[R-390] re: Call for measurements - 100dB carrier reading

[Barry]

>Hi
>
>Just about *any* pot will work just fine. Another approach is to put  
>a very low value pot in series with a fixed resistor.  There's not a  
>lot of power involved here. The only risk is that if the circuit  
>opens up you may loose the meter.
>
>Bob

I think that's true.  Theoretically you could use any value pot that covers the range needed to zero the meter.  The problem would be that a very high-value pot (e.g. 1Meg., etc.) would have such a small area that covers the appropriate range that it would be nearly impossible to zero the meter.  Also, a 1Meg pot probably doesn't have the low-end resistance range needed, but I'm talking about a theoretical pot here.

According to David, the range needed is about 4 to 7 ohms.  A 3-ohm pot in series with an appropriate resistor would probably yield the best range to allow a single-turn pot zero the meter the easiest.

Just my $0.02.  As I said, my 20-ohm, single-turn pot works well enough.  Getting an exact (EXACT) zero is a bit tricky, but given that the meter is not a high-precision device to start with, it really doesn't matter that much.

Barry - N4BUQ