[R-390] panel meters

[wli]

I think that you are correct. Here is some earlier posts re these meters that are of interest to all:
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Date: Fri, 22 Jan 1999 17:27:08 +0100
From: Rolf-Lutz at t-online.de (Rolf Lutz)
Subject: [R-390] R390A Meters

In HSN 22 Page 2 is a specification of those 2 meters. The R390A line meter is 250 µamps full scale, 3360 ohms internal resistance The R 390A carrier meter is 1 mA full scale 17.7 ohm internal resistance. Even a small error (high or low ) in the internal resistance of a carrier meter will cause rather large errors in carrier meter readings ( Dalles Lankford) Spring 1980
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Date: Fri, 4 Nov 2005 19:56:51 EST
From: Flowertime01 at wmconnect.com
Subject: Re: [R-390] Another 390A Carrier meter question

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 whatever 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. 6AK6's will change in gain with any variable you would like to mention. A calibrated circuit it is not. Nowhere 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
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