Changing Frequencies.....danger??

[Holzworth]

I would love to hear from an authorized Futaba repair center on this 
issue. I am sure there are alot of people out there changing crystals. I 
am in the process of changing frequencies myself on all my planes and 
would like an official answer. There are plenty of  other radios that 
have no module and provide access to crystals for changing. Even some of 
the older Futaba radios provide access to the crystal from the exterior 
of the radio. I doubt  if they were woried about someone changing 
frequencies they would have made it that accesible. My guess is the 
manufacturers have seen an opportunity to make even more money by 
telling people that have purchased the more expensive radios that in 
order for them to change their frequency that they have to buy a $50.00 
module in lieu of a $10.00 crystal.

John Pavlick wrote:

>Scott,
> Yeah, what Ed said... Just change the RF module in the transmitter and get
>a new crystal for the receiver. I think the RF module for a 9c is about
>$50.00. Not too bad. You are not supposed to change just the crystal in the
>transmitter - that's why we have modules. Changing just a crystal (in the
>transmitter) requires re-tuning otherwise you might interfere with someone
>else. The module is tuned to the chosen frequency and sealed up. Nothing
>wrong with having a few modules either. The synthesized RF module is another
>alternative. Like Ed said it is tuned to a stable frequency, then a PLL
>(Phase Locked loop) circuit generates the chosen carrier frequency. Also
>acceptable and legal. Bottom line: change the transmitter module and the
>receiver crystal and you'll be within FCC regulations and safe from any
>Homeland Security issues too.
>
>John Pavlick
>http://www.idseng.com
>
>
>  
>
>>-----Original Message-----
>>From: discussion-request at nsrca.org
>>[mailto:discussion-request at nsrca.org]On Behalf Of Ed Alt
>>Sent: Wednesday, June 29, 2005 11:52 PM
>>To: discussion at nsrca.org
>>Subject: Re: Changing Frequencies.....danger??
>>
>>
>>This is quite possibly incorrect.  Depending on how it has been
>>designed, it
>>may not legal or advisable to change crystals in a module unless
>>you are FCC
>>certified to do so and have the equipment to assure that the transmitted
>>energy is within spec.  Think of it: the reason we have modules
>>is because
>>you're suppose to swap the module, not the crystal.  Ofcourse it
>>"works" to
>>swap a crystal.  All this proves is that  you didn't break anything
>>important when you broke the potting seal that you usually find on the
>>crystal.  This potting seal is there to tell the bench technician whether
>>you've been tampering with the module.
>>
>>When you swap crystals, you run the risk of transmitting with
>>less than peak
>>power on your channel, thus wasting energy that would normally
>>radiate out
>>of the antenna and instead heating the module's components more
>>than normal.
>>You also run the risk of transmittnig above legal levels of RF energy
>>elsewhere, in other words, you may contribute to interference on other
>>channels.  You have to realize that you never get a completely
>>clean signal
>>just on your channel when you operate your transmitter.  Part of the
>>procedure of producing an RF module includes tuning, or more correctly,
>>aligning the tuned circuits in the module to the center frequency
>>you want
>>to transmit on and bringing the unwanted byproducts down to acceptable
>>levels.  When you swap crystals on your own, you have undone this
>>work, at
>>least a little bit, maybe by an unacceptable amount. Depends on
>>how broadly
>>tuned the module is designed to be in the first place.
>>
>>Why does a synthesised RF module work?  It's designed with a tightly
>>controlled feedback loop to lock onto the desired frequency and maintain
>>other unwanted RF energy well below specified levels.  Why does
>>it work to
>>swap crystals in receivers?  Because the front ends are broadly
>>enough tuned
>>to allow energy to be received efficiently within a specified
>>pass band, in
>>our case usually much of the 72 mhz band.  The fine tuning, i.e.
>>what makes
>>our receivers highly selective to the channel we want is mostly
>>accomplished
>>by circuitry that is past the stage of the front end.
>>
>>Ed
>>
>>
>>
>>    
>>
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