The amp won't come out of standby unless the power cord is unplugged
for about 20 seconds.

Once you do this it will come on and perform ok.

It can be turned on and off with no problem and will stay running
indefinitely

When turned off for about 5 mins then won't come out of standby again.

I have just got a copy of the service manual looks like it may be the
power good signal.

Anyone seen this before?