Inspecting a newly acquired HP 2392A I found out the chassis was prone to
whiskering all over the chassis.
Whiskers are small metal hairs growing from pure metal (zinc or tin) and can
when they connect to a power line create a shortage and electric arc
damaging the electronics.
Made some pictures and placed them in:
https://www.flickr.com/photos/hp-fix/41897770664/in/dateposted-public/
It's best to remove them by using scotch brite or fine sanding paper, after
that paint or coat the chassis with a clear paint or other nonconducting
paint.
More info about whiskering can be found at:
https://nepp.nasa.gov/whisker/
https://en.wikipedia.org/wiki/Whisker_%28metallurgy%29
-Rik