AM-4979/GR Amplified Speaker

©Brooke Clarke, N6GCE

General Information

This unit has a cable terminated with a U-229 connector that connects to a receiver transmitter.  There is a U-183 audio connector that would accept any of the audio accessories that have a U-229 connector, but something like an M-80 Microphone makes sense.  The PTT line from the U-183 goes to a relay in the AM-4979 that quiets the speaker.


Side Panel

The original cable had rubber insulation on the green, white and black wires that had turned to dust.  Someone had made patches inside the box.
I replaced the cable.  Used two o-rings (1/2" OD x 5/16 ID - 3/32" wall & 7/6" OD x 1/4 ID ) on the inner side of the box in the cable clamp.
The Audio and power connectors as well as the ON-OFF switch can be removed from the panel with their wires attached without removing the speaker.  The power indicator uses a 327 lamp and when the shell is turned CCW three holes allow the lamp to be seen, when turned CW the lamp does not shine.
The Power connector is marked "7204  MSS3114E8-4S"  It has 4 Sockets with A = Ground and C= + vehicle supply.
The origional cable had the MIC wire shielded and the overall cable had another shield.  I am using plain unshielded 6 conductor cable. In this case I followed the color cade already being used:


Speaker is marked with US patent 2755343.  This patent covers a speaker that can be submerged in water and that will not rupture when near explosions or gun fire, it's a real military speaker.
In the lower left is the amplifier module.  It is mounted with two screws on the left and one on the bottom.
The 1 K Ohm resistor is there just for a test, see Problems below.  The plate in the upper left at first looks like a TO-3 socket, but it just holds the 40 Ohm power resistor.

Amplifier Module (MP-9334)

Heat sink is  Motorola MC1554G
other IC is RCA CA3021

Pin #
Internal wire color
External wire color
goes to
 Radio AUDIO B
+ Supply
 Cathode of 12 V Zener
 Relay K1 Blade
 C1 7 R2
 C1, R2 & CR1 Anode
 CR1 Cathode & Relay K1 coil

Circuit Diagram inside lid, Circuit Diagram Modified for Ext Pwr to Radio

The 1N2976 Zener diode drops the military +24 VDC input to 12 VDC to power the amplifier.
What is inside the amplifier module is unknown. if you know what the terminals on this module connect to inside.
This unit should work on normal 12 Volt automotive power.

The components C1 and R2 (10 uF and 820 Ohms) connected to Amp pins 6 7 & and CR1 (1N647 connected to Amp pins 7 & 8 are a circuit that is in series with the PTT switch of a microphone connected to J1 (if any).  When the PTT is pressed it closes relay K1 doing tow things: (1) a contact closes grounding the PTT line on the radio (U-229/U pin C), (2) Grounds the blue wire going to Amp pin 4 Muting the amplifier.

Relay K1

Continous 100 mA, 26.5 VDC, 675 Ohms, Hi-G, Inc., 2K-2299-12, M5757/9-012
A2 blade
Blk Amp 3
Blue Amp 4
Red Amp 8 Coil
Red Coil
R1 & CR2
Blk Amp 3
B2 blade

Data Plate

CX-13143   DC power Cable

This same cableis used for both the AM4979, AM6747 speakers. It is marked:
ICORE 05593,  B4005454,  CX13143/U (6 Ft 0 In), DAAB07-85-C-K546
The yellow tag attached has FSN 5995-01-101-5467

The connector that mates to the AM-4979 is marked: BENDIX 8602 - MS3116E-8-4P
The far end has a Bendix 8602 - MS3111E-8-4P connector (these are both plugs).


After replacing the cable so that there would not be a short, I tried the AM-4979 by powering from inside the box and connecting to a PRC-126 radio.
The radio speaker did not mute.  I checked the DC resistance from the speaker terminal (Audio pin B) to ground and it shows open.  This was true at the input pin on the amplifier module inside the speaker (green wire).  I think this means that the amplifier module is bad.  In order for the radio speaker to mute it must see less than 4.7 K Ohm to ground, and the typical value is 1 K Ohm to ground.  As a check I tacked a 1 K Ohm resistor across the amplifier and tried it . . . the radio speaker now mutes, but the audio output from the AM-4979 is weaker than from the radio by it's self, further confirming that the amplifier module is bad.

I have asked the vendor to replace the amplifier module since I bought this from a retail dealer as  "used checked". (18 Feb 2001). More to come.


This speaker should be great for mobile use of the PRC-68, PRC-126, etc. radios.  It would give louder audio which is needed if you are driving with the window open or in a jeep, etc.

By wiring the audio pin "E" in the new cable to the switched vehicle power, the radio can be powered from the vehicle, saving it's battery.

This unit should provide about the same functionality as the Datron World MT1060DT, except for the interface to a power amplifier.

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