[R-390] R390 NON A IF deck mods in ER
Tom M.
courir26 at yahoo.com
Wed May 18 20:44:36 EDT 2011
I wrote that article. My objective was to replicate the R-725 wiring. It can
be done an alternate way as pointed out but this is how Motorola did it. 73 Tom
Here is the conversion procedure. I use one of these in my 390A. I have not
tested the alternate method.
73 Tom
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Conversion of the R-390 IF Deck for Use in R-390A For Improved Audio,
Or
Make Your Own R-725
Thomas F. Marcotte, N5OFF
As many of the readers of Electric Radio know, the US Army created a modified
R-390A version for direction finding known as the R-725/URR. I wrote about this
modified radio in this publication a few years ago.
The main difference between a R-725 and the R-390A is that the R-725 utilizes a
custom built IF deck that is very similar in construction to the R-390 IF deck.
It has tuned circuit selectivity instead of mechanical filters. The mechanical
filters of the R-390A created distortion when that radio was employed for radio
direction finding use. In an internal Collins engineering report published in
1952, Lou Couillard wrote of the improved R-390A, which at that time was called
the R-390(XC-3), "Although the superior shape factor of the mechanical filter IF
is desirable in most applications, consideration should also be given to an
alternate tuned circuit design for use in special applications. The excellent
shape factor of the mechanical filters precludes the possibility of linear phase
shift across the passband. Where a linear phase characteristic is desired such
as in direction finding equipment, a tuned circuit IF is necessary."
Employment of the tuned circuit IF is exactly what was done in the R-725.
Motorola was awarded a contract in 1956 (476-PH-56-91) to prototype the
R-725. I know of a couple of these Motorola sets existing today.
Packaging of modified sets for quantity DF use were handled by Arvin Industries
and Servo Corporation of America. Approximately three hundred R-390A's were
modified to the R-725 configuration. These sets are relatively tough to find
today. New IF decks were manufactured by the modification companies (actually
salvaging some of the components from the now junker R-390A IF decks) and
installed in existing R-390A's. The new IF decks were named "SERIES 500 IF
STRIP ASSY." They looked almost just like R-390 IF decks (see the comparison
photos), except that the IF connectors were relocated to match the cables and
connectors in the R-390A chassis. The circuits were designed to plug and play
in the R-390A instead of the R-390. The decks are not interchangeable (until
now that is, after performing the modification described herein).
Notwithstanding DF capabilities, a side benefit of the SERIES 500 deck is that
it provides a smoother sound than does the stock R-390A IF deck. Mechanical
filters are said to "ring" and after a while can be fatiguing to the listener.
The purpose of this article shall be to describe how one may "roll his own"
SERIES 500 IF deck from a surplus R-390 IF deck. Please note that I don't
advocate trashing of a good R-390 to do this mod. The IF deck I started with
came from a Motorola junker. I would urge you to likewise find a junker R-390
as a source of an IF deck for this project . Make sure that the deck is in
working condition prior to beginning the modification.
The R-390 IF deck was designed to operate with one 25V filament supply
(unfortunately the 25V supply available in the R-390A is insufficient to power
all of the filaments in the R-390 IF deck). The R-390A deck was designed to
operate with a combination of 6.3V and 25V filament supplies. The task involved
in this conversion is to rewire the R-390 filaments to comply with the voltages
available in the R-390A and provided at the main IF deck connector, plug and
play, without the addition of any new power transformers. Each of the twelve
tubes in the R-390 IF deck must be addressed for full compliance with the
voltages available from the R-390A. Refer to the schematic in Fig 1 as to the
final filament wiring configuration. We will also drop the B+ a bit.
Plug P112 of the R-390A shall be plugged-in to the R-390 IF deck at jack J517.
There is much commonality here, except for the connections mentioned herein.
In general, you will be converting 25V series connections into 6.3V (herein
referred to as 6V) parallel connections for most tubes, and moving the
connections of the BFO/PTO/ballast tube series to a different connection point
in jack J517. The 6V filament supply shall be provided to the R-390 deck by pin
20 of P112 from the R-390A.
General instructions: Refer to the schematic for the original R-390 as the
"before" schematic, and Figure 1 as the "after" schematic for filaments. Use
the best soldering technique you can in this limited access space. Don't
insulate or bind any wires until instructed to do so. You will be utilizing
some of the new 6V supply connections more than once. Make sure you can
recognize your new wires. I used black wire for the 25V supply, red wire for
the 6V supplies, and green wire for new grounds.
The first task is to install a B+ dropping resistor to better match the 180V B+
that the R-390 IF deck is expecting. To do this, locate inductor L503 under the
IF deck. This will be found snapped into a holder right above pin 2 of J517.
Disconnect one end of the coil, and install in series with it (the equivalent
of) a 470 ohm 2 watt resistor. This will tame the B+.
The first tube circuit we'll work on is the ballast tube circuit.
V508 (5749) and RT512 (3TF7) These must be supplied by the R-390A 25V filament
supply. To do this, sever the connecting wire at pin 8 of jack J517 (underneath
the deck) to free this slot up (hint: save access to the connector end of the
wire as you will use it to wire supply to V509). Then, sever the connection at
pin 2 of RT512 and wire this pin to pin 8 of J517 of the R-390 deck with a long
piece of new wire. The filament return connection remains unmodified. This
modification will make the BFO/PTO/ballast tube series connections identical to
the R-390A 25V filament supply connections. As mentioned above, this 25V supply
is insufficient to supply the remainder of the tube filaments, thus the need to
employ the 6V supply for this task.
The following 6V tubes shall have filaments wired from the 6V R-390A supply.
The filament pins of these tubes are pins 3 and 4. Don't sever any connections
unless instructed to do so. The modification will use as much existing R-390 IF
deck wiring as possible (and thus may seem a bit screwy to you until finished).
V504 (6BJ6) 6V will come from its existing connection at pin 4. Ground will come
in the next step.
V503 (6BJ6) Wire pin 3 of V503 to pin 4 of V504 for 6V supply. Ground V503, pin
4.
V502 (6BJ6) 6V supply will come from an existing connection at V503, pin 3.
Ground V502, pin 3.
V501 (6BJ6) Sever ground connection at V501, pin 3 and wire pin 3 to V502, pin
4.
V505 (6AK6) 6V supply will come from existing connection at pin 4. Ground will
come in the next step.
V506 (6AK6) Ground pin 4 of V506. Wire pin 3 of V506 to pin 4 of V505 for 6V
supply.
V509 (6BJ6) Locate the free wire which was cut from underneath J517, pin 8, and
connect it to the 6V filament supply at J517, pin 20.
The following tubes are 12AU7's wired in various series schemes in the R-390.
They must be rewired according to their 6V option for use in the R-390A. Note
two of the connections require dropping resistors on the 6V source of V507 and
V510 to obtain the desired 5.3V filament voltage.
V511 Sever ground connection at pin 5. Connect pins 4 and 5 together for 6V
supply, ground pin 9.
V507 Sever connections at pins 4, 5 and 9, including the two resistors (one 120
ohm and one 22 ohm). Wire 6V supply from your previous work at V505, through
the deck opening for variable capacitor C525, to pins 4 and 5 through a 3.9 ohm,
1 watt resistor. Ground pin 9.
V 510 Sever connections at pins 5 and 9. Ground will come from existing pin 4
connection. Remove 120 ohm resistor between pins 4 and 9. Wire pin 5 to pin
4. Wire 6V supply from J517, pin 20 to V510 pin 9 through a 3.9 ohm, 1 watt
resistor.
There are no changes to any of the other connections in the R-390 deck.
Prior to installing the modified R-390 deck in your R-390A, you must check your
work.
Perform continuity checks from J517, pin 8, with RT512, pin 2.
Perform filament supply and ground continuity checks as follows. Note: There
will be more than one ground connection at various tube sockets, but the
filaments should have continuity exactly as shown.
Filament supply is checked from J517, pin 20.
V501, filament, pin 3; ground, pin 4.
V502, filament, pin 4; ground, pin 3.
V503, filament, pin 3; ground, pin 4.
V504, filament, pin 4; ground, pin 3.
V505, filament, pin 4; ground, pin 3.
V506, filament, pin 3; ground, pin 4.
V507, filament, pins 4 and 5; ground, pin 9.
V509, filament, pin 4; ground, pin 3.
V510, filament, pin 9; ground, pins 4 and 5.
V511, filament, pins 4 and 5; ground, pin 9.
Now insulate any bare connections, and use mini-tie wraps to secure the new
wires to sturdy nearby points.
To enable final installation of the deck in your R-390A, you'll need to make two
adapter cables. These cables shall consist of jumpers (RG-59 is OK,
approximately eight inches in length) with BNC's on each end. You'll also need
two adapters of the type found on the back of the frame of the R-390A at the IF
OUT jack (AMPHENOL 47200). This will provide crossover from MB connection
(R-390A standard)to BNC (R-390 standard). Connect P-218 of the R-390A to J-526
of the R-390 IF deck with one of the cables. Connect P-213 of the R-390A to
J-525 of the R-390 IF deck with the other cable. It is a good idea to label
these cables. See the photo for reference.
Install the deck in your R-390A. You will notice that the screw holes are the
same as for the R-390A IF deck, however the screws of the R-390 deck are of
larger diameter. I did not change these screw as they are captive into the
deck. The BFO, BANDWIDTH, and power connector of the deck will hold it in
place, however I would not install it in a Jeep this way. Changing these screws
is optional.
When you turn on the power, make sure your dial lamps light up normally. If
they don't, you have a filament supply problem so turn off the set immediately
and troubleshoot.
For great sound, instead of using the built in audio deck, I prefer to tap the
audio from the diode load jumper at the back of the set. Through a 0.1 uF or
larger capacitor, feed this signal into your line audio amp of choice, and enjoy
the tuned circuit audio of the new R-725, errrr, R-390A with tuned circuit IF.
You'll get the smooth sound of the R-390 and R-725, but have the parts
availability and support common to the R-390A for the balance of the set. I've
used my modified IF deck in two different R-390A frames, and it worked equally
well in both. In my opinion, it makes the long term listening experience much
more enjoyable.
If you have any questions about the mod, please feel free to write.
73 DE N5OFF
QRT
Thomas Marcotte is a registered professional engineer working in Lafayette, LA.
References:
Cost Reduction Program for Radio Receiver, R-390/391( )/URR, L.W. Couillard,
Collins Radio, 1952.
TM 11-856/TO 31R1-2URR-154, RADIO RECEIVER R-390/URR, January, 1955.
Servo Corporation of America drawings for R-725 series 500 IF deck, Order No.
36-039-N-5-00093(E), September, 1965.
TM 11-5825-231-24, DIRECTION FINDER SETS AN/TRD-23, AN/TRD-23A AND DIRECTION
FINDER SETS AN/TRD-15 AND AN TRD-15A, August, 1973.
TM 11-5820-358-35, FIELD AND DEPOT MAINTENANCE MANUAL, RADIO RECEIVER
R-390A/URR, May, 1980.
Special thanks to Wally Chambers and George Rancourt.
----- Original Message ----
From: "ka9egw at britewerkz.com" <ka9egw at britewerkz.com>
To: Dan Osborne <wb5afy at wb5afy.net>; R-390 at mailman.qth.net
Sent: Tue, May 17, 2011 8:07:28 PM
Subject: Re: [R-390] R390 NON A IF deck mods in ER
Issue 139, Dec. '00
73, Brian KA9EGW
-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of Dan Osborne
Sent: Tuesday, May 17, 2011 2:35 PM
To: R-390 at mailman.qth.net
Subject: [R-390] R390 NON A IF deck mods in ER
Well - anyone remember which issue of ER had the article on the
filament string mods required to use
a NON-A IF deck in an A rcvr ??
The Java code on the ER search page wont run for me...sigh...
tnx
Dan O
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