"geojunkie" wrote in message
om...
I am having a lot of trouble understanding the proper method to align
the crystal filter section (off the output of 3rd IF transformer)in an
SX-71. The manual suggests "rocking" the signal generator around the
IF frequency (455khz) while turning the IF transformer slug and
monitoring the output level to find a null between two peaks. You are
supposed to set the slug to the null. The output tone is to be created
using the BFO set for 1kz at the IF frequency. I don't understand what
is creating the peaks and null... the rocking of the generator or the
adjusting of the slug? There is a huge null created just by rocking
the generator as the beat frequency drops from 1khz to zero and then
ramps up again on the other side. I can't imagine this is the null,
but it overpowers anything else I am seeing. Then how do I set the
slug at the null point, when its impact is hardly discernible?

This is very difficult to do with a standard service type signal
generator. You need one that tunes very slowly around 455kc. The null
you are looking for is very sharp.

I did this a couple of years ago on my SX-62. I found the instructions
in the manual confusing, but basically you are looking for the deepest
difference between the bottom of the null and the top of the peak.
"Rocking" means tuning the signal generator up and down a few tens of
cycles or so to feel out the null and peak. The adjustments will change
the frequencies of the notch and peak slightly. So you are manually
sweeping back and forth, back and forth, and so on.

You don't really need the BFO, if you monitor the voltage of the AVC
line.


I could
be out to lunch, but why not use a sweep generator and scope to
actually look at the wave shape being passed through that section of
the IF.

That would work if you can sweep slow enough. The crystal rings too
much with a 60 cps sweep speed.


Or, why not just adjust the slug to provide the highest peak
at the crystal frequency (which is actually 456.7khz) as is
recommended for the rest of the IF section?

That's probably fine. Getting the crystal filter just right is more
important in the narrowest modes, like CW. It also helps with SSB, it's
not so important with AM. The null can be helpful with hets, but it
takes a bit of practice to use.

It's too bad the single crystal filter doesn't help much with adjecent
channel selectivity, which what is really needed for good AM DXing.

Also, the peak/null of the filter is so narrow that the normal receiver
drift has to be constantly compensated for.


BTW, this is my first communications receiver. Up to now I have been
restoring test equipment, broadcast radios, and a couple of TVs (all
tube gear). Any and all advice would be much appreciated. So far in my
aligning the SX-71, I seem to have increased the ringing caused when
the filter is used to the point it seems hardly useful.

Well, the ringing at least that tells you your crystal is still good.
Doesn't this crystal filter have a switch between sharp, narrow and
broad? The ringing should go down as the filter goes to it's broader
positions. I have a couple of single crystal filter radios. They will
both demodulate CW and SSB with the ringing crystal filter in the sharp
position. It's not a useful feature, at least for SSB. Tuning is
hypercritical, and normal drift will detune it within seconds.



I don't know
if that was caused by misadjusting this particular slug, or because I
aligned the rest of the IF to exactly to the crystal instead of just
455khz, which is what they said to do. I have recapped the unit (just
the papers), replaced a few far out of range resistors and changed a
couple of weak tubes.

Dan

There's good reasons the manufacturers gave up on the single crystal
filter decades ago. The ceramic and multipole crystal filter are easier
to use, and have much better adjacent channel selectivity. But they can
be fun to use, once you get the knack.

Frank Dresser