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Crystal swap questions

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Old 08-23-2004, 12:27 PM
  #1  
picojeff
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Default Crystal swap questions

Two questions.

I know that if you switch a crystal in a tx, especially if you change the frequency by a lot, it can cause problems, and you're not supposed to do it without having an expert make sure it's still in proper functioning order.

First, where does one find a technician to go over my tx when I switch the crystal? Only at the manufacturer, or are there other people who can check it out?

Second, does the same hold for the rx? Or can I switch the crystal in that without worrying?

I'm asking because I've noticed my range is very good with no other tx's operating nearby, but once a couple are turned on at the field my plane starts responding slowly to my inputs, especially when it's farther away. (It's a JR XT8103-DT PCM tx and JR649PCM-S rx.) I believe one fly-away I had was due to this, and I'm too late in realizing I shouldn't have changed frequencies. I'd rather stick to the freq I'm now on, but I'm not flying again untill I get everything thoroughly checked out.
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Old 08-23-2004, 12:43 PM
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Tall Paul
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Default RE: Crystal swap questions

Your symptoms may not go away with a crytal swap.
It sounds more serious.
I'd send the transmitter and receiver back to the manufacturer.
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Old 08-23-2004, 01:00 PM
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Stick Jammer
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Default RE: Crystal swap questions

Right from Tower's website:

Special Notice about Frequency Changes-
*72MHz- Low band channels 11 to 35; High band channels 36 to 60.
Receiver crystal channels (11-35 or 36-60) Within Each Band
Can Be Changed, and Do Not Require a retuning of the receiver.
*72MHz Frequency Band (low or high) for the receiver Can Not Be
Changed from low to high band, OR from high to low band without
sending it into Futaba Services for retuning.
*50MHz, 72MHz, 27MHz and 75MHz Frequency Bands Can Not under any
circumstances be interchanged with each other. 72/75:receiver Must
be sent in for retuning to the appropriate band. 50/27: can't do.
*50MHz, 27MHz and 75MHz receiver crystals Can be changed anywhere
within the respective bands. There are no high or low bands such
as the 72MHz band has.
*Transmitter Crystals Can Not be changed to another channel except
by a licensed service center. It is against FCC regulations, and
illegal for the user to change a frequency determining component
of a transmitter. If the radio is not retuned, it can be a hazard
spectators, modelers or property, and void AMA insurance coverage.
*Check with the local R/C club for the frequencies used in your area.
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Old 08-23-2004, 01:03 PM
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Default RE: Crystal swap questions

I've seen many recommendations for Radio South (http://www.radiosouthrc.com/) for repairs, tuning, etc. I'd definitely send transmitter and receiver somewhere for thorough checkout.
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Old 08-26-2004, 07:31 AM
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Default RE: Crystal swap questions

Stick Jammer, That is Futaba info with low and high, he's using JR.

I think the reciever would be more suspect from what you described. Another question is if the other guys transmitter is accurate or is he the one causing the problem? Maybe when you changed frequencies you got on a channel where someone else is bleeding over. Do the same guys fly when you are having theis problem?
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Old 08-26-2004, 04:09 PM
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Default RE: Crystal swap questions

if your system range checks ok, then your xmitter is ok. possibly your narrow band reciever is not working proprly. more than likely someone elses transmitter is not tuned properly. this happens when people change transmitter xtals and dont retune the xmitter. you might have to have your reciever checked out before you approach the offender, if you can find him.
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Old 09-13-2004, 04:20 PM
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BeauCC
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Default RE: Crystal swap questions

Where are these elusive FCC regulations that prohibit the changing of tx crystals? I have been looking on the FCC regs. website for a couple of hrs and can't seem to find anything. It irks me that everyone that I have spoken with has just "heard" that it is illegal. I would like read this somewhere so when someone asks about it I can say "I read it ".
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Old 09-13-2004, 05:38 PM
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Jim Schwagle
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Default RE: Crystal swap questions

This is an old, old topic that has been flogged so many times that the whip is worn out. Do a search here on RC Universe and you will find enough reading material to last you a long time. I know I've seen the FCC regs posted here as well as other people's responses. It's all here for the search.
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Old 09-13-2004, 05:38 PM
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Mac_Man_UK
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Default RE: Crystal swap questions

Apparantly its in here.......


http://ftp.fcc.gov/Bureaus/Engineeri...98/47cfr95.pdf

Pages 23 - 31 cover RC control.
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Old 09-13-2004, 08:13 PM
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Default RE: Crystal swap questions

BeauCC,
I'm with you! I have been asking for years for someone to show me the FCC rule which disallows changing transmitter crystals. I have read all the rules I can find and the closest I can find is 95.209 (d) which says:

"(d) You must not make, or have
made, any internal modification to a
certificated transmitter. (See R/C Rule
22.) Any internal modification to a certificated
transmitter cancels the certification,
and use of such a transmitter
voids your authority to operate the
station."

On my Futaba 4vf, the crystal is clearly accessible from the outside of the case. I don't see that as an "internal modification".
The 4vf manual talks about replacing the crystal so I presume it is legal.

On my Futaba 8uaf which uses frequency modules, the crystal is sealed inside the module. I would say changing that crystal is an "internal modification" and thus against rule 95.209d.
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Old 09-13-2004, 08:41 PM
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Default RE: Crystal swap questions

the xtal may be accesable but it is internal
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Old 09-13-2004, 09:04 PM
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Default RE: Crystal swap questions

Check 95.222, I think it is, states what you "can" change. Rule 2. You can change a module that is specifed for the transmitter. Does not say you can change a crystal.
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Old 09-13-2004, 09:05 PM
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Default RE: Crystal swap questions

Would that apply to batteries as well? They are more internal than the crystal.
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Old 09-13-2004, 11:32 PM
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JohnW
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Default RE: Crystal swap questions

72MHz RC transmitters operate under the part 15 regs. Part 95 has other info specific to R/C use, but the core is under part 15.

Part 15.1 a,b,c explain the conditions on how one can operate transmitting equipment without liscense. If the part 15 rules for complaince are not followed or met, you cannot operate the equipment.

Part 15.15b states that user controls cannot adjust the device in such a way that it violates any part of the regs.

Further sections list the nuts and bolts of RF criteria for compliance, such as radiation limits, etc. Part 95 list additional info on how, where, etc the equipment can be used.

Here's the problem, the equipment was certified by the maker to meet part 15 compliance when tuned to a particular freq. Change the XTAL, and the device is no longer certified until it is retuned. By changing the XTAL, you have modified the RF circuit, which is not allowed under Part 15 (or Part 95 for that matter), i.e. the device is no longer in compliance.

The XTAL is NOT intended as a user control. The fact that it is sticking out the front does not mean it is a user control. The vendor will state claerly in the manual that the XTAL is not a user control and cannot be changed by the user. But this does not absolve the vendor and the FCC may eventually require the vendors to epoxy the XTAL to the board inside the TX, esp if there are a lot of problems with bad TXs. Other equipment has had a similar fate, most notibly retunable hand held 2-way radios. The FCC recently would not certify certain radios until changes were made such that it was near impossible for the user to reprogram the freq.

Modules are user exchangeable becuase the module is a self contained RF transmitter. Each module is individually tuned to meet compliance.

Cheers
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Old 09-14-2004, 08:31 AM
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Default RE: Crystal swap questions

If the transmitter complies with Part 15, it must be so marked on the transmitter. I can find no such markings on either of my transmitters. I don't believe that 72MHz RC transmitters operate under Part 15 rules.
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Old 09-14-2004, 10:45 AM
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JohnW
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Default RE: Crystal swap questions

Umm.... I don't doubt your radio doesn't have the sticker, but it should. Part 15 applies to all RF devices operated without liscense. Unless your radio is HAM or something it needs Part 15 compliance to operate in the States. Don't need to have sticker to comply as placing the compliance statement in the manual is acceptabe sometimes. There is some flexibility given to the radio maker based on the size of the item. I'd think a TX is big enough for the sticker, but who knows. Besides, small potatoes like R/C transmitters could easily sneak in to the States without the compliance sticker affixed. I see PC equipment nearly every day that lacks the proper Class certification, but it still got in the states just fine.

My radio (I just checked it again) has a Part 15 sticker on it. I just looked online at two other radios, both had Part 15 compliance listed in the manual. I wouldn't be suprised if there are radios without the sticker. However, if people really want to know from the horses mouth, I have FCC sources (both of my parents work for FCC field offices.) I can ask them to be sure, but it will be a week as they are in Iceland on vacation. But, if you read the very first section of Part 15, IMO it should be dead obvious that R/C transmitters must comply with Part 15 as they are unliscensed emitters. Cheers.

EDIT:Oh, was going to mention, my TX is a Futaba 9Z. The two other Futaba radios I checked (via online manuals) were also listed as Part 15 compliant.
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Old 09-14-2004, 11:15 AM
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Default RE: Crystal swap questions

Please do ask your parents when they return from vacation.

The Futaba 9z manual states that the receiver complies with part 15 of the FCC rules.

I am only concerned with changing crystals within transmitters. The 9Z also uses modules and does not have the crystal accessible by the user.
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Old 09-15-2004, 01:25 AM
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Default RE: Crystal swap questions

Your right, the part 15 blurb was just for the RX, surprised I missed that. I deal with Class certification (Digital A & B) at work somtimes and somehow a brain cell misfired and I made a bad connection.

So I looked again at part 95. I think you found the core issue. There isn't exactly anything that exacly says a XTAL swap is prohibited, but if you put togehter several parts it does say that. Look at 95.209, 95.221, 95.222 and 95 sub E. Togehter, it seems the only internal modifications can be made by "authorized" person that can verify the TX is still within operational limits as described in sub E. They did exempt user modules certified by the vendor. They did not exempt XTAL changes. Batteries weren't listed as excluded, but I won't even bother asking my parents about that one for fear they will make fun of me for years. My take is that anyone with the proper skill and test equipment could change thier own XTAL and stay in compliance with Part 95. So, if you have the skills and have the ability to test RF power is at or under 0.75W and verify you are on freq to the 0.002% or what ever Part E said, you can probably change the XTAL yourself. I'm sure most HAMs could do this (or at least fine a bud that has the proper equipment.) But for the rest of us, we need to secure services with a qualified tech to XTAL swap. I will ask my mum when she gets back to be sure.

I also found it interesting that part of Part 95 covered periodic maintenance. I've been shot down by several old and out of tune TXs. Think I'll print that part out and post it in the clubhouse.

Cheers.
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Old 09-15-2004, 07:38 AM
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Default RE: Crystal swap questions

ORIGINAL: JohnW



The XTAL is NOT intended as a user control.
...unless you live outside the US


dave
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