Futaba T6EXAP

[Paul Newell 2]

Peter,

 

The example I quoted of a Graupner crystal in a Futaba compaitible cicuit is an extreme case.

 

However crystal specifications involve a lot more than just the frequency.  In a 35MHz  transmitter the frequency is moved either side of the centre frequency to produce the FM signal modulation.  One way of achieving this is to vary the capacitance of the oscillation circuit as seen by the crystal.  Obviously the amount of this frequency variation is such that it remains acceptable to the receiver.  Just as important is that is does not stray into the 'seeing window' of a receiver on a different channel.

 

Using an incorrect receiver crystal can affect both thecentre frequency AND the amount it changes with the modulation.  This can cause a whole range of problems some of which may not be evident with a range check or even in flight. Some of these are as follows.

 

Occasional glitches, posssibly at only certain battery voltages.

 

Reduced range.

 

Interference with other users on adjacent channels.

 

Interference with other users on channels elsewhere in the band  (due to various combinations of harmonics - too complex to explain on this forum).

 

Might work trouble free with one receiver and not another, either same or different receiver manufacture.

 

At the end of the day everything is designed to work within a specification with a little bit  of safety margin.  If you change something that takes part of the system to the edge of the specification you erode the safety margin and one day it turns around and bites you OR SOMEONE ELSE.!

 

 

[Boots]

Well Paul

                 I shall bear in mind what you have said and will only operate the futaba using the fleet xtals when flying alone, which is the norm and atleast half the people I meet at the field fly 2.4 anyway

 

Do you feel any interferance caused by 35Mhz radios could adversely effect 2.4 Ghz ?

[Biggles' Elder Brother - Moderator]

I've never seen any evidence of 35MHz interfering with 2.4.

 

BEB

[Peter Beeney]

   Paul,
          Yes, thanks for your answer. I’ve always been mildly astonished, in an agreeable sort of way, at what seems to be the high quality on all model radio, certainly at least on 27 and 35 MHz. It’s almost fit and forget, with crystals being extremely low on the list of any suspect areas. Even then the human element can account for a little turbulence here, such as the new pilot who doesn’t know what frequency he’s on, or the experienced pilot whose crystal fell out of the receiver, resulting in a very spectacular high speed i/c ducted fan crash, or my friend, I have to say he’s a real nice bloke, but back in the days of 27, however, he would tend to change his tx pennant but rather unfortunately not always his crystals. This resulted in a couple of interesting situations, but fortunately no harm done.

   I’m also very pleased to say that personally I don’t think I’ve ever seen any of the type of crystal problems you describe, and when we have had the very seldom glitch we’ve usually managed to identify it, vibration has popped up as an offender more than once.

   An ongoing little saga with me is that of batteries, but that another story in another thread. It’s not often you can find a spec. for these anyway, and even when you can on the odd occasion they fail miserably. Then we can be aware of some difficulties.

   Talking of specs., I would say the Spektrum buddy lead would appear to be operating quite near the limit. However, by using Spektrum’s own lead it certainly works ok, but it’s early days, 2.4 is still relatively young, time will tell. As an extension to this theme, within our little club, quite coincidentally and independently of each other, two members have had some persistent problems with one particular make of 2.4 and have decided to cut their losses and change brands. I don’t think I’ve ever known this to happen before! Is this a ‘one off’, or are the standards beginning to slip?

   In the past, the Beach Boys were frequently ‘pickin up good vibrations’, will we also continue to do so, I ask myself?

   PB       

                    

 

[Erfolg]

I guess i am lucky, in that i have never knowingly experienced any problems with crystals themselves. On either 27 or 35.

 

In the days of 27 i just bought what ever coloured crystals. As my transmitter was Waltron, there would not be easily obtainable, that is Waltron specific crystals. Oh i have a 3 channel Sanwa.

 

When moving onto 35 I purchased a Futaba set and then a Sanwa set, both operated together. I just bought Futaba compatible. When i restarted modelling I found my sanwa was dead. I bought a Futaba 6EXA to replace it. The Futaba Gold died shortly after.

 

All using compatible crystals with no apparent problems.

 

Finding that the 6 model memory insufficient, I then purchased a 6EXA, 2.4, I was so satisfied with my 35 version.

 

As for interference of 35 on 2.4 or reversed never witnessed it, or heard of it.

 

I do suspect wet or dampish trees, with leaves, causing some trvial (to me that is ) issues. Where modellers have complained of interference. But you do have to fly sum distance away (I could not see a model that far away, near the ground to actually control it), with a slight hump in that particular field, with very long grass at the time. They blamed the electric cattle fence. I suspect they just lost it. Or was ittttt?

[Old John B]

I cannot stress that preflight checks must be done with the engine running full ahead, this tale of woe might bring an explanation from some of you boffins out there or end up with Ripmax's service department.

I have a Futaba 2.4 EX 6 and three 7channel receivers, please read on.

1)A Super tigre 2000 installed in a Seafury. Did the radio checks, started the engine and when it reached full ahead, the radio started doing odd things and then stopped working. Ah Ha, I said, the battery has failed. Will test again when I have a new battery. Forgot to say that as soon as the engine stopped all returned to normal on the radio front. Still thought that I would get a new battery.

2) Different receiver, spitfire with an SC 91 four stroke. Radio checked before take off, did not do it with engine going full bore. Then 50 foot up prop- nut must have come loose, engine screamed, loss of radio signal, bag full of bits.

3) Flew fun fly with OS 25, same tranny different receiver, no problems.

4) P 40 with SC 91 two stroke. Did a proper preflight check, different receiver, and yes as soon as full ahead was reached the radio gave up, coming back to life again after a committe meeting when the throttle was pulled back to slow. This happened several times.

4) We checked every possible permutation, battery, servos, took the receiver out ,in case vibration was the cause (Actually held the receiver in our hand. Didn't matter. We even bound it to someone elses tranny and still the same problem. The green light would go on and then as soon as full ahead was reached the red one would come on. Sometimes it would flicker and some times would be solid.

It would seem that the bigger engines set up some kind of electrical field that interupts the radio waves, very fanciful and probably very wrong Any explanations from all you experts?Been doing radio control since bang bang days and never had anything like this before.

[Old John B]

I put a bit on the forum this morning about mysterious goings on with my Futaba 2.4 EX6, it came up but seems to have disappeared. Never mind. Three aircraft had been affected.The problem was that when I put the throttle at full ahead the receiver seemed to loose it's signal and this happened on all three aircraft. I have spent all afternoon checking the P40 set up and went through every joint, servo, battery and plug.

I found it, even though the plugs on the extension leads had been taped, the fault was here. All seemed well and then I gave it some serious shaking and banging and here was the fault. One of the pins on the plug had moved back enough to only just touch its opposite pin. Renewed the lead and all seems well. We will never know what happened to the Spitfire, there aint enough left, but I will have a go at the sea fury later and see what I can find there.

I feel a bit of a twit but the lesson is there for all to see. Just like full size planes, do the preliminary checks not a quick twiddleand oh good it all moves. Look ,and if all is well advance the throttle to full and keep looking. I watched yesterday and most just checked that the control surfaces were moving and off they went.. It's all elementary isn't it, but many of us forget what we were taught!

I was blaming Mr. Futaba for nothing, shame on me.