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.

wrote:

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.

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

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 on the
board right next to the crystal, I assumed it had something to do with
tuning the feedback but I don't even know what kind of oscillator it
was so I was hesitant to just start twisting knobs.

On Nov 2, 12:33 pm, wrote:
wrote:
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.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

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.


wrote in message
ps.com...
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.

a écrit :

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 on the
board right next to the crystal, I assumed it had something to do with
tuning the feedback but I don't even know what kind of oscillator it
was so I was hesitant to just start twisting knobs.


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 http://groups.google.com/group/rec.models.rc.air .


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:
a écrit :

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 on the
board right next to the crystal, I assumed it had something to do with
tuning the feedback but I don't even know what kind of oscillator it
was so I was hesitant to just start twisting knobs.


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 http://groups.google.com/group/rec.models.rc.air .


Daniel

wrote:

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 on the
board right next to the crystal, I assumed it had something to do with
tuning the feedback but I don't even know what kind of oscillator it
was so I was hesitant to just start twisting knobs.

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