Baird T5 restoration project.

[radiomarktv]

Hi David, like Peter has said let’s hope the connection is just lost externally to the base and the smell is from the bad connection. A look out for any discolouration around the heater contacts to the CRT socket and tube. Hopefully just a careful re-solder, my fingers are crossed.
Mark.

[high_vacuum_house]

Hopefully it was just a bad connection to one of the heater pins has caused a high resistance and softened the insulation or melted the solder at the joint and it is an easy repair.

I presume that you haven't got the space below the neck to pull off the CRT socket off the base and inspect it without having to remove the tube?

Christopher Capener

[FERNSEH]

Originally Posted by high_vacuum_house

Hopefully it was just a bad connection to one of the heater pins has caused a high resistance and softened the insulation or melted the solder at the joint and it is an easy repair.
I presume that you haven't got the space below the neck to pull off the CRT socket off the base and inspect it without having to remove the tube?
Christopher Capener

Hi Chris,
the end of the CRT is only four or five inches or so from the cabinet base. Still, it might be possible to inspect the CRT base by using a mirror.
This tube is slightly shorter than the original 15MW1, even so the tube is 35 inches long.

DFWB.

[Jac]

Originally Posted by peter_scott

I'm sure we all have fingers crossed that the break is external to the glass envelope. Let's hope that was the cause of the smell.

Peter

+1
Jac

[akuram1]

Sorry if this a stupid idea but would it not be possible to cut a hole in the cabinet bottom to give you access and avoid taking out the crt again. Easy enough to cover up the hole after as no one sees the bottom.

[Panrock]

I haven't got the heater circuit to hand and I don't know from where you measured the heater o/c, but to me the smell and the previous adjustment 'criticality' suggests a scratchy heater pot.

Steve

[FERNSEH]

Originally Posted by akuram1

Sorry if this a stupid idea but would it not be possible to cut a hole in the cabinet bottom to give you access and avoid taking out the crt again. Easy enough to cover up the hole after as no one sees the bottom.

Cutting out a hole in the cabinet base will be rather dangerous with the CRT in situ. It's not too difficult a task to remove the tube from the cabinet.
Just remove the base connector and lift the tube straight up vertically and return it to the specially made cradle. Then, the cradle can be inverted and the base connections inspected.

DFWB.

[FERNSEH]

Originally Posted by Panrock

I haven't got the heater circuit to hand and I don't know from where you measured the heater o/c, but to me the smell and the previous adjustment 'criticality' suggests a scratchy heater pot.
Steve

Hi Steve,
the filament resistance was measured at the side contacts on the tube base. Before this fault occurred the filament resistance was <0.5 ohms.
Will check the filament current pot again.

DFWB.

[FERNSEH]

Have decided to delay the removal and refitting the CRT until the workshop warms up somewhat. The glass bulb must be more prone to implosion in a cold environment.
Play safe and leave the job for a few weeks.

DFWB.

[FERNSEH]

Bad news, the CRT has an open-circuit filament.
The tube was removed from the set this afternoon.
It's doubtful if a replacement Cathovisor 15MW1 or 15MW4 can be found so if the set is going to display pictures the only substitute CRT which will work in this set is the 15" Cossor 65K.
If it is decided to use the Cossor CRT in the T5 a considerable amount of modification work will be required. See second attachment.

DFWB.

[radiomarktv]

David this is the news we didn’t want to hear.
I’ve read before that it’s possible to pass EHT across the heater pins to flash weld the heater together. Is this worth a try?

Mark.

[FERNSEH]

Hi Mark,
that's a good idea. Try EHT first and if no results a hefty spark from a line output valve might just do the trick. But it'll have to good weld because the filament current is 2.5 amps.
At least there is no worries about heater-cathode insultation with this directly heated cathode tube.

DFWB.

[Jac]

That's very bad news indeed David!

Jac

[akuram1]

So sorry to hear that. Have followed this restoration from the start with great interest so hope you can find a satisfactory solution.
Fingers crossed.

[high_vacuum_house]

That is bad news. Did you find the cause of the odd burning smell just before it failed. Could the break be inside the CRT base cap, the solder to the contact has melted with quite a high current passing through it.

I did the HV weld trick on a PENDD4021 valve years ago and it worked well.
Christopher Capener

[dtvmcdonald]

My dearest sympathy for the loss of the CRT. I had a similar event with my 15GP22. It actually impacted my health. I took the extreme step of converting the chassis to a receiver outputting standard RGB 50 ohm signals and fed them to a modern top quality monitor, actually a broadcast-monitor quality picture on the Trinitron! That kept me going until a good 15GP22 turned up.

Good luck finding some sort of solution to get an image again. Those sweep circuits are very very strange.
There are numerous American 16-17 inch tubes with as small as 53 degrees deflection, but that one has a metal cone and is famous for going to air. One of those just might work if the neck is not too big. Possibly use a slightly smaller mask and make a true rectangular picture. Not ideal, but for such an iconic set, better than shelf queen status.
What's the normal high voltage on the Baird? I can't find much on it. I was going to do something like that on my RCA TRK12 until one of those British rebuilt radar tubes with the exact correct size and ultra bright rare earth phosphor turned up.

[Panrock]

From memory, the EHT on this model was 7kV, but David will be able to confirm...

Steve

[FERNSEH]

Many thanks for your sympathetic comments. Today, I tried Mark's suggestion for using the EHT from a TV receiver to weld the open circuit filament. See post 611.
The chosen TV set was a Kolster-Brandes HV40, this set seems to deliver a really robust EHT arc. The first attempt yielded no results. Try again, this time success! Filament resistance is according to the Fluke test meter is 0.3ohms.
That done the next stage will be to connect the CRT to a 2volt isolation transformer and leave it on test to determine how long the repair might last.
My only fear is the emissive material on the filament might have been damaged.
About four years ago I attempted to repair the open-circuit heater in a Mullard AW43-80 CRT by applying EHT to the base pins. Unfortunately, the repair lasted for only two weeks.

Answering Steves question. The Baird T5 EHT is 5KV and is the same in the T23, the set which has a Cossor CRT.

In the T5 the frame scanning assembly is attached to the wooden board which also supports the CRT bulb. In the T23 the frame scanning assembly is attached to the front panel of the cabinet.

DFWB.

[peter_scott]

A brave attempt to save the CRT. I very much hope it does give a lasting repair.

Peter

[radiomarktv]

Fingers crossed David, just one thing comes to mind, does the filament adjusting pot on the PSU chassis need to be adjusted to match the requirements of the tube filament.

Mark.