NC-300 Crystal filter alignment
 

I bought a National NC-300 with a damaged phasing cap in the crystal
filter a while back, so I repaired it and realigned the whole thing. I
thought I had it aligned correctly, but after checking the filter
operation lately, I'm not sure if it's aligned correctly.
On the wider crystal filter positions, it seems to work
satisfactorily, but in the narrowest position, the peak is not
extremely narrow as described in the manual, and the signals are
attenuated quite a bit. I'm not positive if the phasing cap is
actually in the mid-capacitance position when it's in the middle,
where it's supposed to be to align the filter, or not. Also, when it's
in the middle, pretty much all the signals are in the notch, so I have
to offset it some to hear any signals at an acceptable level.
According to the manual, the signals that are peaked are supposed to
be relatively unaffected by the notch, but I don't get a definite peak
anywhere. Just a definite notch. I followed the alignment instructions
in the manual exactly, and re-aligned the filter several times. It
still doesn't seem to work as described in the manual.

I would appreciate any tips on getting this to work as it should.
I do work at a company that has spectrum analyzers with tracking
generators and other similar instruments, so I could probably take the
filter out of the radio and sweep it if this would enable me to align
it correctly.

Thanks,
Al, WA2AS

NC-300 Crystal filter alignment
 

"Al" ) writes:
I bought a National NC-300 with a damaged phasing cap in the crystal
filter a while back, so I repaired it and realigned the whole thing. I
thought I had it aligned correctly, but after checking the filter
operation lately, I'm not sure if it's aligned correctly.
On the wider crystal filter positions, it seems to work
satisfactorily, but in the narrowest position, the peak is not
extremely narrow as described in the manual, and the signals are
attenuated quite a bit. I'm not positive if the phasing cap is
actually in the mid-capacitance position when it's in the middle,
where it's supposed to be to align the filter, or not. Also, when it's
in the middle, pretty much all the signals are in the notch, so I have
to offset it some to hear any signals at an acceptable level.
According to the manual, the signals that are peaked are supposed to
be relatively unaffected by the notch, but I don't get a definite peak
anywhere. Just a definite notch. I followed the alignment instructions
in the manual exactly, and re-aligned the filter several times. It
still doesn't seem to work as described in the manual.

I would appreciate any tips on getting this to work as it should.
I do work at a company that has spectrum analyzers with tracking
generators and other similar instruments, so I could probably take the
filter out of the radio and sweep it if this would enable me to align
it correctly.

Thanks,
Al, WA2AS


They are pretty simple filters. The wider positions are a result of
loading down the crystal to varying amounts (and I think in some receivers,
the broadest position(s) may take the crystal right out of the circuit).
The variable capacitor is there to balance out the capacitance of the
crystal holder.

Maybe the crystal is bad? Certainly people did talk of that happening
in some receivers like the BC-348, but then the holder for that crystal
was unusual. If the crystal is bad, then that would obviously affect
operation. Or if somehow it's one of those crystals that has gone
off frequency with age, and you aligned the receiver to a specific frequency
rather than to match the crystal, then that would affect things. (Assuming
you have the means of generating a variable frequency at the IF and a means
of accurately measuring that frequency, you might check the crystal itself,
either out of the circuit or just feeding the signal into the input of the
filter and looking right at the output, one should see a definite peak
at some frequency.)

Michael VE2BVW