R-45/ ARR-7 Receiver Modifications 

						James M. Stueber W5UOZ-AF5UOZ
						8410 Flower Place N.E.
						Albuquerque, N. M. 


The R-45/ARR-7 receiver is a continuous tuning type covering the range of .55 mc 
thru 42 mc. It is a single conversion superhetrodyne circuit, with two tuned rf 
stages and one untuned preselector rf stage. The design is a modification of the 
well known Hallicrafters SX-28 receiver. The particular modified receiver was 
built under a Signal Corps contract by the Belmont Radio Corp. Inclusion of the 
described changes will provide the user with a good communications receiver, 
overcoming the major disadvantage in the original design, which is the lack of 
bandspread tuning. A schematic of the receiver and a coil layout for alignment 
purposes is included for those who may require them. Appreciation is extended 
to Mr. Amos Peters, AF5LHX, for supplying the receiver through the MARS Central 
Technical Net, thereby making these modifications possible. The changes described 
consist of the following: 

1. Addition of bandspread tuning and dial
2. Addition of necessary wiring to provide ae power supply operation
3. Addition of Send-Rev. Switch
4. Improvement of "S" meter circuit
5. Audio amplifier modification for speaker connection
6. Low impedance antenna input stage 

Bandspread Tuning and Modification for ac Power 

1. Remove the screws holding the cover plate which houses the tuning dial motor 
reversing switch and stop mechanism. Discard. 
2. Cut the wires from the motor reversing switch SW3 and remove the switch. 
3. Remove the stop mechanism from the main tuning dial shaft and discard. 
4. Remove all the knobs from the receiver and the screws from the power connector 
so the front      panel may be taken off.
5. Remove the main tuning dial by loosening the allen-head screws.
6. Remove the cover from the cast aluminum housing containing the 28V dc motor 
and associated       parts. 
7. Remove the filter, motor and misc. parts from inside the box.
8. Remove the fiber gear from the vertical shaft. This may be sealed with paint 
and may be       difficult to loosen, but with the proper size allen wrench and 
solvent to loosen the paint, it    will come loose. 
9. Remove the potentiometer R41 and bracket, located on the top rear of the 
chassis. Cut the wire    leads free near the potentiometer terminals and 
leave them in position.
10. Remove the wires from the POWER switch, tie together, solder and insulate. 
11. Remove the white wire with blue and orange tracer from the MOTOR switch. 
12. Remove the ground connections from pins A and C of receptacle SO-1. 
13. Attach the white wire, with blue and orange tracer, which was removed from 
the MOTOR switch,     to one lug of the POWER switch. 
14. Attach the other end of the wire which was attached to the motor, to pin C 
of SO-1 connector.
15. Cut the white wire with green tracer from the MOTOR switch and attach to the 
other lug of the POWER switch. Pins C and D of SO-1 now become the ac switch-wires 
to turn ae power off and on. The bandspread tuning knob will project out of the 
top of the case, since there is no front panel space available. 
16. A three-eighths inch hole is drilled in the top of the cast aluminum motor 
housing located 11/16 inch from the front of the case, and ¼ inch from the left. 
A shaft bushing is installed  in this hole to accommodate a length of ¼ inch diameter 
shaft and solid shaft coupling. The bandspread tuning will be engaged by throwing 
the MOTOR switch to ON, which engages the electric clutch in the tuning mechanism. 
When fast tuning is desired, the MOTOR switch is  thrown to OFF and tuning is 
accomplished with the TUNING knob.  Parts are now installed to provide de voltage 
to energize the clutch. 
17. Mount a 6.3 volt 1.2 amp. filament transformer under the chassis on the side 
lip near the SO-1 connector. Refer to Fig. 1 for the schematic of this portion 
of the modification. 
18. Connect one of the 6.3 volt leads of the transformer to pin 7 of 6SA7 tube V4. 
19. The center tap connection of this winding is not used, so cut the lead off 
close to the         transformer winding. 
20. Remove the wire with blue tracer from the MOTOR switch. 
21. Connect the other 6.3 volt lead to one con- tact on the MOTOR switch. 
22. Connect the other switch contact to a con- ven,ient ground. 
23. The 500 ohm 5 watt resistor, the 1N93 diode, and 10 mfd capacitor are 
conveniently located near the transformer on a three lug terminal strip and wired 
as shown in Fig. 1. 
24. Connect the wire with blue tracer, which was removed from the MOTOR switch, 
to the junction of the 1N93 diode and the 10 mfd capacitor shown in Fig. 1. 

Addition of SEND-RCV Switch 

1. Drill two holes in the front panel which will accommodate a SEND-RCV switch and 
a pin jack (RCA Phono jack) for a keying relay connection. Due to the close 
spacings and limited panel space available, care must be exercised in location 
of the two holes. A ½ inch hole for the SEND-RCV switch is located 5½ inches 
from the left and 1 13/16 up from the bottom. A three-eighths inch hole for 
the pin, jack is drilled 6 inches from the left and 2 7/16 inches up from the 
bottom of the front panel. Refer to Fig. 2 for the schematic of the circuit.
2. Estimate the length of wire needed to go from pin A and B of SO-1 to the new 
SEND-RCV switch, a SPST toggle switch on the front panel, and install these leads. 
Do not solder these leads at the toggle switch at this time but solder at pin A 
and B of SO-1. 
3. Estimate two lengths of wire to run from the toggle switch to the pin jack. 
The toggle switch wire from pin A of SO-1 goes to the insulated lug of the pin 
jack and the other wire from the    toggle switch to the ground lug of the pin 
jack. Solder all these leads at this time.
4. The main tuning dial, front panel, and knobs may now be reassembled. 

Improvement of "S" Meter Circuit

It was found that the maximum deflection that could be had from the local 
broadcast signals was S-6 with the original circuitry. The following changes 
will improve the circuit. Refer to Fig. 3 for the revised circuit. 
1. Replace the 6SK7 tube V5 with a 6SD7 tube.
2. Change the 100 ohm resistor R22 tb 1000 ohms. 
3. Add a 1000 ohm resistor in parallel with the 330 ohm resistor at he 
cathode of V5.

Audio Amplifier Modification

  Refer to Fig. 4 for the revised circuit.
1. Remove the 68k resistor R39 located on the terminal board on the back 
chassis lip.
2. Tie a jumper wire across the two terminals where R39 resistor was removed 
and solder.
3. Remove the 7500 ohm 10 watt resistor R40 located in a cluster of four 
upright resistors under the chassis near the rear. 
4. Connect the two white wires with red tracers together after removing them from 
R40, insulate and solder them. 
5. Install a 5000 ohm to 8 ohm output transformer in the location on top of the 
chassis near the rear where the R41 potentiometer and bracket were removed. 
6. Remove the white wire with blue tracer attached to pin 3 of 6V6 tube V9. This 
wire leads to a .5 mfd capacitor C52 which is a metal can type and is the bottom 
unit of two stacked ones near the cluster of upright mounted 10 watt resistors. 
This capacitor may be removed or left in, as desired, and white wire with blue tracer 
may be removed from the harness or clipped short and left in. 
7. Install a wire from the other terminal of C52 to a lead of the 8 ohm secondary 
of the output transformer. (There is already a shielded wire attached to this 
terminal of C52, leave it there.)
8. Tie the other lead of the 8 ohm transformer secondary to any convenient ground. 
The white wire with black tracer which attached to the potentiometer R41 may he used 
for this ground. 
9. Connect one wire of the output transformer primary to pin 3 of 6VO tube V9. Do not 
solder.
10. Connect the other primary wire to pin 4 of OVO tube V9. Do not solder. 
11. Remove the 100k resistor R47 connected from pin 3 of 6V6 tube V9 to a terminal 
board on the rear chassis apron. Removal of this negative feedback resistor 
will result in greater output and better communications quality. 
12. Install a .02 mfd 600 volt capacitor from pin 3 to pin 4 of 6V6 tube V9 to 
reduce high frequency response. Solder the connections. Either headphones 
or a speaker may be connected to the PHONES jack. 

Low Impedance Antenna Input Stage

  Refer to Fig. 5 for the revised schematic.
1. Remove the wire lead from pin 5 to pin 3 of 6AB7 tube V12.
2. Connect a wire from pin 3 to pin I of this tube socket and solder.
3. Remove the .0082 mfd capacitor C79 from the terminal board on the underside 
of the chassis near the tube socket of V12.
4. Disconnect the center wire of the shielded antenna wire lead where it terminates 
at the terminal board on top of the chassis near the 6AB7 tube V12.
5. Ground the terminal from which this wire was removed.
6. Connect one end of a .0082 mfd capacitor to the free end of the wire removed 
in step 4 and add on a piece of wire to the other end of the capacitor, long 
enough to go through the chassis grommet. Connect it to pin 5 of the 6AB7 tube V12. 

  A typical power supply is shown in the schematic of Fig. 6. No attempt is made 
to design an exact supply for the receiver in this article, since most users have 
power supplies capable of supplying adequate power to the receiver. 
  A white wire with blue and black tracer will be left over from the modifications. 
It may be cut close and left in the harness under the chassis. 
  A slot is cut in the cover to allow it to be slipped over the bandspread tuning 
knob. This shaft may be cut short enough to allow a knob to fit flush with the 
case cover.
  A dummy knob is installed on the shaft on the front panel from which the stop 
mechanism was removed.
  A bandspread dial is attached with cement to a ¾ inch diameter knob and installed 
on the tuning shaft. This dial is reproduced in Fig. 7 as a guide only because each 
receiver will vary slightly in calibration and the,dial will have to be tailored to 
each case. A suitable pointer for the dial is made from a short length of solid #20 
wire which is attached under the screwhead of the nameplate.   . . . W5UOZ

Parts List

1-Shaft bushing for ¼ inch shaft
1-¼ inch dia. shaft approx. 4 inches long 
1-shaft coupling for 1/4 inch shaft 
1-2 inch dia. bandspread tuning knob 
1-SPST toggle switch (SEND-RCV switch) 
1-Pin jack (RCA phono jack) 
1-¾ inch dia. tuning knob (mount bandsproad tuning dial to this knob) 
1-6.3 volt 1.2 amp filament transformer 
1-1N93 diode 
1-10 mfd 150 volt elect. capacitor 
1-500 ohm 5 watt resistor 
1-8 Ing terminal strip 
1-6SD7 tube 
2-1000 ohm ½ watt resistor 
1-5000 ohm to 8 ohm output transformer, 5 watt 
1-.02 mfd 600 volt capacitor 
1-1 Mh rf choke