so i've been wanting to build something from scratch for a while now and i finally sat down over the last couple weekends and put together a direct conversion receiver for 40m. followed the basic NE602 plus LM386 audio amp design, nothing fancy, got the parts from mouser and a couple caps and resistors from my junk box.

it actually works which honestly surprised me a little. im hearing SSB stations and CW and the audio is decent through a pair of headphones. BUT theres this really annoying hum, not sure if its 60hz or a harmonic of it, and it seems worse when i touch the coax or get close to the board. i built it on manhattan style pads on a piece of copper clad and the layout is kind of a mess if im honest. also the LO is leaking into the antenna port i think because i can hear myself on another radio nearby when i tune around.

before i start adding a second stage or anything i want to sort out these basics. is the hum most likely a grounding issue or should i just box it up and see if that helps. and is LO leakage just something you live with on a simple design like this or is there a buffer stage i should add between the oscillator and the mixer.

the hum is almost certainly grounding — manhattan builds can be great but if your ground returns are all over the place you'll pick up all kinds of garbage. what i usually do is make sure there's a solid ground plane and all the ground connections go back to one point or at least a heavy copper braid running the length of the board. also check your power supply, wall warts are notorious for injecting hum and a small pi filter or even just a 100uF cap close to the supply pins on the 602 and the 386 can help a lot.

as for the LO leakage, yeah a buffer between the oscillator and the mixer input helps but honestly on a simple DC rx with a whip or short wire antenna in a non-commercial setting its kind of expected. a single JFET source follower buffer is easy to add and cuts the leakage down considerably. boxing it up will help some too, even just a tin from an altoids mint although proper RF enclosure is better obviously.

dont stress too much about the LO leakage for now, like if youre just using it at home and not next to anyone it probably doesnt matter a ton. i built a similar thing last year and had the same hum issue and it turned out one of my ground wires had a cold solder joint that i couldnt even see without a magnifying glass. reflowed everything and it got way better. also the NE602 is pretty sensitive to supply noise so def try the bypass caps close to pin 8 if you havent already.