so ive been building a direct conversion receiver for 40m based on a design i found in an old ARRL handbook, like the 1987 edition i picked up at a hamfest. been at it for maybe 3 weeks on and off. finally got audio out of it last night which honestly felt amazing after all the troubleshooting.

the problem is theres this really pronounced hum riding on everything, and its not just a little hum, its almost as loud as the actual signal. tried a different audio transformer thinking that was it but no change. the VFO seems stable enough, im using a colpitts oscillator with a 2N3904 which i know isnt ideal but its what i had. the product detector is built around a NE602 which i grabbed from a kit i never finished.

im running it off a bench supply right now, regulated 12v. replaced the filter caps on the DC rail already. the hum tracks with the audio volume so its definitely getting in through the AF stage somehow but i cant figure out where. grounding is kind of a mess honestly, i did point-to-point on perfboard and its not exactly pretty. would adding a proper ground bus help or am i chasing something else here

ground bus would definitely help, point to point on perf with no real ground plane is asking for trouble especially in the AF stages. but my first suspicion with a hum that tracks volume is the power supply rejection on the NE602 isnt great to begin with and if your decoupling near that chip is light youll hear it. try putting a 10uF electrolytic and a 0.1uF ceramic right at pins 8 and the ground pin, as close to the chip as you can physically get them. like within a cm if possible. also check that your audio amp stage has its own local decoupling, sometimes people decouple the RF stuff and forget the AF board needs it too.

the 2N3904 VFO thing shouldnt cause hum per se, you might get drift but not 60hz garbage. so i'd look at the AF chain pretty hard. what are you using for the audio amp stage, discrete or something like an LM386?

yeah the grounding thing is real, ive built a few of these and the ones that gave me hum problems were always the ones where i was sloppy about where grounds connected back to the main rail. like all your grounds should ideally star back to one point near the power input rather than daisy chaining across the board.

also -- and this might sound obvious -- are you sure its 60hz hum and not something closer to 120? makes a difference in where you look. 60 would suggest magnetic pickup or a ground loop, 120 usually points to rectifier ripple sneaking through. if youre on a bench supply and it's regulated that second one should be minimal but not impossible depending on the supply. anyway sounds like youre close, hum is way better than no audio at all so you're definitely in the right neighborhood