Nothing in an amp would cause anything to heat up to the point where it smokes. You may see 70 Celsius in spots, which is nowhere near to start smoking things like bits of paper or dead bugs.
Are you seeing a wisp of smoke or is it like a burning cigar? Things that overheat or fail will produce a wisp of smoke, cigar-like smoke is usually followed by an acrid smell that indicates burning electrolytic solution or insulation.
On an old amp, I would first suspect a failing film capacitor if it is a wisp of smoke. If you can see the power supply, follow the secondary wires of the transformer to the circuit board. These wires are usually red (red/white usually is the center tap to ground) and are fatter than the primary 120V wires, which are colored black, green or white/black or red/white. Between the secondary red wires on the pc board, there may be a mylar capacitor for EMI rejection. See if that capacitor is bulging, cracked, or soot black. This is fairly common, especially the gold Rifa PMY caps.
Other than that, you can use a bright light and inspect the circuit boards to see if you can see a burned resistor. All good resistors have colored bands and a damaged one will appear burned. Easy to spot. If that is the case, then the amp needs a diagnostic because resistors do not fail on their own, only a very high current causes that, which almost always comes from somewhere else such as a shorted diode or transistor.
I wouldn't even plug it into the wall until it's serviced.