so ive been working on this 40m superhet for about three months now and i finally got something resembling a working receiver last weekend. using a NE602 for the mixer/osc, standard 455khz IF with a couple of those ceramic filters from the junk box, and an LM386 at the end for audio. the thing actually works — i can hear stations — but the sensitivity seems way off and theres this weird low frequency rumble that comes and goes. not 60hz hum, its lower than that, almost like a motorboating kind of thing.

ive already added more bypass caps around the LM386 because that chip is notorious for oscillating if you look at it wrong, and the rumble got a little better but not gone. gain staging is probably the issue but im not totally sure where to start poking around with the scope. the front end just has a simple tuned input with a MPF102 JFET preamp, nothing fancy. IF strip is two stages of amplification between the ceramic filters.

anyone dealt with this kind of thing before? feel free to tell me if my whole approach is wrong too, this is basically my first real scratch build so im definitely learning as i go.

motorboating in that kind of circuit is almost always a power supply decoupling problem, usually between stages sharing a rail. the LM386 fix you did was right instinct but if your IF amp stages are pulling from the same line without good isolation between them youll get that feedback path through the supply. try putting a small resistor — like 47 to 100 ohms — in series with the supply to each IF stage and then bypass each one locally to ground with like a 100uf in parallel with a 0.1uf ceramic. belts and suspenders approach but it usually kills it. also the MPF102 preamp can be a noise contributor if its biased too hot, worth checking the drain current, i usually run those around 2-3ma for receive applications.

the sensitivity question is harder to answer without knowing more about your IF strip gain. two stages with ceramic filters can have quite a bit of insertion loss depending on which filters you grabbed, some of the cheap ones are surprisingly lossy. what part numbers are on them if you can read it?

yeah the 386 is a pain, i built a direct conversion rig a while back and fought that chip for like two weeks. ended up putting a 10 ohm resistor and cap on pin 7 and that helped a lot, theres a note about it in the datasheet if you havent seen it already. also just double checking — you do have a cap on pin 1 and 8 for the extra gain right? sometimes that interacts weirdly with certain layouts and makes things worse even though it shouldnt in theory.