[Hallicrafters] SX-111 alignment clarifications needed


Edward B Richards zuu6k at juno.com
Sun Feb 29 01:33:20 EST 2004


Hi George;

I don't have a copy of that procedure but it sounds pretty straight
forward. What I think they want you to do is peak  the top of T-201 and
T-1 and T-2 for max gain i.e. highest voltage across the speaker, then
slowly tune the signal generator off one side until the meter falls to
it's lowest reading; record the sig gen frequency. Then slowly tune it
through the peak to it's lowest reading on the other side of the peak;
record this sig gen reading. This is your bandpass curve. Now, subtract
the lower freq reading from the higher frequency reading and divide this
difference by 2. Add this number to the lowest frequency reading. This is
the center of the bandpass.  Actually, the bandpass is described by the
difference in frequency half way down each side (the 6 db points) of the
curve.

It doesn't matter if you use a meter or a scope. You are only looking at
amplitude vs frequency. You might find this suitable: set your sig gen
level to show several volts (depending on the speaker impedance) at the
highest reading. Mark a piece of graph paper with frequency across the
bottom and voltage up the side.) 0 at the bottom. Mark it so the highest
voltage will be near the top of the paper. Now, slowly tune the sig gen
from below the bandpass through the band pass to the other side, making a
dots where the voltage and frequency meet as you tune through. You should
have at least 10 points marked. Now, connect all the dots with a line so
you get a parabola shape. Using the 1/2 half max voltage points on both
sides, calculate the bandpass frequency. Set your sig gen frequency half
way between these points, and proceed with the alignment. This will also
show you if the curve is not symmetrical. Touching up the adjustments
should even out the curve. If it is flat on top you are probably using
too much signal. Reduce your sig gen output and run another curve. 

re: Oscillator frequency being higher than the signal frequency: In a
super heterodyne receiver the local oscillator in the radio operates at a
distance from the signal frequency. When these two frequencies are
brought together in the mixer a difference frequency is produced. This is
the intermediate frequency (IF) which is amplified by the IF amplifiers.
A signal frequency the IF away, either above or below, the oscillator
frequency will produce the same IF. Only one of these is peaked by the
antenna, RF and mixer circuits. The unwanted one is called the Image.
Since they tell you the oscillator frequency should be above the signal
frequency, the image will be the IF above the oscillator, or 2 times the
IF frequency above the signal.  You should check this on all bands by
tuning the sig gen 2 times the IF (3300 kc?) above the radio's frequency.
This image should be much weaker than before. If it is stronger, then you
have the oscillator tuned to the wrong response.

I hope this helps.

73, Ed Richards K6UUZ


On Sat, 28 Feb 2004 18:38:02 -0500 George KB2Z
<Thermionic_Emission at earthlink.net> writes:
> Good afternoon all,
> 
> The SX-111 plays pretty good, once the  opposite sidebands are put 
> right it 
> will be a joy to use.
> Adjusting the 50.75 perked it up but did not help the reversed 
> sidebands. 
> Could there more than one peak?
> 
> Can anyone break it down for me. I have read the instructions a 
> dozen times 
> but, there are a couple of alignment procedures I just dont 
> understand. 
> Having never aligned a Hallicrafters rx b4, please bear  with  my 
> not 
> understanding.
> 
> ***On the 1600kc 2nd conv.osc and 1650kc if.
> 1) Says  to adjust for maximum output by adjusting the signal 
> generator 
> freq. w/T-201 top.& T-1, T-2
> 2) Tune through the passband and observe the shape of the response. 
> Set in 
> the middle.
> 
> How much is the signal generator freq adjusted? Shouldnt it be at 
> 1650?
> Do you observe the shape of the response on the meter or a scope?
> If a meter does that mean that if it peaks at say, 20v and drops off 
> at 
> 10v, adjust to 15v?
> If a scope does it go across the speaker load?
> 
> ***For the RF alignment; It says to set the generator and receiver 
> to the 
> same frequency.
> 
> 3) ""The oscillator frequency is higher than the signal frequency on 
> all 
> bands.""
> 
> Could someone please explain that statement?
> 
> Thanks in advance for any info., advice, or explanations. Its all 
> appreciated. George KB2Z
> 
>   
> 
> 
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