Quote:
Originally Posted by Refugee
I am getting the impression that changing the output valve to an indirectly heated type has left the bias circuit floating on the heater supply.
It might well be an abandoned repair job that has never worked after the output valve was replaced.
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The chassis potential is going to be the reference for the RF and detector but it is fed via the 500 ohm resistor and is not properly decoupled from HT.
I would try to contact the owner of the set on the radiomuseum site.
The alternative is to finish off the circuit diagram and redesign the AL1 modification. It looks like the modifications were made a long time ago so it is difficult to tell how extensive the alterations are.