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Forum 141

CRT Heater

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Posts: 1509
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Sadly, I expect one or more of us have encountered a Mullard 14" tube where half the heater has failed short...

Well, whilst enjoying a pint cup of tea in the local pub cup of tea shop, I noticed something....

They have some of those posh looking lamps, with a squirrel cage type filament in it.... one or two were markedly brighter than the others...

The ones which were brighter had several of the v-shaped filaments not quite making it to the bottom of the support wires. They're managed to weld themselves, thus shortening the filament length.

DSC 00771

If you look carefully, you can see the shorted filament ... I wonder if it's the same failure mechanism? 

You could also see the filaments vibrating slightly, presumably as a result of the current flowing.

Posted : 14/04/2018 3:34 pm
Posts: 458
Honorable Member Registered

When I used to look after a large disco installation with many PAR36 "Pinspots" it was a Friday afternoon ritual to turn them all on and go round with a length of wood tapping all the unlit ones which would revive most of them by welding the filament before replacing the ones that didn't respond to treatment.

Posted : 14/04/2018 4:31 pm
Jac Janssen
Posts: 122
Estimable Member Registered

Is the construction of the heater in some way similar to that of the lamp?
I thought is was spiralled.
Do you know where in it's construction the short occurs?
I once has success with rotating the CRT 180°, but I fear it is only temporarily working.
In more than one CRT I had a short between heater and cathode.


Posted : 06/06/2018 6:19 am
Posts: 4005
Member Rest in Peace
Posted by: Jac Janssen

In more than one CRT I had a short between heater and cathode.

That used to be a common fault. RS (Radiospares in those days, of course) did an isolating/boost transformer that was designed to have a very low capacity between primary and secondary specifically to overcome this.

The heater connections were removed and connected together to maintain the series chain and the heater fed by the transformer.

Those transformers served a dual purpose - in the days when Purchase Tax levels of up to 66⅔% were levied and CRT prices were very high, the cost of one of those transformers with the heater fed from the 20% boost tap could give a worthwhile extension of useful life to a low emission tube.

When all else fails, read the instructions

Posted : 06/06/2018 11:47 am
Posts: 4618
Famed Member Registered

Radio Spares CRT isolation/ boost transformers, happy days.  ? 


Posted : 06/06/2018 12:29 pm