[R-390] 390A Line Level meter replacement..?

[Jim Whartenby]

David and the group
I have successfully used Pete Williams' meter modification published on the Milsurplus list back in June of 2006.  I see that you and Dave assisted Pete in fleshing out this modification.  For everyone else, here it is in Pete's own words:

"HiWhile some of you guys have been getting legless from taking the plate caps off 807's, some of us at the other end of the  day have been active in being practical in adapting surplus meters for the carrier level function in R-390A.

The original meter has an internal R of around 10 Ohms and the replacement meter I have (ex W5cab) has an Internal R of around 103 ohms.  It has a  FSD of 1 mA.  The scale plate changed per foto details available on the net.

Altho' it deflects, the reading is a fraction of the original.  Following suggestions made by David Wise and Gary Gitzen,modifications made to the meter bridge circuit gave identical results to that generated with original meters.

Not only was meter deflection approximately the same (Even readings from original meters varied under same test  conditions) but the parameters of linearity and AGC also remained identical to readings obtained with the original meters during the tests.  Time constant values also appeared to be the same as original.

MODIFICATION.  Refer to Fig 22 on page 36 of manual TM 11-5820-358-35.
Replace R524 with a 600 ohm resistor.
Replace R548  with a 160ohm resistor.  Both are easy to get at.
Remove R537 a 22 ohm resistor from the  carrier meter adjust pot.
Remove the shorting link from the center wiper terminal of this pot and connect the white - blue - orange wire to the  "hot end" of the pot.
The  white - blue - green wire stays on the center wiper and it is already connected to the + side of meter M102.  The  white  wire stays going to chassis.
Zero the meter as per the manual and that's it.

NOTE:  Other meters of greatly differing IR's may require modification of the resistor values.  None were available fortest.
Thanks to Dave and Gary for interest and assistance with these  modifications.
Purists can get on with evacuating 807's!

Cheers
Pete D . Williams
METUNG 3904
Australia
jupete at bigpond.net.au"

I always enjoyed Pete's posts, as you can see, he was a character!   He and Nolan Lee made quite a few posts that were both informative and a joy to read.  I miss them both.

The meters I have used in the above modification came from perhaps a half dozen SRR-13's and FRR-23's that I got at a local estate auction many years ago.  I believe that all of the radios at this auction came from the estate of Dr James Black but W5OR certainly has a better memory of this then I do.  Most of these radios were damaged in a fire, some scorched, some with smoke and fire extinguisher residue.  The VU meter was a drop-in replacement, the Carrier Level meter responded well to the above mod.

Let's hope that I have properly formatted the above so that it is readable.  I think that this list does not like my browser!

Regards,
Jim
Too much agreement kills a chat.  E. Cleaver

-----Original Message-----
From: David Wise <David_Wise at Phoenix.com>
To: Jacques Fortin <jacques.f at videotron.ca>; 'Jordan Arndt' <Outposter30 at shaw.ca>; 'R-390 Forum' <r-390 at mailman.qth.net>
Sent: Wed, Nov 3, 2021 10:53 am
Subject: Re: [R-390] 390A Line Level meter replacement..?

Gary Gitzen and I developed a fairly simple IF deck mod that allows use of virtually any 1mA or smaller meter movement for the Carrier function.  No op-amp required.  The mod also improves the action of the Carrier Zero adjuster.  I believe it's in the Pearls of Wisdom archive.

Dave Wise
SWL (inactive)

From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Jacques Fortin
Sent: Wednesday, November 03, 2021 7:46 AM
To: 'Jordan Arndt'; 'R-390 Forum'
Subject: Re: [R-390] 390A Line Level meter replacement..?

Hello Jordan,

Wow ! Good find !
If only there are also some matching style 1mA fs, 18 ohms meters with a
0-100 dB dial for the RF Level, now...
But OK, I am asking too much...
Let us know how they work when you will have those in hand !

73, Jacques, VE2JFE