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Old 20th Feb 2023, 11:52 am   #481
FERNSEH
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Default Re: Baird T5 restoration project.

Still having problems with the local oscillator. The vision receiver chassis is back on the bench and being supplied from a separate power supplier. The oscillator stops working when the HT voltage is reduced to less than 200volts. The waveforms were taken Saturday evening.
Sunday evening. Power supplier switched on and after the allowed warm-up time the local failed to come into operation. Pulling the valve out it's socket and reinserting it shocked the oscillator into operation.
This is the only problem I have with this set. Everything else is working well. Job can be signed off when the vision receiver is working properly again, which it was not all that long ago. Replacing the grid resistor of the oscillator valve seemed to solve the problem.

DFWB.
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Old 20th Feb 2023, 12:44 pm   #482
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Default Re: Baird T5 restoration project.

Could it be dirty pins/socket on the valve base?

Peter
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Old 20th Feb 2023, 6:32 pm   #483
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Default Re: Baird T5 restoration project.

Looking forward to seeing pictures on the c.r.t.

Robin
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Old 20th Feb 2023, 7:37 pm   #484
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Default Re: Baird T5 restoration project.

Peter_Scott wrote: "Could it be dirty pins/socket on the valve base?"
Hi Peter,
the valve pins and socket contacts are OK but I'm thing about the SRBP valveholder material could be lossy at higher frequencies. I will try find to a ceramic B5 valveholder.
Hi Robin,
I have seen a picture on the substitute CRT. It'll be even better hopefully, when the 15" Cathovisor tube is reinstalled.

DFWB.
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Old 20th Feb 2023, 9:36 pm   #485
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
Pulling the valve out it's socket and reinserting it shocked the oscillator into operation.
Is it possible to re-solder the bottoms of the valve pins?

Steve
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Old 21st Feb 2023, 9:58 am   #486
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Default Re: Baird T5 restoration project.

Hi
I had this problem on a home made local oscillator some years ago, I followed the construction details to the letter but the oscillator would never start at the rated voltage. The only cure was to very slightly bias the control grid positive, there after the osc started at low voltage HT every time. Still cannot understand why and may have been masking another issue that for the life of me couldn't find.
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Old 21st Feb 2023, 10:26 pm   #487
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Default Re: Baird T5 restoration project.

Panrock wrote: "Is it possible to re-solder the bottoms of the valve pins?"

Hi Steve, checked the condition of the soldering of the valveholder and the valve pins. All are OK.
The two parallel 50,000ohm HT feed resistors to the local oscillator have risen to 35,000ohms These will be replaced because its most likely the values will increase further over time.
A ceramic B5 valveholder might improve matters.

DFWB.
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Old 30th Mar 2023, 4:46 pm   #488
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Default Re: Baird T5 restoration project.

Arrived today, three ceramic B5 valveholders. One will be used to replace the possibly lossy at HF valveholder presently fitted in the local oscillator circuit

DFWB..
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Old 31st Mar 2023, 3:08 pm   #489
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Default Re: Baird T5 restoration project.

The replacement ceramic B5 valveholder was fitted yesterday evening. Now it's just a matter of reconnecting it into the circuit and test the vision unit again.

DFWB.
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Old 31st Mar 2023, 5:51 pm   #490
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Default Re: Baird T5 restoration project.

The new valveholder has been wired into the local oscillator circuit.
Now a reliable 3volts P - P sine wave is supplied to the cathode of the TSP4 mixer valve.
The HT to the oscillator is supplied through two parallel 25,000 ohm resistors. To maintain reliable operation in the long term the resistors will be replaced.
The vision receiver unit is presently on the workbench and is supplied from a separate power supplier.

DFWB.
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Old 31st Mar 2023, 5:57 pm   #491
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Default Re: Baird T5 restoration project.

Good work sorting out this nasty oscillator David!

Jac
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Old 31st Mar 2023, 9:26 pm   #492
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Default Re: Baird T5 restoration project.

hello Jac,
the vision chassis will remain on the workbench for a few more days until I'm completely satisfied with the results of all this extra work.
It's becoming clear persuading an oscillator to work reliably at 40Mhz wasn't an easy task in 1935 when the T5 was designed. EMI preferred to employ TRF circuits in their first generation TV sets, as did Pye.

DFWB.
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Old 4th Apr 2023, 6:11 pm   #493
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Default Re: Baird T5 restoration project.

I can confirm the local oscillator is working reliably.
No need to shock it into operation anymore, it comes on every time the vision receiver is connected to the power supplier.

DFWB.
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Old 4th Apr 2023, 6:23 pm   #494
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Default Re: Baird T5 restoration project.

That's good news David.

A very nasty fault out of the way!

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Old 4th Apr 2023, 6:33 pm   #495
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Default Re: Baird T5 restoration project.

Hello Jac,
hoping to show very soon a picture on the small Ferranti CRT presently being used for test purposes.

DFWB.
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Old 4th Apr 2023, 6:41 pm   #496
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Default Re: Baird T5 restoration project.

We're all looking forward to that David.

What an enormous job, this T5.

Jac
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Old 4th Apr 2023, 7:54 pm   #497
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Default Re: Baird T5 restoration project.

Yes a marathon! Back from the grave! Looking forward to seeing a picture.

Rich.
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Old 6th Apr 2023, 2:06 pm   #498
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Default Re: Baird T5 restoration project.

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Yes a marathon! Back from the grave! Looking forward to seeing a picture.

Rich.
Me too!

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Old 6th Apr 2023, 6:44 pm   #499
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Default Re: Baird T5 restoration project.

Some results today.
The image on the screen of the 9" Ferranti CRT is Test Card C. It is evident the line timebase speed is too high and there is some difficulty with the frame hold.
After all that work done on the local oscillator, frequency adjustment was necessary to restore the vision circuits to working order again.
Before any attempt to re-install the Cathodevisor CRT I'll continue to use the small tube until a properly synchronised picture is achieved.
Strange, the timebase unit tested perfect in the T23, correct line and frame timebase speeds.

DFWB.
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Old 10th Apr 2023, 3:51 pm   #500
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Default Re: Baird T5 restoration project.

From my previous post: "Strange, the timebase unit tested perfect in the T23, correct line and frame timebase speeds."
Possibly caused by the different scanning coils in use for the Ferranti CRT.
As for the Ferranti CRT, the picture is becoming progressively dim.
This is a tube that has never been used in any other TV set. Acquired it sometime in the 1980s.

DFWB.
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