Welcome to the place. It sounds like one of your output transistors has shorted. If your meter has a diode check function use it to check the output transistors. If your meter doesn't have a diode check function, use the ohm meter instead.
The outputs are the large plastic cased blocks that are mounted to the aluminum heatsink. Each one has three legs. Viewed from the top the legs are from left to right base, collector, emitter. The most common fault is a short from collector to emitter. Because the output transistors are paralleled, if one is shorted, both on that side will test as being shorted.
If the transistors test ok, then the next thing to check would be the power supply diodes for shorts.
Let us know what you find out.