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Old 25th Feb 2021, 5:36 pm   #21
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 2,493
Default Re: MK.123 spyset BFO fault.

Glad to have helped!

I'm not sure the original circuit is meant to produce a perfect sine wave at the output at the anode. A sawtooth is probably normal and it just contains the wanted LO frequency and some harmonics. I'm not sure this will cause audible problems in the detector driven by the BFO.

With this type of oscillator regardless of the active device (could be a BJT or JFET transistor or a triode or a pentode) the active device maintains the oscillation by injecting narrow current pulses into the tap point of the main LC tank circuit.

It's a bit like bouncing a ball where you have to tap it briefly at the right moment to keep it bouncing. The rest of the cycle the LC tank circuit is freewheeling and the active device is biased off by the feedback signal via the grid. Only at the very tip of the sinewave at the grid will the valve be turned on and it will produce a narrow pulse of current before it is biased off again for most of the sinewave.

The big resistor at the anode and any capacitance (C41 30pF) will probably mean you will see a sawtooth voltage waveform at the anode when viewed on a scope via a x10 probe. However, the valve will still be gulping current here with a narrow pulse once a cycle at a 465kHz rate.
Regards, Jeremy G0HZU
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