Hello,
I have a 2A that I was unable to get the passband tuner setup so
that the pitch of the noise was equal on both sides of the center.
I setup a sweep generator, 0.1v pk-pk, sweeping from 40 - 60 KHz, in on pin
5 of V4.
I setup an oscilloscope on pin 1 of V5. A marker output on the sweep
generator is used as a trigger.
Doing this, I can clearly see a narrow passband that is shifted around by
the
twist of the knob. On the 4.8 KHz position, the passband is greatly widened,
as expected.
Now, back to the narrow mode. There is notch on that
passband, and the sides are not symetrical. One side had a slow
rolloff, while the other had a far faster one. That explains the
greater amount of high frequency hissing in one sideband.
I removed and opened the tuner. Four slugs are ganged together and mounted
on a
spring mechanism. Each slug is mounted on a wire, which is
soldered to the brass plate that moves them all together. First thing I
thought was some small drift in the inductance or
parallel capacitance, I unsoldered the slugs and adjusted while sweeping. No
improvement could be made in narrow setting.
Then I thought that the coupling caps might be off. I unsoldered them, and
tested on a precision RLC meter.
They are all within 0.2pF of the expected value. The 0.015 caps are OK, as
is the 150 ohm resistor.
Next place to look is the
Questions:
a. Drake 2A owners - do you get the same audio response on both sides of
center? Am I expecting too much from it? With something this clever it
SHOULD work better.
b. Have I been testing the filter correctly? The output LOOKS high
impedance, so I have not loaded it except with the high impediance input of
my scope. Should I, perhaps, adjust each section for resonance at 50 kHz
then lightly tweak for passband shape?
c. Perhaps somebody out there has a scrap 2A from which I could buy the
passband tuner, in case I ruin this one?
Comments?
--
David Goncalves
W1EUJ
--
David Goncalves
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