Suspect an overload in the main output devices on the heatsink for that channel. One or more of the devices is simply acting like it is a piece of wire. If they are transistors it will be easy to spot them with an ohmeter as it will be just like you have touch the test probes together on touching the terminals of each transistor. It will even do it in circuit, if the fault hasn't blown a hole in it first so you don't need tests. Remember the good channel is the same as the bad so tests on it's transistors etc will be the same as the bad till you find the faulty part! And if it has blown it up the part number is in the good channel so that's one good piece of news.
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Suspect an overload in the main output devices on the heatsink for that channel. One or more of the devices is simply acting like it is a piece of wire. If they are transistors it will be easy to spot them with an ohmeter as it will be just like you have touch the test probes together on touching the terminals of each transistor. It will even do it in circuit, if the fault hasn't blown a hole in it first so you don't need tests. Remember the good channel is the same as the bad so tests on it's transistors etc will be the same as the bad till you find the faulty part! And if it has blown it up the part number is in the good channel so that's one good piece of news.