[Boatanchors] Crystal Effects

[Jim Wilhite]

One other thing to keep in mind is that many of these old sets may 
have had the phasing knobs removed for one reason or another.  If 
that should be the case you are shooting in the dark to determine 
where 0 is.  The knob may say zero, but the cap may not be.

Probably the easy way to find out is remove the cover of the filter 
and look.  Make a mark on the shaft opposite another on the cover so 
you can tell where 0 is.  If it does not peak and null where it 
should, chances are the phasing knob has been removed.

The interesting way to do an alignment is to use a scope on the 
output of the detector and you can see the waveform.  If you use a 
sweep generator then what you see will look a lot like the 
illustration in the manual.  The visual presentation is much easier 
to align than using an analog meter.  Use a sweeper if you can. 
Seeing the center of the crystal filter is much easier.

Jim/W5JO






> The crystal is in a four legged bridge circuit, when you adjust 
> the phasing
> control it introduces capacitive  and inductive elements and thus 
> allows you
> to effect the passband of the crystal, so yes the center of freq 
> will shift
> a little when the phasing control is operated.
>
> However, and this is important, if the receiver is aligned, it 
> must be
> aligned to the center freq of the crystal filter, which is almost 
> never 455
> kc.  It might be closer to 456 or so.  What you do is put just 
> tune in a
> weak to moderate powered station, you do not want a real strong 
> station for
> this.  You take a voltmeter set to AC and put it on the output 
> leads along
> with your speaker, turn the vol up enough to get a meter reading 
> and or set
> your scale so that a meter reading is obtained. Now you can 
> carefully rock
> the tuning knob from side to side while on the station.  Make sure 
> the
> filter phasing is centered or off.  While rocking the station 
> slightly
> notice where the AC meter peaks, that will be the center passband 
> of the
> crystal..  If you use a signal generator make sure it is warm and 
> has been
> on for a long long time and is very stable, you can tune it to 455 
> and
> slowly rock the oscillator from side to side and you will find the 
> center of
> the xtal filter.  That is THE center of the passband of the xtal 
> and you
> align the coils up with that.
>
> But it is common for the freq to shift a little yes when you tune 
> it.