I am very far from being an expert but I have found that poor assembly in mine has caused several problems over the years. This thread is an example of a power supply problem I had.
https://www.vintage-radio.net/forum/...ad.php?t=83064
It might be worth going over the component soldering and connections in general as some strange faults manifest themselves in these amps.
I was going to buy one new myself but when I went to Hardman Radio in Liverpool with my savings to get one all their stock had been returned to Armstrong for inspection as they had received so many complaints and I bought a Rotel instead.