carl24bpool

Hi guys,

ive just bought a brand new trainer off eBay and it came with 6ch receiver and 4ch tx which is a futaba 4ex 35mhz.

i was hoping to pop the crystal from the futaba into my hitec and control the receiver with all 6 channels.

when I put the crystal in the module on the back of the optic 6 and turn it on it makes the test servo start shuddering all ofer the place and it stops as soon as I turn the tx off.

have I got a compatibility issue? Is it due to the module on the optic 6? Does it change the signal or something?

Redback

Carl,

I am not sure (JR man myself) but I don't think Futaba crystals are compatible with Hitec.

Terry

carl24bpool

I always thought crystals were crystals?

carl24bpool

Also if crystals aren't compatible does that mean I have no chance of controlling the futaba receiver with my hitec tx?

its all 35mhz so surely this is possible?

carl24bpool

I've done some reading into it and it looks like I need to use manufacturer specific crystals and I think my hitec tx is dual conversion. Des this mean I need a dual conversion rx?

I guess my easiest option is just to buy a hitec rx like the one in my other plane.

JohnBuckner

My Feedback: (1)

Crystals are definitely not just Crystals. Crystals cut for a duel converstion receivers are not interchangeable with crystals cut for single conversion receivers.

The old best advice was to always use the same brand crystal as receiver for example but just before the major shift to 2.4 some years back some brands (both futaba and hitec) included Rxs that included some dual and some single conversion requiring specific crystals. In the case of Hitec their dual conversion crystals have thin blue stripe on the pull tab. Their single conversion crystals have I believe a thin red stripe on the pull tab.


John

j.duncker

My Feedback: (2)

Futaba and Hitec crystals are not interchangeable.

To do what you want you need to acquire a Hitec crystal for the same freq that your new trainer is on.

rgburrill

If you get the same frequency crystal for the Hitec you should be able to control that Futaba receiver. My Prism can talk to Hitec and Futaba on the normal postive shift pulse and JR and Airtronics with a negative shift pulse.
But I'm pretty sure that Futaba crystals operate at the stated frequency and Hitec crystals operate at 10MHz (the first IFM) less than the stated frequency. Either that or vice versa.

Rodney

Crystals are not necessarily interchangeable. There are Y cut, X cut and other cuts as well, each requiring a different oscillator circuit and different values of L and C for correct resonance with minimum sidebands. Unless you have concrete information on compatibility, best to stay with each manufactures recommendations for that model receiver. Note that the crystal will be different for a receiver with positive deviation than one with a negative deviation even if the stated frequency is the same. The receiver crystals are at a slightly different (about 2 to 5KC above or below) than the transmitted frequency is.

carl24bpool

Wow this gets complicated.

So if I use a hitec crystal in my hitec tx and a futaba in my futaba rx andd both are for hte same channel then it will work??

Does it matter that the Hitec is dual conversion but the Futaba isn't?

MajorTomski

Also the OFCOM may not allow you as a non-trained radio technician to change the crystal. We can't over here.

carl24bpool

I don;t think thats an issue over here really.

How would anyone tell that you had swapped anyway?

4*60

My Feedback: (41)

ActuAlly Jr is pos and Fut is neg.

4*60

My Feedback: (41)

At the price of non 2.4 receivers why not get what you need?

opjose

Because of the ensuing crashes of other flyers using adjacent channels.

carl24bpool

But how do you know nobody is using any channel you happen to have?. Say you have two transmitters with different channels you're telling me you can't swap the crystals between them?

Theres always a chance you could cause such a crash unless you are aware of who is operating in the area surely?

jester_s1

Not only will you cause a crash if you change the Tx crystal without retuning the radio but you'll also cause your range to be diminished. That's why it is illegal here in the states (has been at least since everything went to narrow band in I think 1991). BTW, there is no advantage to using a 6 channel radio with a 4 channel receiver. There could be a benefit by using the programming in your radio or the model memory, but you can't use more channels than your receiver is able to hear.
So now I'll ask you the question that really gets at getting you a good, functional system. Why are you spending money on Ebay buying this older radio gear that you're now having to spend more money and time to make work when you could be investing in a good 2.4ghz system that has more features and is interference free? It may be more money to lay out all at once, but it will save you money in the long run.

carl24bpool

Lid love 2.4 but not got the funds for now. I bout a plane that happened to come with radio gear and was very cheap. 55 dollars for it all brand new. The tx is 4 channel but the rx is 6 channel.

on one of your other points am I right to understand that if I merely swap my crystals in my hitec gear for other hitec crystals of a different frequency then I would have issues? It's is all 35mhz by e way. I thought simple swaps were fine. Crystals can readily be bought in shops over here so I can't see there being issues and what do you have to retune? I thought the swapping of the crystals does the tuning?

on_your_six

My Feedback: (11)

Why even bother responding???? These guys can't be helped.

jetmech05

If you swap chrystals in a transmitter it needs to be retuned on a bench. As stated above it is illegal for us in the States to swap chrystals with out the bench retune. How will anyone know? the sticker on the back wont match the channel. remove the sticker and you can tell by the tuning. but you probably can get away with it. You may have reduced range or possibly interfer with another channel.
Good luck

Charlie P.

Yeah. Anything goes in England as long as it's not sharp or fires a projetile.

When your plane and the two others in the air near you all crash at the same time.

JohnBuckner

My Feedback: (1)

Of course On_Your_Six this gentleman deserves our help. He may be doing things the hardway and ultimately the expensive way but the point is he keeps coming on back and he is listening.

Carl in order to use your Optic 6 transmitter with the 6 channel Futaba Rx. We need to work something out. First does your Optic 6 have a synthesized module on the back or is it a single channel With a fixed frequency on the 35mhz band? They came both ways.

If it is a synth module you need only to remove the module and dial in the frequency the rx uses. if it is a fixed frequency you need to obtain a Hitec Tx crystal that matchs the rx,s frequency. In the US Hitec does not sell transmitter crystals but may there as crystal swaping was quite common and I do not believe illegal as it is here in the US.

Now here is why I believe you have not been able to make it work and the solution is real simple and cost nothing. First a little blurb about something that all FM(ppm) systems do and that is what has been hit on by the fellows and that is modulation shift. All systems do it and some are positive and some are negative. For the tx and rx's to function together they must be on the same shift. Here in the north American market Hitec and futaba systems are by default negative shift. Thats in America , However I think in the europeon market that Futaba sells their product with positive shift.

I suspect that you Got a mismatch in modulation shift. Pretty darn sure that the Optic six has a selectable shift. you may only need to read your manual and look up modulation shift and simply change it from what ever setting its on now to the other option. If I remember correctly they refer to negative as #1 and positive as #2.

John

jester_s1

What i was getting at earlier is that these crystal based systems need to be tuned in order to work right. 2.4ghz is all solid state and doesn't need to be, which is why we don't have this conversation much anymore. What I know about it (not a lot) comes from using 72mhz in the US, so things may be different on 35mhz. With narrow band 72mhz systems, the transmitter can't be manufactured to give a perfectly clear signal at all channels. It has to have a crystal installed and then be tuned to that channel in order to have the strongest possible signal. Receivers don't have to be tuned, because for whatever reason they get good enough reception with just a crystal installation. I don't know more about the technical explanations than that, but it's enough to keep us out of trouble.
And I totally understand needing to build your collection one piece at a time. I started with older stuff too and upgraded as I could. I respect that you're doing the best you can with what you've got, and you're still on here asking every question that you can in order to be successful. You are the exception among guys who go it alone with used equipment because you are willing to listen. I will offer the simple advice to watch your spending on old stuff, especially on fixing old stuff. It doesn't take long before you could have saved for a bit and paid for new stuff that would have been better. If you can get old stuff that works as it is and the price is too good to pass up, then go for it. But when you figure that a good 2.4ghz setup can be had for under $200 USD, it doesn't make sense to drop $30 on something old and then another $50 on getting it to work.

HighPlains

My Feedback: (1)

A lot of incomplete information in this thread. But in post 9, Rodney pretty much hit it out of the park. Crystals have certain circuit responses that the RF designer must understand and design for in order for them to work properly. Besides the type of crystal, quite often they are used in circuits that respond to the harmonics of the crystal, since cutting a crystal for the actual operating frequency would result in a very fragile device.

On the question of heterodyne circuits (where an incoming signal is mixed with a local oscillator), this was first done in RC systems around 1958 when the FCC allowed additional frequencies for our hobby. Traditionally, the receiver L.O. frequency is offset from the transmitter frequency by 455 kHz, either above or below. This worked fine in the early days of the hobby, including the original stuff on 72 MHz (the good old days with two color frequency flags). But as the RF spectrum started getting used up by other operators, we eventually got into the 50 channels of frequency, while they put pagers in between our RC frequencies.

So receiver design had to be upgraded to dual conversion. This meant that the first conversion had the L.O. offset by 10.7 MHz, while the second conversion was offset by the original 455 kHz. What this did was to make the receiver very selective in a very noisy RF environment. One of the problems with a single conversion receiver is image rejection. While the designer puts a filter ahead of the first L.O. conversion, when it was only 455 kHz away from the desired signal, the receiver will also pick up signals on the other side of the L.O. frequency, and since they may be much stronger, they show up as an over riding signal and you end up crashing. Trying to reject those signals with a simple front end filter is just not possible because the filter is not sharp enough when you are only a half MHz and you are operating at 72 MHz. But with the duel conversion working at 10.7 MHz, suddenly the crappy front end filter works like a champ.

So you can see, there is a lot that goes into the design of a receiver. I could talk for hours on the subject and not even hit the highlights of subject. Heck, there are tomes written on just the antenna, or book after book on filter design, and quite a bit more on oscillators. The engineering you can buy for a few dollars is amazing, and the technology developed the past 120 years is stunning. RC is now 115 years old this year, and Tesla is not a car. He is the man that created the future.

j.duncker

My Feedback: (2)

OK yes it is a little complicated

Eg.on say channel 70

For the Hitec Tx there is only one 70 crystal and it has to be Hitec 70 tx

Now for either make of Rx there are two types single conversion and dual conversion so there are TWO possible crystals 70 sc and 70 dc. BUT again Futaba or Hitec.making 4 possible 70 crystals. Just match the crystal to the make and conversion type.

Your Hitec Tx with a Hitec 70 crystal will work just fine with a Futaba rx with the appropriate Futaba 70 rx crystal.

It is quite legal for you to change crystals in the UK. You do not need to retune.

I flew for many years in the UK doing this. BUT you should carry out a range check before flying the first time with any new setup.. With only one section of the Tx aerial extended you should get 100ft ground range minimum.

I used a Futaba Tx with a range of receivers both sc and dc. Of the cheaper smaller options I had good service from the GWS dc rx even in very high threat environments.like 20 up Zagi combat.

I hope this helps and come back if I did not make it clear.