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Old 10th Feb 2018, 10:52 am   #21
Heatercathodeshort
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Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
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Default Re: GEC BT336 possible tube change

Stain.. That is a weird one usually associated with early Mazda tubes only.

My thoughts drift to the CRT but like so many of these faults a 'hands on' approach would probably solve the problem. If the focus drops to such an extent, the only fault that can cause this is dropping EHT voltage but this must drop a considerable amount to have such a dramatic effect.
The second reason would be a slightly gassy tube and this is backed up by 'The Stain'. [That could be a title for a 1950's horror movie]

110 degree tubes are pre focus and require very little in the way of focus potential if any at all. I think you must measure the EHT to narrow this down.

The fact that the raster does not increase in size with reduction of EHT points to the saturation problem with the LOPT. A rough visual test is to observe the brightness of the EHT rectifier heater. If the brightness drops with the reduction in focus, it's a good bet that the LOPT is to blame.

These are good transformers and should dry out given time. The dampness is deep set and has taken decades to creep into the pitch coating and windings. It was never a problem when these chassis were in service. The fault did not exist in the 60's/70's/80's. John.
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