Baird T18

[Michelb]

Hello,

I try to bring back to life a Baird T18 TV set (purchased in France, I'm a French interested by vintage electronic, but not an electronic expert). It's not an easy task as it is incomplete, the bottom chassis is missing.
It means that the missing parts are : radio and tv sound, main power supply, EHT.

For now I left the radio part aside, and concentrated on the TV one. If I succeed to get the TV part working, I will later integrate a vintage Bush Radio to complete the radio feature.

I first did a refresh of the top chassis, I replaced the bad resistors by some new one from the 30's, I replaced the internal content of all the capacitors, changed some wires, purchased missing tubes... And after I built a bottom chassis.
For the main power supply I ordered a custom power supply with all the required outputs, and for the EHT I did it with a chinese transformer. I added some parts found in others TV set (sound output transformer, choke filter), I purchased some genuine tubes and at end I have something that match more or less the original specifications.

I try to summarize but of course It took me few months. And finally few days ago I was ready for the first try
This restart was done supplying voltage progressively with a variator. First the (new) bottom chassis with the loudspeaker connected as the high voltage runs through its coil. Then the Vision part, and the time base part. Finally with the CRT.

So now all is plugged and I am able to push it at full voltage (main home voltage 230-240v).
I did some measurement, I have 4.5-5kv EHT, ~337v main high voltage, my CRT tube heater is fed with 2v, the potential wehnelt-cathode looks coherent (the brightness pot is working as expected, I can reduce until a 0v gap). And the most important : the CRT is still airless, the heater is not opened, I can see it glowing!
But nothing happens on screen, no light.

I also tried to enter an aerial signal on the female aerial plug coming from the vision part (I own an hedgehod converter). I didn't precise it, my objective is to enter an aerial signal for the video, and sound signal directly on final sound tube PENB4 (I put one on my bottom chassis).

I suspect to have an insufficient CRT hater current. To be honest at first with the dedicated CRT heater winding of my custom main power supply I was not able to get more than 1.6v (and the current was at 1.5A). So I did a custom 2V external supply which has a strong heavy copper wire.
So now I have 2v, but the current is only 1.8A. And of course still nothing on screen.

I really don't understand how I can match the expectation of 2v / 2.5A for the CRT heater. To get a such current it will need to provide maybe 3v which seems dangerous.
Do you think it is the reason I can't get any spot on screen?

[Welsh Anorak]

Bonjour - and welcome to the forum!
You have certainly spent a lot of time on this project.

I'd be careful about exceeding the 2v heater voltage. It doen't take much to blow the heater, especially at switch on. Also you have said you see it glowing so I'd expect that to be enough for you to see something on the screen.

Therefore I'd concentrate on the other voltages. You have EHT, you (probably) have sufficent heaters, so have a look at the cathode and grid. Does this CRT have an ion trap? This can mislead if it's not adjusted correctly.

That's just my thoughts - there are members on here who have far more experience with TVs of this vintage.

[Michelb]

Thanks for your reply.

Ok even if I don't have a 2.5a current I will keep 2v max.
Since the beginning I don't switch the tv ON directly at full power, each time I use a variator to raise progressively to max voltage.

I don't think this CRT has a ion trap, I don't have one and don't see one on the technical doc.
From my understanding the ion trap came later.

The voltage of the cathode is set with the brightness potentiometer.
It varies from 0v to 38v.
The voltage of the "Modulator" is null, or near null, the Modulator only carries the video signal through a capacitor.

The CRT only have 3 plugs : 2 for heaters, and one heater plug is also for the cathode. And "modulator" for vidéo signal.
And.of course an anode for EHT.

[Mr Moose]

Hello,

This document,
https://www.earlytelevision.org/pdf/baird_t18_t20_t21_service_manual_v5.pdf

says that you adjust the heater current to 2.5A making sure you use thick leads to the ammeter to avoid voltage drop.

Yours, Richard

[Michelb]

Hello,

To be honest it's one of the part I don't understand well. The current is determined by the need of the final component. The more I supply voltage to the heater the more it needs current, that why I measured 1.5A at 1.6v, then 1.8A at 2v.
The current selector tap of the heater is done by ten sockets linked each others by a resistive wire. So each next one applies more resistance. I reproduce it on my bottom chassis.
And from my understanding it simply applies on heaters voltage and then as a consequence on the heater current.
Maybe if I raise to 3v I will be around 2.5A, but I won't take the risk.

[Heatercathodeshort]

Examine the tube neck with care. Can you see the heater glowing? If not feel the neck of the tube around the gun [turn it off!] If the neck is warm I suspect a faulty CRT. John.

[Michelb]

Yes the CRT heater is glowing, no matter with that, I can see it clearly.

[FERNSEH]

From the Baird T18 service manual details of the Cathovisor 12MW2A CRT and the power supply circuit.

DFWB.

[Michelb]

Hello,

Yes that is the schematic I followed to reproduce the bottom chassis.
I have VR3, VR4, VR5 wired as expected. The HT of the circuit is passing through a resistor (80k) and through the 10k VR4 brightness potentiometer until the CRT heater.

At the end I have around 40vdc max on the CRT heater cathode plug depending of VR4 brightness potentiometer.

I have on the neck of my CRT both heater wires connected, one of these two is also the cathode with the voltage described below (from.0v to max ~40v depending of VR4).

And the third plug on the neck of the CRT is the "modulator" which carries only video signal.

My EHT transformer is supposed to supply 5ma, do you think it may be insufficient?

Regards.

[Michelb]

Hi,

Does anyone has the datasheet of the Cathovisor type 12 MW2?

A friend asked me "why do you want only 2v on the heater as it is written everywhere that it needs 2.5 amp without any voltage precised? Set it to 2.5 amp even if it is at 3v".
And reading again all I can find it is true that it is not clear that the voltage should be 2v, the only given data is 2.5 amp in the technical doc.

What do you think, I am regardless the voltage and set it to 2.5 amp?

Does anyone here own a tv with a working Cathovisor tube? What are your specs?

Thanks.

[German Dalek]

Bonjour Michel,
Please ask a Frank, there are 2:

https://frank.pocnet.net/
http://tubedata.philipse.info/
or here:
https://www.earlytelevision.org/

Good luck!
German Dalek

[Michelb]

Hi,

Thanks I wrote to Frank to ask if he owns the datasheet of this Cathovisor type 12 MW2.

nice Sonora TV3 on your photo, I also have one in working condition (I restored it).

Today I tried to raise the current (and voltage) of the CRT heater. I winded few more turns.
Now at max I have 3vac and 2,37A. I raised it slowly and decreased it slowly before shutdown.

But still nothing on screen, no light. The heater is glowing, there is a 405 lines composite video signal inserted directly on G1 of the final TSE4 video amp tube, and I have the cathode voltage derived from the main HT.

Even if the deviator were faulty I should have something on screen, a spot at least. But nothing, I tried on the dark and set the brightness pot to have the most close potential cathode-wehnelt.

I don't have much more idea. Maybe try to raise it at 2.5 amp as requested, but It's hard to believe it will make a difference with the 2.37A.I had in my last test.

[German Dalek]

Bonjour Michel,
Thanks for the flowers, I have two, both 441 lines sets.
The other one has a "bakelite" colour.

Under normal circumstances, the americans must be able to help you out.
They have the same set like you, please take a look here:
https://www.earlytelevision.org/prewar_british.html

Regards,
German Dalek

[Freya]

Has the tube been re gunned with a non straight gun, this would be very obvious but just an idea.

[Michelb]

Hi,

I don't think the CRT has been re-gunned, it looks like to be in genuine condition, "as is" and dirty since decades.

I think this TV set doesn't work and is incomplete since a very very long time. There was something interesting I noticed at first : a very huge and ancient transformer was attached in the side of the internal wooden cabinet. This heavy transformer is not part of the TV set, as I can't find it in the schematic and photographs of others T18.
I tested this transformer and it turned out to be a 110v <=> 220v converter. What was the use of this thing? I have maybe an explanation. The home voltage was 220v in Great Britain at that time as this TV set was made to run with this voltage, but in France it was 110v until the 1950's. So it is possible that this TV is in France since a very, very long time, why not imported since the beginning. And this transformer has been put inside to make it work on 110v.
In this case the aerial part should have been configured for the frequency of french TV. But back at the time french TV had very few tv show, less than you had in UK. And the war stopped all in France, and it took time to restart at the end of the 1940's.

So if this set is really in France since a such long time I think maybe the CRT is not too bad, it should have been less used than if it was in the UK because you had your 405 lines programs until 1985.
From my experience with the french TV I have from the last 40's, the CRT are always very fresh, it was clearly not used a lot, only few hours.

But it is my theory, for now I really don't know what to think about, I have no idea what to do, I will recheck all again.

[Michelb]

here some crt photographs

[Michelb]

Hi,

I did some others tests, but I am still not getting any reaction on screen.

I have a cathode voltage from 0v to 41v max, wehnelt around 0v, EHT near 5kv.

I join some photographs of my tube heater at 2.2v and 2.135A current.

I start to think this picture tube may be not working.

Regards.

[Panrock]

Hi,

I don't have the schematic to hand for this set so the following comments are rather based on guesswork, but...

With the set disconnected from the mains, have you checked for inter-electrode leaks on the tube using the high resistance range of a multimeter?

What happens if you run the set up with the 'mod' (grid) of the tube disconnected from the video feed and shorted to cathode (HC)?

Have you tried adding a high resistance return path for the video to the cathode where it feeds into the tube after the coupling capacitor?

Is the TSE4 known to be good? They often weren't! Is the coupling capacitor perfect?

Finally, I would try gently tapping the neck of the tube with a screwdriver plastic handle, with the set running under test conditions. This might clear an intermittent internal disconnection.

Always wear eye protection when working on this tube. These Baird sets were very sensitive to mains voltage fluctuations. Even a 10-volt deficiency can dim the picture out.

Maybe this set was modified to receive the 441-line transmissions from the Eiffel Tower during World War 2! Bear this in mind when feeding it with a suitable signal. Anyway, it's a fascinating historical piece. Good luck!

Steve

[Michelb]

Thanks for your reply.

I didn't test all your advices yet.
I tried to gently hit the neck of the tube, and I swapped two TSE4 tubes (I don't have a vacuum tube tester), without any luck.

But before going further I have a doubt on something I noticed. I found that the electrodes are really misaligned. Have a look at the photo attached (I also put one of my miserable work in progress).

To be honest I never paid such attention to a CRT before, because I was not so much annoyed with that in my previous repair. I know it is normal to have a misalignment when an ion trap is used, but as far as I know this one doesn't have one, so it is quite weird.

[Panrock]

Yes, your pictures of the tube internals at different times have been taken from different viewing positions, but there does seem some serious misalignment between (what I take to be) the electron gun and the modulating electrode. It's as if the internal mountings are loose and any movement or tilting of the tube could run the risk of changing the electrode positions. Also, the 'tapping' should have been the last, not the first, thing to try, as I mentioned.

What do others think?

Steve