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Old 10th Mar 2021, 9:22 am   #341
peter_scott
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Default Re: Baird T5 restoration project.

Interesting! I wonder if your T23 originally had standard switching?

Peter
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Old 10th Mar 2021, 10:33 am   #342
FERNSEH
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Default Re: Baird T5 restoration project.

Hi Peter,
although there is no indications that a standards switch was ever fitted the T23 timebase chassis has pre-set controls in series with the front control panel frame and line hold controls.
An interlace diode is present on the chassis. It was a diode connected AC/HL triode now replaced with a Mazda V914 double-diode.
Considering the T5 design was finalised late 1935 was a standards switch fitted to the early T5 sets?
The timebase chassis in the T5 set under discussion omits the interlace diode.

DFWB.

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Old 10th Mar 2021, 5:36 pm   #343
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Default Re: Baird T5 restoration project.

Can't find a date stamp on the T5 EHT power pack chassis.
The 8microfarad capacitor across the negative brightness control supply is a post war replacement, the original capacitor was one of those cardboard cased components. The 0.1microfarad capacitor connected to the slider of the brightness control is leaky and will be replaced. Same will goes for the 8mfd capacitor. The circuit diagram shows a plug and socket arrangement for the brightness control. In this power pack the control is wired direct into the circuit. The T23 has a DC restorer in the brightness control circuit there is no such device in the T5.

DFWB.
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Old 10th Mar 2021, 9:41 pm   #344
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Default Re: Baird T5 restoration project.

All five 2megohm resistors in the EHT bleeder chain have gone high value, same goes for the 1meg resistor at the bottom of the chain.
The 0.1microfarad capacitor is completely useless, it has a 10kohm leak. Actually it is a 0.5mfd capacitor. Same value capacitor is fitted in the T23.
Surprisingly, the 8microfarad electrolytic capacitor is perfect.
The EHT transformer and capacitors are perfect and are delivering a stable 5KV.

DFWB.
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Old 11th Mar 2021, 7:38 pm   #345
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Default Re: Baird T5 restoration project.

Hi David, I might have a suitable 0.5 uF cap here, what voltage?

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Old 11th Mar 2021, 10:59 pm   #346
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Default Re: Baird T5 restoration project.

Hi Ed,
the capacitors are 350 or 400volt types. Two of the original BICC make capacitors in the T5 have been restuffed and are ready to be fitted in the receiver. The maximum voltage across the brightness control capacitor should never be more than 100volts.

DFWB.
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Old 12th Mar 2021, 9:04 pm   #347
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Default Re: Baird T5 restoration project.

Restuff with mustards, or I have some Sprague's here

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Old 30th Apr 2021, 10:10 pm   #348
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Default Re: Baird T5 restoration project.

The EHT power packs from the T5 and T23 TV receivers are now on the bench and are ready to have the old 2megohm bleeder chain resistors replaced.
Apart from providing a discharge path for the EHT smoothing capacitors the resistors also serve as a constant load across the EHT supply which is necessary because the negative return to the transformer secondary supplies the brightness control voltage to the grid of the CRT.
Note the differences in the chassis construction. In the T5 the brightness control is wired direct into the chassis and in the T23 it is by a plug and socket connection.

DFWB.
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Old 2nd May 2021, 9:39 am   #349
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Default Re: Baird T5 restoration project.

What does the switch do on the T5 chassis?

The porcelain insulators are nice!

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Old 2nd May 2021, 10:12 pm   #350
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Default Re: Baird T5 restoration project.

Peter_Scott wrote: "What does the switch do on the T5 chassis?"

Hi Peter,
It switches off the mains supply to the EHT transformer. There is also a plug and socket in series with the switch. The plug serves simply as a link for continuity but it's likely it was used to switch off the EHT on models with radio.
Close up picture of those insulators.

DFWB.
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Old 3rd May 2021, 9:08 am   #351
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
Peter_Scott wrote: "What does the switch do on the T5 chassis?"

Hi Peter,
It switches off the mains supply to the EHT transformer. There is also a plug and socket in series with the switch. The plug serves simply as a link for continuity but it's likely it was used to switch off the EHT on models with radio.
Close up picture of those insulators.

DFWB.
The link plug was originally screwed to the cover of the EHT unit, to act as an interlock.

Cheers
Andy
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Old 3rd May 2021, 9:59 am   #352
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Default Re: Baird T5 restoration project.

They almost didn't need an interlock. Those giant insulators would serve as a warning.

Peter
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Old 3rd May 2021, 1:18 pm   #353
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by peter_scott View Post
They almost didn't need an interlock. Those giant insulators would serve as a warning.

Peter
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Old 3rd May 2021, 4:41 pm   #354
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
Note the differences in the chassis construction. In the T5 the brightness control is wired direct into the chassis and in the T23 it is by a plug and socket connection.
DFWB.
Hi David,
I've just noticed from my picture of a T5 EHT unit with the cover on, that the T5 I restored 20 years ago (which was dated September 1936 inside) did in fact have a socket for the brightness control. So is the set you are restoring earlier than that?

Cheers
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Old 3rd May 2021, 7:45 pm   #355
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Default Re: Baird T5 restoration project.

Hi Andy,
there is no date of manufacture on the T5 EHT power pack but there might be on the other chassis assemblies. Not possible find the date of manufacture of the original electrolytic capacitors because almost all were replaced after WW2.

DFWB.
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Old 3rd May 2021, 10:16 pm   #356
FERNSEH
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Default Re: Baird T5 restoration project.

Beery wrote: "Hi David, I've just noticed from my picture of a T5 EHT unit with the cover on, that the T5 I restored 20 years ago (which was dated September 1936 inside) did in fact have a socket for the brightness control. So is the set you are restoring earlier than that?"

Hi Andy, The T5. The first picture clearly shows that the interlock plug should be attached to the screening box.

The T23. Enhanced picture showing the date on the underside of the EHT chassis.

DFWB.
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Old 5th May 2021, 8:27 pm   #357
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Default Re: Baird T5 restoration project.

Both EHT power units need replacement bleeder chain resistors. All five of the series 2megohm resistors are way out of tolerance. Resistors in the T5 have risen to almost 3megohms and those in the T23 are even worse, one resistor reads 30megohms.

DFWB.
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Old 14th May 2021, 4:56 pm   #358
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Default Re: Baird T5 restoration project.

EHT bleeder chain resistors replaced. First PSU to be tested is the T23 unit.
It's working as it should so now the T5 EHT power pack will receive attention.
The 2.5volt Cathodevisor CRT heater supply will be disconnected and for test purposes a temporary connection to one of the 4volt heater supplies. The Cossor CRT in the T23 has a 4volt heater.
In one of my earlier posts I mentioned that the Cossor CRT was causing concern. The tube seems to be OK now, replacing those resistors in the EHT circuits has stabilised the negative brightness control potential.
The T23 has developed a picture shift fault. Will deal with that later, the priority is to get the T5 electronics fixed.

DFWB.
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Old 14th May 2021, 5:20 pm   #359
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Default Re: Baird T5 restoration project.

Excellent results David!

This CRT looks fine.

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Old 14th May 2021, 6:15 pm   #360
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Default Re: Baird T5 restoration project.

Brilliant!

In more senses than one.

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