Quote:
Originally Posted by Heatercathodeshort
I forgot to add. My example burst into oscillation after a few minutes use, blanking out the picture and sound. The I.F. decoupling capacitors used by Thorn are high quality .002uf ceramic types, that is all but one. The screen grid decoupler of the EF80 R.F. amplifier was the familiar light brown crunchy type and yes it was O/C. Replacement cleared the instability and gave a 100% improvement in gain. J.
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The choice of screen grid decoupling capacitor might be thought to be rather strange considering all the others are high quality components. The paper dielectric Hunts capacitor will have been employed to to introduce a certain amount Cag feedback to increase gain and peak up the Q in the associated tuned circuits.
I met something similar in a Murphy V410. Weird patterning turned out to be a bumble-bee capacitor in the sound IF amplifier having gone faulty. All the other decoupling capacitors were of a different type and like the Ferguson being discussed here the designer had chosen the offending capacitor for it's characteristics which was a tiny degree of self inductance due the rolled foil method of manufacture.
DFWB.