If this is an HCN4226 open the back and locate the convergence board - it's up and to the left mounted on the frame rail. Unplug the power to it, the board is marked with a +/-20v symbol. Now plug the unit back in and try to power it up. If it comes up and has good sound that convergence board board needs to be repaired. The amps on it need to be replaced and both the return and line resistors have to be tested and replaced as needed. Some shops, ours included, have started offering cooling packages to help prevent this circuit from overheating by mounting a cooling fan directly in front of the amp board. It's an inexpensive measure to help cool that circuit.
Bill
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