Dear Chris, LF Group,
Looking at the circuit, there are not that many possibilities for varying
gain with temperature.
The NE/SA 602/612 devices should be quite stable with temperature - the
Philips data shows less than 1dB variation over 0 - 70 degrees. But if the
oscillator level is marginal, gain might vary more.
The 741 output amplifier is working at high gain, and low supply voltage, so
might possibly be marginal. This would be worse if it were driven into
clipping - unlikely if output V less than 1V RMS or so.
The 2N3819 gain might vary quite a lot with temperature, depending on how it
is biased, and the particular specimen of 2N3819.
The RC low-pass filter might show significant variation with temperature if
high-K ceramic capacitors with particularly wild temperature coefficients
were used.
The ferrite rod antenna is a possibility, especially if the tuning is fixed
as shown in the circuit diagram. The Q and temperature coefficient of these
can be quite high, so a temperature change could cause it to drift off
frequency and change the signal level - with a Q of 200, about 1kHz change
in frequency would be sufficient.
I would suggest doing a bit of signal tracing - for example, disconnecting
the RF stage and injecting a signal directly into the mixer would eliminate
the ferrite rod and 2N3819. Another possibility would be to try heating up
components individually, e.g. with a fingertip or a soldering iron, to see
where large changes occur. Or, with the ferrite rod, try re-peaking the
tuning after changing the temperature to see if there has been a shift in
the tuning.
Hope this is helpful,
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Chris Osborn" <[email protected]>
To: "LF Group" <[email protected]>
Sent: Sunday, May 03, 2009 6:13 PM
Subject: LF: Field Strength Meter - advice needed
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