Monthly Archives: June 2022

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.

On on-going tale of two radios, Part 3

By Gordon Bubb G7KNS

To recap, now both sets are receiving with the same sensitivity. My set does not transmit AM and the ex-Kuwait set has a bad intercom output transformer. Both sets have issues with the in-built crystal calibration system.

As my set does transmit PM and CW there is obviously nothing fundamentally wrong so it is time to think about switch contacts and wiring faults.  Ooh look! Our friend Mr. Bodger has been at this set as well, he gets around doesn’t he?

And again…

I also found the two wires going to the tuning meter were disconnected. Maybe this is why the calibration system does not work. Then I saw this…

I think our friend has changed the meter but not finished the job. And why butcher the edge of the mounting plate? We won’t ask about the missing mounting studs; the two on the other side hold the plate in well enough. As to why he cut the co-ax instead of unsoldering it, who knows? The end is only an inch away from the cut.

So repair the co-ax, refit the earth tag and reconnect the meter. We now have AM transmit. Result! And there are signs of life on the meter as well now.

The final stage of tuning shows a nice swing of the meter from left FSD (full scale deflection) to right which is correct, it is tuned for centre zero. But the first two stages do not. They use a 100 kc/s (kilocycles per second, this radio predates kilohertz) crystal and according to my oscilloscope it is not oscillating. Swap the module with the one from the other set and all is now hunky dory. One set now fully operational.

Anyone got a spare 100 khz crystal?