wings

I asked for an example , you gave me one. No arguement here. Does anyone else have any other legitamate examples?

Sport_Pilot

Most have gone to much higher frequencies, most don't even list 72 Mhz. I don't recall anyone advertising spread spectrum tech. That doesn't mean it doesn't exist, but I doubt that they would risk haveing the trasmitter frequency switching fail and stick on one frequency and thus interfering another unit.

Matt Kirsch

My Feedback: (21)

There is way too much confusion going on here. Futaba's hi-band/lo-band only applies to their RECEIVERS. Transmitters are tuned to the frequency of the crystal that's installed in the transmitter at the factory, AFAIK.

JR center tunes their RECEIVERS. That's for sure. Do they also center-tune their transmitters?

Kevin Greene

My Feedback: (85)

The transmitters I'm familiar with and are referring to are the PCM 10/10S/10SX/10SXII/10X....All of these transmitters use plug in MODULES---Which I have been told are center tuned. I even have a receipt for a module that I sent in for a check up---It specifically states that it was center tuned....

Kevin

FHHuber

There's a HUGE difference in center-tuneing a module and the TX which has crystals...

When you swap the module.. you swap out the full RF tuning board. therefore... you will be center tuuned with any module you swap. (if the module has not drifted...) The FCC specificly says swappable modules are fine because of this.

Crystals hacve an allowable error in manufacturing. the transmitter typically has a frequency multiplier circuit, which will multiply the error... then you have the tuning adjustments. If you don't tune the TX to the crystal, you are likely to be off center. If the TX was originally tuned with a high end of limits crystal, and you stick in a low end of limits crystal... you are going to be WAY off center. THAT is why the FCC rule specificly says... don't swap unless you can properly check that the TX is in tune after the swap.

In a factory setting... a guy tuning TX's to the crystals would have the proper instrument.. clip t on.. make a quick tweak of a potentiometer or a tuning capacitor... and drip on the sealing fluid. (notice the little drips of what looks like paint on a couple of things on the tuning board? that's the RF tuning adjustments.) Takes about 30 sec or so to tune one if you do them all day every day.

mr_matt

My Feedback: (10)

Awesome post W.C. You should post it to one of the many similar threads in teh radio forum, can't wait to hear how the chucklheads will deny what you are saying is true. They will blame sunspots or some graveyard curse or something. You should post it over there!

heli001

My Feedback: (5)

Matt,

I didn't want to stir the pot anymore than need be.....sometimes it is impossible to convince a "rock" that there may be other possibilities....this particular thread has been pretty good, with a lot of good information, so I didn't feel the need to spam the other forums...this one is certainly getting enough traffic on its own.

People will believe what they want to and certainly DO what their mind directs them to do. If someone is convinced that swapping crystals is perfectly OK, and has justified it to themselves, it is next to impossible to convince them otherwise. Personally, I like to do research, look at all the options, and make and INFORMED decision....not just shoot from the hip. Granted, there are times when the "hip" approach is the best alternative, but it is done with knowledge of the subject at hand. To me, the subject of swapping crystals in a transmitter is pretty much a common sense issue, and should NOT be done. But that is just ONE opinion of many, and again, I am not a radio expert or technician......if you want to discuss electricity from generation to the receptacle in your house or anything HVAC, I am your man.....

The situation was as Fred described.....Transmitter tagged for an upper end channel and a lower end crystal was installed.

Edit: spelling

bhole74

I swapped tx crystals in my focus radios several years ago because of a dead battery. The tx does not have a frequency label, just the crystal. I have never had a problem with any of them. I guess it's a good thing I didn't have an AMA card at the time.

By the way, does this mean I have to send my Eclipse back every time I dial in the spectra module? Afterall, it is changing the tx channel.[&o]

FHHuber

The Spectra module, as with some other frequency syntesizing systems, is FCC certified for the end user to twist the little switches. That's the whole point of the synthesizer... Its just like a CB radio... you change channels on the dial... you don't have to get it retuned. (Some old CB's used to have one crystal and RF tuning borad for each channel... Some of the up to 5 channel walkie talkie style CB radios STILL use that system...)

And again... it is illegal by FCC regulation. The AMA "rule" is a Notice... if you commit this felony.... you aren't insured. (because you aren't insured when you are doin a stupid thing that is illegal.)

You want it to be legal... yell at the FCC... not the AMA.

bhole74

How is a person new to this hobby supposed to learn of this stuff if not well versed in FCC and AMA rules a regulations? Hitec stated nothing to the effect in their manual or on the TX. I can't find any of this info in the packet I received with my AMA card either, just a general list of rules. I don't feel people doing this is an act of stupidity. I for one don't care to keep up with all the bureaucrats in this country, it is impossible, but if it is such a problem, maybe the radio manufacturers need to print a warning in their manuals, or make the crystals impossible to change rather than putting them on the front panel for the whole world to see. Until I read this thread, I had never hear of this as a problem before.

AMA-69405

My Feedback: (22)

bhole,

I can't speak for Hitec, but I can tell you what is on page 87 of the current JR XP6102 Instruction Manual...

Pretty straightforward... maybe Horizon should write the FCC rules.

Gary L.

FHHuber

Its in every crystal controled transmitter's manual... FCC requires it.

On the Spectra module equipped radios I believe it says something about it being illegal to use a different brand module in the transmitter.

Its all in the "fine print" sections that no one bothers to read... Back when we were required to have an FCC permit to use anything better than 27 mz 1 or 2 channel 150 ft range systems... most people did read that stuff.

Kevin Greene

My Feedback: (85)

My point with my initial postings concerning the center tuned JR Rx and Tx modules is that with them being center tuned is anything really bad happening when you change the crystals yourself vs an "authorized" technician??? (Other than it being an FCC violation) Those of you that are radio savvy know the answer.....FWIW, I have a separate module and receiver for every frequency change I make just to keep legal....

Kevin

FHHuber

The JR TX is center tuned to the frequency WITH THE ORIGINAL CRYSTAL. Takes just SECONDS on the assembly line to do it. you get the same potential for throwing the TX off center by swapping with even the SAME channel crystal as with any other crystal swap. The variance is based on the crystal oscillation multiplier and the crystal manufacturer's allowed deviation.

Multiply the allowed crystal deviation by the frequency multiplier used (some use a 2X... some use up to a 4X multiplier...) then douple it (because if you swap from a crystal at one limit and get a nother at hte opposite limet... that's how far you are off when you swapped.) There's how far you can throw the TX off tuning.

Any brand.... it CAN be clear off where the RX can't "see" the TX signal if you go the opposite direction out with the RX (the same double of max allowed deviation.)

You want to get a 10 ft range check with antennae up... now you know how.

**********

MODULES are different... they are designed to be user swappable and have the RF tuning board and the crystal enclosed. Swap modules all you want.

wings

I am not th at knowledgable about the actual "mechanics" of a crystal. Could some one explain to me how it can mess it up that bad by just removing it and putting another in? Also, what is involved in "tuning" a crystal?

I'm not being argutive, I just don't understand how this could throw things off so badly.

Also, do rx crystal behave in the same manner? If so, then why is not an issue to change rx crystals (as far as the plane is conerned, I realize rx's can't harm others)?

FHHuber

A crystal is a precision ground piece of artificially "grown" quartz.

The precision of the grinding methods is pretty darn good... inside that little can is a chip of rock about 0.05 inch dia and just about at thick as two sheets of paper. However... its not perfect. When you are measuring your accuracy in percetage of a micron its small. I will grant that. but it doesn't take much to drive the accuracy off when you are dealing with such small thicknesses of material. So you have an allowable error before they say the crystal is not useable.

Its kind of like the crankshaft in your model engine has an allowable tollerance on its fit in the bearings. Lets use a bushing bearing type:::: If it fits too tight... the the crankshaft can't turn at all. (too tight can be fixed by hand lapping...) If its too loose you have an air/fuel leak. You will find that some bushing bearing engines off one run have a sort of tight fit... and others are a littlelooser. The tight ones wear in and are fine. The loose ones are a constant headache to tune the low end due to the air leak.

A too thick crystal can be reground a bit... but the cost of regrinding after measureing... it isn't worth it. A too thin one also could be reground to the next higher frequency... (again not cost effective...)

*********

Yes RX crystals behave the same. The RX's are typically tuned with a "perfect" crystal... and then they use a tighter allowable error... to allow you to swap crystals. However... if the TX is off one way and the RX is off the other... it starts to get hard for the RX to understand th signal the TX puts out... so they want the TX DEAD ON.

Queue

Warning...long sorta technical post for Wings

Quartz is what is known as a piezoelectric material. When a small electric current is applied to a thin wafer, it vibrates at a frequency determined by its size and thickness and the electrical characteristics of the circuit in which it is to operate. The oscillation frequency is extremely stable with regard to temperature and can be held to tight tolerances (+/- .003% or better) in mass production. In mass production, quartz blanks are metallized on each side to allow soldering of the leads which are connected to the two pins we see on our crystals. This metalization process is also used to determine the final frequency of the crystal by controlling the thickness of metalization in automated equipment. Crystals are rarely ground to freqency any more as re-grinding would require re-metalization.

Since these rystals are used to accurately control transmit and receive frequencies, the problems with "blind" crystal swapping are several. If one swapped a Futaba channel 22 with a channel 60, everything would probably be OK as the crystals both were manufactured to operate in that particular oscillator. However, TX output power may be reduced and need to be re-tuned to assure maximum range. While I have not verified it, the so-called "center tuned" radios probably adjust to the middle of our frequencies so that power level is more uniform with frequency change. Same goes for the receiver. Sensitivity may be reduced. So much depends on the electronic design of the tx and rx.

If a crystal for another radio brand was used in the swap, it could conceivably operate off its marked frequency by a channel or more as it would not operating in the circuit for which it was designed. That's assuming it would oscillate at all.

All legal issues aside, here's the take home lesson: the average crystal swapper will not have the appropriate test equipment to verify proper operation with regard to operating frequency, power output or receiver sensitivity.

Q

wings

Ok, that kind of makes sense, except for the rx being tuned with a !QUOT!perfect!QUOT! crystal. As you said, everything has a tolerance, nothing is perfect.

I kind of understand what you saying, but I am still unclear about the !QUOT!tuning!QUOT!. And I am unclear about how changing a crystal can cause such a problem. Do you actually harm the crystal when changing it, or is it a calibration thing? Is tuning actually tweaking the tx with the crystal to get the correct frequency? If this is the case, how is it !QUOT!tuned!QUOT!? Do they adjust a potentiometer or something?

Wings

Queue

Q

FHHuber

If you can find an appx 1965 or earlier (some older...) TV and get a look at the channel selctor dial inside the box. you will find that ishas a large number of "plates" spaced kind of looking like a huge heat sink. That is a variable capacitance stack. The stack is adjusted such that each detent you move the selector to corresponds to a freuency for a given TV channel. There is an outer fine tuning adjustment to "tweak" the tuning of the detents. It is ppossible to adjust that outer knob far enough that you can get channel 5 when the dial says channel 7.

The tuning capacitor in your TX operates in a similar manner. It is quite possible to tune a radio with a crystal marked for "channel" 44 to actually transmit on "Channel" 43 or 45. They can make up for that much variance in the crystals.

Now do a bit of math... 72.090 mHz times 100.003%. What's the new frequency? An ALMOST happy... 72.09218 (probably not interfereing with 72.100... yet.)

Take a TX tuned to that max deviation crystal.. and put n one off the opposite maximum. 72.090 (as tuned... because you corrected for the original) pluggin n the maximum off low end... 72.110 crystal ( actual 72.10782...) But wait... you are TUNED to compensate by .00218 high already... so you are transmitting on 72.10564! HALFWAY to the intermediate frequency which CAN be used by someone such as a pager or industrial crane! What if he's off the same amount high? then you are BOTH on essentially the same frequency!

Now make it worse... have yor RX off max high when your TX is off max low (due to crystal swap)... the RX can't see the TX at any range! (thank you narrow band...)

so there you are with your 10 ft range check, antenna extended... and making some industrial crane go nuts.

Kevin Greene

My Feedback: (85)

FHHuber,

From reading your post I don't think that you are very familiar with JR equipment....If you buy a separate JR Tx module you will get it without the crystal installed. You must install the crystal as the module is center tuned to the 72 MHz band. The crystal isn't sealed within the module either. (See the pic) This is the statement that you made that leads me to believe that you weren't too savvy on JR equipment. What this means is that the user decides which frequency is used in this instance.

So, doesn't this throw everything out the window, as far as blanket remarks are concerned, regarding crystal swapping???

To add more fuel to the fire I've included another pic straight out of the latest Tower/Great Planes catalog. Since Futaba doesn't sell receivers with crystals installed then there is NO WAY that any factory technician has tuned the crystal to the Rx!!!! When buying Futaba you simply buy an Rx on either high band or low, and then the crystal SEPARATELY for the upper or lower band as you have for the Rx. I don't know if the crystal comes installed in the Rx when you purchase it....If it does then I'm willing to bet that not a single technician has checked the tuning before it left Tower/Great Planes. My guess is that the crystal has not been installed and it is up to you to install it.

In my opinion, the bottom line is this...Concerning the upper end JR equipment when you buy an extra Tx module or receiver what you get is a center tuned item that doesn't care what frequency it is assigned as long as a factory JR crystal is installed for that specific frequency range. Regarding Futaba equipment, when you buy a receiver from them it is center tuned to either the high band or low band and doesn't care what crystal is installed as long as it is a factory Futaba crystal for that particular frequency and band width. (High or Low) To sum it up, it looks as though frequency swapping in the above cases is not going to harm a thing if you stay within the confinements of the brand you choose. I'm not at all familiar with Futaba's Tx modules regarding how they are purchased or if the crystal comes installed and factory tuned. (Or if it is sealed within it's case and not user accessible)

Additionally, I'm fully aware of the problems with Tx frequency swapping with the lower end equipment. (Tx's without removable modules that are tuned to a specific frequency at the factory) You should NEVER swap crystals in this equipment!!!! This could also include the higher end Futaba Tx modules too if they come pre-tuned with crystal installed and are sealed.

All of this info kinda makes ya think, huh!!!!

Kevin

FHHuber

Unfortunately... citing what may be the only equipment that MIGHT be OK to plug your own crystal in... is prett bad to use as an example for the people wh tend to swap crystals at will.... Generally they are the ones who do buy the cheap stuff.

(I'd hate to see it argued in court if that JR moduel with the user installed crystal caused a problems somewhere... JR and the end user might both be in trouble... THe FCC rule says change the module... but not the crystal.)

Kevin Greene

My Feedback: (85)

I understand what you are saying about trying to argue in court but let me ask you this....Concerning the Futaba receivers being sold without a crystal and relying on the user to install it---What's your take on that??? Both Futaba and JR are forcing the user to install the crystal in some instances.

I know what you mean about guys that swap crystals at will...It all goes back to knowing what you are doing and to educate those that are ignorant.

Please forgive me for coming off as an advocate for crystal swapping as I'm not for this practice. I was playing the Devil's advocate to get some of you to think about the other situations where the manufacturers are encouraging the breaking of FCC regs with their selling tactics....And to also understand that some equipment appears not to be sensative to crystal swapping.

Let me ask you this---Is crystal swapping only illegal concerning the Tx or is all crystal swapping illegal to include the Rx as well?? If only illegal concerning the Tx then this would explain how Futaba can get away with selling crystals separately for the Rx to allow the end user to change them at will.

Kevin

wings

From a manufacturing standpoint, wouldn't it be stupid not to have tx center tuned? It would save tons of time ($) in the manufacturing process. If radio manufactures are smart enough and have the technology to make state of the art radios, it would be foolish not to minimize production time and save money.

How can you find out if a specific tx is center tuned? Is a Futabe 6EXA center tuned?


Wings

AMA-69405

My Feedback: (22)

Kevin,

With respect to the JR transmitter module, I don't know where you bought the one(s) you have, but Horizon does not distribute the XP8103/PCM10X Tx module in the USA without the crystal installed. If legitimate retailers (no... eBay doesn't count) are selling them without the crystal, I'll bet Horizon would be interested, to say the least.

JR Tx modules, as well as JR non-modular transmitters, are tuned to the specific frequency of the installed crystal... they are not "center tuned".

JR receivers are "center tuned" to the middle of the 72 MHz band. At one time, JR receivers were split into "low" and "high" bands (like current Futaba receivers), but JR began center-tuning all receivers several years ago. If I remember correctly, the receivers are tuned to channel 38, which is midway between channels 15 and 60. (JR does not sell equipment on channels 11-14 of the 72 MHz band.)

It is not against FCC regulation to change the crystal in any receiver, regardless of brand, and never has been, to my knowledge. It is against FCC regs to change the frequency of the transmitter, unless the transmitter is type certified otherwise... for example, synthesized modules.

Gary L.