[R-390] Using Surplus Meters in an R390A

[Dennis Pharr]

Barry:

I had the same thought when I started building my version of the op-amp
circuit.  I believe that by not having a resistance that simulates the meter
impedance it may upset the way the bridge circuit works.  Although it may
not make a big difference, I installed an 18 ohm resistor across the input
to my version of the op-amp circuit.  The op-amp then essentially just
amplifies the dc voltage drop across this resistor. With an 18 ohm resistor
shunting the input, the voltage seen by the op-amp is only about 18mv
(assuming a full scale 1ma reading), so the voltages you are dealing with
are quite small.

Actually, the way I understand op-amps, the combined resistance of the two
4.7K input resistors wouldn't make any difference anyway, since no current
flows into these inputs.  Without the 18 ohm resistor, the op-amp circuit
would only amplify the voltage difference seen between the arms of the
bridge circuit.  

Also, the 10K ten-turn pot used on the output of the Skirrow circuit is much
too large for effective use.  I used a 1K ten-turn pot.  This made it much
easier to set the sensitivity of the meter I used (5ma meter movement).

Good Luck
73
Dennis Pharr
WD5JWY

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of N4BUQ at aol.com
Sent: Sunday, August 29, 2004 8:53 PM
To: r-390 at mailman.qth.net
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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