Can anyone identify this CRT

[space_charged]

Hi all,

Recently I bought a couple of CRTs on line. When I collected them (17" Mullard and Mazda) I was also offered two more CRTs, which I bought. They were both small round CRTs and obviously quite early ones at that. One has a label, Mazda CRM 92A, so I have all the information I need on that.

The other one is a mystery, as it has no labels on it whatsoever. I'm attaching a few pics of it in the hope somebody might know what it is. The screen is about 10" diameter.

[Oliver35]

Odd tube- as you say, the overall shape looks old, but the anti- implosion face, metallisation and small all-glass base suggest otherwise. Some kind of radar/fish finding function, maybe? I was advised to use a camera flash (in the dark) to get a brief glimpse of the nature of the phosphor, that would give a clue. Pretty tube, though.

What's the neck diameter, out of interest?

Oliver

[peter_scott]

Looks like an Emiscope 3/16.

Peter

[Argus25]

The bulb & face look old fashioned in design, but I think it has been re-gunned with a modern gun and pin base and possibly got new phosphor with an aluminized coating. Maybe it started out as the Emiscope 3/16 or similar.

[space_charged]

The neck diameter is 35mm. I've just illuminated the phosphor with a UV light and it glows with the same colour as all my b/w CRTs, so I guess its white. The persistence (to UV) was short.

[Oliver35]

Agh, I didn't think of a re-gun. That base did puzzle me!

Oliver

[space_charged]

Yes, I too thought it might have been re-gunned. The silver coating inside does NOT go right up to the phosphor coating though, there is a gap so I'm not sure the phosphor has been coated with aluminium. Actually its not advised to coat an old CRT with aluminium because you need a higher energy electron to get to the phosphor and therefore higher EHT. That in turn needs more deflection current.

If it is or was an Emiscope 3/16 what sort of age would it be?

Without much hope, I will test it (carefully) with an (up to) 5KV EHT supply and see if there is any sign of emission.

[FERNSEH]

If it is an Emiscope tube it will seven pins on the base. I'd say it is a type 3/16.

DFWB.

[space_charged]

I can confirm that it does indeed have seven pins on the base. The pins appear to have "rusted" a fair bit! I've never seen a valve with corroded pins before!

[thermionic]

If that 3/16 turns out to be good, you would have a true gem. I, and many others on here would love to get our hands on one!

My gut feeling is, however that it's as dead as all the others! Good luck and lets keep our fingers crossed!


SimonT.

[space_charged]

You are probably right that it will be as dead as all the others. I suppose there is some hope because it seems to have been re-gunned at some time. It all depends on how well that was done.

Its a real pity that the last re-gunning business has closed. I'm aware there is an attempt to start up a re-gunning facility in the US. I wish them all the best, but I'm not sure they will ever get up and running.

I'm an amateur high vacuum enthusiast and I run a 12" glass bell jar. It is pumped by an Edwards EO2 three stage vapour diffusion pump backed by an E2M2 rotary pump. The system gets down to 1*10E-06 torr and I can readily demonstrate cathode rays. I make my own electron guns and coat my own screens.

Sadly there is a lot more to re-gunning a CRT than where I am now. I lack the glass working experience needed. For a vacuum device to function for any reasonable time after it is removed from a vacuum pump, it needs to be pumped at high temperature. That needs to be done for long enough to remove the multiple layers of water that coat EVERYTHING in the tube.

I do have a 300Watt induction heater, so could heat an electron gun to flash the water off it. Also could flash a getter. I'm going to have a go at making a simple valve (triode) and attempt to seal it off and getter it. It will be interesting to see how long it would work for.

Its one thing to bake out a small valve, quite another to handle a CRT heated to 500C under continuous pumping.

One other thing I've always wondered is if (say my old CRT) has some remaining un-flashed getter material left in the getter boats inside, maybe I could re-flash the getters to improve the vacuum?

[space_charged]

Mmmm, re my Mazda CRM 92A, there are signs of life!

First I ran up the heaters, slowly on a current and voltage limited PSU. The heater volts are supposed to be 2V at about 1.3 amps. Ramping it up slowly, the heater started to glow dull red then a nice colour at 2V and 1.3A. That means the pressure inside the CRT is better than 1*10E-04. Any higher than that and heaters won't reach the correct temperature.

Next, I set up my trusty 0-5000V supply. I just connected grid to cathode for one end and of course the anode cap for the positive. The tube is magnetically focussed, so I knew there wouldn't be a dangerously bright spot to burn the phosphor. I was expecting a fairly large "splurge" not a spot. See the pics attached for what I got (YUM)
The EHT used to take these pics was a shade over 1KV.

Seems like I may have a good tube.
Next I'm going to put the Emitron on the same setup... Watch this space.

[space_charged]

Regrettably, the Emitron is gassy. A blue glow discharge is seen between the gun and the final anode coating. The heater does light but there is nothing on the screen. Because of the discharge, quite an EHT current passes, even when the heater isn't on. So the tube is very soft.

[thermionic]

That's disappointing! At least the CRM92 seems serviceable.

It's a great shame the 3/16 is useless, I had my fingers crossed! It's a pity we can't un earth some mothballed CRT re gunning plant for you to have a play in.....you obviously have the enthusiasm and knowledge. You could be a godsend for all us TV restorers!


Good luck.

SimonT

[Heatercathodeshort]

This is very interesting.
I worked with one of the best CRT rebuilders. I only wish he was still with us. There was nothing that John did not know about re gunning CRTs! The company was 'Central Tubes'.

Water was the biggest problem followed by impurities in the air that were allowed to enter when the tube was opened to remove the old gun etc.

The glass bulb including the screen and the gun assembly had to be heated during evacuation to expel all gas molecules [or as many as was possible] before sealing off.

John fitted large getters that I believe were manufactured in Italy.

He used the original screens and always stated that the best combination was a Mazda screen and a Mullard type gun!

John was a good friend of mine from when I was a young lad in 1964. He could regun anything. Pyrex, metal cone, Rimband, bonded and colour. He had a long standing contract with the BBC to rebuild their monitor tubes that employed the 'rare earth' M5 red phosphors.

I think only Cossor and EMI produced 10" tubes. The EMI examples had a very poor reputation. O/C heaters were very common combined with very short life. The Cossor tubes were excellent and a modified version of the 108K was supplied to EMI coded 2/20 for the later 1807 series. This is recognizable by it's four pin base connected via 'wander plugs'.

An example of a regun carried out by John in 1968 can be seen in the pictures. It is a AW43-88 and still gives a brilliant picture. The black getter can be clearly seen together with the almost invisible neck joint. RIP John you are sadly missed by all that knew you both in and out of the trade.

I would add that another excellent rebuilders was LUBECK with the orange triangular label. I think the guy that owned it was Canadian. was John.

[space_charged]

Thanks Thermionic. Do you know what sort of age the CRM 92A will be? Was it in use in pre-war sets?

Even though its not working and it has been re-gunned, its a great thing to have and I'll keep it.

[space_charged]

Thanks Heatercathodeshort, for a most interesting story. It is sad indeed that we have lost the likes of John and all his knowledge of CRT rebuilding. I would have loved to have met him and watched him rebuild a tube

I have a fair number of TVs and, in all my early colour ones, I had to replace the tubes. On the budget I had back then (early 80s) I always used re-gunned tubes. Its quite likely I used Central Tubes, where in the country were they based?

The colour TVs I fitted re-gunned tubes to were an early partly valve GEC, a single standard G6 and several G11s. I still have them all.

I've got what appears to be a NOS Mazda CRM 171, and I've read that they died like flies going low emission or heater o/c. I'm going to test it shortly with the same setup I used for the Emitron. Yes, I DO know it needs an ion trap magnet, so unless I fit one I'll not see anything on the screen even if it IS good. I've got several sets waiting restoration that have ion trap magnets on their tubes, so I'll pinch one of those for my test. Also got something very similar for use with one of my many Teltron tubes. I've got nearly a full set of these 'cause schools were chucking them out like there was "no tomorrow" at one time. Guess that's because they've sacked all the technicians who knew how to set them up!

Love to chat more with you, but may get "off topic" so will send you a private message.

My HV system is a 14" high 12" diameter borosilicate bell jar. It sits on a 1" thick steel base that has electrical feedthroughs so I can power my various experiments in the vacuum. There is a serious implosion hazard associated with it, so I have purchased a purpose made (very expensive) acrylic cylindrical implosion shield for it.

Pump-down from atmosphere takes about two hours before it gets down to a pressure low enough that tungsten filaments won't burn out. Also the mean free path of an electron needs to be longer than the diameter of the bell jar. Generally I don't start my electron guns till I'm a bit below that, usually 3*10E-05 torr.

The way pressure reduces with time is interesting. The first phase of pumping is with a mechanical pump. That takes about an hour. Then the diff pump can be started and the pressure drops rapidly from the limiting vacuum of the mechanical pump. When it gets down to about 2*10E-05 torr, the pressure continues to decline but very slowly. At that stage the decline seems more or less linear. This corresponds to the "drying out phase" and the diff is fighting a torrent of water molecules reluctantly desorbing from everything (and in particular glass) in the bell jar.

On long sessions when I'm playing with home made valves or electron guns in the jar, I might have the diff on for 3 or 4 hours. The pressure goes on going down and the lowest I've seen so far was 2*10E-06 torr. I've also noticed that when I'm using an electron gun at fairly high beam current (5KV EHT) there is some evidence that this helps improve the vacuum. I attribute this to the gun acting as an ion getter pump. That works by electrons hitting residual gas molecules, ionising them and knocking them into the walls of the bell jar. As ions, they would react readily with the walls and stick (as do gas molecules to a getter). Its a little appreciated fact that the getter in a valve goes on absorbing gas molecules throughout the life of a valve not just when the getter is fired.

Charles

[AC/HL]

Originally Posted by space_charged

Love to chat more with you, but may get "off topic" so will send you a private message.

As long as it's relevant to the Forum, and you think it will be of general interest to members, starting a new thread is preferable to being buried inside another thread where it may disappear without trace. We occasionally get a comment that such and such was mentioned in the past but can't easily be found again.

[thermionic]

Originally Posted by space_charged

Thanks Thermionic. Do you know what sort of age the CRM 92A will be? Was it in use in pre-war sets?

Hi Charles.


The CRM 92 was introduced just prior to WW2, I think. As for the 'A' version, I'm
not sure. I'm sure John can corfirm / correct me!


SimonT.

[Heatercathodeshort]

I think the CRM92 is immediately after WW2. The pre war Mazda triode was the strange CRM91, a much shorter tube with a scanning angle of around 65 degrees. It looks like a large onion. They will interchange if mechanically possible. It does not seem to suffer with H/K shorts and low emission as did the later tubes. John.