An on-going tale of two radios, Part 4

By Gordon Bubb G7KNS

Going back to the bad audio output transformer, I gave some thought as to why it had gone bad. Measuring it, not only was it leaking from primary to secondary but the primary was actually open circuit. The thing is epoxy coated and so looks perfect, no signs of overheating.

Those of you who work with valve radios will know of the infamous component known as “that capacitor”. This is the capacitor linking the audio driver valve, from the anode at high voltage, to the grid of the output valve which should be at zero volts. If “that capacitor” goes short or leaky the grid goes positive which turns the valve on hard causing excessive anode current which flows through the output transformer…. which is not designed for such abuse, and lays down and dies. In the worst case scenario the mains transformer and rectifiers can also be damaged. So let us check “that capacitor” in this circuit.

Now that is what I call a leaky capacitor! It might not even be a capacitor any more, whatever, straight into the bin with it.

These two radios both have an intercom audio system as well as the radio audio and both systems use the same pattern transformer in very similar circuits. The other three capacitors in question all read in the low tens of megaohms leakage so all were changed, it is just not worth leaving them in place when a replacement costs pennies.

Why do they fail? There are a number of possible causes, mainly old age combined with old fashioned materials and construction methods by modern standards. Remember these components are knocking on 70 years old, well beyond their design life. And yes, there will be others in the sets in a similar condition but none in such a critical location.

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