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Old 19th Apr 2021, 8:37 pm   #3
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Decca Bradford Preset Brightness Modification?

Hi!

AH!

I've read thro' the manual again that showed R611 and R612 and it is indeed the all-transistor decoder Bradford 10 Version!

The penny dropped earlier this evening what they were for - I suspect those two resistors may have been added to help with switch-off spot suppression, relying on the principle that the h.t. line takes a second or two longer to discharge on switch-off than the +25V l.t. line, thence taking the common CRT grids a few volts positive of the normal preset potential from VR601 at the moment of switch-off, assisting the e.h.t. to discharge more rapidly and suppress any switch-off spots!

Presumably Decca found the change to the MC1327 RGB matrixing i.c. design in the Bradford 30 decoder rendered special switch-off spot suppression unnecessary because the outputs of the i.c. dropped towards chassis-potential at the moment of switch-off rapidly enough to rapidly raise the CRT cathode potentials towards the h.t. line voltage, and therefore blank the CRT beyond cut-off whilst the e.h.t. discharged!

Chris Williams
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