[R-390] Question about Carrier Level Meter

[Barry]

I did a similar experiment as you describe below when I first got these
meters, but it was quite some time ago.  I placed pots bot in series with
and shunting the meter as well as putting my DVM (in the CURRENT mode) in
series with the circuit.  I don't remember the value of the shunt, but I
think it was pretty close to what would be correct for the stated 100 ohm
value of the meter.

I'm going to do this with the "good" 1 ma  meter in series and see what
values I get when the "new" meter has the 11 ohm shunt.  I'm guessing they
will read pretty close.  If they do, then I'm going to try disconnecting the
existing meter in the working radio and only using the new, shunted one.
I'll try to report my findings.

If this works, I plan to make replacement meter faces for these as I plan to
construct the Line Level meter from one of these as well.  Yes, I know the
Line Level meter will need other modifications, but if I can get them both
to work, they will match nicely.

Thanks,

Barry(III) - N4BUQ

----- Original Message ----- 
From: "Bob Camp" <ham at cq.nu>
To: "Barry" <N4BUQ at aol.com>; "R-390A Receiver List" <r-390 at mailman.qth.net>
Sent: Friday, March 25, 2005 8:02 AM
Subject: Re: [R-390] Question about Carrier Level Meter


> Hi
>
> Here's how to at least begin the process of figuring out what's going
> on:
>
> 1) Grab a D cell and a 20 K ten turn pot ( or something similar)
> 2) Set the pot to max resistance and hook it up in series with the new
> meter and no shunt
> 3) Adjust the pot so you have a full scale reading on the meter
> 4) attach the shunt and see if the meter goes to around 1/10th scale
>
> You won't get exactly 1/10th scale because of the meter resistance but
> you should get close. If you get something lots lower then the new
> meter is well above 100 ohms resistance. If the reading is
> significantly higher then the meter is well below 100 ohms resistance
> (my bet is on low).
>
> If you now rig the new meter and shunt in series with a 1 ma reading
> VOM you can adjust the shunt to give you 1 ma full scale. If that works
> out to a 4 ohm shunt, then that 's what you use. There is only one
> value of shunt that will do the trick. I keep a couple spools of
> resistance wire around for this kind of thing.
>
> The next part is magic. The shunt will always be 1/9th of the meter's
> resistance. If you know the shunt you know the resistance of the meter
> plus shunt.
>
> The meter will do odd things in parallel with an existing meter.
> Putting a second meter in drops the impedances and messes up the
> circuit. Simply measuring the voltage across the meter does not mess up
> the circuit. When you put the new meter with no shunt across the
> existing meter you are using the new meter as a high impedance volt
> meter. Strange to be talking about a 100 ohm current meter as a high
> impedance voltmeter, but that's what it is compared to a 10 ohm system.
>
>     Take Care!
>
>         Bob Camp
>         KB8TQ
>
>
>
> On Mar 24, 2005, at 11:13 PM, Barry wrote:
>
> > 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
> >
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