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Old 28th Jan 2021, 1:49 am   #1
Heatercathodeshort
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Default Television Picture Tubes.

Television Picture Tubes.
A lot of useful information has been written about the various faults and peculiarities that can effect television cathode ray tubes. I will attempt to add my practical experiences having played with very old receivers from the age of around 12-13 [1961] running my own television and electrical shop from 1966 to 2002, and working closely with Central Tubes [Brown and Stanford Ltd.] a very good tube rebuilder.

Most of the early tubes were simple triodes. These include Brimar, Cossor, Ferranti, GEC, EMI, and Ediswan. [Mazda] and of course our trusted Philips/Mullard.

Tubes manufactured by Brimar were on the whole well built and evacuated. The C9A and C12A were manufactured by Mazda for Brimar and were no better than the Mazda equivalents but much rarer in the field.

Tubes by Brimar from the C12B [Cintel] I understand were manufactured at the Footscray works.

Cossor were the first to Aluminize their tubes [10 inch 108K] and this particular tube enjoyed a good life and was fitted in certain versions of the infamous HMV/Marconi 1807 This slightly modified tube was listed under the EMISCOPE number 3/20. [4 wander plug base]

Cossor 15" tubes together with other manufacturers large bulb CRT's suffered a short life and it is my opinion that the huge bulb was very difficult to evacuate with the vacuum pumps of the period.

Cossor continued to manufacture very good tubes until around the mid 50s when I suspect it became uneconomic to do so.

Both Ferranti and GEC produced a range of triode tubes, mostly aluminized that gained a good reputation. The very early 9" GEC types did suffer heater problems mostly dropping or raising their resistance that was not due to a partial short but GEC soon cleared up the problem resulting in tubes that are capable of very good pictures. CRT production for domestic receivers ceased in 1956 and tubes were obtained from Mazda but still retaining the GEC type numbers.

Ferranti tubes were excellent. Again like GEC they were mostly triodes enjoying a very long life. Similar to GEC, Ferranti ceased domestic CRT production in the mid 50's at the time of the transfer of television production to Ekco at Southend who had a contract with Mazda for the supply of valves and tubes.

EMI were the odd one out. They produced their own tubes of hard Pyrex glass some of which suffered from low emission after just a couple of years. Their first AC/DC tube the 3/16 was a disaster suffering from premature emission failure and probably the most annoying and costly fault, O/C heaters! Later EMI tubes were little better continuing through the 4/5, 5/2 and the SE range. They were mostly aluminised tetrodes but the addition of a glittering frock did nothing to encourage a long life. It must have been a great relief to EMI dealers when EMI decided to pass their TV production to Thorn/Ferguson in 1957, the first chassis being identical to the well loved 306T series.

Ediswan Mazda tubes suffered many problems. All the early tubes were 2V triodes that endured problems from day one and disgracefully continued for many years. The arrival of the tetrode gun did nothing to improve reliability. The first tube that enjoyed a good life is the 17" aluminised CRM172 employed in many Ultra, Murphy and Ekco receivers. It gives a cracking picture.

The Mullard tube is probably the best known. The early 9" and 12" types [MW22-7 to MW22-74, the early versions of which were manufactured in the Philips factory at Eindhoven and are bullet proof. They are side evacuated and have a very hard vacuum. Mullard favoured the tetrode gun giving excellent overall focus but the extra electrode did cause problems with internal leaks but this was not a common problem and could be rectified by rewiring the tube as a triode with hardly any noticeable effect on the picture. Mullard were late to aluminize their first domestic tube being the 14" AW36-20 followed by the MW43-69 and the massive 21" MW53-20.

Heater Voltages.
The tube manufacturers have used many differing heater voltages over the years. Basically you will find:
2V 4v 6.3V 7.2v 8.5v 10.5v and 12.6v.
As a very rough guide, Early triode tubes in the Mazda range have 2v heaters. The exception is the 12" AC/DC tube, the CRM122 with a 7.2v .3a heater. Later tubes are 12.5v

Post war Mullard tubes are fitted with 6.3v heater assemblies.

Care has to be taken with EMI tubes. The TA10/15 are 4v but the 3/16 .3a heater was increased in steps from 8.5v, 10.5v ending in 13.5v. There is no indication on the tube of this so take great care when fitting a boost transformer. Exercise caution when experimenting with EMI CRT's.

Early GEC triodes are 6.3v for parallel operation and 10.5v when used in an AC/DC .3a series circuit.

Ferranti produced a large range of similar triode tubes that are are generally 4v but later type for series for operation are rated at 6.3v.

Early Cossor tubes are 4v or 6.3v at varying current. The AC/DC tube is 6.3v .3a.

Early Brimar triodes are 2v with later types rated at 6.3v and 12.6v. An odd one is the 1960 110 degree C17/AA[I think the numbers correct] It has a 4v .3a heater and is the only 110 degree tube with a 4v heater.
There are other odd makers such as Cathodeon , a branch of PYE and very much inferior to the Mullard. The English Electric metal cone was another odd man out, the T909 16" being a direct equivalent to the Mullard MW41-1. They also produced a metal cone 21" circular version the like of which has to been seen to be believed. Usual faults were lack of vacuum due to a faulty glass metal seal and expansion differences between the glass and metal cone resulting in a crack right across the screen.

Take care when modifying chassis for use with alternative CRT's Most can be made to work but double check the heater voltage if it is fed from a mains transformer.
'Normal' CRT faults.

The usually encountered fault with any CRT is low emission. The effects of this are very well known by old type repair guys but can be misread by the inexperienced.

Generally the test card has a very flat appearance lacking in contrast and brightness. Focus is usually OK. Any attempt to increase the contrast or brightness results in the poor quality display turning negative. The picture also has a satin appearance. This is usually not noticed by the customer until they have to draw the curtains and view in total darkness to see any picture at all!

This symptom is not to be confused with a low emission EHT rectifier such as the EY51/U25. When these rectifiers become low emission they are unable to supply the very small current required by the tube. Advancing the brightness causes the picture to increase in size and further increasing the brightness will usually cause the picture to 'blow up' in size and vanish altogether. Decreasing the brightness will restore the raster once again.
25O/C heaters was not a common fault other than the EMI 3/16 but a partially shorted heater was a common defect in Mullard tubes. The heater assembly itself was M shaped and slipped into the cathode assembly. One half of it managed to short out and in a series heater chain dropped only 3-4v resulting in a dim heater and a very poor picture. Giving the neck a tap would often see tiny balls of red hot metal fall out of the cathode assembly removing the short and forming a temporary cure, but not for long!

Mullard tubes also suffered from A1 to grid shorts particularly the MW36-24. A modification involving wiring the tube as a triode would overcome the problem.

Another strange symptom that appears to effect early tubes of the MW22-14/16/18 series is a change of characteristics of the gun assembly. No measurable shorts can be detected but the raster fails to be extinguished with the minimum setting of the brightness control. The picture appears well focused but the contrast is just not quite enough to give a picture that the chassis is capable of.

I have my own way of overcoming this. With Ferguson receivers particularly the 992T [many models in the 99x range] I modify the biasing arrangement of the video output valve to remove some of the negative feedback. A 10uf from cathode to chassis of the video amp usually results in a 30% increase in contrast making viewing more of a pleasure. No change in actual definition is noticeable. I think Thorn may have had a problem with his due to the change in the bias supply arrangements in later models. [negative supply via small rectifier.]

The second is more general and involves reducing the A1 voltage by around a third. This is easily done by a couple of resistors across the supply with the center to the A1 pin. A bit of experimentation is needed but the result is worthwhile. I think the gun has altered it's cut off point, grid to cathode. Just a calculated guess and reducing the A1 potential brings it more into line.
30On the whole Mullard tubes are very reliable and even when a fault develops, they can often be recovered.

Ediswan Mazda tubes were fitted into many of the leading TV manufacturers receivers including EKCO, Murphy and Ultra.

They suffered most of the faults that a CRT could suffer from and it was a great shame that nothing was done about it for many years. The manufacturers appeared to put up with the short life appearing to accept the poor quality control and promises from the Ponder End factory. In the Murphy Service News during 1952 dealers were writing to Murphy complaining why the tubes had such a short life. The reply from Ediswan was that they were aware of the problem and things would change in the future..

The most common fault was a heater cathode leak or short resulting in line slip, cogging of picture or even bits of the picture missing, a very strange effect to observe. Huge numbers of these 2v triodes displayed this fault and could be brought back to life with a low capacity [to earth and mains primary winding] CRT isolation transformer. This was usually screwed to the cabinet timber and wired directly to the tube heater pins and the set side of the on/off switch.

This cured the heater cathode short but low emission was not too far away. Again this could be cured by bringing into circuit the additional tap on the isolation transformer giving a 20% increase in heater volts. This worked for around a year if you were lucky before it was almost impossible to see the picture!

The CRT was a considerable expense especially if you had one of the 15" tubes such as the CRM151 or CRM152B/153 types fitted in your receiver. The tube alone listed at £27.00 plus fitting.

The CRM 171 had so many failures that I was told by mates that were in the trade in the 1950,s that their shelves were full of practically new receivers awaiting replacement tubes many of which were just outside the guarantee period that was only 6 months back then. Customers used to come in and pay 10/- [50p] a week until the bill was settled.

Mazda screens on the older types CRM92/121B would sometimes fail giving a dirty brownish picture. The Emission was OK but the screen material had degraded. I have only met this fault twice, once with a CRM121B in a Regentone Big 12 and a CRM92 in an Ekco TS88.

The later aluminised Mazda tubes most of which still suffered from premature low emission and the odd O/C cathode connection, for all their faults gave superb pictures due to their fine, smooth screen material and aluminising. My mate the regunner always said the best combination was a Mazda bulb and a Mullard gun!

They must have had a problem in the factory maybe with the pumps but their location next to the Lee valley reservoir with all that moisture in the air, probably didn't help. All water under the ridge now of course.

Other odd faults

A slightly gassy tube is not uncommon. Symptoms can vary but usually take the form of blurred images giving almost a static convergence display on a colour tube and very poor focus. After a period of around 10 mins the effect slowly reduces to display a good picture but the whole process will repeat itself on next switch on.

A broken down link between the inner tube coating and the EHT connector. This causes the picture to flutter with a slight discharge sound as the gap is breached by a small arc. The condition worsens with increase in brightness/beam current.

Corner cutting caused by the scan coils not being pushed well up to the tube bowl can be due to an offset gun assembly. It may have sustained damage possibly if the receiver had been dropped at some time in the past.

Ion burn resulting in a perfectly circular dark stain often with a bluish tinge in the dead center of the screen caused by heavy ion bombardment of the screen damaging screen phosphor. Nothing can be done other than making sure the EHT is as high as possible.

Ion trap adjustment

Many tubes manufactured from around 1950 were fitted with ion trap magnets to prevent the ion burn mentioned earlier. The gun assembly was a special one visibly bent at an angle. This was to direct the electron stream away from the horizontal. The ion trap magnet placed on the neck of the tube a short distance from the base connector was adjusted to redirect the electron stream back to the horizontal and towards the screen.

The heavy damaging ions were not anything like as sensitive to a magnetic field and just carried on to the glass wall of the neck.

The correct adjustment of the magnet is very important. Early versions were marked with an arrow and this had to be pointing forwards towards the screen. Later traps were unmarked and it was a simple matter to slip it over the neck. It is placed around a 1/4 inch from the Bakelite base and with the brightness set about 3/4 slowly rotated and moved a little forward and back until a raster appeared. It was then adjusted for maximum brightness ignoring any tendency for corner cutting or poor focus. It is MAXIMUM brightness that is important.

Any focus or corner cutting errors can be corrected with the focus magnets, scan coils [make sure they are firmly pushed up against the tube bowl] and shift plates then a final adjustment of the ion trap can be made. Any stubborn cases of slight corner cutting can sometimes be cleared with a final adjustment to the ion trap but in no circumstances at the cost of maximum brightness.

Incorrectly adjusted ion traps can lead to a starfish shaped ion burn of varying intensity due to the electron stream taking a mis directed damaging path through the gun assembly.

The ion trap magnet may need resetting after any adjustments to the focus or picture shift device or both.

As a final recheck turn the brightness to maximum and check the magnet again for absolute maximum brilliance.

Chassis faults that may give the symptoms of a faulty tube.

There are a few chassis faults that can give symptoms of typical tube failure.

Low A1 supply volts typically caused by leaky decoupling capacitors from the boost rail supply

Faulty video output stage usually the video amplifiers anode load resistor going low resistance giving either a dark screen or very poor contrast.

Washed out picture due to video detector crystal being either O/C or S/C.

Low EHT possibly due to Visconol EHT smoothing cap being leaky [Flyback EHT]

A tube can also be checked by momentarily shorting the grid to cathode. A flash of continuous light should be seen on the screen providing the EHT and heater supplies are in order.

I can't think of anything to add but as I have written this from memory, please let me know! I have written this from memory and probably forgotten some of the early tricks.

Hope this helps. Regards, John.
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Old 28th Jan 2021, 10:16 am   #2
Mike Phelan
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Default Re: Television Picture Tubes.

Incredibly useful information, John, and well worth keeping!
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Old 28th Jan 2021, 10:35 am   #3
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Default Re: Television Picture Tubes.

A great write up, much appreciated. Thanks John.
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Old 28th Jan 2021, 10:59 am   #4
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Default Re: Television Picture Tubes.

Very interesting! Thanks for posting.

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Old 28th Jan 2021, 12:01 pm   #5
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Default Re: Television Picture Tubes.

Hello John , A very ,very comprehensive guide . Thank you for sharing this highly valuable information with us forum members .
I will print this off and save it for future reference .
Best regards ,
Alan.
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Old 28th Jan 2021, 1:07 pm   #6
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Default Re: Television Picture Tubes.

That was a very well put together article John and made for an enjoyable read. I think this is worth being made a sticky, as it will be an excellent reference for CRT faults in the future.
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Old 28th Jan 2021, 2:58 pm   #7
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Default Re: Television Picture Tubes.

Well done John, a very interesting read.

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Old 28th Jan 2021, 2:58 pm   #8
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Default Re: Television Picture Tubes.

With reference to the problems with Cathodeon CRT’s, I only used them for replacement in the middle/late 60’s and I am sure they were just rebadged Mullard, they looked the same and worked just as well. They were the 110 degree type and fitted in place of failed Mazda types.
With the Pye/Philips tie up at that time it would make sense to rebadge rather than have another mono CRT factory.
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Old 28th Jan 2021, 3:15 pm   #9
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Default Re: Television Picture Tubes.

A lot of useful information in there for restorers thank you John.
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Old 28th Jan 2021, 3:16 pm   #10
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Default Re: Television Picture Tubes.

Fascinating! And well-explained.

Regarding Cathodeon CRTs, there is a comment in "The Setmakers" - page 316 - referring to the Pye VT4, demand for which exceeded Mullard's ability to produce the necessary tubes "And some sets were supplied with inferior tubes made by a Pye subsidiary, Cathodeon, - dealers tended to get sets with Mullard or Cathodeon tubes according to whether they were perceived by Pye as up-market or down-market".
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Old 28th Jan 2021, 3:17 pm   #11
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Default Re: Television Picture Tubes.

Mods, John's post should be elevated to a sticky!
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Old 28th Jan 2021, 3:20 pm   #12
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Default Re: Television Picture Tubes.

Yes I think you are correct. The later Cathodeon tubes were Mullard. The early ones in the V4 series had a very 'thick' phosphor and gave a slightly muddy picture when compared with the Mullard.
The tube makers could not supply the quantities demanded by the set makers but I guess by the time the Coronation rush was over and things had settled down a bit it was simply not profitable to manufacture a CRT when Mullard could supply the trade with good tubes at £5 a time. I have read somewhere that deliveries of the V4 were graded. Dealers in posh areas got the ones with the Mullard tube. I think it may have been 'The Set Makers. John.
Sorry G6. Clashed! J.
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Old 28th Jan 2021, 3:21 pm   #13
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Agreed!
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Old 28th Jan 2021, 3:50 pm   #14
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Default Re: Television Picture Tubes.

A really useful and well written reference. Thank you John!
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Old 28th Jan 2021, 4:21 pm   #15
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Default Re: Television Picture Tubes.

In 1958 Cathodean were the first UK manufacturer to make and supply a 17" 110* CRT to the parent company Pye.
The new tube was fitted in the Pye PV110.
The tube had a 0.6amp heater.

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Old 28th Jan 2021, 9:18 pm   #16
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Heatercathodeshort wrote: "Early Brimar triodes are 2v with later types rated at 6.3v and 12.6v. An odd one is the 1960 110 degree C17/AA[I think the numbers correct] It has a 4v .3a heater and is the only 110 degree tube with a 4v heater."
Hi John, the Brimar short neck CRTs were the 110* C17AF and the 114* C19AH. These tubes have 4volt 0.3amp heaters.
https://www.radiomuseum.org/tubes/tube_c17af.html

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Old 28th Jan 2021, 10:16 pm   #17
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I thought it best to read this thread after tea and not rush reading it at work..Thanks John I will have to print it out I just don’t know how you can remember so much information . Andy
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Old 28th Jan 2021, 10:20 pm   #18
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From the 1937 Mullard catalogue: cathode ray tubes types E46-12, M46-12 and M46-15.
The E46-12 is a electrostatic deflection and focus type and the M46-12 and the M46-15 have magnetic deflection and focus. The magnetic tubes have tetrode gun assemblies.
Was the developer Mullard or Philips?

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Old 28th Jan 2021, 10:25 pm   #19
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Default Re: Television Picture Tubes.

Quote:
Originally Posted by G6Tanuki View Post
Fascinating! And well-explained.

Regarding Cathodeon CRTs, there is a comment in "The Setmakers" - page 316 - referring to the Pye VT4, demand for which exceeded Mullard's ability to produce the necessary tubes "And some sets were supplied with inferior tubes made by a Pye subsidiary, Cathodeon, - dealers tended to get sets with Mullard or Cathodeon tubes according to whether they were perceived by Pye as up-market or down-market".
It would be easy to determine whether Cathodeon made their tubes themselves or rather when they stopped doing that if they indeed made their tubes themselves. Just look for codes on the socket or on the bell of the tube. When the tube was made by Philips or any associated factory, there should be a 2 to 4 line code, similar to the codes on normal valves. If cathodeon was still operating under - or associated with Philips, they might have even had their own letter.

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Old 28th Jan 2021, 11:40 pm   #20
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Default Re: Television Picture Tubes.

it's always worth double checking the heater voltage of a rebuilt tube. Example;

A 12V CRM93 would be fitted in an Ekco TMB272, and the series-parallel heater circuit
balance is upset by the 6.3volt gun of the regunned tube.. In this case, the tube was best operated from 12V with a series resistor. The CRT had no label showing heater change.
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