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Old 10th Apr 2023, 9:37 pm   #501
murphyv310
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Default Re: Baird T5 restoration project.

Hi David.
Could you not cobble up a 7BP7 for a test CRT. I have one and it's always to hand for testing sets I don't wish to remove from the cabinet like an HMV 1804 I'm restoring at the moment.
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Old 11th Apr 2023, 11:43 am   #502
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Default Re: Baird T5 restoration project.

Hi Trevor,
7BP4 has a 6.3volt heater which means a separate transformer will be needed as all the valve heater supplies in the T5 are 4 volts. The CRT has a 2.5volt filament.
It's about time the 15" Cathodevisor CRT is refitted in this receiver. But before that is done, the base connector must be removed from the tube.

DFWB.
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Old 18th Apr 2023, 11:44 pm   #503
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Default Re: Baird T5 restoration project.

Hi folks,
David has asked me for some clarification of the Baird CRT base arrangement.

The original CRT socket has sprung side contacts similar to a VCR97 socket, but there are only four connections and the diameter of the socket is much smaller than the VCR97 type of course.
The cathode is internally connected to one of the heater connections. The anode connection is on the CRT base as well, rather than having a separate EHT connector.
The CRT socket is normally permanently connected to the EHT unit, via the EHT cable, the other connections (heaters and grid) have short wires from the socket to inline connectors to enable the socket to be disconnected from the other chassis.

The diagram shows a 500K resistor between the brightness control and the CRT grid. This resistor would have been in the CRT socket and it can be seen in the first photo.

I've included a picture of the bottom of a Baird T23 which clearly shows the tube socket plugged on to the CRT base.

The last photo is the one that David took in 2019 of the base of the CRT from the T5 that this thread concerns. The photo is not all that clear, but it seems to suggest that the socket is missing and that someone has at some point soldered lead out wires directly to the CRT base (yuk!). How did they connect the EHT though? Maybe via a crock clip?

If the socket is indeed missing then the options are, find a suitable way to connect the EHT lead to the CRT (in a way that can be easily disconnected for servicing) or make a replacement socket (not easy).

I hope this clarifies things David. Can you take a clearer picture of the CRT base for us to see please? I wonder what happened to the original socket? Maybe it was breaking down and arcing, so someone decided to remove it...

Cheers
Andy
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Last edited by beery; 18th Apr 2023 at 11:50 pm. Reason: Clarification
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Old 19th Apr 2023, 3:40 pm   #504
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Default Re: Baird T5 restoration project.

Hello Andy,
lot's of useful information in the pictures. Will take a look at the CRT later today and take pictures of the CRT base, it'll be almost certain the tube has been the victim of a vintage bodge job.
However, it may be possible that the 15MW4A CRT required a different base connector?
The second picture clearly shows the EHT cable release button on the side of the CRT base.
Fourth picture That's the first T23 I've seen fitted with the correct Baird Cathodevisor CRT. All the others including mine have the Cossor 15" CRT.

DFWB.
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Old 19th Apr 2023, 10:20 pm   #505
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Default Re: Baird T5 restoration project.

The metal tube fitted around the neck of the 15MW4A CRT.
The metal tube is 155mm long, an internal diameter of 46mm and an outer diameter of 56mm.

DFWB.
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Old 20th Apr 2023, 8:10 am   #506
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Default Re: Baird T5 restoration project.

Good morning David,

The last mile is a very tricky one with that enormous CRT.
Luckily you are always very careful and use good protection.

I'm looking forward to seeing a picture on the 15MW4A.

Jac
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Old 20th Apr 2023, 9:26 pm   #507
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Default Re: Baird T5 restoration project.

Hello Jac,
I've summoned up the courage today to invert the special receptacle which was made to keep the tube safe while service work was carried out on the receiver.
The attachments show the wires directly soldered to the tube base and one of the side contacts.
A connector must be made for this tube. There is no way I'm going to reinstall the tube with those loose wires hanging about.
Three years ago it was a bad enough experience removing the tube along with those wires. One wire did actually attach itself to something while the tube was being lifted out.

DFWB.
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Old 21st Apr 2023, 12:27 am   #508
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Default Re: Baird T5 restoration project.

Hello David I always follow your restorations with great interest and this one is no exception .I have just finished reading about the comments on Nathaniels Dynatron CTV29, on Chris Crustys web page and left a few comments and I have 2 examples a CTV22A and CTV28
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Old 21st Apr 2023, 9:21 am   #509
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Default Re: Baird T5 restoration project.

Hi.
Perhaps a suitable base could be 3D printed and the contacts used from a side contact base of the type used on the MW6-2?
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Old 21st Apr 2023, 12:51 pm   #510
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Default Re: Baird T5 restoration project.

Hi Trevor,
It might be possible to make a connector for the CRT using that great favourite of mine, the 50mm plastic pipe. The inside diameter is 46mm and is a snug fit on the base of the Baird CRT. All that is needed is four spring type contacts and it is job done.
Haven't considered 3D printing because I don't possess a printer or have access to one.
Hi DavyRocket,
I'll take a look at your comments about the Dynatron CTV29, but to be honest it's one set I be happy to pass one to someone else who fancies having a go at it.

DFWB.
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Old 21st Apr 2023, 3:46 pm   #511
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Default Re: Baird T5 restoration project.

Hi David,
it looks like the drain pipe might be the way to go.
I will have a look to see if I have any spare VCR97 type contacts.

Cheers
Andy
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Old 21st Apr 2023, 4:13 pm   #512
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by beery View Post
The anode connection is on the CRT base as well, rather than having a separate EHT connector.
Better avoid using a black drainpipe then (?!?).

David, we're all watching this on tenterhooks. Can't wait to see the results.

Steve
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Old 21st Apr 2023, 4:24 pm   #513
high_vacuum_house
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Default Re: Baird T5 restoration project.

I wonder if you could get some contacts for the CRT base by carefully dismantling a side contact valve base. They look about the correct size and springiness.

Christopher Capener
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Old 21st Apr 2023, 8:38 pm   #514
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Default Re: Baird T5 restoration project.

Hi David, how close in diameter is a Ct 8 (or the rarer Ct5) base?
If close contacts could be easily removed from the higher numbered base

Ed
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Old 21st Apr 2023, 9:34 pm   #515
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Default Re: Baird T5 restoration project.

High_Vacuum_House wrote: "I wonder if you could get some contacts for the CRT base by carefully dismantling a side contact valve base. They look about the correct size and springiness.
Hi Chris, it's going to a Ct8 socket, ideal for the job.
Hi Ed,
was upstairs today searching for a side contact Ct8 valve socket. No such such luck but there is a junk pre-war radio which has those valve holders.
There's no doubt the spring contacts will be ideal for the Cathodevisor tube connector. Take the valve holder from the AZ1 rectifier.
Was kinda looking at the contacts in this Brayhead turret tuner, but better not.

DFWB.
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Old 22nd Apr 2023, 6:04 pm   #516
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Default Re: Baird T5 restoration project.

Attachments show the side contact valveholder which was removed from a pre-war radio set.

DFWB.
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Old 22nd Apr 2023, 8:33 pm   #517
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Default Re: Baird T5 restoration project.

This sort of holder was briefly popular 1938-9 wasn't it?

Steve
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Old 22nd Apr 2023, 8:46 pm   #518
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Default Re: Baird T5 restoration project.

Hi Steve,
Introduced on the Continent in 1934, might have been a year earlier.
The 1938 Pye 815 TV uses side contact valves, EF6, EB4 etc.
Even the CRT has a Ct8 base.
Got to work out how this rare component can be modified for use with the Cathodevisor CRT. Either use the contacts only or cut up the bakelite base and insert sections into the 50mm plastic tube.

DFWB.
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Old 26th Apr 2023, 1:04 pm   #519
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Default Re: Baird T5 restoration project.

There are chinese copies available, so must be some market for them still. I have some nice rectifiers that use that base, and bought a couple of Aliexpress to try them out, but the metal contacts do not flex properly in the grooves and it was hard to insert the tube and a nightmare to release it afterwards.
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Old 6th May 2023, 10:29 pm   #520
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Default Re: Baird T5 restoration project.

Getting close to reinstalling the 15" Cathodevisor CRT. But there's a few jobs to attend to before that happens. First, remove the the 9" Ferranti CRT, it's done it's job proving that the vision circuits work. Next, refit the the on-off switch on the upper left side of the cabinet. That done fit a replacement fuse holder to replace the original heat fuse which is unusable.
The fuse holder is a Belling-Lee type L1045. Its an excellent replacement for the damaged heat fuse.
Slight modification to the mounting of the upper control panel to facilitate easier removal and refitting.
Fitted today, the focus coil spacer block. The 15MW4 CRT must be slightly shorter than the 15MW1. The spacer is a factory made part.

DFWB.
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