Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Radio Equipment => Topic started by: Nige52 on June 11, 2009, 03:31:32 pm
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Could someone please explain the difference between AM and FM 27 meg crystals please? I have a set of FM Channel 4 crystals in 27.145, and another set of channel 6 crystals in 27.095 single conversion.
Needing to buy some spare Rx crystals in channel 6, I looked at what's available on ebay and found lots of 27.095 crystals, but they are orange instead of white and are also AM.....now I'm confused, are they the same or not?
Many thanks
Nige
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The difference is in the transmitter and receiver modulation, not the crystals, although they are NOT interchangeable. If your Tx is AM, you need AM crystals. If you already use both AM & FM crystals in the same Tx, then I suspect one of those sets will not be giving the correct channel, and could cause interference to anybody operating at the same time as you.
There are not many 27Mhz FM sets around, most 2 channel sets are AM.
Barrie
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I was given a 27 FM set, when I tried it at the Lake the 40 fm sets caused lots of servo twitching.
Len.
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Hi Nige, if you 'google' for the crystals you will find them although I saw one set offered at nearly £15, which is rather too much. They used to be very cheap from H.K. and 3 years ago Halfords were selling sets of 6 for a few pounds, wish I had bought some!
Big Ada, I have been using a Futaba 27Mhz FM set for many years with no problems. There should not be any interference unless possibly it is a cheap 40 Mhz set, but who knows?
regards Roy
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I BOUGHT THE SETS FROM HALFORDS, THEY WERE FOR THE LITTLE R/C CARS THEY USED TO SELL,
ABOUT 6 SETS OF 27MHZ CRYSTALS FOR ABOUT £10 AS PART OF THEIR CLEARENACE OF THE RADIO CONTROLLED STOCK-
I AM SURE THESE MUST STILL BE AROUND EITHER ON EBAY OR VIA THE WEB, I KNOW YOU COULD BUY THESE VIA HALFORDS ON LINE TOO,
IT WOULD BE WORTH CHECKING THEM OUT ON A WEB SEARCH, I'LL HAVE A LOOK LATER FOR THE BRAND NAME FOR YOU
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Thank you for the replies.....on looking at what I have, I seem to have one Tx that uses AM Crystals and another Tx that has a large "27mhz FM sticker on the back....the 27FM crystals work fine in the AM TX but the AM crystals don't work in the FM Tx....Confused? You will be...Here's me thinking that there was ONLY one 27 mhz frequency.
Have the Chinese re-invented the 27mhz thingy?
Confused of southern Spain
:o :o :o
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Thank you for the replies.....on looking at what I have, I seem to have one Tx that uses AM Crystals and another Tx that has a large "27mhz FM sticker on the back....the 27FM crystals work fine in the AM TX but the AM crystals don't work in the FM Tx....Confused? You will be...Here's me thinking that there was ONLY one 27 mhz frequency.
Have the Chinese re-invented the 27mhz thingy?
Confused of southern Spain
:o :o :o
You should get on well with Footski, he's confused down there as well. %)
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Here's me thinking that there was ONLY one 27 mhz frequency.
Have the Chinese re-invented the 27mhz thingy?
Confused of southern Spain
:o :o :o
There is only one 27Mhz range FM and AM are just different ways of modulating the signal. You cannot operate FM and AM on the same frequency at the same time. They will interfere with each other.
HTH
Doug
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Thanks Doug,
I understand that, but what I really need to know is, will a pair of FM crystals work in an AM transmitter, and visa versa? I realise that AM and FM crystals are not individually changable. It's just that I want to buy some off the Bay and am unsure which pairs to buy, AM or FM.
Cheers
Nige
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People who manufacture crystals print the cans with the frequecy of the crystal. In the case of the TX, this is the frequency transmitted to air. Because of the way RXs work, the RX crystal is offset by 455KHz, and will be printed as such.
In order to create confusion, the marketers of the crystals (ie set manufacturers) stick little labels to them. Very often, the RX is LABELLED with its matching TX frequency rather than the frequency of the crystal itself. Sadly, the only information I have been able to turn up on the web details the TX frequencies - the manufacturers are very reclusive about giving useful information, probably a facet of their continuing effort to lock you into their particular range.
If the FM sets use the same IF strip as the AM sets, the offset will be the same and same crystal will be correct for both. The difference will come in with dual conversion receivers.
Any pointers to websites with useful frequency listings gratefully received.