[R-390] Carrier Level Meter findings

[Bob Camp]

Hi

The problem with the "high impedance" meters is that the drive circuit 
for the carrier meter is a bit odd for a tube circuit. We tend to think 
of tubes as being high impedance devices. In this case the meter forms 
part of a low impedance cathode driven bridge circuit. The impedance of 
he meter actually makes up part of the cathode impedance on the driver 
tube.

I'm sure they had their reasons for doing it this way. With the right 
meter in the circuit the carrier meter is amazingly linear.

	Take Care

		Bob Camp
		KB8TQ


On Mar 26, 2005, at 5:08 PM, Barry wrote:

> Last night, I did some more experimenting.  It turns out the internal
> resistance of these meters is nowhere near 100 ohms.  It took 140 ohms 
> shunt
> resistance for these to read 1ma.  By my calculations, this would make 
> the
> internal resistance about 1260 ohms.
>
> In any case, the meter will read 1ma full scale now.  I disconnected 
> the
> meter in my working R390A and jumpered this one in place.  The results 
> were
> somewhat dissappointing.  It works, but where the original meter will 
> show a
> 60 reading, this meter will only show about 20.  I'm sure it has 
> something
> to do with the fact that the internal resistance is so high, but I 
> don't
> know why that affects things like it does.  I can even remove the shunt
> resistor an the meter still doesn't give much more reading.
>
> Looks like I'll either be building Jan's OpAmp circuit or finding some
> originals.
>
> Barry(III) - N4BUQ (who is going to have my own "intelligence agency" 
> black
> panels painted soon...)
>
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