Much of the following operational and engineering information is from analyzing how the radio works and does not appear in any tSEC/KYV-2(A) Secure Voice Module the manuals. 1 Purpose of Equipment The U.
Marine Corps accepted the “A” version in 1983. This was the second in this series of radios. Instead of the eight modules used in the PRC-68, the “A” version uses two modules, one on the front side and the other on the back side. The frequency synthesizer has a microcontroller that converts the Frequency switch positions into serial data for the synthesizer IC and generates the 150 Hz tone and battery saver waveforms.
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The “A” version adds a EEPROM so that the channel frequency assignments no longer need to be in a sequence. The number of unusable frequencies for the PRC-68A is much lower than on the PRC-68. Four Antenna matching networks numbered the same as and covering the same frequencies as the above filters. All the channel assignments must be in only one of the frequency bands.
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On the PRC-68B and later radios these switch functions will be combined and the switch located on the front panel of the radio with the ANT label. The physical and electrical characteristics of the radio set are given in 1. Special capabilities and features of the radio set are as follows: a. Later model radios moved the ANT matching switch to the outside of the radio allowing any frequency on any channel. Warning Tones – The PRC-68A generates a warning tone for low battery. Antenna Matching Switch – The PRC-68A does not have an external ANT matching switch.
The internal switch needs to be set for the frequency band in use. Liquid Crystal Display – The PRC-68A does not have an LCD. COMSEC Operation – Provides 16 kb VINSON compatible secure voice operation by simple attachment of the KYV-2,-2A NSA approved COMSEC device between the radio set and battery. This may be because of some problem with the KYV-2,-2A or maybe just some other reason like the KYV-2. When the battery voltage goes down to 10 volts the speaker will sound a low battery warning.
This may be heard by an enemy that is near by. Figure2-3 showing A, B and D frequency programming switches, LOAD-NORM switch and Low Battery Adjustment. E on the AUDIO connector is for vehicle power. When using pin E for power the radio operates with the switch set to OFF! This is the way they decided to save the batteries.
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If someone hands you a PRC-68A radio there is no way, of determining what frequencies is has been programmed for. On all the later radios there is an LCD that displays the Rx and Tx frequency as the band switch is changed. The internal switches are only active while programming a single channel and can be changed so that even the last channel programmed may not be given away by the switch positions. On the PRC-68 you can tell all of the channels by looking at the programming switches. On all the later models you can see the frequency in the LCD, but on this radio you need a spectrum analyzer or scanner to find out the programming. Maybe this was the major reason for the PRC-68B?
Bottom – battery snap connectors and SVM plug. If they are selected during testing in Section 5, erroneous readings may result. If they are used in the field the reception range will be greatly reduced. 50 Ohm cable using 914598-801 adapter. Selects one of 10 preset operating channels. The spring loaded SQUELCH DIS position will disable the squelch as long as it is held.
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Enables radio set to transmit when pressed in normal operational mode. Provides interconnection between SVM and radio set. Jumper Plug allows radio to operate when SVM is not installed. When an external handset or headset is connected the internal speaker will go into mute mode. 1 Short Antenna The Short Big Base Rubber Ducky antenna is used for communications to 300 meters. A needs to be tested with the PRC-68 and PRC-68A. 7 Handset The H-250 Handset can be connected to the AUDIO connector.
This allows the Squad Radio to be carried in it’s Carrying Bag attached to a standard pistol belt. There is a flat on the handset connector. 9 Manual Channel Frequency Programming If you are handed a PRC-68A radio, there is no way to determine what frequencies have been assigned to each channel without using another radio or test equipment. This may be the major drawback of the PRC-68A. When in the PROG mode the CHAN knob determines the channel to be programmed.
There are 4 switches used to control the frequency of operation of the PRC-68A called A, B, C and D. The C switch is the 10 position CHAN switch on the top of the radio. The A, B and D switches are located on the Synthesizer module. These switches are labeled directly in frequency and do not follow the PRC-68 definitions. T FILTER and ANT MATCH switches to the corresponding band number. I have not yet worked on this.
Use the small screwdriver that is held under the module housing to set the “A”, “B” and “D” switches to the desired channel frequency. Use the CHAN control to set the “C” frequency. NORM position the channel is programmed into the EEPROM. T FILTER switches and the SQ ADJ pot.
If the Band was not changed proceed to step 3 below. 5 – Install module housing onto radio by tighting the two captive screws. Difficult to Get Correct It is difficult to get the desired frequency. 0 MHz on Band 3 I got slightly high result.
Even when using Band 2 for 51. 0 MHz, it took a number of tries to get it correct. The last time I rotated the CHAN switch on the top of the radio to CHAN 0 and back to CHAN 5 then switched to NORM. Without another radio or a spectrum analyzer it would be very difficult to set up this radio. Maybe mine is different from the rest, but I don’t think so.
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10 Jan 2002 – it may be that I did not fully realize the significance of the two modes of the CHAN control. Rx Frequency Channel Reprogramming The PRC-68A does not support split Tx and Rx operation. Channel Data The PRC-68A does not support data transfer. Back Photo – has two standard military pistol belt clips and will not attach to a standard civilian belt.
The bottom of the main pouch has an insert so that when removed will allow for the extra length of the Secure Voice Module. It may be the case that the original Operator’s manual would also fit into the main pouch with the radio, but that’s just a guess based on the possible manual size. 5″ x 11″ because that’s the standard copier paper size. 16 150 Hz Tone Disable The PRC-68A does not support disabling the 150 Hz tone during transmitt. When testing a seperate 150 Hz filter needs to be used. MIC Mute Circuit This is the circuit that puts a DC voltage on the handset speaker in order to determine if there is an external handset. You may want someone to hear your transmission that has only a commercial receiver.
You may not want someone with a commercial receiver to eavesdrop on your transmissions. In areas where there are TV stations you may want to avoid using the frequency band for active channels. By the same token you can transmit to a TV receiver tuned to one of these channels, and although not clear speech is understandable. Many hobby scanner radios cover 29 to 54 MHz but not 54 to 88 MHz because of the above TV allocations. Another factor is the use of loop antennas for Direction Finding. This may be a reason to use or not use the 76 to 88 MHz range. 20 Speakers It is possible to use the military speakers like the LS-454 with the Squad Radios, but it would be awkward to transmit by using the radio’s built in microphone.
There are aftermarket CTCSS tone generators that may be adapted for this use. 1 8 Modules The PRC-68A and all the later Squad Radios use two modules. AF module that also contains the microcontroller. The way the modules on the PRC-68A work is different from the newer radios. KYV-2A Secure Voice Module SVM This optional unit is issued to authorized users and is installed between the bottom of the radio and the battery.
It has two captive screws that hold it to the bottom of the radio and it has battery latches like are on the bottom of the radio for attaching the battery. MIL-STD-188-220 the Estelle data link protocol. The Rx audio has a low pass filter to reduce the noise that comes with non secure audio in a wide bandwidth. PCG-68 – Programmable Code Generator is used to load the . The SVM attaches to the bottom of the Squad Radio with two screws and the battery latches onto the bottom of the SVM using the same latches on the battery as are normally used for the radio. The Jumper Plug which must be connected to the radio for normal operation needs to be removed so that the SVM can make connection to the radio. AF module There is no LCD display on the PRC-68A.
It’s probable that the microcontroler uses 8 bit words sinse that would be approiate for the time frame when the radio was developed. The PRC-128 uses the Motorola XC68HC805C4P so it’s likley the same basic chip is in all the radios from the PRC-68 onward. U5 – NMC9346C EEPROM 64 words each 16 bits long – manual only talks about 20 words of freq data, is the rest used for something else? 0 Maintenance Instructions For repair advice you can contact Jim Karlow, KA8TUR He might do a repair, but it is the third priority for him after his job and family. This test set contains a frequency counter, power meter for forward and reverse and a comb generator to test receiver squelch as well as a field strength meter so that the PRC-68 antenna circuits can be adjusted.