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Pictures showing Heater Cathode Short on Tube

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freya
(@freya)
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As some interest was shown regarding the appearance of the Mazda CRM121 tube with the heater cathode short as fitted to my Ekco T161, i have put these pictures on a new thread so they can be moved if required,

No controls were altered during changeover between the circuit driven heater supply and the external transformer.

The good picture is showing the tube being driven from an external 2 volt transformer.
The bad picture is showing the tube being driven from the internal 2 volt supply.

Stephen

 
Posted : 03/05/2013 7:27 pm
Anonymous
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excelent one of those faults thats difficult to discribe but your picture leaves no doubt.
rob t

 
Posted : 03/05/2013 7:47 pm
valvekits
(@valvekits)
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A bit too short for Behind the counter - Tales from the Trade, but I can tell you that as an apprentice this was a fault that once eluded me, especially as other symptoms were general loss of sync with the picture eventually fading out after a short time.
The pipe smoking guys who enjoyed Practical Television test cases were all scratching their heads as various courses of action were suggested and implemented without success.
We had someone in for a job interview and the boss was showing him around the workshop. When they arrived at my bench he was shown the problem and asked for his opinion to which he suggested the CRT.
He got the job.

Eddie

 
Posted : 03/05/2013 9:15 pm
6.3volts
(@6-3volts)
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H-K shorts have fooled me more than once. In my experience they can give very different symptoms in various sets. I can remember : Very negative picture as if the spot limiter was faulty; Poor focus on an electrostatic tube; as well as one like your example. Has anyone got any more examples which might help with diagnosis?
Jonathan

 
Posted : 05/05/2013 11:31 am
Anonymous
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you can put a couple of same value RF chokes, or a smaller common mode choke like used in a mains filter (it can even have the capacitor on input and output) then the current doesn't saturate it, in series between heater and transformer and then only isolation matters. With a series RF choke it's a 100% cure as the series choke blocks all higher frequencies.

 
Posted : 05/05/2013 5:45 pm
Anonymous
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Depends purely on the size of the chokes :) Lower inductance filter will be also lower resistance and it's likely only above 400 to 500kHz the capacitance of the transformer starts to be significant. Even on a low capacitance transformer 20 turns bifilar wound on a ferrite ring will make a good difference and won't affect the heater 50Hz due to low resistance.

I admit I've not tried one off a washing machine :w00t:

I've done a lot of power over RF coax projects for the day job. Similar problem with identical mathematics. The larger value chokes could have self resonance in Video band. That improves the "trap" at that frequency. Smaller value ones always help the higher frequencies best and for size always have less heater voltage drop. My last two work project DC over RF feed had to do 850mA DC and flat 5MHz to 65MHz as well as flat 110MHz to 862MHz and another 1.5A flat 5MHz to 65MHz as well as flat 110MHz to 960MHz

Bifilar wound on a single core always works better than two separate chokes.

It's a problem with same solution using direct filament valves (battery or not) in common grid or cathode follower configuration as you need signal isolation from the DC (or 50Hz AC on power valves) supply on the cathode.

Essentially with a HK short you are in the realms of Direct Filament design and the CRT is like a common grid amplifier to drive.

As an aside are there any direct filament CRTs as these would be better for a Battery set as Indirect heated cathode always needs about x4 power minimum for same emission.

 
Posted : 05/05/2013 6:16 pm
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