If memory serves me correctly, the IN acts as the power amount signal. That could explain the issue. I’ll do some more digging to refresh my memory. Had a similar issue with loss of power, but I had a tube failure on my 60 watt LS1630, twice! My new HP2436 150 watt machine wasn’t reading where it should’ve when I got it a few months ago and requested a new tube. The meter I bought already paid for itself.
If for some reason, it is the power supply, you might want to check these folks out:
https://www.lightobject.com/Search?CategoryId=15
Let me know how you make out.
Ken