so i've been poking at this thing for about three weeks now and i finally got some audio out of it last night. its based on the WB4HFN design, one transistor regen, wound my own coil on a piece of pvc pipe because i couldn't find the right form. picked up 40m signals pretty clearly for about an hour then something changed and now it just kind of hisses and the regeneration control barely does anything. like it either oscillates hard or nothing, no sweet spot anymore.

i haven't changed anything that i know of. temperature in the shop got pretty cold last night, maybe that did something? the trimmer cap i used was pulled from an old AM radio so who knows what tolerance that thing actually is. anyway if anyone has built one of these and ran into similar issues im all ears. also curious if the coil spacing matters much, mine is pretty tightly wound.

coil spacing absolutely matters on a regen, yeah. tight winding increases the distributed capacitance and that can mess with where the feedback sweet spot lands, especially if you're also dealing with component drift from temperature. the trimmer cap from the AM radio is probably NP0 or something close but who knows, those old units sometimes had ceramics that drift like crazy with temp changes.

what i'd try first is just warming the whole thing up with a heat gun on low from a distance and see if the behavior changes. if the sweet spot comes back when its warmer, you've basically confirmed its a thermal drift issue somewhere. could be the transistor bias point shifting, could be the cap, could even be your coil former expanding slightly. pvc is actually not terrible but it's not the most stable thing either. if you wound it yourself try loosening the turns a bit if you can, or just wind a second coil with slightly more spacing and swap it in, takes maybe 20 minutes.

ive had the exact same thing happen with a similar build and it drove me nuts for like two weeks lol. turned out one of my solder joints on the feedback capacitor was just... barely making contact. cold solder. everything looked fine visually but when i reflowed it the regeneration control came right back. worth checking all your joints especially around the tank circuit before going too deep into the coil stuff.