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All Forums >> Radios, Batteries, Clubhouse and more >> RC Radios, Transmitters, Receivers, Servos, gyros >> changing tx crystals
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changing tx crystals - 4/11/2003 5:14:57 PM   
Pete the Geek


 

Posts: 21
Joined: 3/3/2003
From: Russellville, AR
Status: offline 		Ladyflyer,

Not a problem, I too apologize if I seemed a bit insincere....the ignorance scenario I came up with was a bit of a joke, and I hadn't realized people felt so strongly about this issue. There are differences between the ignorant and misinformed..I have to agree that if someone did swap a crystal on the 72 Mhz in a very public area and caused some form of catastrophe, knowing that this could happen....well, there is no excuse there.

Strato seems to be right. There is no mention of frequency seperation...they all seem to be subject to crystal swapping penalties....yet you are about the fifth person who has told me that there is nothing illegal about swapping crystals on 27 Mhz frequency. I am not concerned any more about my own set up, I am just curious as to why there are so many varying opinions on what constitutes a legal radio....seems like this should be a lot more clear cut.

Thanks

(in reply to flyboy11)
       Post #: 51

Whooooa there - 4/11/2003 7:28:59 PM   
FlyinBrian


 

Posts: 54
Joined: 4/8/2003
From: NorthamptonNorthants, UNITED KINGDOM
Status: offline 		I don't think Ladyflyer was pointing fingers just giving the reasons pretty suckcinctly - (u probably notice my spelling is er -unique) Thanks LF

I fully agree that information is the way forward - the RC manufacturers should give "writ LARGE" the do's, dont's and the WHYs. However - Hitec and other RC manufacturers operate world wide and have to cope with regs in many countries so they try to produce a "catch all" r/c set that meets the needs of all. Hence if a blob of glue on the xtal makes the set sellable in the USA (as opposed to taking out the socket on the PCB and soldering the xtal direct) then thats what the will do.

Remember who meets the cost of any aditional work required to make a set legal in the US - The Customer!

It is also quite possible that some sets sold in the US were actually produced for other markets but someone bought em cheap and brought them into the US.

Just for clarification this is my understanding of XTAL vs MODULE

XTAL = a small 1/2 x 3/8" or so square silver metal can with two prongs on the bottom that usually has a little tag attached displaying the band/frequecy number

MODULE = a Plastic box c 2" x 1" x 3/4" with clips and a multi pinned socket that mates with a plug in the rear of the Tx case.
May have a sticker on it denoting the frequency (before it becomes fuel soaked and falls off - unless u fly electric)
When installed the module in effect becomes part of the case - you dont need to open the case to change a module, you normally but not always have to open the tx case to change a xtal.

I'll leave this thread now as it shows some signs of getting personal - a great primer on all things R/C can be found at -

http://www.torreypinesgulls.org/Radios.htm

I'm going flyin

(in reply to flyboy11)
       Post #: 52

changing tx crystals - 4/12/2003 12:58:23 AM   
Ladyflyer



Posts: 1035
Joined: 1/13/2002
From: North Am, PA, USA
Status: offline 		[QUOTE]Originally posted by Lynx
It's funny that the only reason this whole thread exists is because the FCC made illegal something that people did before (and still do now anyways) illegal because there's a slight chance a detuned xmitter will broadcast on the channels next door, . [/QUOTE]


A detuned transmitter is not the primary concern. An improper crystal CAN and wiil cause interference to industrial users. It has never been legal in the US to swap crystals on the 72 / 75 mHZ band as far back as I remember. The rules were in effect LONG before there were any fees in place . Remember our use iof the frequency could go away with the stroke of a pen . The vigilance is up to us.
The CB (27 mHZ band is covered under different rules than the shared 72/75 mHZ frequencies . Any interference there would be to a CBer who may be running 1000 times the legal 5 Watt limit himself.

_____________________________

LF
When they have to say "NO FEAR" it really means NO SENSE.

(in reply to flyboy11)
       Post #: 53

changing tx crystals - 4/12/2003 7:23:40 AM   
Lynx



Posts: 1374
Joined: 9/19/2002
From: Rochester, NY, USA
Status: offline 		If a detuned xmitter isn't the concern then why is it legal to swap crystals if it's properly tuned by a tech? I mean that's WHY it causes interference to industrial users because it's slightly tuned off channel when crystals are swapped without retuning the capacitors in the xmitter to center of that slightly different frequency. The same thing happens with receivers which is why they can lose range if it's not peeked for the crystal that's in it. It's just not illegal in a receiver because they can't cause interference. Makes me wonder why they don't just mandate that the transmitters require circuitry to determine if they were in tune or not, and adjust for it. Oh wait.. Don't they call that a PLL circuit? =) Why isn't the RC industry so far behind the rest of the world? You can find better tuners in 50 dollar walkmans =\

_____________________________

-Curiosity may have killed the cat
-But that's why they have nine lives

(in reply to flyboy11)
       Post #: 54

changing tx crystals - 4/13/2003 4:31:49 AM   
Ladyflyer



Posts: 1035
Joined: 1/13/2002
From: North Am, PA, USA
Status: offline 		Not really Linx ,the MAIN problrm is operation with an improper crystal. The tolerance of tuning will keep the frequency within the bandspace the normal signa; would occupy. Many RF decks have NO TRIMMERS for "pulling "the crystal and there is little hope of pulling it 10 kHZ off. Harmonics from a mistuned TX would likely be too low power .

The tech is to make certain the crystal is the proper crystal ,on a legal RC frequency . From many of the posts here it should be quite apparent that people come up with old color code channel crystals that are spot on industrial freqs. They try to use RX crystals without any idea where they will be transmitting . THOSE are the concern. A mistuned transmitter's threat is only a fraction of the potential problem. Again I said MAIN concern .

The main thrust of tuning is to ensure the oscillator actually starts every time. The successive stages MAY need a little tweaking. A transmitter operating well on one freq will usually be quite happy when shifted to another without tuning. More harm is likely to be done by an inexperienced hand TRYING to tune the stages than could ever be done simply putting a PROPER crystal in place.If the oscillator is peaked for max output there is a chance it will not start every time. Most tuning requires backing off just a touch from peak output. All of the other stages that require tuning are present in the synth RF decks the same as the crystal RF decks. The common misconception that the TX will be off freq is NOT the root of the concern .The real off freq threat is from an illegal crystal. And it certainly wasn't money as so many assert ,since the rules were in effect long before fees were charged.

If you really MUST swap ,MAKE SURE you have the right crystal .
The tuning is a minor issue by comparison.

NOTE the other countrie's transmitters make the swaps without tuning just fine . Those folks run closer together than we do. They use the same RF deck design we use . IT WORKS ! We don't allow it for the reason I stated , the likeyhood of problems from some one taking the 72.160 crystal out of an olf MRC transmitter they bought at a garage sale and keying up on the crane in the steel yard at the end of the street . The receivers are a non issue that merely serve to prove they (FCC) don't really care how well your transmitter and receiver talk to each other.

I remember ,I was there 20+ years ago

< Message edited by Ladyflyer-RCU -- Apr 12 2003 11:46PM >

_____________________________

LF
When they have to say "NO FEAR" it really means NO SENSE.

(in reply to flyboy11)
       Post #: 55

changing tx crystals - 4/13/2003 7:56:33 AM   
Pete the Geek


 

Posts: 21
Joined: 3/3/2003
From: Russellville, AR
Status: offline 		On a semi-related note, my friend brought his airtech stealth over today for me to have a look at....when he was operating the transmitter (49 mhz), my computer speakers started making this weird humming beeping noise...transmitters are curious things indeed!

Pete

(in reply to flyboy11)
       Post #: 56

changing tx crystals - 4/13/2003 7:39:11 PM   
Ladyflyer



Posts: 1035
Joined: 1/13/2002
From: North Am, PA, USA
Status: offline 		As are non receivers when they act like a receiver . Many audio devices will demodulate a strong RF signal . Ham operators have to deal with disgruntled neighbors who pick up their signals on teephones ,answering machines ,TVs ,Stereos and loose dental fillings. Once the RF is rectified it can be easily heard and amplified. The old style telephones were pretty bullet proof but the new cheap imports have plenty of solid state devices built in that are prone to problems.
The FCC states that the interference is the fault of the receiver in most cases. They almost always side with the Ham operator .

The same basic concept applies to unlicensed RC operations. We CANNOT generate any interference . We must stop operating when we are found to be generating interference with LICENSED operations. At the same time we must accept any interference generated by a licensed station.
Being a squatter has it's downside. The bright spot is we can get Ham licenses and operate on 50 mHZ.

_____________________________

LF
When they have to say "NO FEAR" it really means NO SENSE.

(in reply to flyboy11)
       Post #: 57

changing tx crystals - 4/16/2003 3:22:50 AM   
quemazon


 

Posts: 64
Joined: 4/16/2003
From: Northern New Mexico, NM,
Status: offline 		It looks like I'm just getting in on the end of the discussion here, but I wanted to put in my $.02 anyway.

When I lived in Champaign, IL, one of my good friends was a Futaba tech who literally spent 8 hours a day retuning TX's that had been sent in for a channel swap. He told me that off-the-shelf TX's weren't really tuned to the specific frequecy that they came on. A channel-specific tuning only happens when you send your TX in for that specific task. Before that point, your stock TX is tuned the same as the rest of the TX's coming off the line. It just has a differnt crystal in it and a different sticker. He viewed his task as more of a general TX tune-up than a frequency specific tuning. Of course he did set it on the desired frequency when doing the tune up, but he said that it didn't really make any difference.

As far as I'm concerned the no-crystal-swapping rule is an artifact from a time when manufacturing processes and circuit design were not as refined and robust as they are today. AFAIK my channel 16 TX was never actually tuned to channel 16, so putting a channel 38 crystal in it doesn't increase the probability of it being de-tuned. Crystal swapping doesn't cause radios to interfere with other frequencies.

Is crystal swapping illegal? Sure. Will your radio cause interference because of it? No.

(in reply to flyboy11)
       Post #: 58

changing tx crystals - 4/21/2003 8:57:39 PM   
Ladyflyer



Posts: 1035
Joined: 1/13/2002
From: North Am, PA, USA
Status: offline 		[[COLOR=blue]QUOTE]Originally posted by quemazon
It looks like I'm just getting in on the end of the discussion here, but I wanted to put in my $.02 anyway.

When I lived in Champaign, IL, one of my good friends was a Futaba tech who literally spent 8 hours a day retuning TX's that had been sent in for a channel swap. He told me that off-the-shelf TX's weren't really tuned to the specific frequecy that they came on. A channel-specific tuning only happens when you send your TX in for that specific task. Before that point, your stock TX is tuned the same as the rest of the TX's coming off the line. It just has a differnt crystal in it and a different sticker. He viewed his task as more of a general TX tune-up than a frequency specific tuning. Of course he did set it on the desired frequency when doing the tune up, but he said that it didn't really make any difference.

As far as I'm concerned the no-crystal-swapping rule is an artifact from a time when manufacturing processes and circuit design were not as refined and robust as they are today. AFAIK my channel 16 TX was never actually tuned to channel 16, so putting a channel 38 crystal in it doesn't increase the probability of it being de-tuned. Crystal swapping doesn't cause radios to interfere with other frequencies.

Is crystal swapping illegal? Sure. Will your radio cause interference because of it? No. [/QUOTE] [/COLOR]


................ I'm not sure you have been reading what has already appeared here on the subject.
The no swapping law is not a mere holdover. Again,it was and does serve to prevent the WRONG crystal from getting into a transmitter and causing interference to LICENSED operations.
There has been no modernization of the RF decks in transmitters that make the old rule obsolete. The RF decks in the current crop of transmitters is essentially the same as the old pre 1991 breed. The modulation (deviation ) has been limited to decrease bandwidth ,otherwise if anything the RF decks have been cheapened but not materially or functionally improved..

You are correct about the tuning aspect. The tuning is not the main concern.

A month or so ago I posted a list of SOME of the LICENSED users in Cook County Illinois that we SHARE the band with.The list had more than 200 users. The radios are used to control various industrial processes. They control alarms,emergency pumps and overhead cranes among other uses. Many of those users were spoit on the OLD RC freqs. If one of those old crystals or some other odd crystal were to be used in an RC transmitter we could generate harmful interference.

Responsible operation does not involve exempting oneself from the rules. No matter where you live or fly the law applies equally to everyone. No ,it is not yesterdays law,it is still with us .

_____________________________

LF
When they have to say "NO FEAR" it really means NO SENSE.

(in reply to flyboy11)
       Post #: 59

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All Forums >> Radios, Batteries, Clubhouse and more >> RC Radios, Transmitters, Receivers, Servos, gyros >> changing tx crystals
Page: <<   < prev  1 2 [3]





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