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[6705russell]

OK guys, I have a fleet setup in my majestic class and i am picking up bad frequency off other transmitters? I checked that we were all on different frequency's so when i got home i checked with one of my futaba transmitters on a different frequency and that too was giving the fleet set-up grief?  The receiver aerial is away from all power cables so no interference there, do the fleet transmitters/receivers have special crystals, ie. not Futaba, i wonder if there are futaba crystals in the set up? 

Any help would be appreciated

Russ

[Stavros]

Seems to me that you might have suggested the problem,electronics to me is quite like painting a boat stick to one make of paint don't mix and match and may I suggest you do the same with the crystals

Stavros

[6705russell]

Yes thanks for that Stavros, i have put a post on "Wanted" , i think they might all be obsolete now?

Russ

[barriew]

Russ,
I carried out checks with our frequency scanner recently to see which crystals I could use in which Tx - I have Futaba, Hitec and Fleet .
All the Futaba and Hitec crystals gave accurate frequency readings in the Fleet Tx. I have yet to try the Fleet Rx with Futaba and Hitec crystals.  You do sometimes get Fleet 40Mhz crystals appearing on Ebay but they are expensive when they do appear.

Barrie

[ids987]

If it is a crystal problem, it is more likely to be with the Rx crystal. Two reasons: 1) If I understand correctly, your receiver is picking up other transmitters - not just your own. 2) The Tx crystal is "on frequency" ie the crystal frequency and the Tx frequency are the same. The Rx crystal is deviated from the transmitter frequency by an I.F (Intermediate Frequency), which can be above or below (Tx + or - I.F), can be dual conversion (2 I.F frequencies - various further permutations), so there are various permutations for I.F crystals.
Could it be an AM Rx crystal in a FM Rx ? Faulty (misaligned) Rx, or Faulty Rx crystal are others that spring to mind.
Or I may just have completely got the wrong end of the stick......

Ian

[wombat]

I think that there is a good chance that the problem is more than just errors in the crystal frequency, I think it is more fundamental than that. It may be a limitation of the first stage of the reciever - there is a variable gain amplifier (AGC - automatic gain control) that controls the level of signal fed into the rest of the reciever. It is entirely possible that this is being overloaded by the more modern transmitters - sort of like trying to listen to someone in a very noisy room.

Wom

[rp6705]

Thanks for the information guys, mind boggling!  I have trawled the internet sites and forums and i think if you havent got Fleet crystals in Fleet transmitters/recievers you,re stuffed.

Russ

[barriew]

Thanks for the information guys, mind boggling!  I have trawled the internet sites and forums and i think if you havent got Fleet crystals in Fleet transmitters/recievers you,re stuffed.

Russ

Russ, Not so! I have just been checking and using genuine Hitec crystals in both the Fleet Tx and Rx (XP-FM) I had no problems connecting without interference. This was on the bench not installed in a boat. I also tried Futaba crystals - these did not work.

Have you tried the set-up out of your boat. It seems muchmore likely that the problem are not with the crystals, but it may still be a problem with the Rx.

Barrie

[malcolmfrary]

If the problem ONLY happens when another TX is transmitting, it must be an RX problem.  Probably a sensitive receiver not filtering out the unwanted signals and then doing its best to resolve them.  The simplest way is to substitute a known working receiver.
The crystals should be marked with their rated frequency, the TX crystal will show the frequency transmitted to air in MHz, the corresponding RX crystal will be offset by .455 MHz because thats how superhet receivers work.  If the numbers match, and the pins are the right spacing, it does not matter whose radio they were made for.  The value pf .455MHz is a common value for a single conversion RX, dual conversion is different and some manufacturers may have their own ideas.

[rp6705]

Will the Fleet transmitter work with Futaba reciever or is the Fleet gear paired?

[rp6705]

I have just been looking at an earlier topic on reciever aerial length, the aerial length from my reciever is only about 200mm then it connects on a short piece of thicker cable soldered to one of the masts on the ship, the mast is light gauge brass, would this have a impact on the interference?

Russ

[ids987]

If the problem ONLY happens when another TX is transmitting, it must be an RX problem.  Probably a sensitive receiver not filtering out the unwanted signals and then doing its best to resolve them.  The simplest way is to substitute a known working receiver.
The crystals should be marked with their rated frequency, the TX crystal will show the frequency transmitted to air in MHz, the corresponding RX crystal will be offset by .455 MHz because thats how superhet receivers work.  If the numbers match, and the pins are the right spacing, it does not matter whose radio they were made for.  The value pf .455MHz is a common value for a single conversion RX, dual conversion is different and some manufacturers may have their own ideas.

Malcolm,

Agreed that 455KHz is standard offset for Single Conversion. It fits in with all the nice, off the shelf 455KHz filters. I'm not personally sure though, if (amongst RC manufacturers), there is a standard for whether the Rx oscillator runs at Tx Freq + 455Khz, or Tx Freq - 455Khz. Either permutation is equally valid for a superhet receiver, but would require slightly different circuit optimisation (as well as a difference in the Rx crystal frequency of 910KHz). If the Rx crystal is the correct value of offset (ie 455Khz) - but the wrong side of the Tx frequency, it may work, but if it does work, it may be problematic.
With regards to crystals, there are also other design parameters, which affect a crystal's resonant frequency. The resonant frequency for a given crystal is specified at a given load capacitance. If the load capacitance is different, the crystal will be pulled slightly off frequency, so (again, unless RC manufacturers have standardised these things), putting a crystal designed for one Tx or Rx, into a different Tx or Rx, will not necessarily result in exactly the same frequency.
My point with regards to FM vs AM is that crystals designed for FM Transmitters need to be lower 'Q' than crystals for AM transmitters usually are. The nature of Frequency Modulation requires that the Tx frequency is "pulled" by the signal. This is really only significant at the Tx end though, so not really relevant here.
I'm interested to know of any standards for these things - which may be used amongst RC manufacturers.

Ian

[Telstar]

Ian I know 455KHz is the "normal" IF for many commercial radio manufacturers, but I remember checking  one of the Japanese RC systems (some time ago) and found they used 477KHZ. Obviously Xtals matched for this make of equipment will not work in other makes.
If the Xtals are for dual superhet RXs the the IF is normally 10.7MHz.
Also as you mentioned. Xtals made for AM TXs are not certain to work in FM TXs.

The initial problem of cross channel interference is most likely a faulty RX, but remember early 27MHz equipment was designed to run at 50KHz channel spacing and selectivity was not as critical

cheers Tom

[malcolmfrary]

If the setup works at all, it follows that the bits are all pretty much working.  However, if other users on different (possibly adjacent? we don't know for sure) channels are affecting the system, my money goes on the receiver not performing as it should and resolving information that it should be rejecting.  I would try a different receiver to see what happens as the quickest way of eliminating some of the confusion.
Once you know what the actual problem is, you can easily look back and see how the symptoms happened in the first place.

[rp6705]

Yes thanks Malcom but are any other receivers compatible with Digi-Fleet?

Russ

[HS93 (RIP)]

You may find it cheaper to ditch the fleet Gear ,any part that you can get are years old fleet do not meet modern standards and as far as I know are not CE approved , that is why they went out of the TX Game as they could not get the type approval,

Peter

[rp6705]

Yes i think i will be going the 2.4ghz way, just had a quote for transmitter/6-channel reciever and 4 servos...£150.00 which i think is pretty good.

Thanks all

[ids987]

Ian I know 455KHz is the "normal" IF for many commercial radio manufacturers, but I remember checking  one of the Japanese RC systems (some time ago) and found they used 477KHZ. Obviously Xtals matched for this make of equipment will not work in other makes.
If the Xtals are for dual superhet RXs the the IF is normally 10.7MHz.
Also as you mentioned. Xtals made for AM TXs are not certain to work in FM TXs.

The initial problem of cross channel interference is most likely a faulty RX, but remember early 27MHz equipment was designed to run at 50KHz channel spacing and selectivity was not as critical

cheers Tom

Tom,

Thanks for the info - I think I have seen 477KHz I.F filters in the past. Pretty sure you're right about 10.7MHz as first I.F for DCRxs as well - usually with 455KHz (or maybe 477KHz) as second I.F.
Sorry, I didn't mean the 455KHz to be statement of fact, so my wording was a bit off. My main point was that even if the Tx frequencies and IFs (of two systems) are the same, it doesn't necessarily mean that the corresponding Rx crystals are the same frequency (as one may be Tx + 455 or 477KHz, and the other may be Tx - 455 or 477KHz).
I was also trying to say that; even if two crystals are nominally the same frequency, they may be specified with slightly different external factors (eg load capacitance). These factors change the resonant frequency of the crystal - slightly.
There are many factors which could cause poor cross-channel rejection - especially with close channel spacing, and especially where strong signals exist. Although most of these are at the Rx end, it is possible for a transmitter to pick up a signal from a nearby transmitter, mix it with it's own, and transmit various products of the two.........

Ian

[barriew]

Will the Fleet transmitter work with Futaba reciever or is the Fleet gear paired?

Mine works with Hitec, Futaba and Micron radios.

Barrie

[6705russell]

Ok i have been testing all day with frequency checkers etc and it looks like the reciever as gone belly up, took apart and resoldered a couple of dry joints but to no avail, so i am replacing everything with Futaba gear, best to be on the safe side, down to the model shop today and picked a 6exa 6 channel set, looked at the 2.4ghz but the additional recievers were £65.00 each!, pricey when you can only use the transmitter for one boat.

Thanks guys for all the info 

Russ