[R-390] R390A Carrier Level Meter Question

[Bob Camp]

Hi

One of the amazing things about an R-390 is just how accurate the 
carrier meters are. Every one I have checked has stayed pretty darn 
close over the whole range above 20 db. They have been pretty good at 
10 db. When you take a look at what they had to do to make that happen 
there's a lot that went into it.

All that said there is another way to go. The good old back panel 
connector has the AGC voltage on it. No big surprise that the AGC 
voltage also follows the carrier level. In a number of situations the 
AGC voltage was used instead of the carrier level for accurate signal 
monitoring. If you are going to rig an external setup there should be a 
way to do a LCD display that would show signal level to a tenth of a db 
over a 100 db range. You'd be the only one in town with one of those 
.....

By using the AGC voltage you would also get around the normal problems 
with the carrier meter zero pot. That alone would be worth the effort.

	Enjoy!

		Bob Camp
		KB8TQ

On Aug 23, 2004, at 2:27 PM, David Wise wrote:

> 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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