Moreplay left channel crackle then out completely

[dyrstr8s]

My Moreplay, been using with no problems for weeks (months maybe) since upgraded.
Recently had an intermittent crackling, popping in left channel, then it went out completely.
I performed the low voltage test from the upgrade manual. Both sockets measured at 6.27-6.3v.
I performed the high voltage test. Measured 127v at U6, 180v at B+ Reg TP, but less than 1v at U16.
What to check next?
Thank you.

 

[Paul Birkeland]

I would swap tubes just to see if that makes any difference, but you're likely up against a broken wire or a flaky solder joint. The 0V at 16U makes me wonder if the wire connecting from there to the upgrade board is broken.