Hi guys,
I have a problem on my rc heli. Actually it is a cheap chinese one 'marchand x-5000', but it is good for beginners (i think ). The problem is that even with new batteries in transmitter and charged heli battery, it will not work untill I touch antennas. On heli sticker says 40MHz, and transmitter says 27MHz. What should I check? I checked receiver disconnected from heli (i thought it was because of motor noise disturbance), but It wont work either, untill connecting antennas.

Thanks in advance

you have mentioned the reason for the problem in your post

"On heli sticker says 40MHz, and transmitter says 27MHz" the frequencies do not match........ get your self a 40 mhz tx for the same or vice-versa..

regards.

Buzz, just being curious, if that were the case (mismatched Tx / Rx) - would it maifest in such a way - Heli works only when antennas are touching? Why I am asking is because I have a small RC car which exhibits the same problem.

For now I suspect on receiver. I think that if mismatched frequencies it shouldn't work at all. Telecommunications are not my occupation, so I am asking for help. Anyone else having similar problems?

In general in Mhz range stray capacitance plays a major role and it is easily picked as u saw. Just touching the antenna induces our body capacitance. This changes the output waveform if the circuit is not properly compensated for.
Sorry i cant be of more help for your particular case as i am not aware of the circuitry. but it is curios to know that the Tx/Rx are of so different frequencies and they still work when antenna is touched.  implies that is by how much the frequency is shifting. please ensure that the actual crystals in the Tx/Rx are same. If not I would advice to do so.

The mismatch can be just a labeling issue too.  Can't see how they can work otherwise.

Yeah, that makes me wonder. How it works at all if frequencies are different. I'll try to take photos of circuits and post them today.

+1 .. I totally agree with MPP. If you touch pre-amp input from Tape-Head, you may listen to 'Vivid Bharati' AM Radio channel in your speakers! At least this used to happen in North India. So the Leaking PPM or what ever encoded control signal just jumping through the physical connect and fed directly to the stage after the RF -demodulated and finally (correctly) decoded by the receiver is entirely possible.

Argent, did the Tx and Rx combo ever worked normally ? Then they might have been packed wrongly. China exports different Freq Tx/Rx for same toy to different companies. You could have been just unlucky.

I know that situation, I had similar with my old walkman, when I touched tape head I could also listen to some radio station. It is, that body presents some kind of antenna. But I dont touch antenna by my hand. I touch helicopter's antenna by remote control's antenna. When they are connected everything's fine. If I wanted, I could make a long thin wire to make connection between antennas and fly helicopter. But then it wouldn't be wireless heli

And, yes, tx and rx worked normally.

If it is a labeling issue (which in my opinion would be very rare) then you might have to tweak the coils in the final amp stage to maximize the power output...... open the tx and check the freq. of the crystal, this will make things a little clear...the rx does not have a crystal controlled oscillator, hence no crystal so frequency cannot be determined in a way similar to that of the tx.  In case of these cheap Chinese toys, if there is an issue with the rx then you might have to take a small phillips screwdriver and turn the ferrite bobbin in the coil (wear rubber gloves while doing so). If this works then gradually increase the distance between the tx and the rx till you get a descent range.

 If it is a case of mismatch then too, upon touching the two antennas, the system will work sometimes because these super regenerative receivers are very noisy and as there is no crystal controlled osc and hence no IF,AGC,etc. there is a good chance the system will work......(VC,I hope this answers your question......).
I have fixed alot of such toys for IHC Bombay, If you want to manually tune the system, then let me know, I will guide you through the process.....


regards.

Thanks a lot buzz,
let's see what we can do. I am posting photos of transmitter and receiver.
This is receiver

receiver from other side
(blue wire is antenna)

and transmitter
(it uses 8*1,5V AA batteries)

One more. This is crystal from transmitter.

if it is mismatching frequencies case, would it be crazy to try to change one or another circuit to match frequencies?

From the pictures one can easily make out that the tx has a freq of 27.145 mhz and the rx has a freq of 40.675 mhz. your best chance of getting this system to work is to get the tx replaced from the dealer of the product......the only other way is to completely modify the transmitter circuit (a lot of effort and without a sound knowledge of electronics and soldering virtually impossible!!)

thank you for your help.

I have knowledge and experience about electronics (and soldering) but just not about rf circuits, but I see that it would be a lot of job to do. I'll see what I can do.

Thanks a lot.

Good suggestion...  do that and you will not have problems 

I think I'm gonna try something I have on my mind, first

I'll post results

Hi, before experimenting, I am interested if there is a way to do modifications to the transmitter to match frequency of receiver? I know i need crystal, but what else should I consider? I am afraid I won't be able to return it to seller.

hello guys,
well as I said I'll post results, so I am posting
Maybe it will sound crazy, but...
I have found a crystal with label 40.320 MHz (in an old modem) and replaced existing 27.145 MHz in transmitter with it. Guess what Helicopter flies. I can not control it 100%, but it works, without wires
Receiver frequency is 40.675, so it differs from transmitter frequency and I think I should do some fine tuning. As it can be seen on photos, there are variable capacitors (or resistors) both on receiver and transmitter, so I am experimenting with them. Now, if someone knows (from experience) will I be able to make it as it would be from factory setting? Without changing other elements. I know also that for really fine tuning maybe I should change some capacitors or coils. Just tell me am I doing good?
Thanks in advance

Without charting out the circuit of the RX and TX you are pretty much shooting in the dark... Ya, sometimes shooting in the dark is fun.. enjoy!

yeah, right

I'm afraid you will never make a marriage between the transmitter and receiver no matter what crystal you put in. The circuitry just isn't designed to work that far apart (in frequency) A major rebuild would be required to make them compatible. Yes you may get a small reaction more from harmonics and stray RF than being matched.

You will never be happy with the pair and just a small distance will cause dropout and instability. As mentioned, either get a receiver to match the transmitter or a transmitter to match the receiver. Or a matched pair altogether. At least you know it will work reliably. The range on those units is very poor. We used to say throw a stone and the distance it goes is a bit more than the distance the radio will work. Remember, the lower the frequency the more wire you will need dangling from the model. So try for the 40mhz gear. A shame you live so far away. I have dozens of the darned things lying around. See if you can find someone with a wrecked toy heli and ask them for the radio equipment. You may get lucky. BUT please don't be tempted to try to get better range by tweaking things which turn on the board, you could lose the lot. PLus you will have to avoid anything but the correct size and maybe even model due to different wiring, setup of the board, size of the board and many other matters. Just a slight mismatch may stop the heli from leaving the ground due to excess weight. If it's too light it may be unflyable, if you can figure out where all the wires go and if the plugs are the right ones. Companies don't like people trying to do what you are attempting. They want you to buy a new one.
I agree that shooting in the dark is fun but it's not much fun when there's nothing there to hit.

Sorry to dampen your spirits but it simply won't work without the right gear. If you have a friend in Australia and they are returning to India. Let me know and I will give them a brand new helicopter and transmitter in the box to bring back for you. You have my word on that.

Sincerely Stu

+1'd!
But "argent" is in Bosnia (& Horzegovina).

Oh dear, my sense of direction is atrocious. I assumedhe was in India. Sorry Argent.

Stu