[R-390] Using Surplus Meters in an R390A

[N4BUQ at aol.com]

David,

I agree with this.  I was trying to come up with a solution using the existing (hi-ohm) meters with a resistance network.  As I was drawing it up and attempting to come up with shunt resistor values, etc., it occurred to me that I would need the voltage across the original meter.  As I started looking at this, it became crystal clear that a network of resistors would not work.  The voltage across the meter could not be increased by the network (hope that makes sense).

Maybe I'll try your carrier level mod on my next radio.  I replace the one in my current radio with the 10-turn wirewound.  Works fine.

Thanks again!

Barry(III) - N4BUQ

<snip>

>By the way, I suggest that instead of referring to this meter as a
>"1mA meter with 18 ohms resistance", we call it an "18mV meter".
>Given the low-impedance circuit it's living in, this expresses the single
>real requirement as a single number, which is a lot easier than carrying
>around a table of equivalent resistances for replacement meters of
>various sensitivity.  Just my $.02 (in 1953 dollars).
> 
>73,
>Dave Wise (SWL)
> 
>-----Original Message----- 
>From: N4BUQ at aol.com [mailto:N4BUQ at aol.com] 
>Sent: Sun 8/29/2004 7:53 PM 
>To: r-390 at mailman.qth.net 
>Cc: 
>Subject: [R-390] Using Surplus Meters in an R390A
>
>
>
>    I downloaded Jan's article about using surplus meters.  Looking at the
>    schematic, it appears that inserting the OpAmp where the meter used to be will
>    result in a minimun of 9.4K ohms where (according to what I remember posted on this
>    list) the original meter's DC resistance is somewhere around 30 ohms.
>    
>    Can someone comment on how/if this affects the performance in this part of
>    the radio?  The text of the article indicate it is necessary to reset the
>    carrier meter adjust control for the proper readings.  I assume that the difference
>    of the resistance will cause this setting to be somewhat different than with a
>    meter with 30 ohms resistance.
>    
>    I haven't looked at the entire circuit yet and perhaps resetting the carrier
>    meter adjust control to compensate the difference in resistance causes the
>    circuit to become "normal" again, but not sure.
>    
>    I have some replacement meters I am interested in using, but their internal
>    resistance is around 600 to 800 ohms so I was looking for a way to use them. 
>    It appears Jan's method is easy enough, but I was wondering what the effects
>    are when using it.
>    
>    Any comments?
>    
>    Thanks,
>    
>    Barry(III) - N4BUQ
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