Vidor CN396:Hum Poor Gain
The symptoms of this fault are that a nasty hum is present and the output power is low. The hum can be reduced by turning down the volume control, and a good strong hum can be produced by touching the control grid of the DAF91 valve (V3 in Trader sheet 981) so the fault is obviously in front of the volume control circuit.
An i.f. signal injected at the control grid of V2 produces a good output from the speaker, but there is little or no increase when the signal generator leads are transferred to the control grid of the DK91 frequency changer. The windings of the first i.f. transformer are found to agree with D.C. resistance values quoted but if the tuning capacitors are tested, the primary one C3 is found to be open-circuit. This has been found to be the case in several of these receivers.