[R-390] R-390A meter substitutes, repair, or sales.

[Charles Steinmetz]

Chris wrote:

> I have never attempted to take one of these meters apart so would someone
> who has performed meter surgery please enlighten me if it possible to open
> these meters up.  The circular "nut" may not be a nut at all.. But if it is,
> it looks like it requires a unique spanner wrench.or ?

Back in August, Larry posted comments about opening a line level meter, 
with links to photos he took along the way and a detailed description of 
the process:

<http://mailman.qth.net/pipermail/r-390/2016-August/056145.html>

You can find an article about using 1mA FS meters with higher resistance 
here:

<http://www.qsl.net/kh6grt/page4/R390meters/meters.htm>

This requires adding a meter amplifier.  Schematic here:

<http://www.qsl.net/kh6grt/page4/R390meters/CarrierMeterAmp.pdf>

NOTE: The LT1012 and OP97 opamps are improved functional replacements 
for the obsolete LM308, but their overcomp capacitor uses Pin 5, rather 
than Pin 8 (as the LM308 uses).  The appropriate overcomp capacitor 
value could be different, as well (but it is extremely non-critical).

Best regards,

Charles