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Difference between high and low freq. bands

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Old 03-04-2006, 10:34 AM
  #1  
siuhunter
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Default Difference between high and low freq. bands

Can you interchange crystals on receivers or do they have to run in the high or low freq. channels exclusively? Are transmitters set up the same way?
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Old 03-04-2006, 11:00 AM
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carrellh
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Default RE: Difference between high and low freq. bands

The high/low band issue applies only to Futaba and only to 72 mhz receivers.
Low is 11-35 center is halfway between 23 and 24
High is 36-60 center is halfway between 48 and 49
The receivers are optimized for reception at the center of each band.

I personally wouldn’t change a receiver to the far end of the other band but probably would be willing to try it down to 30 or up to 40. If it range checked good I’d fly it. My transmitters are on 47 and 49 so I’d never try using a low band receiver with them. It might work perfectly but I’ll never know.

Transmitters are a different story. All of the manufacturers say it is illegal (in the USA) to change a transmitter crystal without having it checked, and retuned if necessary. This statement nearly always starts an argument about the regulations and the way they are interpreted. I’m not a lawyer and don’t care to read the regulations. If I need a different transmitter frequency I’ll either send one in to get it changed or buy a new one. Always need a good excuse to buy something new.

If the transmitter uses RF modules instead of crystals you can change modules at will and any channel from 11 to 60 will work just fine.
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Old 03-04-2006, 11:01 AM
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Default RE: Difference between high and low freq. bands

Siu,
Please be a little more specific about the band you are talking about and the brand of radio.
Futaba has a high and low receiver within the same band. I would guess this MAY prevent crossing xtals.
From my HAM radio days I don't think there would be a problem. The tunning would be more specific and increase sensitivity some.
Hitec does not play this game.
You can't change the crystals in a transmitter. This must be done by a licensed technician.
When they change the xtal they also re-tune the radio for maximum output.
I hope this helps,
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Old 03-04-2006, 11:05 AM
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Default RE: Difference between high and low freq. bands

If you are speaking of the high and low channels for Futaba receivers in the 72mhz range, you are best staying within a channel that the receiver was tuned for. Futaba does this to give their receivers a bit more accuracy. While it is possible to use a low channel in a high channel, it would be best to not do this. If you really need to, send the receiver to a service center to have it retuned.

As far as transmitters go, within the US and many other countries, crystal changes can only be done by qualified technicians who have the necessary training and equipment to test and retune the transmitter. Radios with pretuned RF modules are an exception, since the module has already been tuned.
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Old 03-04-2006, 11:13 AM
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Default RE: Difference between high and low freq. bands

ORIGINAL: KW_Counter
You can't change the crystals in a transmitter. This must be done by a licensed technician.
Actually post #2 answered the question very well, I'll start the arguement though. The regs state a qualified person as there is no liscense to change crystals. It is somewhat vague as to what a qualified person is and that is where the arguement starts. Most would agree that the average end user is not qualified and probably does not have the equipment to test the transmitter. While you may get a good range check after switching crystals you may also be interferring with other channels and hence the need to test.

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Old 03-05-2006, 09:18 AM
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Default RE: Difference between high and low freq. bands

Thanks for the responces. I have a channel 34 Futaba FB-T4NBL transmitter w/ an extra Rx crystal and saw a used reciever at a good price with a channel 40 crystal in it. I was just hoping I could change them out. By what Carrol said, it seems I can, it's just not the most recommended for optimal signal reception.
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Old 03-05-2006, 09:37 AM
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Default RE: Difference between high and low freq. bands

Basin Bum,
When I was involved in the electronics industry, 30 years ago, the FCC did have a licensing program.
This was especially evident during the CB craze. We didn't have all of this high band unlicensed usage we have now.
I understand things have changed a lot since then.
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Old 03-05-2006, 10:10 AM
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Default RE: Difference between high and low freq. bands

ORIGINAL: siuhunter

Thanks for the responces. I have a channel 34 Futaba FB-T4NBL transmitter w/ an extra Rx crystal and saw a used reciever at a good price with a channel 40 crystal in it. I was just hoping I could change them out. By what Carrol said, it seems I can, it's just not the most recommended for optimal signal reception.
I would try it without hesitation. You can hook everything up and do a preliminary range check without installing it in a model.
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Old 03-06-2006, 12:48 AM
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Mike Baldwin
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Default RE: Difference between high and low freq. bands

Last summer I had a crash with my Hobbico Extra. It was on its madien flight. I had set up a few different planes with serveral differnt radios, all Futaba 127 receivers. I did not know about the low and high desination on the receivers. The plane fucntioned fine on the groud, I did a radio check at about 30 yards with the anntenna down and every thing seemed fine. I put the plane up with a nice long slow climb and as it got about 75 yards away it sudenly banked to the right and dropped to the earth, it was like I had not imput into the plane after this. No one at the field could figure out what happened. Im not saying that it wasnt pilot error, but I dont know. Later upon going through the plane I noticed it was one of my high band receivers and I was using my channel 24 radio. I did some testing with the two systems and when using a low channel in a high band receiver your get twitches and glitching at half the distance than with the proper high band in the receiver. I still dont know if this is what caused the crash but it is all I could think of. Any one else thy this.
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Old 04-18-2006, 10:31 AM
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Default RE: Difference between high and low freq. bands

I just got a Sukhoi 26 from a Goldberg kit from my dad. He had never flown it It has a gas engine in it (18cc) and every time the engine fired up, the control surfaces and throttle would fluctuate when doing a ground test. He had it on channel 39. When we tried it, we replaced his crystal with my channel 18 crystal and there was no fluctuation at all. I'm currently looking at buying a computer transmitter so I can have all my planes on one transmitter. The other two are on channel 24 but there are a few guys who are normally at the field at the same time as me that also use 24 so I'm thinking of moving to 17 or 27 where there should be less need for the freq. card between members. I thought of going to 54 since they seem easier to find crystals for but that's when the thought of the high/low stuff came into play. Does anyone know if the Hitec Electron 6 receiver does the same high/low designation?

Andy
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Old 04-18-2006, 10:47 AM
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Default RE: Difference between high and low freq. bands

Only Futaba uses the high/low designation on their receivers.
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Old 04-18-2006, 10:55 AM
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Default RE: Difference between high and low freq. bands

For 10 or 15 bucks you can have the receiver tuned to exactly the center of the channel you are going to use. There's no physical difference in the high/low recievers, just in thier tuning and they can be center tuned to any channel in the 72mhz band.

This serves 2 purposes and is something I do with EVERY used reciever I purchase regardless of high/low band (which has been done away with in Futabas recent offerings by the way)

First it optimizes performance.

Second its a good check to make sure there isnt something wrong with the reciever hence the reason it was being sold off.

I've had 2 come up with bad boards or loose components by doing so and in spending 15 dollars probably saved a couple of hundred (i fly helos, they arent cheap to fix)
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Old 04-18-2006, 04:16 PM
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Default RE: Difference between high and low freq. bands

Ditto with other posts. You can change your RX xtal all you want. Only Futaba uses the high/low tuning on RXs, but even there, you could put a low band xtal in a high band RX. While totally legally, it isn’t recommended without retuning. TX crystals cannot be changed legally, hence Futaba doesn’t offer high/low band TX’s since each TX or TX module is tuned for one exclusive frequency.

Basin and others... the regs are very clear on who can change the xtal, and in a way you answered your own observation. The actual “qualified person” is vague as it was intended cover a broad range of qualified persons who could change the XTAL; however, the specs to which the TX must be tuned to is not vague. If the user cannot verify their TX stayed in spec after the XTAL change, they are not qualified to make the XTAL change. This is pretty clear, if you cannot verify the TX stayed in spec, you are not qualified to make the change. It is almost one of those things that if you have to ask “What are the specs?” or “What equipment do I need?”, you’re not qualified. It is much easier just to do what I did, get a synth module… problem solved.

Cheers
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