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-   -   Baird T5 restoration project. (https://www.vintage-radio.net/forum/showthread.php?t=152005)

FERNSEH 31st Aug 2019 9:35 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
Hi John,
Funny you should mention 15" narrow deflection tubes. I'm considering fitting the Cossor tube you gave me in the T23. My friend Kevin called in today and he suggested making a new adaptor plate to mount the tube in the Baird. The Cathodevisor CRT in the T5 is the most fearsome tube I've ever had to remove from a TV set. Close runners up in the scary stakes has to e your 15" Emiscope 3/6 and the Mazda CRM151, but what about that 16" tube that was fitted in the pre-war GEC BT8161?

DFWB.

Argus25 31st Aug 2019 10:42 pm

Re: Baird T5 restoration project.
 
Quote:

Originally Posted by FERNSEH (Post 1172518)
Hi Argus,
I've redrawn the circuit of the DC restorer to show the 500Kohm resistor in parallel with the WX6. Note also the 200Kohm resistor in series with the diode.
The third attachment show the underside of the T5 EHT unit. No DC restorer was fitted in this receiver.

The purpose of the resistor in series with the diode is to isolate the diode's capacitance from the video signal. If it is too large the restorer's effectiveness is diminished. The typical value RCA chose was around 10k to 47k as I recall, it needs to be at least as big as the anode resistor in the plate of the video output valve and not much bigger in value. Maybe, looking at the photo you posted, its a 22k not a 220k ? Check it on the meter, if it reads 200k, maybe its a gone very high 22k ?

Argus25 1st Sep 2019 12:42 am

Re: Baird T5 restoration project.
 
Quote:

Originally Posted by FERNSEH (Post 1172249)
The CRT has leakage between the anode and grid, possibly a piece of cathode material has got lodged between the electrodes.

I have been thinking about this remark.

The spacing between the anode and grid inside the tube in the CRT gun structure is very wide, quite different between say the heater & cathode or grid assembly and cathode which are closely spaced.

Not only that, inside the vacuum environment tube, unless there was a lot of loose internal aquadag, which it may not have, it is unlikely any leakage would develop between the lead out wires at the pinches, although some form of metal vapor could possibly have condensed there, but if it was getter overspill you would probably easily see it.

I think it is much more likely the leakage between the anode and grid connections is on the outside surface of the CRT, in the base, where the lead wires exit the bulb, I have seen this before.

What happens is the lead out wires corrode over time and it creates a subtle greenish film on the glass between the wires where the pinches are and between where they exit the base of the neck. This film is not soluble in contact cleaner.

If it is this problem, it needs to be sprayed with de-ionized water and the glass around the lead out wires gently stroked with a soft tool, a wooden cut down match end attached to a handle, being careful not to touch or bend the wires where they exit the bulb, as they can be very fragile, possibly corroded and break off. Then dry it with a hair dryer on low heat.

I'm not sure what the base on this tube is like or how easy it could be to remove if that was necessary to check it. Obviously before attempting it you would have to be 100% certain each wire was de-soldered and free from the pin as those lead out wires are easily broken when they are aged. And if they are going to break it will tend to happen close to the glass pinch, then its a real problem.

Heatercathodeshort 1st Sep 2019 9:20 pm

Re: Baird T5 restoration project.
 
I was thinking about that GEC 6101 16" glass tube just yesterday! They are very rare and I think 70 degree, maybe aluminised, so the picture would be poor if installed in the T5.
The Mazda CRM 151 again requires high EHT but is very narrow angle. 15" tubes are a problem and always were. Something always turns up! John.

FERNSEH 3rd Sep 2019 3:25 pm

Re: Baird T5 restoration project.
 
Argus25 wrote: "The purpose of the resistor in series with the diode is to isolate the diode's capacitance from the video signal. If it is too large the restorer's effectiveness is diminished. The typical value RCA chose was around 10k to 47k as I recall, it needs to be at least as big as the anode resistor in the plate of the video output valve and not much bigger in value. Maybe, looking at the photo you posted, its a 22k not a 220k ? Check it on the meter, if it reads 200k, maybe its a gone very high 22k ? "
The resistor in series with the WX6 is indeed 22Kohms and is well within tolerance for such an old component.
Yesterday evening the EHT cable to the tube anode was disconnected and voltages and waveforms on the CRT grid checked to see if there would be any changes. Results were just the same as with EHT. So it's has to be leakage between the grid and cathode that's causing the brightness control and DC restorer not to work correctly.
The CRT will be taken out later today and that wire link between the cathode and heater will be removed. The tube cathode connection will be free for the brightness control as it is in the model T20.

DFWB.

FERNSEH 3rd Sep 2019 10:20 pm

Re: Baird T5 restoration project.
 
Just to compound the horror of this set the tube won't come out. After removing the safety glass and mask pull off the anode connector on the CRT cone. Disconnect the three wires on the tube base. Disconnect the line scanning coil plugs. The tube should lift out, but it doesn't, and of course you can use any form of force when handling these fragile tubes.
This job is not going very well.

DFWB.

FERNSEH 3rd Sep 2019 10:34 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
There is an insulating ring between the focus coil and the line scanning coils. See attachment.
It's possibly this part that is preventing easy removal of the tube.

DFWB.

FERNSEH 3rd Sep 2019 10:57 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
Of course the tube won't come out. The metal adaptor plate must be unscrewed and using this device lift the tube straight up and away from the cabinet.
Let's see if that works.

DFWB

davyrocket2 3rd Sep 2019 11:17 pm

Re: Baird T5 restoration project.
 
It would seem you have created a very large type of Meccano jigsaw puzzle but, I am sure your many years of servicing will get it sorted. Probably when first built the cabinet was built around the tube etc?

FERNSEH 3rd Sep 2019 11:22 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
The tube is at last out from the cabinet. Now I can remove that wire link between the heaters and cathode.

DFWB.

FERNSEH 4th Sep 2019 1:06 pm

Re: Baird T5 restoration project.
 
Quote:

Originally Posted by davyrocket2 (Post 1173413)
It would seem you have created a very large type of Meccano jigsaw puzzle but, I am sure your many years of servicing will get it sorted. Probably when first built the cabinet was built around the tube etc?

Hi David,
As awful as the tube mounting assembly looks now it was far worse when I acquired the set in 1993. The focus coil was mounted on three long pieces of 2BA threaded rod, the whole assembly was, well, just wobbly. Without delay wood spacer blocks were made to raise up the focus coil so that it was close to the line scanning coils. It's difficult to believe that this CRT substitution job was done by Baird Television Ltd. Or was it?

The tube was refitted this morning and the set is ready to be tested.

DFWB.

FERNSEH 7th Sep 2019 10:43 am

Re: Baird T5 restoration project.
 
2 Attachment(s)
At last I've found the 9mm plywood needed to match the veneered surfaces on the original side panels. So yesterday I set about making two new panels. The attachments show the surfaces will require more dark wood stain to look like the originals.
It will be recalled the side panels made earlier this year are a perfect fit but it was all too obvious these were nothing more than dark stained plywood, not a good look.
After this project is completed we'll find that there will be almost enough spare cabinet parts to make another T5 or T23.

DFWB.

peter_scott 7th Sep 2019 11:19 am

Re: Baird T5 restoration project.
 
They look nice.

Peter

Hunts smoothing bomb 7th Sep 2019 11:51 am

Re: Baird T5 restoration project.
 
Well done David, it's getting there, those 15" CRT's scare the cr:censored:p out of me!

Cheers

FERNSEH 7th Sep 2019 1:07 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
Hi Peter, I've applied more wood dye to the panel surface and as the attachment shows it's starting to resemble the original part.

Hi Lee, it's a scary business removing and refitting those 15" tubes. In my T23 it's easy to lift out the tube but refitting it is a real hairy job. The tube bowl is placed on a special metal plate which must be aligned with the four threaded spacer rods and these never line up first time so you are holding the tube with one hand and fiddling about with the rods with the other. Not a pleasant experience.

But if you think 15" and 16" all glass tubes are scary just check out the 20" Dumont tube.
From the ETF: http://earlytelevision.org/dumont_20AP4.html

DFWB.

beery 7th Sep 2019 9:13 pm

Re: Baird T5 restoration project.
 
Hi David,
those bits of wood for the side panels are a real find. The veneer pattern is even similar to the original panel shown in your picture.

Cheers
Andy

FERNSEH 7th Sep 2019 10:23 pm

Re: Baird T5 restoration project.
 
Hi Andy,
Those new side panels have got the cabinet renovation restarted. Now the newly made front panels must be veneered, that's no problem as veneer sheets are readily available.
As for the T23, the CRT has gone dim, so before it was too much brightness, now there isn't enough.

DFWB.

FERNSEH 6th Nov 2019 9:04 pm

Re: Baird T5 restoration project.
 
3 Attachment(s)
It was suggested to me that the cabinet top panel could be salvaged. I had my doubts but was willing to have a go. As the first picture shows the plywood panel is in just same bad state as all the others.
I was not entirely satisfied with the first attempt at making a new top panel so a new one is being made. The second picture shows the first stage of the construction of the new panel.
The book matched veneer strips arrived yesterday so a start can be made applying the veneers to the new front panels.

DFWB.

Jac 6th Nov 2019 9:36 pm

Re: Baird T5 restoration project.
 
Good evening David,

Quite a big job!
Luckily in expert hands.

Jac

FERNSEH 6th Nov 2019 10:14 pm

Re: Baird T5 restoration project.
 
Hello Jac,
The first attempt to make the top panel didn't go very well. The router tool kinda went wild and shaved off too much wood in places. I did some corrective work using wood filler but this job has to be right. All the new parts must replicate the originals.

DFWB.


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