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Old 14th May 2019, 8:12 am   #161
peter_scott
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Default Re: Baird T5 restoration project.

I wonder if the Baird line timebase was more stable than EMI's or just didn't rely on having line syncs or whether they just perceived a little disturbance at the top of the picture as a minor problem relative to the types of issue raised by Cossors concerning the Baird transmitted signal.

Did the Cossor line timebase make any special arrangement to maintain line speed in the absence of line syncs?

Peter
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Old 14th May 2019, 10:10 am   #162
FERNSEH
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Default Re: Baird T5 restoration project.

The Baird 240/25 system sent only a single broad frame sync pulse so the possibility of hooking at the top of the picture was a real possibility if the line hold control was slightly off frequency. But there was no circuits to compensate for this.
The Cossor engineers also noted that the deep sync pulses limited the picture contrast. Baird sync pulses were 50 per cent of the video waveform, the reason for this was allow for the poor black level referencing which resulted in the picture information being present in the sync pulses.
It was a hopeless TV system with no potential for improvement.

And yet the Baird TV receivers were excellent.

DFWB.
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Old 21st May 2019, 7:52 pm   #163
FERNSEH
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Default Re: Baird T5 restoration project.

The old lacquer on the cabinet mirror lid surface has been removed to expose the condition of the veneers. Some minor corrective work is required but the veneer pattern isn't very nice. The book matching isn't perfect, but that's how it was made over eighty years ago.
Later today I'll rub on some French polish and then we'll see how the veneer really looks.

DFWB.
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Old 22nd May 2019, 10:07 am   #164
FERNSEH
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Default Re: Baird T5 restoration project.

Here's how the cabinet lid looks after French polish was dabbed on yesterday evening. It doesn't look too good. I'd suggest that the old veneer should be removed and replaced with something better.

DFWB.
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Old 27th May 2019, 6:34 pm   #165
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
Here's how the cabinet lid looks after French polish was dabbed on yesterday evening. It doesn't look too good. I'd suggest that the old veneer should be removed and replaced with something better.

DFWB.
Hi David,
if the veneer is not damaged in any way, but just not very nice, then surely it would be best to keep it as it is the only original panel.
Mind you, if I was the original customer, paying 85 Guineas in 1937, I might be a little annoyed at the poor veneer matching, but then again, who would really look at the top of the lid...

Cheers
Andy
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Old 28th May 2019, 8:42 pm   #166
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Default Re: Baird T5 restoration project.

It would look better if the dark marks were first bleached out. And toner applied to the non-veneered sections.

Steve
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Old 28th May 2019, 9:25 pm   #167
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Default Re: Baird T5 restoration project.

Also (sorry to be a pain) but it looks like patches of the old lacquer remain and more preparation is required.

Steve
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Old 26th Jun 2019, 11:01 am   #168
FERNSEH
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Default Re: Baird T5 restoration project.

It's evident some wee beastie has chewed the top of the line output transformer but I don't think all is lost because the damage is only the outer turns on what turns out to be the sync injection winding. See the attached circuit diagram.
The transformer has been removed from the chassis for inspection.
The primary is the inner winding, the secondary serves as the oscillator feedback winding and supplied the scanning coils.
The capacitors have been tested and all are useless. All the electrolytic capacitors are of TMC make and were fitted in 1947.

All the other chassis assemblies are working and just need the capacitors replacing or in the case of the waxies - restuffing. Most of the resistors are close to the indicated values.

DFWB.
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Old 28th Jun 2019, 12:51 am   #169
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Default Re: Baird T5 restoration project.

Hi David,
Was it attacked by a woodworm that had lost its way?

Any info you might work out in terms of turn ratio etc could be useful for the Garrick restoration.

Cheers
Andy
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Old 3rd Jul 2019, 9:52 pm   #170
FERNSEH
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Default Re: Baird T5 restoration project.

Hi Andy, as the damaged outer winding is only for injection of sync pulses and it's possible that the loss of a few turns shouldn't have much effect on the line hold.
The receiver uses a fair quantity of 0.5mfd 400V B. I. capacitors. It goes without saying every capacitor is leaky as one would expect in an eighty year old set. New Hi-Viz capacitors have been ordered from RS components and these will be used to restuff the original capacitors. It is essential that the original appearance of all the chassis assemblies is maintained.

DFWB.
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Old 9th Jul 2019, 10:38 am   #171
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Default Re: Baird T5 restoration project.

Work continues on the timebase chassis. Last night I discovered a selenium rectifier which I assume to be an interlace diode. It's most likely this component was fitted in 1947 along with all the other components that were replaced to return the set to full working order.
My Baird T23 also has an interlace diode. The component used for the purpose is a diode connected AC/HL triode or in my set a Mazda V914 double-diode.
Note the very vintage radio valve holder, looks like something from the 1920s.

DFWB.
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Old 10th Aug 2019, 11:37 am   #172
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Default Re: Baird T5 restoration project.

Two timebase units. See attachment. The unit on the left is from the T5 and the other is from my T23.
The latter I have to confess wasn't restored in a sympathetic manner, simply made to go only. That one will receive attention, but only after the T5 timebase is fully restored to working order.
The BICC 0.5mfd capacitors have been restuffed and now the TMC electrolytic capacitors will be attended to.
The two timebase units are fully interchangeable which means the T5 timebase can be tested in the T23. The same goes for all the other T5 chassis units.
The Cathodevisor 15MW4 can't be tested in the Baird T23 because the CRT mounting arrangements were modified in order that a Cossor 3265 CRT can be employed.
The cabinet restoration is held up because I'm still looking for walnut veneered 9mm plywood boards for the side panels. Been touring about Newcastle and Gateshead visiting junk shops and secondhand furniture stores. No luck so far.

DFWB.
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Old 10th Aug 2019, 11:45 pm   #173
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Default Re: Baird T5 restoration project.

Hello David is there no other source of 9mm ply say from a wood supplier or alternative source, is this not used on boats like marine ply or is just that the walnut version hard to find regards David
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Old 11th Aug 2019, 12:31 am   #174
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Default Re: Baird T5 restoration project.

Hi David,
The problem is that the original side panels are in such a bad state that there is no way of saving them.
Next week I will visit a timber firm in Gateshead and take a look at what's available. And it's only those side panels which are holding up the job. I've considered applying veneers on the newly made panels but past experiences of doing this have proven disappointing.
So in the meantime I'll concentrate on the electronics. The timebase chassis employs a large number of electrolytic capacitors and all are unserviceable. It was the same with the T23.

DFWB.
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Old 27th Aug 2019, 11:41 pm   #175
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Default Re: Baird T5 restoration project.

The plan was to employ the Baird T23 to test the chassis units from the T5.
But first the Cossor CRT had to be repaired, it will be recalled there was a bad grid connection on the base. That's been put right.
The T23 still doesn't display a picture. A bright raster appears on the screen but the brightness control doesn't have any effect. In these early Baird sets the tube cathode is at chassis potential and the positive going video is supplied to the grid. The brightness control supplies a 0 to 100V negative bias to the tube grid, that's missing.
Pull out the grid CRT plug and connect up the oscilloscope, the video returns. Refit the plug and the video disappears again. Faulty CRT? Hope not, it's more likely the missing grid bias is causing the tube to go into grid current and bypassing the video signal to ground.
So I've got to get the CRT grid bias working before the T23 can be used to test the other set's modules.
The attachment shows the 15" Cossor CRT installed in the T23.

DFWB.
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Old 28th Aug 2019, 2:31 pm   #176
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Default Re: Baird T5 restoration project.

Hi David,
how does the brightness circuit work?

In the T5 the negative end of the EHT was used to develop a negative voltage across the brightness control. The grid was connected to the wiper of the brightness control via a 500K resistor that enabled the grid to modulated from the output of the vision receiver (via a coupling capacitor). The CRT cathode is connected to one side of the heater and that is then connected to ground via a low resistance pot in the HT PSU, which is used to set the heater current. See the attached circuit.

Your T23 is a T5/T20 hybrid. I've attached the circuit of the T20's TRF unit. If the grid bias is adjusted in the same way as the T5 then R18 in the TRF circuit would have to be deleted (point 'M' in the TRF circuit is connected to the CRT cathode).
However, does your T23 control the brightness via the cathode as in the T20?
I've attached the circuit of the T20 PSU, note the CRTs heater/cathode is connected to brightness control VR4 which is connected to HT via another R18.
Also your set has a DC restoration diode as well...

All this might well mean that you have to test the T5 EHT, HT PSU and vision receiver all together rather than mixing them with bits of T23...

Cheers
Andy
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Old 28th Aug 2019, 5:28 pm   #177
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Default Re: Baird T5 restoration project.

Hi David,
I was looking through some pictures on Utube yesterday and discovered this capture of a Baird T5 operating in Waterloo Station in 1936. It looks genuine by the light reflection in the mirror. I have read about the Waterloo demonstrations but this is the first time I have actually seen a picture.
John.
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Old 28th Aug 2019, 6:31 pm   #178
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Default Re: Baird T5 restoration project.

Hi John,
Although the cabinet the Baird T5 is not as well made as the contemporary EMI models the T5's electrical performance is as good or even better than the HMV models. The T5 cabinet is a bit flimsy compared with the solid and heavy EMI sets. And the Baird T5 has that big fifteen inch CRT.

Meanwhile my T23 is still giving problems. Glasnost: it turned out I'd connected the video lead to the wrong connector on the tube base. That's been put right and the video signal is now supplied to the CRT grid connector..
So there is now a picture on the screen, but the brightness control still doesn't work!
The oscilloscope proves that the tube grid voltage can be varied by the brightness control. However, it is not the sync tips that are setting the black level reference, the negative brightness control voltage is referencing to the peak whites of the video waveform. I'd say the DC restorer diode needs reversing so that it conducts on the sync tips.
I reckon the CRT will have to come out again so that the base connections can be checked again.

DFWB.

Last edited by FERNSEH; 28th Aug 2019 at 6:37 pm.
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Old 28th Aug 2019, 8:01 pm   #179
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Default Re: Baird T5 restoration project.

Beery wrote: "Your T23 is a T5/T20 hybrid. I've attached the circuit of the T20's TRF unit. If the grid bias is adjusted in the same way as the T5 then R18 in the TRF circuit would have to be deleted (point 'M' in the TRF circuit is connected to the CRT cathode).
However, does your T23 control the brightness via the cathode as in the T20?"

Hi Andy,
I haven't checked to see if R18 is present in the RF chassis but it's safe to say that it has been removed by the manufacturer. The 15MW1 CRT in the T5 has a directly heated cathode whereas the Cossor 3265 tube has an indirectly heated cathode. The cathode is connected to one of the heater connections so like the T5 it is at chassis potential. And like the T5 the brightness is controlled by varying the grid voltage which is derived from the negative return in the EHT supply. The resistor bleeder chain across the EHT supply provides the constant current for the voltage drop for the negative CR tube grid bias.

DFWB.
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Old 29th Aug 2019, 4:42 pm   #180
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Default Re: Baird T5 restoration project.

The sketch shows the DC restorer in the T23. There is something not right about this circuit. The positive going video is supplied to the CRT grid. The function of the DC restorer diode is to conduct on the sync pulse tips to the DC reference level set by the brightness control. Normally the DC reference will be a positive voltage but in the Baird T23 it is negative and in all accounts this is the wrong polarity for the circuit to work properly.

If it is at all possible the brightness control circuit will be rearranged so that the CRT cathode voltage will be controlled in the same manner as the in the model T20. The video drive to the CRT grid will be DC restored with the reference level at a fixed potential.
See the third attachment in Andy's post No. 176.

The second attachment in this post shows the overly bright picture on the screen of the Cossor CRT.

DFWB.
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