...The DC output is 22 volts or so on the output plug and on the ACCY connector on the rear chassis. I measured 22 volts on the top of C1 as well.
1) At least one of the large pass transistors on the heatsink is "welded up" which is another way of saying "a hard collector to emitter short". If a diode check measures near-zero Ohms from C to E in both directions that's your proof. A working transistor will flow current from C to E in one direction only when there's enough base voltage to turn the junction on. Otherwise, no voltage on the base = no current flow from C to E in either direction. *
2) The regulator circuit has failed in such a way that the base voltage is being held high enough to drive the pass transistors into full saturation.
Failure mode #1 is far more common than #2. If it was case #2, the output would be approximately half a volt less than the voltage on the big filter cap as there is always some voltage drop across a good bipolar transistor junction.
* FET's are an exception as a healthy example can remain turned on for several minutes after the base voltage is removed. Very unlikely you have MOSFET or HEXFET pass transistors.