[R-390] Question about Carrier Level Meter

[ToddRoberts2001 at aol.com]

In a message dated 3/24/2005 11:10:06 PM Eastern Standard Time, N4BUQ at aol.com 
writes:
I have some 100uA meters that I'd like to use as replacements for the meters
in my R390A.  They are supposed to have an internal resistance of 100 ohms.
Supposedly, by shunting with an 11 ohm resistor and then placing an 8 ohm
resistor in series, this will make them effectively 1mA meters with
approximately the same internal resistance as the original meters.

I experimented a bit tonight with this and I didn't get the results I
thought I would.  I shunted one of them with 11 ohms and placed it in
parallel with the existing meter.  I expected both meters to read
approximately 1/2 of what they normally would in the circuit.  Instead, the
shunted meter barely moved and the original meter hardly noticed the new
meter in the circuit.  I removed the shunt and the new meter read a bit
more, but not nearly as high as the original meter that it was in parallel
with.

I then placed an original meter in parallel with the existing meter and I
got the action I expected.  For example, a 60dB signal would show as about
30dB when they were connected in parallel.

Is this happening due to the fact that the internal resistance of the new
meter is so much higher than the original meter and the current is simply
taking the path of least resistance?  If I remove the original meter from
the circuit, can I expect the new meter to perform better or will it be low
reading no matter what?

If I have to, I'll build Jan's circuit and tune them up that way, but I was
hoping to simply use a resistance network and use them as is.  Maybe not?

Thanks,

Barry(III) - N4BUQ

Unless both meters have EXACTLY the same internal resistance and full-scale 
readings they will not read equal when placed in parallel. Have you tried 
placing the meters in series when testing them? If you place the meters in series 
it will assure equal current in both meters. Do you have a way to measure 
exactly the internal resisitance of the 100uA meters? It sounds like they may not 
have an internal resistance of 100 ohms from your results. I would try a 
low-value variable resistor across the 100 uA meter to see if you can 'dial in' an 
equal full-scale reading. It won't work to place an additional resistor in 
series with the 100 uA meter AFTER it has been set to read 1mA full scale, that 
will throw the full-scale reading off again. You would have to set the 100 uA 
meter for a reading somewhat less than 1mA full scale and then add the series 
resistor if you absolutely had to have the same series resistance as the 
original meter. Again I would play with a variable resistor across the 100 uA meter 
to see what results you can get. 73 Todd Roberts WD4NGG