AM verses FM Transmitters?

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20 December 2008 at 08:22 #17626
David Meier
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@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.
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20 December 2008 at 08:36 #17627
Paul T
Participant
@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
20 December 2008 at 09:03 #17629
David Meier
Participant
@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.
20 December 2008 at 11:27 #17633
Paul T
Participant
@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
20 December 2008 at 11:36 #17634
Bob Abell 2
Participant
@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
20 December 2008 at 15:05 #17640
60watt
Participant
@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
20 December 2008 at 23:08 #17660
David Meier
Participant
@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.
20 December 2008 at 23:22 #17661
60watt
Participant
@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
20 December 2008 at 23:51 #17664
David Meier
Participant
@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.
9 January 2009 at 19:20 #5018
David Meier
Participant
@davidmeier28154
23 June 2010 at 18:20 #27405
Geoff Cropper
Participant
@geoffcropper73342
Hi All, I think the original question was , What’s the difference between AM and FM ? regards Foggy.
23 June 2010 at 20:49 #27407
Dave Milbourn
Participant
@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
24 June 2010 at 21:11 #27414
Geoff Cropper
Participant
@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.
24 June 2010 at 23:06 #27416
Dave Milbourn
Participant
@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
25 June 2010 at 10:22 #27421
Geoff Cropper
Participant
@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
28 June 2010 at 07:43 #27457
Rick Devonshire
Participant
@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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