From: zimmer@calvin.stanford.edu (Andrew Zimmerman)
Subject: Re: Crystals AM & FM: Factory Adjustment???
Organization: Stanford University
Date: Sat, 25 Jul 92 08:55:23 GMT

In article <1992Jul24.112417.13418@lmpsbbs.comm.mot.com> bobwo@ecs.comm.mot.com (Bob Woodhouse) writes:
>In article <1992Jul23.222724.6449@informix.com> robertw@informix.com (Robert Weinberg) writes:
>>Here's another question you R/C gurus can answer while we are on this thread.
>>
>>My Futaba radio kit literature (Attack 4-channel AM) says I can buy another set  
>>of crystals, but I'd better send the set back to Futaba for them to adjust the  
>>crystals. True? Why?
>>
>>--
>>+==============================+===============================+
>>|       Rob Weinberg           | Workstation publications tech |
>>| Email : robertw@informix.com |       Graphic designer        |
>>| Voice : 415-926-6754         |    <imagine your ad here>     |
>>+==============================+===============================+
>
>My Magnum Sport instructions said the same thing.  Well, it's not quite
>the case that they would actually adjust the crystals;  perhaps they
>are implying that the would tune some circuitry or something.  At any
>rate, the common and effective practice of just pulling out crystal#1
>and substituting crystal#2 seems to have given nary a problem to me
>and to any racers who find that they need to use an alternate frequency
>in an A-main due to conflict with another racer.  In short, I personally
>would disregard this particular Futabian instruction.
>
>Regarding my earlier post which asked if AM and FM crystals are the same
>thing with different prices, if I ever get an FM radio, I will certainly
>try 'AM' crystals before I pay the 'FM' crytstal price.
>
>Stay tuned,
>Bob Woodhouse

Futaba might just be protecting themselves by telling you to send it back to
the factory. The FCC has different rules and regulations for each frequency
band in the the spectrum.  
For example:
 53 MHz band - This band can be used by Amateur radio operators for
radio control.  Output power must be under 1 watt, and the amateur's 
call sign, name, and address must be on the transmitter.
 72 & 75 Mhz band - We don't need a special license to operator on these
bands, but you do need a license to work on the transmitters.  Changing
the crystals is working on the transmitter. (The Futaba manual mentions
that it is illegal to change the crystals on these frequencies.)
Some of the new radios get around this problem by having radio modules.
Since you are not working on the transmitter, you can change frequencies
by changing the radio transmitter module and the crystal in the reciever.
 27 Mhz band - It appears to be legal to change the crystals in this 
frequency band.  (Futaba doesn't mention that it is illegal, so I am guessing
that it is legal.)

FM vs AM crystals -  I doubt that AM crystals will work in an FM radio.  
I would guess that they use a different IF.  AM typically use
455 kHz.  FM typically uses 10.7 Mhz.
I've seen some schematics for recievers that use the 455 kHz for AM.  The
FM IF is based on other radio circuits, not RC circuits.

High cost of crystals and tuning the reciever - National makes IC's for
both RC transmitters and receivers.  In the section on Reciever Alignment,
they state:
   This is because the crystals which are commonly used with these systems
may have tolerances as loose as +/- 0.01%.  At 49 MHz the resultant +/- 
5 kHz deviation could easily put the incomming signal out of the 3.2 kHz
receiver IF bandpass.
   Therefore, there may be some argument that you would like to have the
reciever factory tuned for a particular crystal.  (This would especially
true for aircraft.)
   It might also explain why the crystals are expensive.  ie they might
have tighter tolernences then those mentioned above. (Or, the radio
companies want to make a lot of money. :-))

Futaba vs Airtronic crystals -  It appears that Futaba and Airtronic
use the same Frequency crystals for their transmiters (no surprise there)
and different frequnecies for thier recievers.  Why you buy a set of
crystals, but the set that matches the type of reciever you are using.
This also means that you can use a Futaba transmitter with an Airtronic
reciever or vis versa.  (I have tried Futaba Transmitter with an 
Airtronic receiver and an Airtronic transmitter with a Novak Futaba J
reciever.  Both worked fine.)

Hope this helps,
Andrew
zimmer@calvin.stanford.edu