[R-390] What is inside R-390 meters?

[Richard]

This may help a little although  targeted towards R390A.

Check http://www.skirrow.org/Boatanchors/how%20to.htm for 
http://skirrow.org/Boatanchors/TechTalk2.pdf Adapting Surplus Meters for 
the R-390A

R390A meter faces are at http://www.r-390a.net/faq-refs.htm

The line level meter I fit into my radio used a DC meter movement and an 
external bridge rectifier with a repro scale.

73 de Richard W5SRB