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Old 2nd Jan 2007, 10:04 pm   #7
Heatercathodeshort
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Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
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Default Re: TV adjustments - another quick NOOb question

Hello again,
I am working from memory here but I think the tube heater supply to the Sony KV2912 is derived from the line output transformer in the normal way but is rectified by a small diode mounted on the tube base but not smoothed.
This ‘half’ AC/DC arrangement appears to have an odd effect with some valve and crt heaters resulting in them dropping less volts than is required as they age, more so if supplied at line rate.

Many years ago [1965] Ferguson produced a 16” mains portable using a special ‘folded’ version of the very popular 950 chassis. In an attempt to reduce dissipation four valve heaters were wired in series with the H.T. supply. The set had a strange warm up sequence producing a raster and then slowly as the I.F. and video output valves warmed up, a picture and sound.

As the set aged the warm up time became longer and longer finally resulting in a service call. Observation would prove that the four valves in series with the H.T. were hardly glowing at all. The only way to get it working was to replace all four valves but these would work perfectly normally in a conventional series circuit.
Ferguson recalled huge numbers of these for modification to normal mains dropper configuration.

I don’t know the tube heater voltage for the crt used in the Sony but would imagine it to be the usual 6.3v. It would be interesting to disconnect the heater pins on the tube base and feed it separately with a 6.3v supply and observe the results.

I never managed to improve the crts by reactivation. The cathodes were probably well poisoned by then. I would add that some of these sets were very clean and appeared to have had little use. The 21” version also had a somewhat premature life, but nothing like as bad as the 29” version.

Regards John.
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