Re: Eddystone 840C
I notice from your photos that it's got a couple of the Plessey electrolytics with red-and-black ends and a yellow label round the middle.
They're bypass-capacitors for the cathode-resistors on the UAF42 1st-audio and the UL41 audio power-output stage.
These capacitors are known to quietly transform themselves from being a capacitor to being a low-value resistor with a bit-of-capacitance as a side-order.
If the UL41's cathode-bypass capacitor (C73) goes leaky it means the valve will pass excess current and have a short/unhappy life.
Given the current cost of a new UL41 of reputable-origin - typically £18 - , it would to me make sense to replace these capacitors as well as "that capacitor" before powering-up.
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