Baird T5 restoration project.

[matspar]

Truly marvellous work David. I've followed this restoration with enormous interest.
It will be incredible to see the completed set.

[FERNSEH]

Last week wasn't so good for the T5 restoration. The plan was to have all the units installed in the cabinet in order to carry out tests.
To do this holes must be drilled in the shelf which supports the timebase and receiver assemblies.
First, drill the holes for the screws which attach the units to the shelf. So far, so good. Next, cut out the rebates for the screw heads and washers, again, job done.
Time to implement my policy of glasnost (openness)
It didn't take long to realise the rebates have been cut out on the wrong side of the shelf.
So what can done about this? No other choice but make another shelf.
Fortunately, another cut to size shelf was made at the same time as the one presently fitted in the T5 cabinet.
A 165mm hole must be cut-out in the centre of the board. A new hole cutter tool was ordered on Friday and it arrived on Monday, that's fast service.
All the large holes have been cut-out in the board and it has been test fitted in the cabinet, it's a perfect fit.
The next task will be those chassis mounting screw holes, followed by the rebates which are cut-out on the underside of the shelf.
Rest assured I won't make the same mistake this time.

The attachments show the the stages of construction of the new shelf.

DFWB.

[FERNSEH]

Construction of the new cabinet shelf was completed today. Those rebate holes now on the underside side of the panel.
Once everything has been finalised the panel will be painted black.
Those chassis units look a bit rough but are serviceable.
The second attachment shows how the original panel looked before the restoration started.

DFWB.

[murphyv310]

Hi David.
The old panel is covered in woodworm, reminds me of a certain HMV that collapsed.
You must be near completion now.

[FERNSEH]

The new panel. Paint it black.

DFWB.

[Jac]

Making good progress David!

I am curious how the picture will be on such a very large CRT.

Jac

[peter_scott]

Jac,

If I remember correctly this photo was of a T5 that Andy (Beer) restored. I guess there's no telling how good the CRT in David's set will be but there's no doubting that they can be impressive.

Peter

[FERNSEH]

Assembly of the controls panel started earlier this week.

DFWB.

[line sync]

Looking great david.
What do the numbers mean on that wheel ?

Robin

[FERNSEH]

"What do the numbers mean on that wheel?"
Hi Robin,
the numbers indicate the position of the local oscillator tuning control.
Might be a legacy from the earlier experimental versions of the T5 when it was not absolutely certain the frequency of the vision transmitter was to be.
Also, to correct oscillator drift.

DFWB.

[FERNSEH]

The first picture shows the position of the mirror lid components before restoration started.
At last the exact position of the mirror lid mechanism components has been determined. The Baird T23 was used as a reference. The purpose of that powerful spring is to keep the mirror lid open in the 45* viewing position.
The third attachment shows the scimitar shaped bracket.

DFWB.

[Jac]

Making good progress David.

Indeed it is a scary spring so close to the enormous CRT...

Jac

[peter_scott]

The lid prop is patented GB475735.

Peter

[beery]

Originally Posted by peter_scott

Jac,

If I remember correctly this photo was of a T5 that Andy (Beer) restored. I guess there's no telling how good the CRT in David's set will be but there's no doubting that they can be impressive.

Peter

Yes Peter,
That was a very long time ago now. I hope to visit it again once I've got my car going. Apparently the T5 is still working, but the sound has drifted off tune. Unfortunately the fine tuning control only works on the vision signal.

Cheers
Andy Beer

[FERNSEH]

Jac wrote "Indeed it is a scary spring so close to the enormous CRT..."

Hello Jac, the first attachment shows the bulbous Cossor CRT installed in the Baird T23. Some protection from the mirror tension spring would be a good idea for this set. As the set stands now there is no lid mechanism present in this set. It was removed in order a pre-amplifier unit could be fitted above the vision receiver unit. Strange, why not just fit it on the other side of the cabinet?
It should be possible to replicate the swinging bar.

Hi Peter, thanks again for your research.
A picture in the December 1935 Television and Shortwave magazine shows a Baird T5 having only a simple strut lever attached to cabinet right side to support the mirror lid in the viewing position.

Hi Andy, the regenerative demodulator employed in the TRF sound receiver requires critical adjustment. Several FC4 octodes must tried until best results are obtained. My T23 has a M.O. MX40 pentagrid in lieu of the FC4.

DFWB.

[FERNSEH]

The first attachment shows the cradle used to support the CRT bulb.
Massively over-engineered, the plywood is 15mm thick. Note the pole pieces which are attached to the frame oscillator transformer. A similar arrangement was employed in post-war Baird and Ambassador TV sets.
The original loudspeaker has been cleaned up and refitted.

At last, the control panel is permanently fitted in the cabinet.
The missing control is the brightness and unlike the other controls is wired directly into the EHT power supply unit.
In the T5 and T23 models the brightness control is in the negative return from the EHT transformer secondary. A chain of 2.2megohm resistors maintain a constant current across the EHT source.

DFWB.

[FERNSEH]

The attachments show the video waveforms about the vision demodulator and video amplifier stages.
Signal source is from my own make standards converter. Image is test card C.
The first attachment is from the control grid of the video amplifier valve, a Mullard
TSP4. The negative going video is from the full wave demodulator valve, a Mullard 2D4A.
Second attachment shows the positive going video to the grid of the CRT.
Third attachment: the eight broad frame sync pulses. 'Scope set to "delay" to display the frame sync.
The rise and fall of the sync pulses edges is not as good as it should be. I've seen much better sync pulses displayed in this set.

DFWB.

[FERNSEH]

By chance I discovered when the sync lead is shorted to chassis the rise and fall of the sync pulses are sharpened up considerably.
See the attached video circuit diagram.
Note the video amplifier has a variable anode load resistor.
Oscilloscope set to 10V division. AC input.
The sound receiver continues to cause problems, as does the same in the T23.

DFWB.

[FERNSEH]

The attachment shows the vision chassis underside around the detector and video amplifier stages. The video HF peaking coil has the appearance of a 2200ohm resistor.

DFWB.

[line sync]

Are you saying that someone in the past has fitted that resistor in place of the HF peaking coil ?