so ive been building a simple direct conversion receiver for 40m, nothing fancy just the classic NE602 mixer feeding an LM386 audio stage. got it breadboarded last night and i can actually hear stations which is exciting but the audio is really noisy and there's this constant hum that i cant seem to shake. the VFO seems stable enough, im using a colpitts oscillator with a 2N3904 but honestly im not 100% sure my coil winding is right. wound it on a T50-2 toroid, maybe 25 turns or so, i kind of eyeballed the inductance.

the hum is pretty clearly 60hz related i think, goes away a tiny bit when i touch the ground plane with my hand which tells me something isnt right with grounding maybe? im powering it off a wall wart right now which is probably not helping. im wondering if anybody has gone through this with the NE602 before because everything i read says its a pretty sensitive chip and easy to overload. anyway not sure if its a shielding thing or a power supply thing or just my layout being terrible

the wall wart is almost certainly a big part of your problem, those things are terrible for receiver work. even a regulated one puts out garbage on the rails compared to a battery. throw 4 AA cells at it temporarily just to see if the hum drops, if it does you know where to focus. also the LM386 is notoriously sensitive to power supply noise, you really want a bypass cap right at pins 6 and 2 as close to the chip as possible, like 100uF electrolytic plus a 0.1uF ceramic right next to it.

touching the ground and having it change suggests your ground plane isnt continuous or youre missing a connection somewhere. on a breadboard this stuff is a real pain because the ground rails dont always connect end to end depending on the board. worth checking with a meter before you go crazy chasing other things.

yeah i had almost the exact same issue with an NE602 build a while back, turned out my bypass caps were too far from the supply pins. also worth checking if your audio stage is picking up RF from the oscillator, if the LM386 input lead is too long it acts like an antenna basically. i ended up having to shield the VFO section with a little tin can just to clean things up. not pretty but it worked lol