[R-390] Panel meters again

[David Wise]

In 2007, Gary Gitzen and I worked out an IF deck modification that permits use
of just about any panel meter in place of the special 17-ohm meter originally used.
Any 1mA meter of 100 ohms or less will work, including the original.  Most meters
with lower FS current can also be used.  As an added benefit, the zero adjust action
becomes smooth and linear.

Here's a textual description.  Full writeup and schematic available on request.

Disclaimer: This mod may rub some people the wrong way.  If you hold the R-390(*)/URR
design sacred, then read no further.  Get an original meter, or emulate it with external
circuitry a la Jan Skirrow.  I don't mind, it's a matter of personal taste and an attitude
I sympathize with.

But the carrier meter circuit is a bad design.
I REALLY get off on fixing bad designs.  If you do too...

0. This mod assumes you have the original 100-ohm R523 zero adjust pot.  Don't worry,
the mod also fixes the problems with that pot that may have driven you to replace it.

1. Add a 1N914 or similar Si diode, with cathode grounded and anode connected
to the junction of R544, R546, and R547;

2. Change R549 from 82K to 75K;

3. Change R548 from 27 ohms to 150 ohms;

4. On R523, cut the link between slider and clockwise end, leaving R524 connected to
slider, and R537 and M102 connected to CW;

5. Change R537 from 22 ohms to 68 ohms;

6. Change R524 from 680 ohms to 620 ohms.

The circuit is now calibrated for a 100-ohm 1mA meter, and can handle
lower-resistance meters by the simple expedient of adding a series resistor.
Most meters with lower FS current (500uA, 50uA, etc) can also be used,
again adding series resistance depending on the meter.

7. Mark the deck or otherwise record the mod.
WISE/GITZEN 2007 M102 CARRIER METER MOD - CALIBRATED FOR
100 OHM 1MA METER.  [Or whatever yours is.]  TO OPERATE WITH
ORIGINAL METER, ADD 82 OHMS IN SERIES.  [Or "CHANGE SERIES
RESISTOR TO 82 OHMS."]

HTH,
Dave Wise
________________________________________
From: R-390 [r-390-bounces at mailman.qth.net] On Behalf Of Cecil [chacuff at cableone.net]
Sent: Wednesday, October 29, 2014 6:02 AM
To: Grayson Evans
Cc: R390A
Subject: Re: [R-390] Panel meters again

They may fit the holes but will require drilling the panel for the attachment screws.  That coupled with the mods necessary to make one function as an "S" meter properly and the cost ($60 + s/h ea.) puts my vote squarely in the Find an Original pair column.
Their still out there...a bit hard to find at times and no more expensive.

Just my 2 cents worth..

Cecil

Sent from my iPad

> On Oct 29, 2014, at 4:05 AM, Grayson Evans <wa4gvm at gmail.com> wrote:
>
> I am looking for replacement panel meters for my 390A.  Not original, but more modern meters that will fit the holes.
> I notice there have been very long threads on this subject many years ago (the thread I have is dated 1999, hundreds of posts), which really didn’t seem to resolve much except the specs for the meters.
> Wondering if anyone found a source they like lately.
> (the meters on the restoration I am working on are in pretty bad shape, but I am rebuilding them to keep as spares)
>
> I found this company that makes a great looking replacement for the line level meter, but I am not sure they are actually available:
> http://www.sifammeters.com
>
> ebay has a zillion sellers of various conventional analog meters, most very inexpensive, but didn’t see one I liked.  It is fairly easy to modify the meter for the correct scale, and put a new printed scale on it (done it many times), if I can figure out the actual range of the meter movement (for example a labeled 0-100V meter is probably a 0-1 ma meter movement with a scaling resistor inside the case).
>
>
> Grayson
> TA2ZGE - Ankara, Turkey
> KJ7UM
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> hollowstatedesign.tumblr.com
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