crystals

[Kevin Flack 2Participant @kevinflack2]

After a number of years I suddenly discovered that there crystals that need to be installed as the TX / RX as per indication or they don’t function at all.

I am sure that I used them on some of the boats without this distingtion? or was I just lucky?

Do all 27Mhz crystals work with all sets?

 

[Kevin Flack 2Participant @kevinflack2]

[neil hpParticipant @neilhp]

i often used to reverse my crystals on 27mg if someone was on the same colour at the lakeside……..they gave a slightly different frequency, and enough to allow both of you to sail.

don’t think it works on 40mg though.

[Mike DavidsonParticipant @mikedavidson22772]

Hi Kevin, crystals aremost important as the home office has laid down accuracy figures that all radio control sets must conform tothe transmitter crystal set s the frequency that is radiated into the ether by your transmitter, and the receiver crystal tunes your receiver exactly to matcgh the transmitterthe two crystals make an exactly matched pairwithout which your radio would not workeach crystal would work in a radio of a different frequency, but the output would be very low indeed and the receiver would belooking for a very different frequency,so it too would not workThe frequencies not milligrammes but MegaHertzwere laid down by the AmericanFederal Communications Commission (Fccand were interlaced with the CB radio frequencies at exactly 10 KiloHertz apartso one could operate without being interfered with by an adjacent transmitterand if you care to look it up, the 27MHz channels are seen as gaps in the AM CB channelsand as far as I know there are two or three channels where CB and models were allocated identical frequenciesI think that the black channel was allocated to a CB channel and hospital paging systems accounting for the number of aero modellers claiming that a CB user caused a model to crash, it certainly did for me when I was giving a demonstration to disabled children in their theraputic swimming pooland I had sailed along one wall to give one section of kids to get a close viewand I was heading towards the other side to do a lengthfor the other kidswhen the boat dug her stern right in and accelerated flat out in a hard turn to starboard, sadly it collided with the tiled concrete wall knocking one propellor right off, the other engine took over propulsion and I brought her to a halt and steered her towards me so I could recoverethe wreckagewhich was not funny. I knew that there was nothing wrong with the boat so I switched off the batteriesand lifted the boat out of the water and on to her stand where she would be safeI looked out of a window, and saw a ford capri with a six foot long CB antemnna clamped to its roof parked outsideI went utside to speak to the driver and asked him what power he was using. it turned out he was using a 1000 watt linear amplifier andI warned him that he was breaking the law on several counts, but it turned out that propagation effects meant that a4 watt legal CB could reach the eastern seaboard of USA easily so lots of CB’ers were talking to Americansand loving every minute of itso I contacted the chairman of the CB club and asked if I could speak to the attendees of their next meetingat which I explained the things they could do legally without crashing radio controlled aircraft of boats, and I did the same at the model club of which I was chairman. After that, things ran a little more smoothly.I have just realised one thing you said in your post was somewhat incorrect,which wasthat you reversed your crystals if another person was using the same colour frequencythis would mean that your transmitter would send at a frequency 455Kilohertz lower than his and your receiver would produce an intermediate frequency455Kilohertz above, and not 455 below the transmitted frequencyas it should to match the settings of the tuned circuits withinthe combination could have worked I guess, but not nearly as well as if the crystals had been fitted to the correct units. I think the best way to eliminate interference is to use the peg system where an antenna would be mounted o the controller’s table with a load of coloured pegswhich you could take if you wanted to use that coloured frequencyWhat I did was to buy a nearly complete set of crystals so I could always fit a different pair of crystals to my rig and sail off into the sunset, no worriesIf you would like me to send you my paper which I wrote explainig how radio control works, send me your e-mail address, and I will send it to you as an attachment.Mike Davidson

[TelstarParticipant @telstar]

Hi Kevin

To function properly Tx and Rx Xtals  must be a ‘pair’.  Since not all manufacturers stick to the same standard, some pairs of Xtals (say Futaba) will not work for a different radio system (say Acombs) so for a given set use the correct pair. furthermore when Futaba changed to FM from Am systems they changed the matching on the Xtal pairs, so Futaba AM Xtals (normally 2 ch) don’t work on Futaba FM sets (usually 4 ch and above) .

Reversing/ swopping  Xtals is strictly speaking illegal and can cause interferance with other radio systems.  

40Mhz systems don’t have the same irregularities but still need ‘pairs’ of Xtals to work, while 2.4GHz  use a completley different system of frequency controle

www.ukrcc.org   can give british model frequencies which have nothing to do with the American system

 

Cheers Tom

[Kevin Flack 2Participant @kevinflack2]

Wow thanks for the replies I seem to have opened the proverbial can of worms with this one!

I suppose the real problem for me was having a number of different TX’s from  cheap Chinese to Acoms and better. I normally took a single set to the club prefering to change each boat setup to run each individually. This helped with the congestion due to the limited slots available at the club, at 27Mhz..

Does it also mean that each set should be kept paired for optimum use especially regarding the make?

Obvioisly there is a club board with all the available frequencies to have the required overall controls. Never had much noted interference with the best availabilty on a split frequency which no one else had. Fortunately with boats, mostly scale problems are not as serious as flying!

Now the new set is 40Mhz dual conversion FM (Hitec focus 6) not too sure of the term dual as I see single types. This set will be used for the bigest kit still to be built.

Locally we do seem to follow the British systems in South Africa, although I am not too

sure how regulated they are?

Mike contact on:  [email protected]

[TelstarParticipant @telstar]

Hi Kevin

Sorry I didn’t check your country of origin when quoting UK regulations.

 

Still with all decent radio controle combos the Rx has a build in electronic filter that blockes all frequencies exept the IF frequency, as Mick  mentioned above this is usually 455KHz  ( exept for some far east systems which use 477KHz)   so Tx and RX xtals differ by this frequency. E.g  Red/orange    Tx 27.025Mhz           Rx 26.62Mhz

This is on single conversion (single superhet) systems, (IF 455Khz)

With dual conversion, the Rx has two stages of filter, the second one is the same as for single conversion.   But in front of this the first filter runs at 10.7Mhz, This means for a dual conversion  on Red/Orange        Tx 27.025Mhz            RX 16.325Mhz.   

Dual conversion allows better selectivity (less chance of interferance form ajacent channels)

this means single conversion and dual conversion Xtals can’t be used in single conversion sets and vise versa

 

I don’t think there are 27Meg band dual conversion systems other technicalities make it impractical

Cheers Tom

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