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Old 16th Sep 2020, 12:26 pm   #48
Join Date: Jan 2011
Location: Croydon, London, UK.
Posts: 262
Default Re: FRG-7. Very quiet

TP104 is connected to the drain of the mixer, Q105 via a 22pF capacitor. Q105 is a simple mixer, the output of which will be composed of the signals made up from the input into the gate, the input into the source, the sum of these, the difference frequency plus the many harmonics and their sums and products.

The source load of Q105 only a choke, not a tuned circuit, so you will see the whole lot of frequencies when you look at TP104, although some may be attenuated a little by the tuned circuit formed by T401, VC2 and TC401 which is fairly loosely coupled to the output of the mixer. The difference frequency of 2-3MHz is selected by the tuned circuits later on (T401, VC2 and TC401 then T402, VC2 and TC402)

I expect the dominant signal to be the one from the oscillator side chain, which presumably is 53MHz. The good news for you is that all is as it should be.

frsimen is offline   Reply With Quote