Rx Xtal; Can I use it in a Tx? [Archive]

XXWoodmanXX

09-02-2006, 05:19 PM

I'm probably asking a silly question, regarding this:

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LH700470&P=1

Now, it states that it's a 75Mhz AM RX crystal. I bought an Airtronics Rx off of Ebay w/ a Ch#70 xtal in it, so now all I need is a Tx crystal to complete it.

Can I put this "RX" crystal in my Tx without any ill effects?

ambiotter

09-02-2006, 11:18 PM

No, you cannot.

Demon-TC3

10-08-2006, 12:57 PM

No, you cannot.
actually...you can...but its illegal....
well im sepakin on behalf of 27 and 40mhz...it is possible technichally speaking but for some reason it is actually illegal (by law not just BRCA). but also signal quality wont be as good and you probably wont get thesame range and also more glitching...
Enclosing, DONT try it...
Hope this helps...

rezenclowd3

10-10-2006, 10:32 PM

I also just found out that one is not even allowed by FCC law in the US able to change crystals. One can only change the channel if the radio is module based. If you want to change channels in a non modular based radio, you must send it in to the manufacturer. It is legal to change crystals in some other countries though. CHeck here: http://www.rcgroups.com/forums/showthread.php?t=94580

Demon-TC3

10-11-2006, 04:17 AM

no i think changing crystals cassifies as a plug in module.
having said that i think in the US that 40MHZ is illegal? not sure but here 72 and 75 are both illegal (at least for cars) i think 75 may be for planes here...

rezenclowd3

10-11-2006, 12:23 PM

Ya in the US 40mhz is illegal. Gound in the US is 75 and Air is 72. A module is classified as something that also houses the oscillator (I believe) A module is supposed to change the out the whole Transmitter board of the TX. (all the associated frequency tranmission electronics) One is not supposed to JUST change out the crystal. (in the US) I think it would be a good idea to write the magazines about this for clarification maybe. I just may do that if I have time today.

XXWoodmanXX

10-11-2006, 07:10 PM

Neon_Dave

10-11-2006, 07:18 PM

Yeah... I don't really get it either - so what it's saying that you CAN'T change your RX/TX crystals without the manufactuer doing it for you?! What's the point of that?

I don't see what's so illegal about it. I understand if you're using a frequency that may interfere with other things such as radios between emergency services and such, but this? :rolleyes:

That's crazy.

XXWoodmanXX

10-11-2006, 08:00 PM

I know that the airport nearby uses alot of low-band AM. I already stay clear of 27Mhz AM as it is. Too many glitches and runaways in the past. :p

AreCee

10-11-2006, 09:11 PM

LOL That's crazy. Sorry, but to me, this law holds about as much water as tearing the tag off your bed mattress. :p
It is legal for the buyer, after the mattress is purchased, to tear the tag off not the seller.

However, the strict FCC rules are that you can't change frequencies without retuning the transmitter and that includes swapping out crystals. It's perfectly legit as long as the radio is retuned and certified. This does not hold true with synthesized channels because every time you redial the frequency the radio is still in tune.

Now when was the last time you saw the FCC police at a track arresting people for changing crystals? :teacher:

slink

10-11-2006, 09:23 PM

Yeah... I don't really get it either - so what it's saying that you CAN'T change your RX/TX crystals without the manufactuer doing it for you?! What's the point of that?

I think the point of the manufactuer doing it is retuning the radio system to meet the new channel.Does it matter....I've never noticed problems changing crystals but apparently the FCC thinks it matters for some reason.I would certainly love to hear the reasoning behind the law.

rezenclowd3

10-11-2006, 09:27 PM

AreCee explained it alot better than me. It also doenst hold true if the TX is module based where the crystal is built in. I found out about this matter this month, as I am getting into rc airplanes and helis, and guys would say "oh hes on my channel" and Id retort, "WHY DONT YOU GO BUY MORE CRySTAls!" They would look at me like im crazy! I wondered why some guys had 2-3 TX's even though most dont have more than 10 planes (model memory maybe?)

Neon_Dave

10-11-2006, 10:06 PM

*sigh* like they say - Laws are meant to be broken :D

Be it as it may, I'm not gonna bust my @** to send in a TX to some company on the other side of the world for a crystal change - legal or not.

I would certainly love to hear the reasoning behind the law.

^^Ditto :rolleyes:

To me, this just seems like another pointless law that's just made to take up space in the rulebook :roll2:

:D

AreCee

10-11-2006, 10:51 PM

Actually it came about many years ago before there was computer controlled digital synthesis. Back in the early 1900s all radios were crystal based and there were many conflicts especially on the public bands. This is when the FCC was formed to help regulate and control who got to use what channel and where. I used to work in the area of public safety and we guarded our few channels so carefully that if any interference broke in it was reported immediately to the FCC. Now I know that we are playing with toys on very low powered radios but we are broadcasting over the public airwaves and need to remember that we are not alone in the radio universe. :teacher: Enough preaching, now go have fun and please avoid radio conflicts; change your channels accordingly. :)

Think of it as traffic control and what would happen if there were no traffic laws and rules. As it is there are thousands of accidents on the roads each year because many feel that rules and laws are to be broken.

Rules and laws are just a way to insure that we all get along together and not hurt or damage each other by acts of carelessness.

rezenclowd3

10-11-2006, 11:02 PM

So if you dont want this hassle of sending the radio in for a crystal change, get a spektrum or nomadio:-)

Neon_Dave

10-11-2006, 11:16 PM

I just asked my dad's outlook on this - his response was this:

All the frequencies available on the market for RC (whether they're crystals or modules) have been previously approved by the FCC or other body; making sure ahead of time that nobody else (such as emergency service radios, for example) is on the same channel. So, with that in mind the channels you can change to by swapping crystals or modules are already approved.

They may mean swapping to crystals or modules that are not approved (i.e. 40 mhz) are not allowed, unless through the manufacturer.

Now, that being said this law, from what my dad gathered from it, would state that you can only use freqencies within a certain bandwidth, approved beforehand by the FCC, not specifically crystals. 27.195 is not much different than 27.225 - it's still the same bandwidth.

This way, it makes sense now :rolleyes: Maybe this has shed some new light on any confusion?

-Dave:)

rezenclowd3

10-11-2006, 11:38 PM

Not really, the FCC specifies that one may not change the crystal...

Neon_Dave

10-11-2006, 11:48 PM

Not really, the FCC specifies that one may not change the crystal...

...crap... :rolleyes:

Well, to tell you the truth I've never even knew this law existed until a few days ago when I found this thread, and I never heard anything about something like this before.

Had no complaints, conflicts, interference with anything outside of somebody else's RC (same channel :D ), or anything like that. It just doesn't really seem to be an issue...

I really don't know what to think....

z-man280

10-11-2006, 11:51 PM

Not really, the FCC specifies that one may not change the crystal...
going by that, then how can manufacturers sell crystals for radios? It would be agaisnt FCC rules to sell them.

rezenclowd3

10-12-2006, 12:04 AM

I was thinking about that too, but then again, it seems that it is not policed.

z-man280

10-12-2006, 12:10 AM

right. they must know that compaines make them changeable, but...heres the loophole i think...you cant put 27 mhz in a 75 radio...so you have to stay within specs of THAT radio, i.e., within the span on that band allowed...from 26.9 whatever to 27. whatever, and same with 75's... or its rendered useless. it will only work in the frequerncy ranges set by f.c.c. ....sound feasable?

rezenclowd3

10-12-2006, 12:24 AM

yup. Although it may work in a non module based system, but the main frequency (ie 72, 75, 40 mhz) has changed so much that in reality your transmission will be out of tune. That HAS TO be sent it for re-calibration.

highroller

10-12-2006, 02:39 AM

First to answer the RX crystal will only work in the receiver, if the crystals are installed backwards the system won't work. One send the signal (TX) the other is meant to receive(RX). I had to once changed channels as a fellow racer was also on 70, I inadvertingly installed the RX crystal in radio and the TX crystal in receiver. The Futaba crystal will not work in a Airtronics systems even thought the two are on 75 or 27 mhz they operate on a different shift.
There was a radio compatibilty lists I found somewhere that listed which radio brands where interchangable.
From as far as I can remember we have been switching channels in radios when we had conflicts with another user who was also on the same channel.

rezenclowd3

10-12-2006, 02:59 AM

Highroller, ya, your supposed to use crystals made by (or for) the TX and RX brand. IE dont use an airtronics crystal in a futaba 3pk. A RX crystal in a TX will work with a TX crystal in an RX. BUT dont do that, as its a one off frequency then, and still illegal, and may cause problems with others.

rezenclowd3

10-14-2006, 03:33 AM

Demon-TC3

10-14-2006, 07:11 AM

I AM WRONG!!!! I asked my LHS owner, and he said that FCC law is for air units. Its OK to change the Crystals for ground RX and TX!!! He said the air guys once upon a time could also change at will, but that was changed....

It still seems completely stupid!
Why should anyone care about changing frequency...they have all been tested and cross refferanced against frequencies used for emergency services(and other frequencies in use) and crystals are made accordingly...there is no risk at all in changing the frequency of any radio (so long as the frequency as a whole is legal in the country) so why should you need to send the entire radio (or radio module) and receiver to the manufacturer just to get the crystals changed onto another freq?
im pretty much possitive noone will actually do it..but still would like to know why such a stupid law exists...