Repairing an Onkyo HT-R380 Amplifier stuck in Standby

A typical fault with most old amplifiers and receivers is that they switch into protect or standby mode at switch on. In the case of the Onkyo HT-R380 the unit switches off after a few seconds and sets the red standby light flashing.
This video investigates the fault on a unit I picked up from eBay. It starts off with a look at how the power supply works which is sort of relevant to know what's happening in the unit.
I have taken the HDMI board out of it to save me keep taking it out every time I needed to look for a component on the main board.
Note that some components are on the underside of the main amplifier board so you can't easily get to them. The protect signals are easy to get to on the processor board by the connector that connects to the main amplifier board.
If you don't know what you're doing, poking around in any mains powered equipment is dangerous. The terminals on the mains transformer have the mains supply on them and it's all too easy to touch or rest your arm on them and get a shock.
The main amplifier also runs on +/-49vdc so you have 100v between some points that's backed up by powerful smoothing capacitors.
An update: I brought a replacement STK module from AliExpress and it was more faulty than the original faulty one. I swapped it around with the good one and the faults moved with the new one so on that basis the circuitry on the main board was ok. I'm looking into replacing the module with something more modern..