[R-390] Carrier Level meter R-390 A

[Gary Gitzen]

Hi R-390,

Pete from Oz wrote of problems finding original 17 ohm
carrier level meters, and notes that available 100 ohm
meters give at best grudging needle deflection.

This prompted a look at the schematic of the carrier
meter circuit for the 390A. It's essentially a bridge,
balancing currents between the cathodes of V504,
the 6AK6 final IF, and V506A, the AGC time constant
tube. The resistors in the bridge allow a voltage to
be developed across them, based on the currents.
Any voltage difference flows through the meter,
moving it.

Why not, I asked myself, change resistors R537 & R548
to larger values to allow larger deflection using
100 ohm meters? A quick guesstimate is that
removing R537, the 22 ohm 1W across the carrier adj
pot, and increasing R548 from 22 ohms to approx 160
ohms should give reasonable results. It might
even be appropriate to add a small resistance
(22 ohms???) in series with R523, carrier adj,
to get adequate adjustment range.
Possibly also reduce R524, the 680 ohm 6AK6
cathode resistor, to 600-620 ohms or so. Shunting
R524 with 5.1K or 7.5K would give that result.

Can anyone shoot reasonable holes in this idea?
Caveat #1: I haven't tried it.
Caveat #2: If you do this, _please_ document it
on both the chasis and the IF module.

Gary, glad both his R-390As have original meters