[R-390] Another 390A Carrier meter question

[Flowertime01 at wmconnect.com]

DW Holtman, WB7SSN future212 at comcast.net

In a message dated 11/4/05 7:20:30 PM Eastern Standard Time,  writes:
It takes a pretty strong station to get any readable meter movement.  I have 
acquired the correct meter. My question is about the sensitivity of the meter.
Any ideas or suggestions will be greatly appreciated.

DW,

Sad things I have to say. The R390 carrier level meter is un scaled to any 
standard. Never was. Never was taught in school to be related to any thing. It 
was just a nifty tuning indicator that let you sort of know when you had the 
signal in the band pass or the receiver.

The zero point was never an easy thing to set. It was the only specification 
for the meter circuit. We never tried to even get them to zero for the 
operators. We left them laying a bit off the left peg just so that what ever signal 
did come alone was not lost before the needle got off the peg. Some of the guys 
have used 10 turn pots to get a better zero. Some have used 10 Ohm pots and 
made up the rest of the resistance with fixed resistors to make the zero a 
little easier to set.

If you have a signal that puts the needle up about mid scale, you likely can 
bite down on a chewing gum wrapper and detect the signal in your mouth on one 
of your fillings. Sensitive the circuit is not. Calibrated is not used in 
reference to the meter circuit.

One side of the meter is set to a small positive voltage by the current drawn 
through R548. This is a 27 ohm cathode resistor for the AGC time constant 
tube. When you change the AGC speed. the change in voltage causes the meter to 
peg. This is all normal. The 5814 sections draws a little less current than the 
6AK6 so its 27 ohm resistor balances some where with the resistance in the 
6AK6 cathode circuit.

On the other side of the meter the stock 100 ohm resistor shorts a 22 ohm 
resistor. So the carried meter adjust varies from 0 to 18 ohms. The circuit is a 
voltage meter to measure the voltage drop across the bottom end of the 6AK6 
fourth IF cathode resistor. The tube always conducts. There is always some 
voltage drop. Placing the other side of the meter against the AGC tube cathode 
resistor just offers a zero point for zero signal.

Once a signal hits the grid of the 6AK6, the tube conducts a bit harder. A 
little more voltage is developed across the metered section of cathode resistor 
and the meter will move up scale.

6AK6s will change in gain with any variable you would like to mention. A 
calibrated circuit it is not. No where is the developed signal in the receiver run 
across a fixed resistance and the magnitude of power metered. The line level 
output meter is as close as you get to a metered signal. This is just the 
audio level developed and has no calibrated relation to the RF signal strength.

The meter is nice and it functions as designed. A S meter it was never 
intended to be.

Roger KC6TRU