so i built a crystal radio about two years ago when i first got into this hobby and it actually worked pretty well, pulled in a couple AM stations with just a long wire out the window. figured since that went okay i'd try something more ambitious and started working on a superhet receiver from a design i found in an old ARRL handbook from like 1978 or so. im using a 455khz IF strip and the first IF transformer i wound myself on a torroid which probably wasnt the smartest move to start with but here we are.

anyway the thing sort of works — i can hear something on 40m but theres this constant birdie around 7.1 that i cant seem to get rid of no matter what. shielded the IF can, tried moving the oscillator section further from the mixer, even wrapped the whole board in foil for a test and it's still there. my gut says its oscillator radiation getting back into the antenna input but honestly i dont really know enough yet to diagnose this properly.

has anyone gone through something similar building their first superhet from scratch? starting to wonder if i should've just bought a kit instead of rolling my own completely.

the birdie at 7.1 is almost certainly your local oscillator leaking — if you're receiving 40m and using a 455khz IF then your LO is sitting right around 7.555 or 7.045 depending on which side of the IF you're on, and even tiny amounts of radiation from that get picked up by the front end before any selectivity can do anything about it. foil on the board helps a little but you really want a proper grounded metal enclosure for the oscillator section, not just a shield — actual tin or copper can soldered together and grounded solidly at multiple points.

also worth checking if your RF gain stage (assuming you have one) is oscillating slightly, that can cause all kinds of weird birdies that move around when you touch things. poke around the front end with an insulated stick and see if the birdie shifts frequency at all, that'll tell you something. first superhet is always kind of a mess honestly, dont get discouraged — i redid mine three times before it was clean enough to actually use.

yeah i had almost the exact same thing happen when i built a 40m direct conversion receiver, not a superhet but similar problem with LO leaking out the antenna port. what finally helped me was adding a bandpass filter right at the antenna input — nothing fancy, just a simple LC filter tuned to the band. cuts down on the junk getting in AND stops some of the oscillator from sneaking back out. might be worth trying before you tear the whole board apart.