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RC transmitter transmits on first harmonic

I've got a JR 72 MHz transmitter module that was transmitting a very weak signal on the correct frequency. I replaced the power transistor and the module worked for a few minutes, but now it transmits a very strong signal on 144 MHz. There is no signal being transmitted on 72 any more. I imagine I've got to get the crystal back into its fundamental mode but I could use some hints. Does anyone have a service manual or schematic for the module?

A 2m RC system would be really neat but I don't think it'll be easy to find a receiver.

Reply to
stickyfox
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On Thu, 02 Nov 2006 07:54:56 -0800, snipped-for-privacy@gmail.com Has Frothed:

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That's because it would be illegal to operate there.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794
Reply to
Meat Plow

2nd harmonic, 1st is the base frequency! IIRC 144 is ambulance service band, here at least, so maybe not so neat. Need to show us the oscillator circuit, otherwise can only say the osc cct needs to be tweaked so it prefers the lower frequency. Can probably be done with a little capacitance added to reduce closed loop gain at 144.

NT

Reply to
meow2222

FWIW.. not that it helps:

144 MHz is the American 2m ham band, for which I have a license.. I left that out. It's not exactly in the band plan for RC aircraft but it's not like I'd be causing real aircraft to plummet to the earth.

Thanks for the tip about capacitance. There is a trimmer cap > snipped-for-privacy@gmail.com wrote:

ambulance service

Reply to
stickyfox

if its a cheap transmitter, chances are it uses a crystal oscillator at a low fundemental frequency, and uses a frequency doubler tank to get your desired frequency. something is wrong there.

a frequency counter and spectrum analyzer would be good for situations like this.

Reply to
Mike

snipped-for-privacy@gmail.com a =E9crit :

It is very difficult to get schematics/ service manual in the RC hobby. Some kind of trade secrets, maybe.

Probably the trimmer cap is there to adjust the frequency shift at

+/-1.5kHz from center frequency. This is the FSK system used in RC equipments.

Did you used the correct replacement transistor?

Try posting on

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.

Daniel

Reply to
Daniel

Yeah, you've got it there. The trimmer is for the freq. shift, according to a tech who works on it. I'll have to do that next. :) I've got the right transistor.. it looks like I'll have to adjust the inductors, but there're 3-4 of them and I have no idea what they do. The project kind of got sidelined anyway.

Daniel wrote:

Reply to
stickyfox

I doubt that has anything to do with it, all those do is pull the xtal slightly. Could be wrong tho.

You'll nrace the cct diag for the osc, and tweak or even add some rc somewhere

NT

Reply to
meow2222

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