Static, left channel from B&K EX-4420

I looked at the schematic. Looks like the relay you need is twT9as 24VDC.

Your amp takes two since they are single pole relays. One for the left and one for the right channel. I would replace both while you are at it, since they are electromechanical devices and have a life cycle. The good one is likely not far from giving out also. They are very cheap, and you can find them all over the internet by typing twT9as. Check eBay. Would love to know the outcome! Best regards.

There also a relay RL3, looks like it's a 12V DPDT relay with a coil resistance about 900Ω.

It was a slo-blow fuse, All fixed!