[R-390] R390A Carrier Level Meter Question

[David Wise]

Wondering what will happen if you put in
any old 1mA meter?

The R-390, R-391, and R-390A use the meter as one
leg of a bridge circuit, and the meter was wound
with unusually heavy wire to get the exact resistance
required to give the correct calibrated response.
Most 1mA meters have coil resistance higher than 17 ohms.
If you install such a meter, the radio won't be harmed,
but the meter will not go upscale as far as it should
for a given carrier level.

There is info around which explains how to modify the
radio to use a given meter, and other info on building
an amplifier to make a meter compatible with an unmodified
radio.  I don't know if the following is mentioned anywhere.
It probably is, but it doesn't take much space to explain,
and if you can find an appropriate meter it's the "smallest"
mod possible.

You are not restricted to a 1mA meter per se.  You can
use a more sensitive meter along with a shunt to give
a 1mA full-scale response.  This does not escape the
basic problem of coil resistance, but it does widen your
options a bit.  For example, if you had a 50uA meter with
340 ohms coil resistance, you could shunt it with 17.9
ohms.  The problem is, 340 ohm 50uA meters are just as
rare as 17 ohm 1mA meters.  Still, you never know what
will pop up in the junk box.

By the way, there's nothing sacred about the exact 17 ohm
value, that's just what they found would give about the
right cal most of the time.  If you really care about
this and have a signal generator where you can trust
the output level, you can calibrate the meter to match
your particular radio by tweaking its resistance slightly.
On the other hand, this goes against the idea of interchangeable
parts, which was a fundamental tenet of the radio's design.

73,
Dave Wise (SWL)

> -----Original Message-----
> From: Roy Morgan [mailto:roy.morgan at nist.gov]
> Sent: Monday, August 23, 2004 10:16 AM
> To: R-390 at mailman.qth.net
> Subject: Re: [R-390] R390A Carrier Level Meter Question
> 
> 
> At 08:33 PM 8/22/2004 -0400, Les Locklear wrote:
> >
> >FS = 1 Ma. Full scale 1 Milliamp.
> 
> 
> Yes, but there's more.
> 
> Not just any 1 ma full scale meter of the right size will work.  The 
> original meters had an internal resistance of 17 ohms or some 
> such.  Hard 
> to find. Not impossible to have made but they's cost some 
> $100 each if we 
> ordered over a hundred.
> 
> There have been a number of articles and web pages that 
> describe how to use 
> other meters instead of the original one.
> 
> Roy
> 
> 
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
> 
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