MK-2137/PRC-68 Maintenance Kit
NSN 5820-01-117-7664

©Brooke Clarke, N6GCE

Background

The TS-3354 is a field test set that allows doing the alignment that's associated with changing channel frequencies.  This is a kit of parts that facilitate bench testing of the PRC-68.  There are not any stimulus or response measuring functions as part of this kit.

At one time the carrying box contained foam rubber of the kind that turns into black goop.  If you know the arrangement of the parts in the foam let me know so that I can restore it with better quality foam.

Label

CII.08502A   Ser No 47
Contract No M9545081POIV416
MK-2137/PRC-68
Maintenance Kit
Mfr: 01365    Wt 7 Lbs
Part No. 81A5015A0000
NSN 5820-01-117-7664    US
Ind Graphic Arts Rep Div Alb GA

The CAGE code 01365 is Marine Corps Logistics Base, Albany, GA 31704 (not Magnavox)

Manual

TM 11-5820-882-23 is a PRC-68 Manual but has a couple of pages about the MK-()/PRC-68 Maintenance Kit. i.e. the TM does not use the 2137 value.  The bench testing in this TM is based on using the contents of this Maintenance Kit.

Contents

 
Item
Magnavox p/n
Name
Use
Photo
1
914877-801
Audio Adapter Assembly
Provides means of keying transmitter and interfacing test equipment through the audio connector on the radio set
2
565460-801
Power cable Assembly
Provides regulated 15 +/- 0.5 VDC to radio from external power supply (20 to 30 VDC)
3
565461-801
RF cable assembly
Provides means of connecting test equipment to the test points on the radio set circuit board
4
914876-801
RF attenuator assembly
Provides means of connecting Wattmeter and frequency counter to radio set
5
565462-801
Jumper plug assembly
Provides B+ continuation in radio set TP18 <-> TP19 when transmitter module is removed
6
565462-802
Jumper plug assembly
Provides B+ continuation in radio set when IF/AF module is removed
7
914598-801
Antenna adapter
Provides means of connecting antenna of radio set to test equipment
8
914878-801
150 Hz filter assembly
Provides filtering of 150 Hz squelch tone during distortion checks
9
565463-801
Test lead assembly
Provides means of connecting digital voltmeter for voltage checks on radio set printed circuit board
10
 
carrying box
Provides a box to carry the test kit
(this is the same as the box for the PRM-34)

Analysis of the Contents

Audio Adapter Assembly

The Audio Adapter Box has three BNC(f) connectors labeled Receive Output, Transmit Input, and Transmit Meter.  A standard U-229 (5 pin) connector on a cable connects to the PRC-68 AUDIO connector.  There is a "Push to Transmit" button on the lid.

AUDIO pin A connects to the box ground
AUDIO pin B connects to the Rx Output center contact and there is a 62 Ohm resistor to ground (mutes internal speaker)
AUDIO pin C connects directly to Tx Meter center contact and then goes through a PI attenuator to the Tx Input connector.  The attenuator is about -46 dB.  This allows connecting an audio generator directly to the Tx Input connector and ending up with a mike level drive to the radio.  The actual drive signal can be measured at the Tx Meter jack.

For receive testing the Rx Output is designed to feed the TS-723 Spectrum Analyzer.  For transmit testing AF generator URM-127 feeds the Tx input and an audio voltmeter monitors Tx meter.

There is no DC blocking in this box.  I need DC blocking to eliminate the + 5 to +6 Volts on the B pin prior to feeding my 4395A spectrum analyzer.

The box itself is a cast metal Pomona model 2901. It's like the 2428 but with mounting flanges

Power cable Assembly

I think when this unit was designed digital metered power supplies were expensive.  Its purpose is to take in DC from 20 to 30 Volts (which includes a military vehicle battery) and output 15.0 Volts. Maybe the "bench" could be located in a com van?  Inside there is an LM7815 in a TO-3 package.  The positive lead has a 0.5 Amp fast acting 1/4 x 1 1/4" fuse and there is a spare fuse.  The output goes to a Squad Radio Battery Snap for connection to the battery terminals of the radio (no provision for powering through the AUDIO connector).

It is housed in the same Pomona Electronics model 2901 box as is used for the Audio Adapter Assembly.

RF cable assembly

This RF cable is made from a 3 foot length of RG-174 coax with a BNC(m) connector on one and and two pins on the other end.  Both pins have a o.040" diameter point to fit into the PRC-68 printed board test points.  The ground lead is about 7" long.

RF attenuator assembly

This assembly takes in the RF from the radio by means of the antenna adapter and feeds it through a 0.01 uF cap directly to the Wattmeter output.  The Wattmeter center then goes through a 2.6 k Ohm series resistor to the Test Output connector which has a 62 Ohm resistor from its center pin to ground.  The two resistors form an attenuation network.

Power Jumpers

These are plastic blocks with a metal strap on the bottom and a couple of 0.040" diameter pins to plug into the fixed printed circuit board.

The one for the transmitter module has the pins 0.472" C-C.  The PCB has a dashed line between TP18 and TP19 in the area where the Transmitter Module 1A4 fits and this is where the jumper needs to be inserted when the Tx module is removed so that some of the other circuits will receive DC Power.

The one for the IF/AF module has the pins 0.895" C-C.  The jumper is connected between E7 and E14.
 

150 Hz Filter

This is made up of 3 resistors and 3 capacitors in a twin-T arrangement.  From the left connector there are a couple of 22 k Ohm resistors in series going to the right connector.  At the center there is a 0.2 uF cap to ground.  From the left connector there are a couple of 0.2 uF caps in series going to the right connector.  At the center there is a 10 k Ohm resistor to gorund.

The box is a Pomona model 3230 that comes with two BNC(f) connector installed.

Antenna adapter

This is the first Magnavox Antenna adapter I have seen that works properly.  As you screw it down you can feel the spring fingers contacting the hot antenna terminal and it screws down until the housing bottoms.  I have seen about a dozen of these (including some after market units) and the spring fingers all hard bottom before the housing bottoms.  You can see that it probably was reflow soldered rather than using an iron because there is not a pile of solder around the center pin.  This is the standard Magnavox design with no DC return.
See my Antenna Adapter that has a built in DC return.

Test lead assembly

This appears to be made up of stock Pomona leads.  The Black lead is a model 1959-36 male banana plug to aligator clip.  The red lead is a model 4853-36 male banana plug to 0.080" diameter probe.  Maybe they no longer make test leads with the exposed banana plugs or offer individual test leads?
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