Repairing a Deaf Softrock Transceiver

My Softrock Ensemble RXTX transceiver has gone deaf a while ago. It was working very well, allowing me to to experiment with WSPR and PSK31. But at some point it gradually got deaf, until it could receive nothing. It could still transmit fine.

I tried to fix it, trying the easy things. I checked that the Si570 oscillator was working and changing frequencies; it was fine. I also inspected the board visually for bad solder joints, but didn’t see anything obviously wrong. I don’t have a signal generator, so I could not easily check where the input signal is lost in the receive path.

Yesterday I decided to give it another go. I used a 1-transistor crystal oscillator that I put together as a signal generator. It had a 10.116MHz crystal. I hooked its output to an oscilloscope, verified that it was oscillating, and that the signal was not too large (it was around 200mV peak-to-peak). I hooked its output to the input of the Softrock and started tracing with the scope’s probe.

I could still see the signal at the antenna side of T4. So the low-pass filter was fine (the signal was also much cleaner at that point, because the low-pass filter removed the harmonics from the oscillator’s output). Next, I checked resistors R54 and R53, which feed the RF signal to the detector. No signal. This narrowed down the search considerably. Testing both sides of inductor L4 showed that it was blocking the signal. From the top side of the board, the joints did not look good; there was no solder creeping up the wires. I unsoldered it and discovered that one wire was not tinned properly. I scraped the enamel off the wire, tinned it, and soldered it back in place. Now I was able to see the audio-frequency signal at the output of the receiver on the scope, indicating that the receiver was now working. I’m using it right now on 14MHz WSPR.

Robby’s building instructions tell you to remove the enamel very carefully off enameled wire, and that not doing this leads to many of the problems with the kits. I did not heed the warning carefully enough, I guess. I still find the evolution of this fault interesting, in that the radio was working fine for a few months before the soldering fault showed up.