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Old 23rd Feb 2021, 10:41 pm   #6
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: MK.123 spyset bfo fault.

It's probably too soon to start analysing the rest of the BFO circuit (as an oscillator) but I did put the tank circuit into a simulator using a simple gm model for the valve. As long as the various capacitors and the inductor L10 are healthy then it looks like the valve only needs to produce a gm of -0.3mA/V to be able to generate enough negative resistance to start up the BFO at 465kHz. This assumes all caps are healthy and the inductor L10 (237uH?) has a Q of about 100. Even if the Q of L10 is only 45 the circuit should just start up with a gm of -0.3mA/V.

This circuit does look like it should be keen to start up but the initial power up conditions need to be such that the valve is able to produce that minimum gm for long enough for a complete/full startup to occur.

I think it might latch into the hard switched on state if it can't start and maybe it can get stuck like this if something in the circuit is no longer as it should be.
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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 23rd Feb 2021 at 10:47 pm.
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