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Old 2nd Nov 2022, 5:06 pm   #441
FERNSEH
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Default Re: Baird T5 restoration project.

Despite its appearance it is a coil. It has a DC resistance of 1ohm.
Actually, it has one end painted black, so if it was a resistor, it would be 2000ohms. Body, tip, dot or band.

DFWB.
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Old 4th Nov 2022, 10:52 am   #442
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Default Re: Baird T5 restoration project.

Still having problems with the sound section of this receiver.
This time the field coil in the loudspeaker has gone open circuit.
Found upstairs among a collection of unrestored Defiant radios a Jensen branded loudspeaker which fits perfectly in the Baird T5.
However, the loudspeaker is 15mm deeper than the original Rola unit. The magnet assembly might be too close the CRT bulb.
The vision unit continues to work reliably.

DFWB.
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Old 7th Nov 2022, 4:24 pm   #443
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Default Re: Baird T5 restoration project.

Cables supplying power to the sound unit needed replacement. See attachment. This was done yesterday and the unit tested today.
Nows the time to supply the unit with 41.5Mc/s signals again.

DFWB.
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Old 7th Nov 2022, 5:19 pm   #444
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Default Re: Baird T5 restoration project.

Nice job.

Steve
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Old 8th Nov 2022, 10:44 am   #445
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Default Re: Baird T5 restoration project.

Hi Steve,
thanks for the compliment. The cable forms supplying power to the timebase and vision receiver units will also need replacing.
The sound unit. There is some activity in the form of hiss and microphony but still no modulated audio. Adjustments to the regenerative demodulator are critical. The unit will have to be bench tested again.

DFWB.
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Old 8th Nov 2022, 10:53 am   #446
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Default Re: Baird T5 restoration project.

Hello David,

Making good progress.
It certainly is a restoration of unimaginable proportions.

I particularly envy your cabinet making/restoration skills!

Jac
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Old 8th Nov 2022, 11:19 am   #447
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Default Re: Baird T5 restoration project.

Hello Jac,
It'll not be long before the scary task of reinstalling the CRT will have to be faced.
Actually, in terms of size the Cathodevisor 15MW4 isn't much different from the 15" Cossor tube fitted in the Baird T23. The Baird tube is about 100mm longer than the Cossor.

DFWB.
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Old 12th Nov 2022, 11:39 am   #448
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Default Re: Baird T5 restoration project.

The vision receiver unit continues to supply 50V P - P video but on occasions the local oscillator stops working. The oscillator can be shocked in operation by pulling the valve out and plugging in again.
The oscillator valve is a Mazda AC/P, a low power LF triode.
https://frank.pocnet.net/sheets/146/a/AC-P.pdf
In common with many pre-war superhet TV receivers there is no RF amplifier before the frequency changer stage.
Better news is the fact the sound receiver is now working. It's very lively with plenty hiss when no signals are being received.

DFWB.
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Old 18th Nov 2022, 9:16 pm   #449
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Default Re: Baird T5 restoration project.

As soon as the troublesome local oscillator is working reliably again and the cable form between the signal unit replaced it will be time to reinstall the CRT.
The third attachment shows the oscillator tuning capacitor and the worm drive mechanism.

DFWB.
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Old 23rd Nov 2022, 4:01 pm   #450
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Default Re: Baird T5 restoration project.

Arrived today from China, two 10Kohm 5watt potentiometers.
These will replace the existing focus controls in the T5 and T23 TV receivers.
The focus control in the T23 has burnt out and the one in the T5 is suspect so it will be replaced also.
Also replaced, the cable form between the vision unit and the power supply.

DFWB.
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Old 1st Dec 2022, 9:39 pm   #451
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Default Re: Baird T5 restoration project.

T5 and T23 focus control modification. The new 10Kohm 5Watt potentiometers are of good quality manufacture but I'm considering for long term reliability replacing them with a switched resistor chain and a 1Kohm potentiometer for fine adjustment. On the T5 the fine focus adjustment will be on the upper control panel and the coarse adjustment on the timebase controls panel. On the T23 the two controls will be located on the timebase control panel.
More switched positions the better as this means more resistors in the chain for better heat distribution.

The local oscillator has now given up altogether.

DFWB.
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Old 7th Dec 2022, 10:59 am   #452
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Default Re: Baird T5 restoration project.

The T5 certainly does it's best to give you a long headache!

Jac
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Old 21st Dec 2022, 9:44 pm   #453
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Default Re: Baird T5 restoration project.

Hello Jac,
it certainly does. The vision receiver unit is on the work bench again. HT and the 4volt heaters supply from a separate bench power supply unit. Oscilloscope connected to the oscillator. The waveform shown in the attachment. Oscillator squegging effect. 'Scope set to 0.5V division with X10 probe.
Timebase 0.5uS/div.

DFWB.
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Old 21st Dec 2022, 10:21 pm   #454
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Default Re: Baird T5 restoration project.

Looking under the chassis there seem to be quite a few very long leads. I wonder if the decoupling is quite as good as it appears on the schematic.

Peter.
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Old 22nd Dec 2022, 12:04 am   #455
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Default Re: Baird T5 restoration project.

Hi Peter,
there's no doubt the HT decoupling can be improved. I do have some 0.01uF mica capacitors. One or two can be added to the oscillator HT supply.
The grid resistor was checked not long ago and found to be in tolerance. Tested the same component this evening and found the value has risen to 36Kohms. Will replace the resistor tomorrow and report back my findings.

DFWB.
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Old 23rd Dec 2022, 12:32 am   #456
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Default Re: Baird T5 restoration project.

Replaced the local oscillator grid resistor. Used one with a value of 22Kohms.
Now the oscillator is working again.
The HT voltage is 250V. Heater volts 3.9V.
Oscilloscope set to 0.5V/Div with X10 probe. Timebase 50nS/Div.

DFWB.
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Old 23rd Dec 2022, 7:28 am   #457
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Default Re: Baird T5 restoration project.

Good results David.

A nasty problem out of the way now!

Jac
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Old 23rd Dec 2022, 8:51 am   #458
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by Jac View Post
Good results David.

A nasty problem out of the way now!

Jac
+1
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Old 23rd Dec 2022, 6:52 pm   #459
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by Jac View Post
Good results David.
A nasty problem out of the way now!
Jac
Hello Jac and Peter.
retested the oscillator today, it's perfect.
The vision receiver is reinstalled in the cabinet. That's all five chassis assemblies in working order.
Tempted to fit the CRT, but it might be safer to wait until the workshop becomes warmer. As mentioned earlier, when the CRT is refitted in the cabinet, it remains in there once and for all.
Workshop is not a pleasant place to do any kind of work at this time.

DFWB.

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Old 23rd Dec 2022, 8:35 pm   #460
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Default Re: Baird T5 restoration project.

Looking forward to that better weather and a fully working T5.

Peter
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