View Single Post
Old 13th Oct 2015, 1:38 pm   #18
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
Default Re: 'Beau Decca 131 projection receiver 1950'

Summoned up the courage today and connected the Decca 121 to the mains.
The sound is OK but all that can be seen on the screen is a defocussed raster, the focus control has no effect whatsoever. The focus control system does need explanation and the official Decca service devotes a whole page column about this. Other makers of projection TVs such as Ferranti and the Philips 1400A employ a focus stabiliser valve, a power pentode like the EL41 or PL82.
The Decca does have a valve in the focus circuit, one half of V19, a 6SN7GT. This valve receives a controlling voltage from the cathode of the EHT generator valve. I'd imagine variations of the cathode volts of the EHT valve relate to the demands on the EHT current taken by the CRT.
I'm sure it will all become clear as time is spent on study of the set.

DFWB.
FERNSEH is offline