This topic raised in the "TRIMCAPs don't peak" thread.
I think it's worth its own thread. Here it is.
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In the S-85 there is no obvious coupling between the RF amplifier tube (v1) and the MIX/OSC tube (v2) in band 1.
The output of V1 seems to get lost in the R26/C29 couple.
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Someone argued that the signal path could go through the S1B wafers.
Other argued that there is some other stray coupling in the circuit wiring.
Someone noted that the S-40B has an identical circuit but it is equipped with an additional capacitor (C62), positioned just between the V1 output and the v2 input.
The following picture sketches the S-85 circuit with the possible signal path and the phantom C62 highlighted.
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So I've made some measurements on my S-85.
Using my 9640B sig-gen connected to the ANT A2 input via a RTMA dummy antenna (A1 shorted to GND),
AM modulated at 30%,
I got the following:
Below the indicated value the signal gets lost in the noise.
As you see I also made a test shorting R65 in the input circuit as someone argued that it could impact (as it does) the receiver sensitivity in the higher part of band 1.
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Thoughts?
--
Emanuele (IU1KNR).
Emanuele Girlando