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Interchangeable Crystals

Old 12-30-2005, 11:19 PM
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Default Interchangeable Crystals

Ok, let me preface this post by stating that I actively flew in the 70's and early 80's. I used Kraft, Orbit and Royal radio gear.

Now that I've returned to the hobby in a big way, I have 1 Futaba 7 channel computer transmitter with 1/2dz. airborne paks. All my receivers are high-band on 42-43 and 44 channel.

When I fly multiple planes on any given weekend, I simply take the 3 transmitter crystals along and swap out each to correspond to that particular aircraft. I change frequency pins as needed altho our "club" is informal with only 1 pilot in the air at any given time. Safety is always paramount as our field is in close proximity to the local airport.

My question involves the practice of swapping out the transmitter crystals. I just read in the latest MA that this is a "no-no"? I'm confused. Why have interchangeable capability if this is detrimental to range???? I'm not so dense as to realize that you don't move more than 3 channels either direction and never from low band to high band. But if the crystals oscillate properly and are made by the transmitter manufacturer, where is the range-limiting variable entering the picture? I flew all season in this manner and had absolutely no impact on performance.

If this is the case, why even offer this feature and advertise it as a benefit? Guess I should have stayed with 6 meter stuff, (altho it's been over 25 yrs since I held a tech's license and Morse code? Long forgotten,lol).

Any thoughts on this one?

Thanks!
PointMagu
Old 12-31-2005, 12:32 AM
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Default RE: Interchangeable Crystals

Try to buy a transmitter cyrstal?

Many of the new ones have RF modules in the rear. No cyrstals. You can buy different RF packs or buy a PLL one that you can make any channel you want.

Here's what they are like...
http://www2.towerhobbies.com/cgi-bin...&I=LXHKB1&P=ML
Old 12-31-2005, 01:18 AM
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Default RE: Interchangeable Crystals

Though most don't want to hear it, it is technically illegal to change out the Tx Crystal to another channel without having a tech retune it. They sell the tx crystal for replacemnt purposes only. It'll work and you'll probably never have a problem, but it's a risk I wouldn't take, and if you loose control of your model or worse and more likely you shoot someone else down and something happens, one thing that is investigated is if the radio matches it's factory settings, i.e. if it was tuned to ch54 and its got a ch56 crystal in it will be held against ya. At that point the AMA would more than likely deny your claim. It's of course a radio circuit, and a low tech rendition of one to boot, despite the computer manipulation of the digital pulse position. As with any radio circuit they need to be tuned. The problem is the entire circuit is part of the tuning, not just the crystal. The crystals themselves, even the best one are pretty low quality, the point is the entire system need to be tuned in order the achieve the published specs. If you really need to change channels look for a Phase lock loop set up or an interchangeable module set up.

I've never seen an advertisement of a Transmitter with interchangeable crystals, changeable modules and Phase lock loop systems but not crystal based. I'd sure like to see that advertisement. Also any radio build before around 94 would be considered a wideband radio and is illegal to operate under FCC rules ham band or standard rc band. Rx is a different story. It deals with signal rejection to over simplify it. There is no power involved no stray signals to interfere with others. I'm not sure of the physical difference in the newer rx that allow the high and low band of crystal interchangeability, it does allow a wide crystal range. Rx that are not marked as high or low band are tuned to a given channel and are also not to be changed. Futaba gave me a stern note when I sent them an rx tuned to channel 46 with a ch 40 crystal in it.
Old 12-31-2005, 08:27 AM
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Default RE: Interchangeable Crystals

Are you actually swapping only a crystal? or you pulling a module off the back of the case and swapping that? The module is fine, the crystal is technically verboten...
Old 12-31-2005, 08:58 AM
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Default RE: Interchangeable Crystals

It's an older Futaba with the crystal socket in front..no module.

So, three transmitters for three airplanes. Guess it's time to buy a newer transmitter or two.

I appreciate everyone's input. The advertisement I alluded to wasn't a recent one but rather the ORIGINAL one when the transmitter was purchased new in the late 90's. So, "no", I won't be able to show it to anyone but I do recall reading that as a feature I wanted at that time, (and still do!)

This is a great forum for anyone and everyone that loves the hobby as I do!

ps, I have a 7 channel Kyosho system that doesn't have interchangeable crystals still in NIB. Guess this Spring it will see sunshine!


Thanks again!
PointMagu
Old 12-31-2005, 10:24 AM
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Default RE: Interchangeable Crystals

Hi!
In Sweden and in the rest of Europe it is common practice, and legal too , to swap both transmitter and receiver crystals.
Over here we use 35 MHz for airplanes and 40 MHz for both air, water and surface models.

Regards
Old 12-31-2005, 12:55 PM
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Default RE: Interchangeable Crystals

no you don't need 3 radios. . I stressed to never change the Transmitter frequency. that's a big no no. The modern Rx on high and low band are ok to chnage Rx crystals in. get a couple more recievers and a couple Rx crystals that are on the same frequency as the Tx and use the one radio for all the models. If your doing to pop for a new radi get either a futaba or JR with interchangable moduals if you really want to change frequency here a couple options

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXJDG0&P=0]futaba 9c[/link] you set it to any frequency you want. you then get a couple Rx's

I highlt recommend the above.

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXAHK8&P=0]hi-tec[/link]

I've surcome to using hitec recievers and servos. this is a cheap way to get a tx with the abity to change frequency
Old 12-31-2005, 01:20 PM
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Default RE: Interchangeable Crystals

So the Rx's aren't tuned as the transmitter?

Well, that certainly would be alot cheaper and easier to manipulate!

Guess that explains why most receivers I see advertised are without crystals...well, duh...that makes sense!

Here I've been changing the wrong crystals! One transmitter on it's "tuned" channel and multiple receivers. Well now, that's like back in the AM days when I flew on 27mHz.

Who sez an old dawg can't relearn old tricks?

:-)

Thanks!
pointmagu
Old 12-31-2005, 01:35 PM
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Default RE: Interchangeable Crystals

right!
plus the modern computer system like
[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXKAJ1**&P=0]6texa[/link] you have model memorys so you can save trim setting , servo direction and mixes for each model. also revers the servos so you don't have to think about the orientation of the servo when installing them. except duel aileron servos and flaps of course.

I have this

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGAF0**&P=0]7cap[/link] great radio. The only thing it lacks that ticked me off is no duel elevator support. you can do it but the trim only works on one servo
Old 12-31-2005, 10:42 PM
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Default RE: Interchangeable Crystals

Ok, I took your advice!

I placed a 7CAP on layaway at my LHS this afternoon. At $239.00 (with 3004x4 servos) I couldn't pass it up,(Channel 35, PCM/1024).

I got home and downloaded the manual when I came across this on PAGE 7, Bullet point #18:

In North America it is against FCC regulation to change the crystal within the transmitter to a different channel. All such transmitter crystal changes must be performed by a certified radio technichian. Failure to properly tune a system to its new channel may result in decreased range and may also result in interference to other types of frequency users on adjoining channels. Doing so also voids your AMA insurance.

So, I was right when I stated that this forum ROX!

Many thanks for setting my record straight..(AND, I got a new radio at a great price!)

pointmagu
Old 01-01-2006, 12:54 AM
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Default RE: Interchangeable Crystals

You learn something every day, right?

Notice the above post where the guy mentioned that he had Futaba transmitters and Hitec recievers. I'm that way too. Most people at the flying field wher I go have Futaba transmitters be their plane is more likely to have a Hitec reciever because they are cheaper and you don't have to worry about high band and low band.

I swap my Hitec recievers around to different fequencies on occasion.

Enjoy,

Jim
Old 05-30-2024, 05:24 PM
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While I know this is a OLD post, it did come up in a search. And no one above mentioned the actual reasons for prescribed operations. So here is a brief summary:
In (nearly all) RC transmitters, a crystal is used to establish the output frequency. (It should be noted, a crystal may be cut to oscillate on a harmonic. EG 20 Mhz oscillator, to produce 2x or 3x the frequency). But.. the channel frequency is amplified, and modulated by a pulse stream.. but then goes thru tuned circuits and output transistors, and another tuned circuit to the antenna. Past 1993 the bandwidth was narrowed to 10Khz.. so that more channels could work, next to one another, without interference. Both AM and FM produce side bands.. utilizing the given bandwidth per channel. (Very old stuff, might have had a CW (continuous wave tx, but most often even those were modulated AM). A crystal frequency is verified to be within a tolerance, inserted into the TX circuit, and they are tuned to NARROW the output as much as possible. IE it is TUNED to ONE frequency. If you start swapping crystals, it's basically in conflict.. trying to pass the wrong frequency thru a narrow band amplifier! Thus the reduced range. And it's possible to have some energy output on the original frequency. NOT a good plan, if you have a crowed field, and multiple people flying at once. There are a lot of ways of getting by, breaking the laws in life, and no lightening occurs typically. But it can!

For a receiver? Different end, different structure. If you test the crystal, it will likely oscillate at 455Khz above or below the given channel frequency. Why? Signal enters via the antenna, then usually a broad band RF amplifier. The crystal oscillator, and the incoming RF go thru a "Mixer", and it produces the sum, the difference, and both original frequencies. But that signal, (the IF Intermediate Frequency) goes thru a very narrow filter, and tuned circuits, to become A) VERY selective, and B) amplified, and typically has a AGC circuit, to keep from being swamped if nearby the TX. SO.. the antenna sees ALL the RC transmitters, and lots of other RF.. but Immediately, it is mixed, with the chosen crystal, and the output IS selective. All incoming signals are "sorted" right up front! So the rest of the radio is operating on 455Khz.. NOT 72mhz. We transmit a half to .8 watts on the vintage systems, and typically fly very nearby (line of sight).. so the 72Mh is not highly amplified.. but the IF frequency is, and made very selective. That's why you can change the RX crystal, with very little loss. It will mix with a new channel frequency, and only the 455khz difference is utilized. The reason that proper crystals be used, is because some brands go above the channel freq, some go below, but both are 455Khz off.

I hope this helps with anyone else, who like me, still enjoys the "vintage" radios on 72. 2.4 channel hopping, is much more like dialing the right phone number.. even though the frequencies are jumping around all over the place!
Old 05-30-2024, 06:01 PM
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And the reason for a tech to do the tx crystal change? The type of crystal will be checked, the frequency of it, (as some crystals have different characteristics). The TX stages will be aligned around that new frequency, and they the power output will be checked. And most will use a spectrum analyzer to establish that the resulting transmission is properly narrow.

And yes, above, there are dual conversion and single conversion receivers. (a DC has two intermediate frequencies, where SC has only one. DCs are more selective... but lots and lots of SC receivers have been used successfully.

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