AN/PRC-10A
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Portable tactical FM transceiver, 38-55 MHz
The AN/PRC-10A is an evolution of the tactical FM Radio Set AN/PRC-10, first introduced in the 1950s to replace the SCR-300 (BC-1000) radio. The AN/PRC series comprised the PRC-8, PRC-9 and PRC-10 sets and the subsequent versions with the "A" suffix. Except for different frequency coverage the 8(A), 9(A) and 10(A) models are identical in structure, function and circuit, The differences of the A series are a simplified circuit using two less tubes and one quartz crystal instead of two. The front panel and the tuning knob are also slightly different. The Radio Set was in use in the Korean conflict and at the beginning of Vietnam war, to be soon replaced by the transistorized AN/PRC-25 and -77. It also saw service in several NATO countries, where it has also been manufactured under license, My PRC-10A comes from the italian army and it was serviced (perhaps produced) in Italy by a local firm. The date printed on my radio shows that it was still in use in 1974 and probably later.
The AN/PRC-10A in manpack configuration was powered by the BA-279/U battery. The battery case CY-744A/PRC holding the battery was clamped to the bottom of the radio.The AM-598/U power supply and audio amplifier was reserved for vehicular use.
AN/PRC-10 denotes the Radio Set, comprising the transceiver proper and the accessories. The transceiver itself as part of the set is named RT-176A/PRC-10 (no A suffix affter 10 in this case). The old version is the RT-176/PRC-10.
- Dimensions 9.5x3x9.5 inches, weight 9 lb. (radio only)
- RX: single conversion superhet, 4.3 MHz IF
- TX: frequency locked to the RX local oscillator via an AFC circuit
- 13 subminiature tubes, plus one miniature noval tube for the transmitter
- Calibrator: 2.15 MHz quartz crystal
- Power supply battery (BA-279/U): 67.5 V and 135 V for anodes, 1.5 V and -6.0 V for filaments (nominal voltages)
- Output power ≥ 0.9 W giving about 5 miles coverage
- FM deviation: ± 15 kHz
- Sensitivity: 12 dB S/N with 0.7 μV input
The first version of the apparatus, the AN/PRC-10, had two more tubes and a slightly different circuit
An overview of the radio sets and accessories is shown below
First tests
Battery connections
First tests using alkaline batteries for filaments and anodes. Note the eight 9-volt transistor batteries series-connected to provide the RX anode voltage.
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Left: My reproduction of the original BA-279 battery. The weight is 2.2 kg, considerably less than the original one (8 lbs.) Perhaps less weight does also mean less punch? Right: The innards of the battery. The anode voltage is supplied by 16 9V alkaline batteries (cost ∼ 20 €) . I followed K4CHE's design where the 9V batteries are directly plugged one into the other, dispensing with a lot of wiring. Battery holders are used for the D cells.
The battery socket is made from a piece of PCB and contacts come from a discarded octal socket
Mains power supply
I built a mains power supply to provide all the four required voltages. The power supply uses two transformers: for the anode voltage (B1 and B2} I used a salvaged Heathkit transformer used in reverse, i.e. the 230 V mains voltage is applied to a 250V secondary and the original 110V primary winding provides the B voltage. I connected the 6.3V secondary in series with the 110V winding to boost the final voltage. Two transistor regulators are used to provide the +B1 and +B2 voltages. The transistors are HV types (VCEO , VCES, VCBO at least 200V) otherwise there is the risk of destroying them with a momentary short circuit on the output. I did not include current protection assuming that the transformer will not allow too much current. For the filaments there is a second transformer (12 + 12 V) with two monolithic regulators. The voltages are adjusted to be the same as in the AM-598/U supply, as shown below
| B1 | +63 V |
| B2 | +130 V |
| A | +1.35 V |
| C | -5.7 V |
These voltages are slightly lower than those from fresh batteries. Batteries are safe because they quickly decay, but it would not be a good idea to run 1.25 V tubes on 1.5 V all the time.
The schematic diagram is shown below. Notice that a Schottky diode (3 A) is used to drop the voltage at the output of the 7906 regulator. Protection for the 1.35 V filament supply is obtained by two series-connected high-current diodes in parallel to the output that limit the voltage to 1.6 V. The 1A fuse should blow if too much current goes through the two diodes, in case the LM317 develops an internal input-output short. The diodes and the fuse are not shown in the schematic (to be added).
Since July 2023 both HV regulators employ the BU806 output transistor. The only difference between the two boards is the Zener diode: 51 V in the 130 V version, and 27 V in the 63 V one. Power dissipation is modest: a small radiator is used only on the 63 V regulator.
Schematic diagram
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130 V power supply board (left) and components layout on PCB
Power cable colors match the binding posts on the power supply.
The PRC-10 power connector has eight pins but is not octal, even if somebody tried to force mate the two.
Power cable wiring
Output power measurement
I use an Agilent 8508A Vector Voltmeter connected to the sampling output of a Dummy Load. The sampling output has been measured (using the same VV) to have an attenuation of -52.65 dB at 51 MHz. The 8508A is setup to measure μW on 50 ohm. The output power in W is obtained by multiplying
μW reading x 0.18408 = W output
Results
- fresh dry batteries Po = 1.73 W
- used batteries Po = 0.72 W
- AC power supply Po = 1.1 - 1.25 W
- specifications Po = 0.9 W minimum
Last updated 29/6/2023
The IF Module AM-427A/U
The PRC-10A uses four plug-in modules (called IF cans) in the 4.3 MHz IF amplifier. Each module comprises a 5678 tube and the associated tuned circuits. The IF modules are color coded yellow.
The interior of a module and the schematic diagram are shown below.
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IF modules are hermetically sealed but the TM explains how to open them for repair. Breck, K4CHE has an excellente Web page where he explains in detail how to repair a broken module.
The photo below shows a module where I replaced capacitor C102B which was shorted. On the right a fixture to test IF modules.
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Documents
- TM-11-4065A Manual for PRC A models
- TM-612A More info
- AM-598/U Manual Vehicular power supply
- TM-11-6625-823-15 Battery Test Set AN/PSM-13
