so ive been working on this thing for about three weeks now, pretty much based on the classic rock-mite design but i kind of went off-script with the oscillator stage because i couldnt source the right crystal and figured id try a colpitts with a variable cap to get it close enough to 7.030 or so. got the PA stage mostly working, im seeing around 800mW into a dummy load which isnt bad for what it is.

the problem is the oscillator drifts like crazy for the first 10-15 minutes. i mean it settles down eventually but when i first key up its just wandering all over the place. i put a small piece of foam over the crystal area thinking it was thermal but that didnt really do much. supply voltage is stable, checked it on the bench supply and its flat. not sure if its the crystal itself or something in how i have the tank circuit loaded. the caps i used are ceramic disc which i know arent ideal but its what i had in the junk box.

anyone dealt with this before? im not sure if i should just order some NPO/C0G caps and swap them out first or if theres something else going on. the drift direction is always the same - goes low then comes back up if that matters.

yeah the ceramic disc caps are almost certainly your problem, or at least a big part of it. the temperature coefficients on generic ceramic discs are all over the place, Y5V and X7R stuff can drift hundreds of ppm per degree C which in an oscillator circuit is brutal. NPO/C0G will make a huge difference, seriously night and day. i went through the exact same thing on a 20m QRP rig i built a couple years back, swapped out maybe four caps in the oscillator tank and the drift basically disappeared.

the direction of drift being consistent (going low first) does kind of point to thermal as the culprit too so you're probably on the right track with that diagnosis. worth pulling the ceramics first before you go chasing anything else. also make sure your oscillator stage isnt getting heat from the PA - if they're physically close on the board that can cause exactly this behavior, the PA warms up fast and the oscillator just gets cooked slowly for the first few minutes of operation.

not super experienced with homebrew transmitters but i built a 40m receiver last year and had drift issues that turned out to be my voltage regulator getting warm and causing the supply rail to sag just slightly - like not enough to see on a meter but enough to pull the oscillator. probably not your issue since you said the bench supply is flat but just throwing it out there in case