PRM-32 (TS-20) Test Set, Radio

© Brooke Clarke 2001 - 2007

Introduction and General Information

This test set allows two PRC-90 radios to be used to test each other.  The radio that is transmitting has some of it's energy rectified and used to drive the transmit signal strength meter thus confirming that the transmitting radio and battery are working.  The PRM-32 will operate with transmissions in the civilian aircraft band around 121.5 MHz as well as the military distress frequency range around 243 & 282.2 MHz.  Inside there is what amounts to a waveguide below cutoff attenuator that reduces the transmitted signal by about 104 dB.

Note: The PRC-90-1 also should work since it has a 50 Ohm type antenna connection (1/2 wave with matching circuit at base), but the PRC-90-2 probably will not work since it uses a much different antenna (1/2 wave & no matching circuit).

The antennas are removed from the radios under test and adapters screwed into each radio.

Unlike the TS-24B, this set does not test the antenna along with the radio.  The good thing about not using the antenna is that the test will not generate a false alarm.

This is the government test set for the PRC-90, -103 and -106 Survival radios. "Requires one radio as receiver and another as transmitter. Tests 243 MHz frequency for beacon, transmitter MCW-voice and receiver outputs. Also Tx-Rx voice outputs for 282.8 MHz (PRC-90) and 121.5 MHz (PRC-106), earphone & battery test. Has 1.8" square meter with Red-Green scale, TX-Rx connector adapters for hook-up & operator's info. 3x5x4, 3 lbs."  at Fair Radio. - Photo

 eBay Photo, Accessories - The TS-20 is made by ACR.
another eBay photo -
ETM on line Manuals:
036228.pdf  TM 11-6625-2632-24P-  RADIO TEST SET, AN/PRM-32A (NSN 6625-01-013-9900) (THIS ITEM IS                TM 11-6625-2632-14
037755.pdf  TM 11-6625-2632-14-1  RADIO TEST SET, AN/PRM-32A (NSN 6625-01-013-9900) (THIS ITEM IS
TB 43-180 has calibration instructions for this test set.

The -24 manual is just the parts list for the test set.

Image of Box lid with Label -

Maintenance Information

The one shown here from eBay came with a broken meter movement.  It fits in a 1.5" diameter hole and is a 50 uA movement.
9 Sep 2002 - I received a 50 uA meter movement from Ken's Electronics - after installing the meter (I left the 0 - 50 uA scale becasue the origional red (< 25 uA) Green (>25 uA) scale would not easily retrofit to the new meter, although it is close.
Connecting the output of my Agilent 8648A sig gen and turning the power up all the way (13 dBm (20 mW) gives the following readings (after setting 50 uA using a couple of PRC-90 radios:
 
uA
2
3
4
4.5
4
3
2.3
MHz
1
45
69
250
623
767
1000

Use for Other Frequencies

The Transmitter input is terminated in 50 Ohms and has about 100 dB loss to the Receiver Output that is also 50 Ohms impedance.  The PRM-32 only has a calibration at a single frequency and the PRC-32A has two calibrations, one at 121.5 and the other at 243.0 MHz.  As you can see in the Transmit input to Receive output frequency plot the response has about + and - 6 dB of ripple about a nominal 100 dB of loss.

The PRM-32 could be used as a general purpose power meter for 1 Watt type hand held radios if it had a meter scale with graduations and a calibration table.  I think this can be done by adding a pair of test jacks to allow a DMM to measure the voltage across the meter and making a calibration table.
 
 

Here are some delta dB numbers relative to -100 dB:
 
Freq
rel dB
ref -100 dB
30
-1.44
88
+2.69
121.5
+3.0
138.9
-3.9
144
-2.56
222.5
+8.69
243
-1.05

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