flyboy11

Hey guys I'm curious, why can you not change tx crystals without having someone retune it but you can change the rx crystals yourself? Also, why is it illegal?
Thanks

Phil Cole

It's illegal because the FCC wants it that way.

Most likely the "retuning" just involve checking that the transmitter is still on frequency and the modulation is OK. Most of the time it will be OK, but there is no guarantee, and the FCC wants to be sure that any transmitter let loose on the aether is functioning properly.

Malfunctioning receivers won't pollute the airwaves, and the FCC is not concerned about crashing due to faulty radios.

strato911

As Phil said, it is illegal to change Tx crystals because the FCC says so. The reason is because RC users are SECONDARY users of our radio frequency bands. The PRIMARY users (industrial) pay big $$$ for the right to can use their frequencies without interference. If a Tx is slightly out of tune, it could interfere with industrial equipment long before it affects the neighboring RC channel, since the Primary channels are in between each RC channel.

The reason it is legal to change the Rx crystal is simple - the Rx doesn't transmit, so if it's out of tune, it doesn't affect anyone except the RC pilot (and his/her plane).

CAP232CM

My Feedback: (8)

I don't understand this at all. I have a Hitec radio with a spectrum module and I can change it to any ch. I want. Why is this not illegal or is it? I have a JR radio with 2 different modules. Is it illegal to switch them to change the ch.? This makes no sense at all.

Flyfalcons

Modules are cool. They are certified to be changed out. Same goes for synth modules. From what I understand they are 'self-tuned' in that the tuning of the Tx isn't critical but the tuning of the module is. It's just 'standard' crystal swapping that is illegal here.

CAP232CM

My Feedback: (8)

Oh I see thanks for the heads up.

strato911

Sorry to confuse you. What you are doing is the proper way to change channels.

Changing modules or changing channels on a synthesized transmitter is legal.

Lynx

The spectrum module is designed from the ground up to be able to switch channels like that. If the module uses a crystal it wasn't, it was ment to work with that one crystal, if you change it it needs to be checked for tuneing to avoid spurios emitions on nearby channels (don't wany to hit primary users of the 72mhz band)

strato911

Lynx - He's talking about changing the entire module, not the crystal within the module.

Modules contain the entire RF circuit, and are tuned for the crystals contained within them. That is why the FCC allows modules to be swapped - they are already tuned for the crystal.

Pete the Geek

So what you're saying is that if I took a Hitec 3 channel am focus that originally came with a removeable 26.995 crystal and swapped it out for a 27.045 crystal it is illegal?!?! Why even make the darned crystal removable then?

Steve Lewin

Because it's not illegal in most of the world and Hitec are not going to make a special version just for you .

Steve

FlyinBrian

As a point of interest.

In Europe it is both legal and common practice to change tx xtals, this is both a boon and a pain as you can not be sure from one day to the next what freq. someone is using.

I guess Synthersisers will make this easier and even more common therefore good Tx freq control becomes even more imperitive.

MPX PROFI systems with "Channel check" will not allow the tx to transmit any RF if it detects the frequency is in use - I think this should be the norm for all new Tx's especially synthersised units.

Pete the Geek

I imagine this is common place as alot of us newbs don't know this information. I seriously doubt the effects of changing out a transmitter crystal on this frequency will cause major cataclysmic events....sorry, I just can't believe that LOL. It works for the English

Jim Schwagle

My Feedback: (1)

It's not that your plane will crash because of the crystal change. Other, primary users, are spaced between our channels. They're much closer to you frequency-wise than another RC'er. If your swap puts you a little off and an RC crane drops a pallet on someone you're going to get a visit from the FCC, and lots of lawyers and if enough people do this the rest of us will lose the 72MHz band because the FCC doesn't give a rip about the Hobby industry. We're secondary users, not the primary users and the radio makers must swear on a bible that the radios won't cause interference on the primary users channels. We won't if we follow the rules. Oh yeah, swap a transmitter crystal and forget your AMA coverage, or your homeowners either. Do something illegal and all coverage bets are off.

By the way, it is my understanding that the British and other European RC'ers have their own dedicated band, that's why they can swap crystals. We don't.

strato911

And boy are those fines steep! quote from FCC:

Pete the Geek

According to a hitec rep, this only applies to the 72 mhz frequency channels on crystals, not 27 mhz, which is why they sell the crystal for the 27 mhz tx and not 72...now I'm really confused LOL.

MO_Radio_Tech

The reason that changing crystals throws off tuning has to do with the tolerance and accuracy of crystals. If tolerance and accuracy are the same between two crystals, it is possible to move from CH16 to CH60 without disturbing tuning at all. However, if one of the crystals is a bit "off", it can mess up tuning pretty bad. Often you can throw off tuning exchanging crystals on the same frequency!!!
The radio tech will tune the RF section, or module, to the crystal and verify the device for proper freq and good output.

The average consumer does not have the equipment or knowledge to determine or evaluate these conditions, so only "authorized" or licensed techs are supposed to do crystal changes on transmitters.

This FCC rule is for our own good, because receivers will range much better if the transmitter's signal is in the center of the receiver's window (also called bandwidth). Also, if the transmitter's signal is not near the center of the receiver's window, at distance the receiver may even mistake signal for interference and reject it. This would cause you to crash!!

An old radio tech saying that still holds true is "always tune to the rock". Thus, two modules on the channel can be identical as long as they are both properly tuned.

The new synthesized radios and modules still have a crystal. At the factory they are tuned to this crystal. With the advanced circuitry inside, the module can convert to different channels using the same crystal. The tuning never has to change between channels because the crystal oscillates at a constant rate.

Pete the Geek

Thank you for your reply...I am still being bombarded with images of men in blacks cars driving up to my apartment to take me away because of a crystal change. Can anyone verify the legality of the 27 MHZ issue? Thanks

strato911

Over the last few months I have read through the FCC regulations governing RC use (Rules 95.2XX) several times, and have yet to see where it allows crystal swapping on ANY of the RC frequency bands (27Mhz, 72Mhz, & 75Mhz).

Click here to download a copy of these FCC regulations from their FTP site.

strato911

Also, read Rule 95.645 regarding certification requirements of the radios...

MO_Radio_Tech

Pleae note that I just edited and added more info to my previous post.

Pete the Geek

Ok Strato, either Hitec are selling black market crystals, or I'm actually using a module..sorry, this is all very new to me The crystal (or module) is easily accessible on this radio and obviously meant to be removed and or replaced. Otherwise, they are kind of giving the suicide case a loaded gun, ya know? Thanks for all the info.

MO_Radio_Tech

Most manufacturer's crystals are easlily accessible and swapped. JR glues them in to discourage, but not prevent people from swapping. Just because you can do something does not mean that you should.

Pete the Geek

Ok then, Hitec is in violation of FCC regulations, as well as me then.

MO_Radio_Tech

I don't care if you break the LAW, just don't crash your trainer into my car.