Re: QCX+ Not Transmitting


John Laidler
 

I've done a bit of fault finding this morning after taking it out of the case. There is certainly something weird happening on transmit.

Starting to work through the fault finding guide for the transmit circuit the voltages on IC3 when receiving were exactly as they should be. Things changed which I set it to transmit. As my oscilloscope hasn't arrived yet I can't check the waveforms on transmission but pin 11 was not giving the right voltage. The guide says it should be +5V but it fluctuates in time with the dots and dashes showing around 2.5, 3.5 and 4.9. It is hard to read them as they change quickly but it certainly isn't the steady 5V the guide says should be there. The supply voltage was 11.9V when on receive and this dipped to about 11.7 on transmit. As the radio is supposed to work between 7 and 16V I don't think the supply is at fault.
I also measured the output voltage using the dummy load and found that was fluctuating too in time to the dots and dashes.  This time the fluctuations were between 7 and 13 volts. Last night I didn't measure any voltage on the dummy load but I think that was because I put the probes on the top board, not the bottom one as the dummy load instructions say you should - RTFM!

I've also checked the voltages around T1 and they were correct.

So, I'm stuck. I've found something wrong - the fluctuating voltage on pin 11 of IC3, but what might cause that?

As before, all suggestions will be most welcome.

John

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