As @DrDrill said it really sounds like an effect loop problem. Just put a (small length) patch cable from the send to the return : if the problem disappears you can leave it so and put some contact cleaner or have the two switching connector jacks replaced.
It's a very common problem, especially if you never use the effect loop, but too often forgotten by techs.
And NEVER cut and start immediately anything with power capacitors or you'll blow them. Just wait 10 to 30 real seconds to empty them totally and start again.