AM verses FM Transmitters?

[David MeierParticipant @davidmeier28154]

Could some kind person please advise me the pros and cons of AM and FM transmitters for use on fast electric and scale model electric boats? I was given some old 27Mhz AM recievers and a transmitter to put in a fast electric boat I am fitting out and I have just discovered that the transmitter is dead. I need to at least buy an AM transmitter but am wondering if I would be better to buy an FM transmitter and reciever instead.

Regards.

David.

[Paul TParticipant @pault84577]

Good Morning David

The 27Mhz system is beset with problems such as crossover with other users (CB radio, taxis, car remote locking etc)

As in all things you should buy the best that you can afford, I don’t know what your budget is but I would suggest that you look at the new generation of frequency hopping systems such as the Graupner mx-16 or at the very least buy a 40Mhz system

Paul

[David MeierParticipant @davidmeier28154]

Thanks Paul. I can get hold of a reasonably priced 35Mhz Transmitter from an e-sky helicopter. Could I fit 40Mhz crystals into that and use it for my boat?

Cheers.

David.

[Paul TParticipant @pault84577]

Hi David

Sorry but I don’t know but I would love to know the answer myself as I have a great 6 channel 35Mhz set just sat in the shed.

Paul

[Bob Abell 2Participant @bobabell2]

Hello Paul

I think it`s against some sort of law to use 35 meg on a surface model………..35 is for aircraft only

But I was told by a expert club member that 40 meg crystals will work in a 35 meg tranny?…………..but I`ve never tried it myself………………having given all my 35 meg trannies away!………….Grrrrrrrrrr

Bob

[60wattParticipant @60watt]

Hi David.

There is a member on this forum who has serviced rc sets but he hasn’t been replying to pm’s.

I’ve repaired many a faulty set and what I can tell you is  a high frequency scope will show if  a 27 am transmitter is working.It will pick up most faults without direct connection.

By "dead",if you mean a battery meter shows zero then that is the easiest fault to pick up.A bent or corroded battery contact could be to blame.A dial indicator on a 27AM transmitter is usually a battery meter rather than an RF power meter.

Tell us what make and model your transmitter is.Do you have a voltmeter? If it is a 2channel replacement you need then they are inexpensive from Howes – one of the Modelboats advertisers.Older models can be expanded to 4chan.

The difference between some 35 and 40fm single conversion receivers is only the label.

Tom

[David MeierParticipant @davidmeier28154]

Hi Tom.

The transmitter is an Acoms AP-227 Mk3. Yes the battery meter is not indicating. I have just checked the battery terminals for corosion and they are all clean. I have checked the on / off switch with my multi meter and it is working. A visual inspection of the back of the circuit board doesnt show any obvious burnt bits or dodgy looking components. No broken wires. Rightly or wrongly I tried touching the wires of a 7.2 volt battery directly across the battery meter solder lugs and didnt get a flicker of movement on the needle.

The plot thickens. I put the batteries in the transmitter with the circuit board unscrewed from the main body so I could get to the switch and meter contacts. the batteries were charged to 1.35 V and I measured 10.2V across the 8 batteries . I switched the transmitter on and measured across the back of the battery meter and only got 2.3V. Switched off the transmitter and measured across the 8 batteries 9V!!. Measured across the back of the switch with it turned off 9V then when switched on a few hundred mV now the batteries are down to 8.1. Looks like there is a short somewhere doesn’t it? Could it be the Battery meter do you think?

David.

[60wattParticipant @60watt]

I doubt it.The battery meter probably has a resistor in series.

A battery meter (as a voltmeter)reading zero is a sure indication of an open circuit at the supply as that is where it is connected.

A working set should draw around 100mA

Tom

[David MeierParticipant @davidmeier28154]

I think I will take up your earlier sugestion of picking up a cheap 2 channel unit. If they made mechanical transmitters I would be able to fix them.

Dave.

[David MeierParticipant @davidmeier28154]

[Geoff CropperParticipant @geoffcropper73342]

Hi All,        I think the original question was , What’s the difference between AM and FM ?   regards Foggy.

[Dave MilbournParticipant @davemilbourn48782]

AM = Amplitude Modulation

FM = Frequency Modulation

I bet you wish you hadn’t asked……..

Imagine a radio wave; it looks like a graph of a sine wave, for all you schoolboy technocrats. If you want to change it in any way then you have to vary either its amplitude (the height of the wave) or its frequency (the distance between the high points). If the wave were an audible sound then increasing its amplitude would make it louder, while increasing its frequency would make it higher in pitch. By varying these properties of the radio wave a Tx can effect a change in response to the signal at the receiver end and thereby move your servos. OK – there’s a lot more to it than that but essentially that’s the difference.

By the way, FM is more resistant to interference, just like your domestic radio set.

Dave M

[Geoff CropperParticipant @geoffcropper73342]

Hi Dave,        Many thanks for that.      Is that why AMs are cheaper than FMs?        I’m looking for a 40mhz setup for my boat.        regards   Foggy.

[Dave MilbournParticipant @davemilbourn48782]

 Is that why AMs are cheaper than FMs?

     

Not really – I think it’s that they made a lot more of them. The difference in cost over many thousands of sets would be marginal.

Go for 40MHz FM every time. I use and recommend Hitec gear (but not the Zebra – it has issues over quality).

Dave M

[Geoff CropperParticipant @geoffcropper73342]

Hi Dave,        Thanks for the advice, I’ll look for a Hitec 40mhz FM setup.        I bought a Laser 6 35mhz last July, so i’ll have to sell it or maybe just get a 40mhz Tx and Rx to go with the servos i’ve got.          What do you think ?           Foggy

[Rick DevonshireParticipant @rickdevonshire]

There is a debate on the RCM&E web site over the possibility of a 35meg system working, legitimately with 40meg crystals and vice versa. Is there anyone with the electronic/telemetry know how who can give a difinitlve answer to this question?

I have Hitec Focus dual conversion set and if this is possible it would save me some money.

Rick.

 

 

 

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