The Fault

No transmission – pretty simple really.

Testing and Repair

Turning it on and all seemed OK. The small radio scanner showed that there was no signal transmitted; at least not in the 40MHz band.

Maybe the crystal or part of the transmitter circuit had failed.

Next, open it up and run it from a bench power supply (10V). Current draw was about 30mA; not enough if the transmitter was running. Check the TX board for power (10V) and PPM signal. No PPM signal suggested maybe there was no output from the main board.

There was a PPM signal in there. But the open collector output had no pull-up so it must be on the TX board. Yes, and there is a 5V regulator for the first stages of the transmitter. There was 10V in, but no 5V out of the regulator.

I replace it with a similar device (didn’t have the same package). Now we have 5V and PPM. Plugging in the crystal and the current draw rose to about 180mA; much better. The radio scanner now picks up the signal on the correct frequency.

We run it for about 30 minutes to see that it keeps working and nothing gets hot; specifically, the new regulator.

This is a 78L05 in a TO-92 mounted to replace the original SMD regulator.

Is it really working correctly

The battery needed a good charge, so that happened over night. The current draw of just over 180mA seems a bit low based on similar 35MHz transmitters I’ve worked on. Time to look at the signal level or field strength and do a quick range test.

Using the Icom PCR1000 receiver with a minimal (untuned) antenna we get some comparative signal level readings.

The first image shows the Futaba signal at 40.650MHz (where it should be). The TX was 4 meters away and with the antenna fully collapsed. About -79 dbm.

Note: every -3dbm is half the signal strength.

Ignore the other signal/noise. This test was done inside.

The Next image shows the signal with the Futaba transmitter 10 meters away with the antenna now fully extended. About -67 dbm.

This shows that fully extending the antenna makes a big difference. -97 up to -67 dbm. About a 1000x improvement.

Range Test

A walking test along level ground provided reliable control out to 500 meters. This should be much better in the air. It was done using a low cost Dynam single conversion receiver.