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Old 17th Aug 2021, 2:50 pm   #16
turretslug's Avatar
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,407
Default Re: AR88 BFO problem.

The BFO is decoupled from the 150V stabilised supply by a 47k resistor and 4.7nF Micamold (economy Bakelite-cased paper capacitor, and truly infamous for going leaky)- it's quite conceivable that this capacitor starts off a bit leaky and the leakage snowballs with heating of the capacitor itself and the set overall, starving the BFO of its already constricted supply. It could even go dead short without the rest of the set being aware.

The BFO coupling to the final IF amp grid is described as "electrostatic", i.e. very light proximity effect, OK for CW sidetone with appropriate control settings but making SSB reception a bit finicky. The post-war SC88 version got a 10pF BFO coupling capacitor to improve things, but this sort of thing is a fine line between adequate carrier injection and excessively stimulating the AGC.
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