Dear Chris, LF Group,
...>>>>
The problem's before the mixer.
As advised, I disconnected the RF stage and injected a signal into the
mixer and it gives a constant output even with a blow heater playing on the
insides.
...
So that narrows it down to the ferrite rod antenna circuit or the FET
preamp. Injecting the signal directly into the preamp input should tell you
which it is.
...>>>>
I've spent a few more hours on the F/S meter today and still have not found
the problem.
I can blow with a drinking straw on any component and there's very little
output change yet with time there's a slow drift with temperature - the
output goes up
with increased temperature.
...
At the moment, my bet would be on the ferrite rod antenna. The
transconductance of a FET (JFET or MOSFET) certainly varies with
temperature, but generally decreases with increasing temp, so the output
level should fall as the temperature rises, and probably not as much as you
are observing. Since the output of the ferrite rod ant depends mostly on the
bulk properties of a fairly large chunk of ferrite, it would take an awful
lot of blowing down a straw to have an effect ;-) But a change in ambient
temp would give a slow change in output as the ferrite warmed or cooled.
Cheers, Jim Moritz
73 de M0BMU
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