[R-390] Panel meters again

[Tisha Hayes]

if you know the internal resistance of the existing panel meter you could
make a little circuit that has the same shunt resistance, tap either side
of a resistor in series and run that into an op-amp, zero and span that to
your hearts content and drive a different meter. (even, Gasp! a digital
meter <joke>). Add a little bit of capacitance to the op-amp circuit, make
the thing smooth out the values, etc...

You could steal power from the 6.3 VAC filament supply, run it to a little
rectifier/regulator and mount that on the same board. Maybe even add a
little internal meter LED light. (or a red flashy LED in the corner to show
on peaks, that would be neat).

Sometimes I think we get too wrapped around the axle of maintaining an
absolutely perfect reproduction of the panel meters. If you do not have the
meters or you have bad meters there still are scavenged meters that show up
from time to time on ePay. Heck, there is an entire industry of people who
cannibalize R-390A's and sell them for their parts value, well in excess of
the cost of an entire radio.

I do not advocate breaking up radios for parts but I have been the
purchaser of many R-390A parts from a variety of sources. This is to the
point where I could build complete radios just out of parts; just so I can
have a spares supply.

The form-factor of the panel meters is pretty standard. I have seen them
used in other radios of the same era. If someone was really motivated they
could put just about any meter movement in one of those cases. If you had a
shallow enough replacement meter movement you could install the little
breadboard op-amp/scaling circuit behind it. Or you might be able to find a
meter movement case that is deeper.

Thinking of a DIP-8 Op-amp package, three lead regulator, tiny little
bridge rectifier, a few resistors and caps, two small pots for scaling,
yea, that would be neat. It could be mounted on a breadboard the size of a
50 cent piece.

-- 
Ms. Tisha Hayes. AA4HA

*""It is not because things are difficult that we dare not venture. It is
because we dare not venture that they are difficult." -Seneca"*