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Old 30th Aug 2019, 5:15 pm   #192
Heatercathodeshort
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Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 7,666
Default Re: Baird T5 restoration project.

I have exactly the same problem with a 7" EMI triode type 3/2. Switch on, picture appears for a few seconds then the screen floods with light. Switch off for four seconds then switch back on and tube will be OK for hours. Sometimes a second switch off is necessary but it always comes back. Disconnecting the cathode lead under fault conditions still leaves the tube brilliantly illuminated with no picture of course.
I have tried every trick in the book to clear the short, all to no avail. At first I thought it may have been some conductive grot at the glass pinch where the lead out wires emerge from the gun. They are very close together, brittle and delicate. The EHT for the anode also enters at this point. All OK but the wires in the pinch are VERY close together, not really nice practice. At least it does give a picture.
It sounds like your Cossor tube has a similar problem. I believe these early EMI tubes were prone to shorts. Hope you get it sorted David. John.
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