Crystal confusion! Why isn't anything simple!?

[spiff84]

hi

 Until recently i have used Futaba and Acoms AM 27mhz radios with Futaba and Acoms AM 27mhz receivers and a Futaba 40mhz FM transmitter with Futaba 40mhz FM receivers. I thought it was simply the case that 27mhz is always AM and 40mhz is always FM, with all transmitters, recievers and crystals on these respective bands happily working together. Unfortunately, like most things in this world, it looks like it's more complicated than first impressions.

 

I thought it was about time i got some decent radio gear, and due to reports that brushless setups and 40mhz can lead to interference have just bought a 27mhz Losi Sport Radio (rebranded JR XR3i) and a LRP Phaser 27mhz receiver (the older type, not th e newer Competition or Sport models). When they arrived i noticed that both say FM on them. The receiver is labled FM, but no signs of what mhz.

I tried my AM 27mhz crystals and both my AM 27mhz transmitters with the LRP receiver with no luck. So i slapped in a FM 40mhz crystal and used my 40mhz radio and it worked fine.

 

As for the Losi Transmitter, i tried it with AM crystals and a Futaba AM receiver and it seems to work ok, i couldn't get the Acoms AM receiver to work with it, it would do some strange things like go half throttle when i turned left with trigger on neutral [:S]

 

My questions are:

1. If i buy a set of FM 27mhz crystal, should my LRP receiver work fine, or does the fact that it works on 40mhz mean it's been labled wrongly by the Ebay seller and is actually 40mhz only? (in other words, do some/all FM receivers work on both 27mhz and 40mhz FM, depending on what crystals you put in them?

 

2. Is using 27mhz AM crystals and receivers with my Losi FM transmitter going to damage it or lead to reception problems?

 

3. Depending on answers to the above questions, anyone got a 27mhz FM crystal set for sale at a resonable price or does anyone want to buy a 40mhz Phaser or 2x Futaba FP-R103F? []

 

 

 

Sorry for the long winded explaination. Thanks in advance for any help []

[fatboyslim]

In the UK 27mhz is only available on AM as 27mhz FM is the legal CB band

40mhz is available in both AM and FM

The fact that it works ok with 40fm crystals would make me think the gear you've bought is 40FM as it's not possible to just swap crystals as the 'set' is tuned to a particular frequency band

[terry.sc]

In the UK 27mhz is only available on AM as 27mhz FM is the legal CB band

A section of 27mhz is allocated to radio control models. Whether AM or

FM makes no difference on the exact frequencies allocated to models.

27mhz FM radios are becoming more common and the narrower band means

less interference than the old wide band radios.

27mhz is also allocated to CB radio, but restricted to FM and the exact

frequencies are offset from the R/C frequencies so in theory they

should not interfere with each other.

[terry.sc]

I can't help with the exact problem, but from my experience 27mhz AM

radios can use any crystals but 40mhz FM radios usually need the

manufacturers specific crystal to work properly. I would therefore

presume that 27mhz FM transmitters need the correct crystal.

FM and AM work in totally different ways, they aren't compatible at all and will never work together.

[spiff84]

Thanks for that info you 2. I think you're probably right fatboyslim: the newer Phaser Competition is sold as a 27mhz and 40mhz model separately, so i doubt this older model has the abiltiy to span both bands. Looks like im stuck with a transmitter i like on 27mhz and my 3 good receivers on 40mhz....... d'oh.

I've never used a pistol grip radio before, so if i take to it i'll have to sell the receivers. I chose the Losi Sport 'cause it was dirt cheap on Ebay but lots of features like digital trims and 4 model memory. About the only stick radios i fancy are the KO Vantage and the Futaba 3VC, both are quite costly, even on the 'Bay.

 

Anyone know if using 27mhz AM crystals in a FM 27mhz transmitter is an issue? I would have thought it would be, but as i said above, it works fine on the little practice i've had so far........

[theshopkeeper]

If it works, it'll work.  But the key thing is that you couldn't be sure exactly which segment of the band you're using.  Sometimes you can use a 40mhz AM crystal set in a 27mhz AM radio - provided you use both tranmitter and receiver, it works but you'll never know exactly what segment of 27 you're really running on.

Depends if you believe that a crystal is an intellegent bit of technology that actually knows which band it's transmitting on or just a frequency shunt.