The typical Onkyo (well most amplifiers) fault. The amplifier switches into standby after a couple of seconds.
This is caused by something triggering the protection circuits, more often than not something in the main amplifier output stage has failed such as a power transistor going short which puts DC on a speaker output.
In this instance three transistors have failed on the centre channel, presumably one failed and took the other two with it. Easily done when you have +/-50 volt power supplies!
There are a lot of boards in these things, taking them apart without having to remove all the connecting cables is a challenge but it makes reassemble a easier.
New parts ordered. If they go pop again this would be a good excuse to fit a modern TDA7293 based 100w amplifier board as shown below in place of the failed channel.
Home theater amplifiers have at least 7 individual power amplifiers in them and a failure in any of them will shut the amplifier down. A lot of the transistors in older units are obsolete, making them difficult to source.
These TDA7293 boards cost about £5 which is cheaper than buying individual transistors once you’ve factored in shipping costs from multiple suppliers.
Obviously fitting it into an existing design isn’t that simple as the power supplies in big amplifiers are often in excess of what the TDA7293 can handle plus you have to attach it to the existing heatsink with an insulating washer.
A bit of work, but cheaper than buying a new receiver.