@till Sorry to hear that David after all the work.
Hi Frank, the only course of of action now is to modify the Baird T5 to accept the 15" Cossor type 65K CRT. The Baird T23 in my collection was modified by the factory to employ the tube made by the rival manufacturer. Of course the Cossor CRT is not a drop-in replacement. For starters a metal adaptor plate must be made for the differently shaped CRT bulb. The frame oscillator transformer with it's extending pole pieces and the focus magnet will have to be raised up 5 inches or so to accommodate the shorter CRT. So now I have task of dismantling the T23 in order to measure up the positions of the new components needed to completer the Baird T5 job.
Worth mentioning the Baird T23 might have been subject to a works CRT replacement but it was one the roughest conversion jobs I've ever seen.
Till Eulenspiegel.
The Baird T23 in my collection was modified by the factory to employ the tube made by the rival manufacturer.
It wasn't unusual to see Cossor valves specified in certain positions pre-war Bush radios though - quite often the Cossor MVS/PENB would be used as an RF amplifier and there were several other examples to be seen. I'm sure the 41STH makes an appearance too.
Today, I gave the Cathovisor CRT another blast of EHT. Once again this action seems to have done the trick. But how long will the filament hold up?
Till Eulenspiegel.
Time will tell...
The one I did recently, that would go O/C as it was warming up, then cycle on and off (like an old-fashioned Xmas tree lamp), responded to 250 volts DC, current limited to the 600mA heater rating using two huge incandescent lamps.
The first two times we tried it, it failed again after a few minutes, then after a day switched off. So we hit it again with 50% more current in a kill-or-cure attempt and so far it has held up for over a month.
So it does seem to matter how much energy you can put into the welding arc.
I wonder if it need to be DC? We used the TV's HT supply (transformer sourced voltage doubler) but you'll need a few amps, so maybe direct mains in series with 2 or 3 amps worth of lamps (bathroom heat lamps?) would do the trick.
I think, if it fails again, what do you have to lose by trying a more aggressive approach?
Just re-reading this fascinating saga.
I recall that, when the filament failed for the first time, there was a burning smell.
This smell could not possibly have come from a hot joint inside the glass. It follows that the fault is outside the glass and hence it can be fixed.
The blistering of the phenolic base seems to be evidence that this has happened before. Maybe that's why the base wires were soldered to the pins?
Anyway, assuming you can remove the solder from the base contacts, could the base be removed? After soaking in a suitable solvent?
Failing that, cut the phenolic around the filament contacts and remove them. Mechanically clean the wires emerging from the glass and re-solder.
(what is the best flux for the alloy used to match the Pyrex? Some-one out there must know!)
Then test for resistance!
Re-assemble, using epoxy to hold the cut-out contacts in place.
You came so close, don't stop now!
I agree that the problem is most likely a bad connection inside the cap into the pin. I would have that cap off or cut open.
This had happened so many times especially with high current heaters.
As a start have you tried reflowing the pins with fresh flux and solder?
To quote Rocky, of Rocky and Bullwinkle fame:
"But that trick never works!"
Not in my experience anyway. You may think you've fixed it but you've only extended the time when it will strike again!
Have a look at the picture of the wire that was inside an octal yoke plug pin in the post I made about the 68 year old intermittent fault. That wire survived several attempts to re-solder it over the years.
I cheat slightly.
If the pin is open ended as most octals will be because they are soldered after the wire is threaded through, I heat the pin, flux it, insert a thin wire, heat it, flux it, solder it. Cut off the wire.
This method has never failed me. Lead tin solder of course.
