old 15th May 2021, 11:01 am   #364
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Default Re: Baird T5 restoration project.

Hi Andy,
There is three Cossor 15" CRTs upstairs and one in an English Electric 1550M TV set, the model with FM radio.
I've been reluctant to boost the tubes fearing that the cathode surfaces might be ruined completely.
Nothing to do with the T5 restoration but the preamplifier unit that was fitted in the T23 was found yesterday. The preamplifier was not made by Baird Television Ltd., but it is nicely made and was fitted on the right side cabinet panel. Uses a single Mazda SP41 pentode.
It begs the question should the preamplifier be refitted into the T23? After all it's part the set's history.

DFWB.
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Old 15th May 2021, 12:37 pm   #365
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
It begs the question should the preamplifier be refitted into the T23? After all it's part the set's history.

DFWB.
I guess it would be best to refit it, but leave it electrically disconnected.

Cheers
Andy

Last edited by beery; 15th May 2021 at 12:38 pm. Reason: Spelling
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Old 15th May 2021, 6:56 pm   #366
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Default Re: Baird T5 restoration project.

If you decide to fit the preamp it could always be removed in the future but it’s all part of its history if it stays.
Mark.
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Old 15th May 2021, 7:31 pm   #367
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Default Re: Baird T5 restoration project.

Hi Mark,
the pre-amplifier was rather crudely wired into the receivers circuits.
A special plug can be made for the pre-amp which will permit the unit to be plugged into the power supply unit. A B5 valve base can be employed as the plug and a B5 valve socket attached to it for the TRF unit power supply plug.
The wires to the pre-amp will come out from the side of the valve base. A neat plug and socket solution.
The T23 employs secondary emission valves in the RF amplifiers, these were well known for going low emission so it's possible the preamplifier was made to compensate for the poor performance of the RF amplifiers. The same TRF unit was fitted in the T18 and T20 models. Uses three TSE4 valves.
The T5 is a superhet and employs Mullard TSP4 pentodes in the IF amplifiers.
No RF amplifier stage before the frequency changer.

DFWB.
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Old 16th May 2021, 1:09 am   #368
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
The same TRF unit was fitted in the T18 and T20 models. Uses three TSE4 valves.
DFWB.
It looks like there is hardly anything in the Baird TRF unit. It can't have been very sensitive.

Cheers
Andy
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Old 17th May 2021, 10:30 am   #369
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Default Re: Baird T5 restoration project.

Hi Andy,
TSE4 secondary emission valves have a very high mutual conductance figure, 15mA/V, same as the EF184 frame grid valve.
However, but it's not just the gm, also important is the input resistance at VHF.
The T5 employs conventional RF pentodes in the receiver unit. Safe to suggest that the TSP4 is a rebased Philips 4673.
https://www.radiomuseum.org/tubes/tube_4673.html

DFWB.
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Old 7th Jun 2021, 8:24 pm   #370
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Default Re: Baird T5 restoration project.

The T5 line output transformer was refitted today. I'm hoping to have the unit working by the weekend. Unlike the EHT unit the timebase unit can be tested in the T23 without any modifications.
The attachment shows the two timebase units, the T23 timebase is the one with new electrolytic capacitors, well they were new in 1993.

DFWB.
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Old 7th Jun 2021, 9:09 pm   #371
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
the T23 timebase is the one with new electrolytic capacitors, well they were new in 1993.
Capacitors from 1993 still seem new to me and thinking about that makes me feel old!

I hope the line output transformer holds up.

Cheers
Andy
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Old 8th Jun 2021, 5:33 pm   #372
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Default Re: Baird T5 restoration project.

Beery wrote: "Capacitors from 1993 still seem new to me and thinking about that makes me feel old!"
Hi Andy,
I was forty-eight then!
Working my way through the T5 timebase circuits today. The line timebase circuit is almost identical to the T23 but the frame timebase differs considerably. The scanning coil assemblies of the two receivers will have to be compared, there is a distinct possibility the T5 scanning coil has four windings.
An additional electrolytic capacitor is connected to the frame scanning coil plug.
Also, the circuit around the Westector is more complex when compared with T23 which has the valve interlace diode.

DFWB.

Last edited by FERNSEH; 8th Jun 2021 at 5:40 pm.
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Old 9th Jun 2021, 4:40 pm   #373
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Default Re: Baird T5 restoration project.

Confirmed today that the scanning coils assembly in the Baird T5 which the subject of this discussion has four windings. The extra winding is used for sync pulse injection.
Further study of the frame sync circuit is called for but what is clear is the AC/HL triode is employed as a frame sync amplifier rather than as a sync separator valve.
It's possible that the TSP4 pentode functions as the line and frame sync separator.
The T23 has two sync separator valves. The TSP4 pentode for line sync and the AC/HL triode is the frame sync separator.

DFWB.
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Old 9th Jun 2021, 10:26 pm   #374
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Default Re: Baird T5 restoration project.

The sync separator works in a similar manner as the Plessey Mark 1 chassis. That is, the line sync pulses are taken from the anode of the valve and the frame sync from the screen grid. Initial observations of the frame sync circuit reveal the Baird or was it Bush designers had paid special attention to achieving good interlace. The T5 was considered a large screen set in it's time.
Unusual to find in a 405 line receiver with a double integrator to filter out the frame pulse train.

DFWB.
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Old 10th Jun 2021, 7:16 am   #375
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Default Re: Baird T5 restoration project.

I think that Merdler and Gilbert were the designers of that part of the scanning circuitry along with Tingley and Pugh. All working for BTL.

Peter

Last edited by peter_scott; 10th Jun 2021 at 7:21 am.
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Old 10th Jun 2021, 5:41 pm   #376
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Default Re: Baird T5 restoration project.

Peter_Scott wrote: "I think that Merdler and Gilbert were the designers of that part of the scanning circuitry along with Tingley and Pugh. All working for BTL."

Hi Peter,
The attached circuit diagram shows an example of their work. It would seem that the Westector, if that what it is, performs the function of sync separator and the triode valve is the sync amplifier.
More work needs to be done on the timebase unit before it can be tested in the T23.
Note the double frame sync integrator.
It might be possible that the selenium diode doesn't follow the convention red indicates the cathode.

DFWB.
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Old 10th Jun 2021, 9:23 pm   #377
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Default Re: Baird T5 restoration project.

Hi David and Peter,
Compare the two circuits attached.
The T23 timebase circuit seems to have been drawn by a Mr Boynes. I'm sure I've heard of him

Cheers
Andy
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File Type: pdf T23 Timebase.pdf (538.3 KB, 19 views)
File Type: pdf T5 Timebase.pdf (448.8 KB, 14 views)
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Old 10th Jun 2021, 9:46 pm   #378
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Default Re: Baird T5 restoration project.

Thanks Andy,

Much Appreciated,

Peter

p.s. The 0 views is untrue.
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