Chris van den Berg
the information that I posted before, does contain a major mistake!
I was forgotten that the 10 dB attenuator was still connected while
measuring the output of Q14. So the output was not 0,3 mw but 3 mw.
However, now I measured again and the output of Q14 is 1,2 mw and
the output of the 2N2219 (2N2218) is now 80 mw (gain of 18dB).
I wonder where the 3 dB went (may be this is the negative aspect of
adding shielding ;-( )
Increasing of the oscillator level did not result in better
functioning mixer (it can be tested easily by adding a 0,1 uF
capacitor over the 1K resistor between Q8 and Q9).
By the way, adding a RFC on the 100 ohm potmeter (in the mixer) gave
a better reduction of the AF component, however, the AF component
will not pass the filter.
Sorry for posting the wrong information,
some additional measurement results (all loaded with 50 ohm):(with much LF in the microphone stage). This means a gain of 27 dB
(but I am sure you should not measure it all with 50 ohm load).