My dryer had a terrible burning smell. A repairman cleaned out the lint and filter. For a few days all was OK, then the dryer circuit breaker would trip when the dryer was not in use and now that burning smell is back. What is the problem?

All the following is with the power OFF.
Tough to guess, but would follow the incomming Line wires for resistance to Ground and resistance to other Line wire, Disconnect components until the low resistance is gone, unplug Timer as well to see if that removes the short. You can also test the element and thermal for resistance to ground with wiring from each diaconnected.

You may be able to see the damage visually.

If the problem is present with the dryer turned off, the dryer controls are certainly suspect. Until this problem is solved or you buy a new dryer, leave the breaker off when any time the dryer is unattended. There have been many recalls of appliances which have caused fires even when not in use, and the problem is often in the control panels.

My guess is that he did a half-way job of cleaning and servicing it. You have to tear a dryer down to the individual pieces and wash out all of them and them grease all bearings and check all safeties and the element. I would guess that it is still dirty which caused the element to burn out and touch the side of the dryer. Just tear it down and you will be able to see what is wrong with it.