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I have an old Graupner 35Mhz transmitter and receiver which I would like to get running again. Bought batteries and connected power and a servo to the receiver but nothing happens when I use the transmitter. Both have the same frequency quartz.

Do you have any tips on how to verify that both the transmitter and the receiver are functional or how to debug? I have a basic oscilloscope and a multimeter at hands. Both batteries are loaded and connected properly, I think. When I connect the servo to the receiver I sometimes get a tiny movement of the servo as I am plugging it in.

Any helpful input would be highly appreciated

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  • $\begingroup$ Cool! Are there any LEDs on the TX/RX to identify whether or not the devices are sufficiently powered? $\endgroup$
    – ifconfig ♦
    Commented May 11, 2020 at 17:11

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AFAIK servos are usually driven by a PWM signal (which the Graupner RX seems to put out as well) with usually 50Hz. So you could basically check if the RX is putting out PWM signals on the 7 outputs. Just connect your scope to one ground pin as well as one data pin. Then you should see a square wave on the scope. If the square wave has a duty cycle of 1000µs, it means 0° (so the left most position of the servo). 2000µs means that the servo is supposed to rotate to the right most position, usually 180°.

There are some exceptions with some servos (continuously rotating servos - similar to stepper motors, or some servos have more than 180° of rotation), they probably don't affect you though.

If you are unsure about the waveforms or something else, I'm happy to help. I wouldn't say that the tiny movement of the servo you described means anything. It's probably just the servo "snapping" into it's position after getting power or something to help the servo to find out its current position.

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  • $\begingroup$ I too was thinking about this, but it doesn't confirm which part (TX or RX) is to blame if this doesn't work and there is no PWM signal on the output of the RX. $\endgroup$
    – ifconfig ♦
    Commented May 11, 2020 at 18:51
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    $\begingroup$ Well, if the PWM is permanently at 1000µs, it is probably (not safely though) the transmitter since I'd say that transmitting is most of the time more prone to error/failure than receiving. If there's no output at all, then it is either the power delivery (batteries) or the RX. If the output is changing correctly, it's probably the servo or else the power delivery struggling to supply current as soon as the motor (and therefore load) is attached (causing battery sag) $\endgroup$
    – Bastian Springer
    Commented May 11, 2020 at 18:56

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