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

[Drew Papanek]

On the topic of using a meter having a different internal resistance to 
replace the  original carrier level meter, Gary Gitzen wrote:

>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?

Fair Radio performed that type of modification when they installed 
non-original meters in R-390A's for sale a few years ago.  If someone in 
this forum has one of those radios perhaps they might comment on the carrier 
level meter's performance.

Possible change in response of the AGC integrator (AGC LONG) when changing 
bridge resistors has been mentioned.  Those who have performed the complete 
Lankford AGC modification need not worry; his complete mod does away with 
the integrator and substitutes a 20 uF (IIRC) capacitor in the AGC LONG 
mode.

In another posting, Gary wrote:

>The specific action of the AGC-variable suppressor
>grid voltage on V504's (6AK6) cathode current is
>unknown. Can anyone provide insights? Will it result
>in any significant cathode current change, thus
>meter action; or is V504 merely a convenient current
>source?

Perhaps that was done in providing for another carrier level meter function, 
one that many of us have forgotten: indication of IF last stage (6AK6) 
overload when in the MGC mode.

Dave Wise wrote:

>My carrier pot mod
>(posted once or twice in the archives) uses the
>original pot, wired as a pot, but backwards; that is,
>the 6AK6 cathode current goes to the wiper, and
>the meter goes to the top and to a shunt resistor
>to ground.

I performed this mod and like the stability it has given the s-meter zero 
setting.

A few years ago someone posted an op-amp circuit  which permits the use of 
many different types of meter without having to modify the original bridge 
circuit.  It was constructed on a small piece of perfboard and mounted on 
the meter terminals.  It borrowed power from the nearby frequency readout 
pilot lights.  It presented a resistance to the bridge circuit that was the 
same as the original meter's.

The op amp circuit got me to thinking of variations on the 
board-mounted-to-the-meter theme.  The op-amp could be configured as a 
buffer with high input impedance, and be connected to the AGC line instead 
of the original bridge circuit.  The overload indication would be lost but 
how many of us would care and how many of us even use the radio in the MGC 
mode enough to require that indication?

The main advantage would be flexibility in meter selection and stability of 
meter zero setting.  Unless, of course, the no-signal AGC voltage  varies 
because of drift in that delayed AGC B+ bleed into the AGC line...

Drew  (who also has genuine original glow-in-the-dark meters in his radios)