so after about three weekends of messing around i got my direct conversion receiver for 40m actually pulling in signals. based loosely on the NorCal 40A design but i changed a few things because i didnt have all the parts and honestly i was impatient. the VFO is a colpitts using a 2N3904 and it drifts like crazy for the first 20 minutes but after that its actually pretty stable. audio sounds decent through my old computer speakers.

the problem is i'm getting this really annoying hum, like 60hz hum, even when nothing is connected to the antenna port. ive triple checked my power supply filtering, got a 1000uf cap right at the board plus some smaller ceramic bypass caps. tried running it off a 9v battery and the hum is still there which makes me think its not the power supply at all but im not sure where else to look. maybe RF getting into the audio section somehow? the board layout is kind of a mess honestly, i just used ugly construction on a piece of copper clad.

anyway im pretty happy it works at all for a first attempt. heard a bunch of SSB on 7.2 last night which was cool. just want to kill this hum before i try adding a band switch for 80m.

congrats on getting it going, first working homebrew receiver is a great feeling. the hum on a DC receiver is almost always one of two things in my experience -- either you have a ground loop somewhere in your audio chain or your mixer is picking up mains frequency through the enclosure or nearby wiring acting like an antenna. since it persists on battery power id lean toward the second one. try building a small Faraday cage around the detector/mixer stage, even just some copper tape shielding can make a huge difference. also make sure your audio wiring is short and twisted if you have any runs of more than a few inches. ugly construction is fine but audio leads running near the VFO or LO section will cause you grief every time.

the drift thing is normal for a Colpitts off the shelf like that, you can help it a lot by thermally coupling the main frequency-setting capacitors together and keeping them away from any components that get warm. some guys wrap the VFO section in foam after warmup just to stabilize the temperature. not elegant but it works.

im also building a DC receiver right now, different band but same basic idea. i had the exact same hum issue and for me it turned out to be my audio amp chip, i was using an LM386 and those things are weirdly sensitive to layout. had to add a 10uf cap from pin 7 to ground and keep the gain-setting stuff really tight to the chip before it settled down. probably not your exact problem but worth checking if youre using one of those.