[codefox1]

Agree with all said. Lead dress and placement of components is very important. Also make sure the screen and bias voltages are correct. If this set has 2 XXL's swap them. Microphonics in a tube take on a whole new meaning when they are in IF/RF service, but sometines you can still tap them gently with a rubber eraser and get a clue.

Blast the switch with deoxit, work it back and forth several times, and let it dry out overnight. I like to take an old dry tootbrush to clean out any crud while it's still wet with the solvent. I'll also swab a tiny bit of WD40 on the bushings and detents with a Qtip to make sure the action is free and easy. If these places are really dried out, I apply a miniscule amount of lithium grease with a toothpick and then wipe a little more WD40 on them.

Many cases of motorboating ultimately come down to a problem in the B+ chain; one stage not being properly isolated from the next. If either the resistor in the voltage divider has changed value, especially if it mearures low or has arced over, this can be it. Rarer, is a replacement filter cap that is open or too small in value, or mis-wired.

Finally if the problem is caused by a very strong local station, something needs to be done to reduce the signal coming in through the antenna.