so ive been sitting on a pile of parts for like six months and finally got around to building a direct conversion receiver for 40m. used the NE602 mixer chip which i know some people have opinions about but it was what i had on hand. built it ugly style on a piece of copper clad and its actually picking up signals which honestly surprised me more than it should have.

the problem im running into is theres a ton of audio hum and i cant figure out if its coming from the power supply or if its RF getting into the audio stage somehow. im running it off a wall wart right now which probably doesnt help. also the BFO drifts pretty bad for the first 10-15 minutes which i expected but its worse than i thought it would be. like i can barely copy CW in that window.

anyone built something similar and got it stable? wondering if adding a voltage regulator on the oscillator supply would help or if theres something else i should be looking at first. not trying to make this into a contest radio obviously just want it to receive decently on 40m SSB and CW

the hum almost certainly your wall wart. those switching supplies dump garbage everywhere and a direct conversion rx is just gonna pick all of it up especially if you dont have good bypassing on the audio stage. throw a 7812 or 7809 whatever youre running and some decent filter caps in there and the hum will probably drop way down. i had the exact same issue with a NE602 build a while back and just adding a proper reg cleaned it up like 80 percent.

the BFO drift is trickier. if youre using a ceramic resonator itll always be sketchy at startup. an LC VFO with a decent quality coil wound on a form thats not gonna expand with heat will be more stable but it takes more tweaking. some people just let it warm up before they operate which isnt great but it works. regulating the oscillator supply does help, definitely worth doing regardless.

oh man i went through this exact thing last year. my first dc receiver was an absolute hum machine. ended up being a ground loop between my audio amp board and the main board, i had them connected with a long wire and it was acting like an antenna basically. once i tied the grounds together at one point instead of daisy chaining it got a lot better. might be worth checking if youre doing something similar.

also the NE602 is fine dont let anyone tell you otherwise for a starter build. yeah it overloads easy but for 40m at home its totally usable. have fun with it, theres something really cool about hearing stations on something you actually built