M0NTV

After completing all the parts to my receiver I took it to the club this week for some more help testing. I knew that the receiver worked when I injected a signal from my NanoVNA, but testing in the real world didn’t yield any results. The club has a signal generator which I used with much assistance to test the product detector.

It turns out that although the product detector works it has horrendous sensitivity, of the level that would require something like a 1KW station across the street to ‘hear’ anything, so more work ahead to diagnose that fault.

In addition to this something went pop in my audio amp yet again. The amp was working, I turned to focus on the product detector then I heard a spark, saw some smoke and the chip was very hot. There is definitely some kind of fault in that audio amp, I think as it’s has so much desoldering, resoldering and adjustment there is probably an intermittent short, or something along those lines…more work to diagnose this fault too.

After chatting through the noise issues with my audio amp with a club member I’ve made a couple of changes, firstly I’ve replaced the AF input cable with a jack socket (and using a shielded jack to jack cable for the input audio) and secondly I’ve grounded the cases of the potentiometers.

After adding the audio in socket I was having issues, the noise was much reduced and I was getting output when playing audio through my laptop but I wasn’t hearing anything when testing the output of the product detector, eventually I realised that my issues and confusion was because I was using a stereo cable and stereo jack socket in the audio amp but a mono jack socket in the product detector. For the audio amp of the laptop out it was fine, the ring output from the laptop socket went through the stereo TRS cable to the ring of the socket in the audio amp and then out mono through the (correctly) wired AF out socket. In the case of the product detector the audio went through the tip which was connected at the audio amp end and the ring was grounded, so no sound unless I pulled it out a little, once I rewired it the issue resolved.

For the grounding of the potentiometers I connected the three together using some old desoldering braid, perhaps a bit odd and used a lot of solder but pretty effective, I then connected the end one to the ground on the voltage regulator. This all worked fine, but then when trying to track down some remaining noise I unplugged the AF in cable and it went silent, I then replugged it and the same and no sound… after a bit of checking over the board I replaced the LM386 chip and it worked, so once again I’ve somehow fried a LM386!

Regarding the remaining noise I’ve realised that it is only present when my KSGER T12 is switched on, so I’m guessing that the noise is from the switched mode power supply and is coming in via the power in on the amp, so I’ll need to do some decoupling there, to be continued…!

Testing my Direct Conversion Receiver

More Audio Amp “Fun”