There are several NPS200s in the market you might want to look into. They aren’t worth much so you may consider replacement vs fixing. I can certainly help you diagnose the problem, its likely a loose wire or bad cap somewhere. I m in Sunnyvale. don’t want to do upgrades cos there’s a world wide parts shortage (eg nichicon KX and KG gold tunes are mostly out of stock) I’m also not in the business of swapping a cap and a few resistors and charging people a fortune sprinkling fairy dust. The basic upgrade addressed a few sources of bad sound in the original amp. But it’s not worth dumping $ into
It. and the resale value of the amp is just not worth dumping $$$ into it.