The PRC-47 HF SSB/CW/RATT Transceiver

Updated 6/12/2024

Some notes regarding the AN/PRC-47.  This radio was designed by Collins and 1423 sets were produced by them (Contract NOm-72362, 15 December 1961).  The Department of the Navy also contracted with Bendix to produce additional sets in a competitive bid.

All the Collins sets were identified by the Federal Supply Code for Manufacturers (FSCM) on the data plate as 13499. Those built by Bendix have the FSCM of 06845. I have found no obvious differences between them; mine is a Bendix-built set.

The Operators manual is TM 11-5820-509-12 The Depot manual is TM 11-5820-509-35

It seems that the PRC-47 was primarily used by the US Marine Corps in Vietnam but there are many references to its use by the USAF and US Army as well.  They were also used by the Navy during Market Time coastal patrol operations as a base operations radio supporting Swift boat comms.

I believe their use was widespread as it is a versatile and reliable radio that is particularly weather resistant unlike the FRC-93 (Collins KWM-2) that also saw widespread use in fixed installations.

The Australian Army also used the PRC-47 in Vietnam.  Photos are on the web of the Australian 104th Signals Squadron at Vung Tau operating a PRC-47 in a vehicle on CW.  That radio also had the CV-2455 RATT converter installed. Thanks guys – we remember.

I will focus on actual field use in the Navy and later on in use as a transceiver on Ham Radio freqs in the field. My Navy unit used these radios during many training exercises in the 1970’s and early 1980’s. The Marines had begun the transition to the PRC-104 and the Naval Amphibious School at NAB Coronado was “surveying” their PRC-47’s.

They had been using the PRC-47 for many years prior and theirs were pretty worn out. We were authorized to draw excessed Property with the standard “1149 Form” and soon we had a few radios on our books. The unit ET’s (Electronics Techs) got them back in shape after some work and we were in business.

The photo below shows U.S. Navy personnel in a coastwatcher communications training exercise on a Pacific beach around 1982.  Upon landing, they made a radio check to indicate safe arrival prior to moving inland to a more favorable OP location.

In this case the PRC-47 was being powered by two 12 volt lead acid batteries in series because we apparently had problems with either the silver-zinc batteries or their charger.  (Improvise, adapt, overcome). 
Authors Photo:

This was a typical training exercise conducted by Mobile Inshore Undersea Warfare Unit 1920 (later renamed as MIUWU 103).  Coastwatchers would communicate back to the primary surveillance site with the PRC-47 if they were out of range of the PRC-77 that they were also equipped with.

In this exercise, we were operating within ground wave range and therefore used the standard whip rather than the low horizontal slant wire or inverted “L” wire antenna (NVIS-capable) which we normally used.  The use of wire antennas were entirely dependent upon available cover, concealment and supports at the surveillance site.

We had assignments of several low frequencies to take advantage of the propagation/time situation as necessary.  With military operations it’s nice not to be constrained by Ham-Band frequency limits.  We just get assignments from the Navy frequency coordinator.  It allows you to think “differently” about establishing comms….

These 2 sailors are a Chief Bosn’ Mate and a Gunners Mate – not radiomen.  They were  cross-trained in tactical radio operations among many other skills – and could make things happen.

Below is another shot of our PRC-47 set up on Mt Rose, Nevada during a cold weather training exercise. MIUWU 1920 was working with Charlie Company, 4th Recon Battalion Marines based at the Stead Facility (home of the Reno Air Races) north of Reno. A few days in the snow doing cross country skiing, snowshoeing, cold weather survival, land Nav and of course, Comms.

I note that this radio is tuned to 6970, our assigned 6 Megger freq for this exercise – the same freq we used on the PTF boats back at Great Lakes. It was a commonly assigned Navy HF training freq back then. Notice the lack of ground radials when the photo was taken – hopefully a “teachable moment”.  We were on about 20 feet of snow – a long way from “ground” !

When using an unbalanced antenna such as the Whip or single wire antenna, the radio (ANY HF radio) chassis MUST be RF grounded with a suitable ground wire array to keep the chassis at “RF ground”. Not needed with a dipole – its’ “ground” is the “grounded” leg of the dipole itself.

The transmitters’ output impedance matching network requires the ground to function properly when using an unbalanced antenna. In addition to not tuning properly, the chassis will he “hot” with RF power and that will burn you when you touch the handset or chassis (YOU are now part of the antenna matching circuit).

While not lethal, you will probably make this mistake only once…. Consider the thickness of that fat foam antenna base insulator. The chassis can become hot with a similar RF voltage – but no insulator to protect you! Ground it!

I recall that keeping the BB-451 battery insulated from the snow extended its charge life.  Not surprising. We were using it to maintain a comms schedule, 170 miles back to our base at Treasure Island CA.  Worked great.

PRC-47 supporting cold weather survival training, US Navy, Mt Rose Nevada

Below is a poor photo showing LT “G” in the back of our 2 1/2 ton truck set up as a mobile Command Post during an exercise at Camp Pendelton CA.  He had the mid-watch apparently. That’s the PRC-47 back in the shadows with its H-33 handset coil-cord dangling.

He was the OOD and received coastwatcher reports from the deployed team via HF.  That and other information was radioed to the exercise umpires aboard ships in the Amphibious Task Force at sea.  Great training – but that is another story in itself.

Also, that’s a Yaesu FRG-7700 Ricebox HF receiver we would bring along to monitor other HF circuits.This was the Jimmy Carter era and the military – and especially the Reserves, had little funding for anything – sometimes we brought our own gear to get the job done.   Improvise, Adapt, Overcome. HOOYAH!  Authors Photo:

Moving on to “Civilian” applications. Although PRC-47’s look deceptively like a “turn-key” radio for a Ham to use, I would not consider them to be a “beginners” radio if there is such a thing. Unless of course if you got one that works straight away. There is a lot going on behind that panel…

Ham Radio RATT With the CV-2455 Converter: I captured a NIB CV-2455 converter many years ago from Fair Radio but I don’t have a suitable terminal unit.  Long ago I built an interface to run AFSK with an FT-101 with an old AST Advantage NB-SX25 notebook PC running some software I got from a Canadian ham, VE3LNY.  Worked great.  Looking for a 28 VDC Mite if you have a spare that needs a good home.  I can’t run MMTTY on the Laptop – can’t handle it!

But now that I have the PRC-47 going I needed an interface to convert the CV-2455 loop current into TTL levels for the old notebook PC.  I designed a circuit using opto-isolators and level converters to interface the 20 ma loop current to the serial port and vice-versa and it works great as well.  Trouble is, I can only work RATT stations running 850 cps shift, Space high, USB AFSK, 60 WPM, the US Military standard MIL-STD-188-110B.  But that’s the whole point!

Below is a photo of the PRC-47 with the RATT system mounted in the back of the Bronco.  It’s not permanently mounted on shock mounts so I can take it out and use dismounted when necessary.  The small box with the blue front panel is the interface between the CV2455 and the ancient DOS notebook computer. A really old computer but it runs the system perfectly – and is therefore not obsolete!

I had been powering it with a second deep cycle 12V battery in series with the vehicle battery for 24 volts. I had it switchable to charge them both in parallel and then into series for operation.

I have since installed a 12V-24V DC-DC converter rated at 480 watts as a trial.  It does the job running the radio on High power, the switch provides fused 12 VDC for the input.  So far initial tests find it to be RFI quiet.  Still checking various freqs/bands.

PRC-47 12-24 Volt converter

Works great.

However, turns out I am not alone in running the military standards.  The RATT  Clatternet formerly running in the Pac Northwest was running such a net with their own Mil gear, including GRC-106’s, PRC-47’s with Mite terminals etc.

In the 1990’s I worked them many times on their 10.137 MC freq and then on 40 meters as well.  That included W6GER, W7LFK, K6FIB and N7EPI. Worked a number other guys in San Diego on that net too.  However, they QSY’d down to 80 meters as that supported their local (NVIS) situation better than 30 or 40 meters for that time of day.  I had listened for them a few times on 80 meters but no joy.
—————————
FLASH 8/9/14 There was renewed interest expressed at the May 2014 West Coast Military Radio Collectors Group gathering to revive the Clatternet (II).

Since then, we have run a RATT net for close to 2 years with several other people getting “re-interested”. Look for us on 3587, 7087 or 10137 Kc on Sunday Evenings around 0330Z, most likely on 7087 (at 0100Z on 3587 in the winter) for now. Plus/Minus in 1 Kc increments as QRM dictates. 850 cps shift, 45 baud, Mark Low, (USB AFSK. fldigi/MMTTY if you need to.) Hopefully it will move to entirely mechanical TTY terminal machines as time goes on. But at least military radio equipment and emission type for now.

FLASH: 3/28/20: CLATTERNET III is back in casual but sporadic operation on the West Coast after a years’ stand down. H-Hour is sometimes 0100Z Sundays depending upon propagation and daylight savings time follies.  Informal skeds are often arranged via EMail first.

In late 2022 we are now operating at 1730Z Saturdays on 7087 kc (window frequency), 850 cps shift, Space high, 45 baud. I’m still running the PRC-47 with the CV-2455/PRC-47 RATT AFSK converter but sometimes with a Signalink interface. Mark is on 7088.575 kc, Space is on 7089.425 kc to accommodate the PRC-47 or other SSB-based AFSK system.

Our East Coast MRCA military radio op “cousins” are also running RATT ops with these setup specs as well.

A recent experiment is to run the stock USB PRC-47 on AFSK RATT using a Signalink USB interface with a laptop PC “glass terminal”.  Software is FLDIGI.  Wide or narrow shift and M/S polarity set by the software.

All it took was to fabricate a cable with an RJ-45 on one end, a U-77 on the other.  I also squeezed a DC blocking capacitor inside the U-77 to protect the Signalink from the DC mic bias used by the PRC-47 for the H-33 carbon mic handset.  The blocking cap could have also been installed on the jumper header Mic connection inside the Signalink box.  Works great, however:

One significant issue with my PRC-47 is the 400 cps HV power supply switching noise is audible in the signal.  This is also true while running RATT using AFSK to generate the MK/SP tones.  The output is not clean and I need to address that.  Sorry for any QRM up until now.

[Note to self:  When powered by DC, it is probably the 400 cps square wave in the PA and other filament circuits coupling into the signal.  That may be unfixable.]

[OT]  I can also operate on the Clatternet RATT circuit using the PRC-174 as shown below:

Tailgate Teletype Ops

The above setup is a little simpler to use than the PRC-47 for dismounted or portable Ops.  Here using the Signalink USB interface with a Win10 Laptop running FLDIGI software on the Clatternet from a remote campsite. Inverted L Antenna up in the big trees.  Tailgate Teletype.

This setup could also get me into the Internet EMail system via a WINLINK HF Gateway node if necessary.

The PRC-47 drives the MP-57/MS whip or a dipole/wire when stationary.  It works well on SSB but I have not connected with anyone running Mil standards via the whip yet.  I had it out to the Military Vehicle Collectors of California (MVCC) Camp Delta in September as well but there was no one up on RATT then.  Next year!

It’s a good system.  The PRC-47 is stable and the CV-2455 has very sharp filters so I can read signals barely above the noise.  It drives the transmitter fully, of course with the USB (standard) filter module installed.  The Convertor utilizes AFSK to drive the transmitter mic circuits. I carry another Amplifier-Modulator module so I can work LSB voice with the PRC-47 as necessary.

The CV-2455 cooling blower is serious!  Runs TX at 100 Watts with barely a detectable change in the output air temperature.  Great design – but it’s LOUD!  Sounds like a Shop vacuum.

PRC-47’s at the Military Vehicle Collectors of California rally

Above: Three PRC-47’s set up and operating at our “Battalion Communications Center” at a Military Vehicle Collectors of California rally at Camp Delta. We had one on 40 CW another on 40 USB and the third on 30 meters CW. (Plus a 4th one in the Bronco set up for RATT.) Phased array anyone? You’d just have to be real fast at connecting the batteries to the proper set at the proper time to establish the directional radiation field. This setup drew a curious crowd….

PRC-47’s

More PRC-47 testing. The battery jumper cables run off to the water faucet “ground system”. OK, which one sounds better – this is Radio # 1…..Stand by…..

PRC-47 paired with a GRC-9

Above: A PRC-47 and GRC-9 covered the HF comms requirements on this mountaintop near Lake Tahoe CA. A very quiet radio location a LONG way from any RF interference sources.

PRC-47’s at the ready in The Commo Bunker

Above: Night Ops: PRC-47’s standing by in the Commo Bunker. The upper radio is for USB, the lower radio is for LSB – just move the antenna lead-in to the radio of choice.

The PRC-47 can also be operated remotely with the Control Group AN/GRA-6. It is a field telephone system connected by up to 2 miles of WD-1A/TT commo wire.

AN/GRA-6 remote control set for the PRC-47

AN/GRA-6 remote control at the campsite

Above: My camper buddy operating the PRC-47 remotely from his tent. The radio was about 100 meters back into the forest. This system can also power the radio ON and OFF without a need for a local operator at the radio. Very handy!

Campsite Comms: The AN/PRC-47

When deployed “by the book” these are great campsite radios. A low dipole antenna usually works much better than the whip for regional comms inside of a few hundred miles. Here working some ARRL Field Day LSB cross-country contacts from the mountains.

Below is a shot of my buddy, caught in a night FLASH while sending our SITREP traffic. Or was it trying to order a Delivery – Pizza? We were up in some remote campsite and were using his PRC-47 “dismounted”.  In this case powered by two 12 volt deep cycle Marine batteries.  Ground radials deployed, vertical whip, these radios work great.

Below is another shot of the same site during the day.

Below is a shot of my PRC-47 operating “semi-dismounted” on the tailgate of the Stealth Bronco up in the mountains someplace. Powered by its own deep cycle batteries as before. The GRC-9 is peeking out from the shadows inside the truck.  I got my PRC-47 from Gerba-Narramore-Phillips Technical Equipment in San Francisco and it was more expensive than Fair, but no shipping.

I later bought a USB (for LSB operation) filter from a guy advertising in Electric Radio Magazine.  Combined with a spare Modulator module from Fair, I got it going on LSB for Ham voice comms when there were few “Mil” configured USB stations to talk with.  I can switch sidebands by switching modules, choosing not to modify the radio to do both via a panel switch.

PRC-47 transmit spurs: Anyone who has listened to a PRC-47 powered from 28 VDC knows it can emit several “other frequencies” aside from the desired signal. This is primarily from the 400 cps power oscillator which drives the HV circuits. I have tried a lot of shielding and bypassing of these signals before they get amplified, but to limited success. Since it is mostly a square wave, lots of harmonic content in those switching edges.  [See Note to Self above.]

Also, in CW, Oscillator Q5 in the Audio Frequency Amplifier Module generates an 800 cps tone that is injected into the mic circuits to provide a CW frequency 800 cps above the suppressed carrier “Window” frequency. This produces the desired CW frequency power, sounding like an 800 cps CW tone in the target receiver tuned to the same Window frequency. Following this?

Without a spectrum analyzer available to me, I used another receiver to tune around to analyze the output, concluding that the second and third harmonic of 800 cps was getting transmitted as well. I believed that Q3 and Q4 were being overdriven by the oscillator tone despite the fact that CR5 and CR6 (between them) was supposed to clip this signal – in fact they add to the problem by generating harmonics.

This mess appears at J-8. Transformer T2 cleans this up a bit however the 1600 and 2400 cps signals are inside the filter passband so they get amplified and transmitted as well as the desired 800 cps signal. Combined with any undesired 400 cps power supply buzz (present in the audio circuits) which is also inside the filter passband you can have a basic harmonica sound in CW mode.

Looking at the RF with a scope, it looks like this undesired 1600 cps signal accounts for about 20% modulation on the carrier on my particular radio – not good. What to do?

Note: To get around this problem, the Collins KWM-2 and other similar commercial and military SSB radios of that era use a higher frequency tone to generate CW. The KWM-2 uses 1450 – 1750 cps depending upon the serial number of the radio. That higher frequency tone ensures that any undesired tone generator harmonics fall outside the filter passband and are therefore not radiated. I don’t know why Collins did not do that with the PRC-47.

Setting the mike gain to produce a proper drive level on voice and then figure out how to attenuate the 1600/2400 cps harmonic turned out to be an easy (no new holes) band-aid. Simply putting a resistor in series with the CW Key “hot” line reduces Q5’s output, therefore reducing this Q3-Q4 overdrive problem on CW. (In the Tune mode, this resistor is out of the circuit so the signal still sounds multi-toned).

The value of this resistor band-aid is a tradeoff – clean output versus somewhat reduced power output. Recall that the TX RF output meter reads total power output – which includes the noise. On my radio, a 5.1K resistor produced the best tradeoff and the output now sounds pretty clean.  I used a 1.5K resistor on another radio.  Just connect up a 10K pot and adjust for cleanest key-down signal with acceptable power output, then install that value as a fixed resistor at the Key.

I have installed “LSB” filters and performed alignments/tuneups on 5 different PRC-47’s, both Bendix and Collins. They all have this problem to one extent or another, before and after conversion. I expect that this radio was designed primarily as an SSB voice radio, this problem was probably noted but not considered worth the expense to fix it for CW ops in the long run. Incidentally, the 400 cps switching oscillator noise on SSB voice signals was present on all 5 radios that I worked on, some worse than others. From barely noticable to pretty loud.

A more elegant fix would be to monitor the output on a spectrum analyzer, make some real measurements and devise a sharp filter in the Collector circuit of Q4 to clean up the 800 cps signal when only in the CW mode. Ah, but space is at a premium…. On the to-do list.

BT

Aside from its complex analog frequency synthesizer, this radio has an interesting signal flow.  The receiver is all transistorized except for two miniature RF amplifier pencil tubes in the (bullet-proof) receiver front-end. The transmitter has a PA driver tube and the PA output tube.  What is unusual is the 2 tubes in the receiver front-end RF amplifier stage are also used to drive the PA driver tube in the transmit mode!  Twenty four volts B+ on the plate on receive, then switching to 150 volts on transmit.

Never seen anything like that in any transceiver. The Collins guy who presented that at the Initial Design Review surely got some puzzled looks.  But it works.  However…..

PRC-47 Repair N6CC

EMBRACE THE SUCK……

Above: This radio can be a bear to work on – especially if you don’t have the various extender cables (which I don’t).  Those 2 subminiature pentode pencil tubes are buried in there – plus another PA driver tube.  Almost all the interesting circuitry is in well shielded modules plugged into the main chassis with multi-pin DC connectors with integral coax connectors.  All the interesting signals are in coax and you can’t probe them at the chassis connectors.

There is very limited access to most signals, even if you remove the module dust covers, or the modules themselves. Pretty difficult, again without the extension cables. Even my Multimeter was telling me “Oh Well”…….

You have to disassemble everything, apply external power and control voltages and then apply surrogate signals – then look for the outputs.  Here, a faulty sub-miniature transmit-receive relay (K1) within the receiver RF amplifier board is being isolated.  No signals making it from the RF amplifier to the receiver mixer stage.  Used to work great.

Rare for a hermetically sealed relay contact pair to fail.  Tiny currents involved!  I found and then adapted a small 12 volt DPDT relay to do the job – then shoe-horned it into the tiny space available.  Back on the air…

PRC-47 Receiver T/R relay replacement

Above: The culprit 20V SPDT relay on the left, the replacement JunkBox 12V DPDT relay on the right.  Those 2 Amp contacts will never wear out.

PRC-47 bench testing

You have to do a lot of “working around the problem” and then inferring what’s happening.  I guess Collins figured these would not be maintained in the field by a bunch of Hams with minimal test equipment..  Or, an Integrated Logistics and Supply System consisting of a large pile of miscellaneous parts that “might come in handy”.  Improvise, Adapt, Overcome.

Again, as a cautionary tale, I would not consider the PRC-47 to be a “beginners” radio, troubleshooting-wise.  I really like them when they work – I really hate them when they don’t!

PRC-47 Power Amplifier Tube

Above: Another weird problem (aren’t they all?), this time with a friends’ Bendix PRC-47. No drive to PA or PA driver tube. Looks like no transmit signal getting through the Amplifier Modulator circuits but works fine on receive. But most Amp/Mod circuits are common to both TX and RX.  Oh great….

Symptoms?  Incorrect ALC control to the Amp/Mod ALC amplifier, Q1. Turns out the PL-177WA PA tube was massively shorted – filament-to-grid-to-screen-to-suppressor. The filament lit up!  But there was enough “grid current” flowing through that mess via the ALC current sampling resistor to fool me into thinking the PA was probably OK so I probed elsewhere. NOT.

First time I have encountered a blown PL-177WA PA tube.  Not sure why it blew up – no peripheral circuit problems or damage.  The owner said it had been working properly some time ago.  Possibly left in the TUNE mode, – off resonance – too long?  Mechanical shock?  Thermal shock? Wonder why Collins didn’t make that a spring-loaded switch to force you out of TUNE if you got distracted.  Hmmmm.

Here’s a hint: When checking a problem like this (or any other TX problem) work in low power mode if possible and use a 5 amp versus the 20 amp panel fuse in the 28 VDC input circuit until you’ve got things figured out. Could save your power supply transistors. (That trick saved my second set of transistors!)

Hoover – Tail-less Kitty

Above: Hoover the tail-less Cat. Got caught up reading the latest Fair Radio catalog on the workbench; time for a nap.  Take your PRC-47 someplace else…

Now what? Move over Hoover.

PRC-47 PA Compartment

Above: Another PRC-47 was producing no RF output. No screen voltage was found at the power amplifier tube socket. Culprit was the screen bypass capacitor, C-139, inside that PL-177WA base assembly. It was a shorted 0.01 uf 1KV disc, the only failed capacitor I have ever found in the 6-7 radios I have revived.. Replaced with a 0.01 uf 6KV ceramic. Another day on the bench.

Mechanical:  Another potential problem with the PRC-47 is the alignment of the tuning shafts between the Oscillator and Translator modules and their respective drive shafts from the tuning knobs.  Mechanical tolerances.  All design dimensions have +/- tolerances associated with them.  Then there are the machining deviations actually resulting.  Then there are the assembly deviations.  Then there are the as-built system deviations.

Collins dealt with this by using tapered dowel pins on the modules and zero-clearance holes in the chassis to force parts into designed alignment.  But Murphy’s Law says that all deviations will all add up in the same direction in order to drive you crazy.

So Collins utilized the white nylon Oldham shaft couplers to permit some axial mis-alignments between drive and driven shafts.  But at the end of the day, some radios just don’t have smooth tuning knob rotation.  (When the tuning rotations are smooth, they are VERY smooth) But some sets bind at some or all settings.  Time to get out the shims, and maybe files and assorted implements of destruction. (Last resort to make it functional.)

After fine tuning the parts, most sets I have worked on can be made to tune smoothly if they didn’t initially.  Over the operational lifetimes of these radios, some modules may have been swapped between Collins and Bendix-built radios. It happens. This probably didn’t help.  You’ll get there.

One Interwebs commenter advised lubricating everything in the PRC-47 as if it were an Automatic Transmission.  My advice: DON’T DO IT!  None of these moving parts rotate at 3600 RPM.

I normally don’t lubricate anything in PRC-47’s except maybe a very thin film of silicone grease on the Oldham coupler nylon contact surfaces.  It shouldn’t need it;  lube attracts dirt and dust, gets into switching surfaces, gums up anti-backlash gearsets etc. and that causes problems.

If you DO “lubricate” it, you will likely be doing that often – to clean up the unnecessary congealed, sticky dirty mess as the more volatile components evaporate away. Avoid if possible.  If it “needs” lubrication, something else is wrong and “lubrication” will just mask the problem. The Collins engineers knew what they were doing by leaving the gear sets “dry”.

However, don’t forget to put some silicone grease on the panel gasket and O-Rings under the 5 machine screws that hold to the case to the back of the chassis. These prevent water seepage when the radio is placed on the wet ground.

The USMC issued a modification to disable the VOX circuit in the PRC-47, making it PTT – only.  When used with a regular H-33 handset the VOX operates AFTER the handset PTT switch is depressed in a stock radio. Kind of redundant if not confusing.

[UPDATE] PTT MOD: In Audio Amplifier Module A1 connect a diode (like a 1N4005) anode to Pin 3 (coil) of relay K1 on circuit board E2.  The cathode of this diode is then wired to the unused Pin 11 of the Audio Amplifier Module chassis connector. (That diode was already factory installed on some late-model PC board, E2).

The modification then requires a jumper under the main chassis from that Audio Amplifier A1- Pin 11 connector to the PA Module Filter box, FL2, terminal 12.  The wire already on terminal 12 is the PTT line in the radio. This new wire now carries the PTT Ground signal to the VOX relay K1 via the new diode.

Keying the PTT handset now grounds the cold end of PTT Relay K1, operating the relay and turning the transmitter on.  Next, turn down the VOX Gain pot, R-40 all the way down.

VOX was probably handy when the operator was wearing a headset with attached microphone – frees up the hands for other work. Like plotting targets for incoming Naval Gunfire…..That feature would also increase the battery operating hours, probably the main reason for the early VOX function.

I disabled the VOX, per the Marine Corps mod, and now the handset PTT works normally in keying the transmitter. Simple, worthwhile (and authorized) modification.

H-33 Handsets: My buddy’s Collins PRC-47 developed a crackling, random static noise during transmit. Classic carbon microphone element problem. Disassemble the H-33 by unscrewing the Mic element. Take Mic element and drop it on a hard surface a dozen times, reassemble, test. Shakes up and rearranges all those carbon granules in the Mic element. Noise all gone, back in business.

Some more PRC-47 observations in answer to some questions I have received:

Antennas?  It depends:  As a tactical field radio it is designed to work with a short (in terms of wavelengths), low impedance whip or a wire antenna at either 15, 25 or 45 feet in length.

Those antenna impedances will be capacitive with a low value real component at the operating frequency.  Note that the transmitter will not tune properly with antenna lengths between quarter and half wavelength long.  Those will present  a high value impedance and will be inductive, something the transmitter is not designed to drive.

On Tuning the PRC-47:  First, read the manual*.  Then pre-set the Tune and Load controls for the frequency and antenna to be used.  While proceeding with the Tune process do not leave the transmitter in Tune for more that 3-4 seconds while adjusting to minimize heating and possible damage.

See the tuning chart on the radio front panel below – it will give you lots of insights on how the output tuning network operates.

It’s an L-C-L TEE network.  The TEE input inductance is a variable roller set by the TUNE control to resonate with the shunt C; fixed capacitors selected by the Megacycle control.  The output branch has lots of variable series inductance (3 series-connected roller inductors adjusted via the LOAD control) on the output leg.  That inductance designed to tune-out/resonate the short (capacitive) antenna load component with those antennas.

PRC-47 Tune-Load Chart

Note that the lower end of the “45 foot” antenna curve intersects the frequency axis at appx 5.2 mc.  That’s the frequency where a 45′ antenna is a quarter wavelength long, is resonant and therefore does not require any series inductance to tune it via the Load control.  (Likewise with the 25′ antenna at around 9 mc.)

You would set the Load control at Zero as indicated on the chart for that antenna and frequency.  Tune would be initially set to around 16 as indicated on the chart for 5.2 mc operation.

That panel chart does not address any other antennas besides those issued and therefore does not address the Load control setting for others.  But any other antenna that is a quarter wavelength long at the operating frequency or otherwise near 50 Ω will tune properly with the Load control set at or near zero.  Then just quickly (!) adjust the Tune control for maximum meter deflection under tuning conditions.  See paragraph 3-11, TM11-5820-509-12, the Operator and Organizational Maintenance manual for specifics.

*The tuning procedure in the Operator manual for the issued antennas has you tune for maximum meter deflection but then to further increase the Load control until the meter drops 3 units and then readjust the Tune control for maximum deflection.  Then followed by additional “tweaking”. 

These further, minor adjustments beyond the initial “tune” are done to increase the PA Loading which improves amplifier linearity.  This is good engineering practice but it significantly increases the time the transmitter is in Tune mode which is tough on the PA tube, power supply and power source.  I have found in practical use I can sacrifice a little PA linearity (which I or the receiving station can’t detect) to protect the transmitter components from overheating, damage etc.  YMMV.

Note the below accessory Tuning Chart provided with the AN/PRC-47/AS-2259/GR “crossed dipoles” antenna system.  This is the one that mounts a rigid “50 Ω” mast to the front panel and held vertical by the 4 dipole wires in a double inverted Vee configuration.

AS-2259/PRC-47 Tuning Chart

Note that this antenna is also capacitive over the 2-12 mc tuning range as indicated by a Load control setting requirement of between 8 and 1.  The 2 dipole leg lengths were chosen in conjunction with the transmission line mast length to insure that the antenna is always capacitive (and therefore tuneable) over the frequency range of the set.

The AS-2259 antenna approaches resonance above 12 mc, outside the tuning range of the set.  But with a Load control setting of 1, it illustrates the L-C-L tuning network operation with low impedance, capacitive antennas.

The PRC-47 will work well with a dipole or other resonant, low Z antenna.  I have used mine for 30 + years on dipoles and double/fan dipoles (often in the boonies) and they work very well.

In the field, the Whip and Wire antennas also tune/load OK (use the right antenna for the freq/time/distance you want to optimize).  I use a 14 foot high (tip to ground) center loaded Whip when mobile.

The radio has more that adequate receiver sensitivity since it was designed for the short whip – which delivers a “small” amount of signal to the receiver.  With a larger antenna like a dipole, strong signals can overload the receiver because its AGC is audio-derived. (It is not AM so there is no carrier signal strength reference available to control gain of the previous stages.)  As a result, you get some distortion. It’s a combat radio, not a Hi Fi.  Design compromises were made.

Overall receiver performance on HF like this is generally limited by atmospheric noise rather than receiver sensitivity/noise floor anyway.

LSB Conversion: An LSB conversion is necessary if you want to talk with  stations operating on Lower Sideband.  Obtain a suitable 500 kc Upper Sideband filter to replace the stock filter.  Then just do a careful re-alignment for maximum output of the Amp/MOD circuits with the new filter per TM11-5820-509-35, page 6-17.  I’d recommend contacting Fair Radio Sales regarding a possible replacement filter.

If you are putting a new USB filter into the radio Amp-Mod module (to make it work on LSB) follow the instructions on Page 6-17.

I just use 2 different modules for LSB/USB to avoid any permanent changes/damage.  I mostly run CW, RATT (USB Mode) and USB voice with my buddies…

The 2N1653 PNP power supply switching transistors have shown to be a potential weak spot with these radios if abused by mis-tuning, extended-time tuning or other transmitter fault.  Mis-tuning causing excessive PA  current and sometimes a transistor(s) will fail before the 20 amp DC panel fuse goes. Not good.

In the early 1960’s they were the best high current/power transistors available but Collins did what they could with the technology at the time.  High current/high power NPN’s were still off in the future.

I have not needed to evaluate substitute P/S oscillator transistors.  (My 1963 USMC manual specifies 2N1653 germanium PNP transistors rated at 25 amps collector current, among other specs)  TM11-5820-509-35 (Nov 1974) calls out 2N2287’s. Others have commented on possible replacements:

Transistors I have seen “mentioned” are 2N5884, 2N5684, 2N2955, MJ2955, NTE-180, 2N1166. Chuck reports success with the silicon NTE-180’s. I don’t know if using modern silicon versus germanium transistors would require a slight DC base bias increase to overcome the higher silicon emitter-base voltage drop. Probably. Comments anyone?

If and when my power supply transistors quit I will probably reconfigure the power oscillator to use modern silicon NPN power transistors. That will require reversing the collector-emitter wiring, transformer center tap wiring, one capacitor and 2 diode polarity reversals and a small base bias change. Relatively small stuff. Much more robust with spares plentiful and cheap.

Batteries? It depends.  I use 2 small “deep cycle” “Garden tractor” batteries in series and that works well for a weekend in the woods doing casual SSB and CW ops.  High power RATT will kill them pretty quickly of course… Bigger batteries=better obviously, unless you have to carry them.

My buddy uses the larger Marine Deep Cycle batteries (below) – they go for a long time but are quite heavy. Low SSB/CW duty-cycle is your friend. At home, I use a 20 Amp 28 VDC regulated supply.  With 28 volts at the panel connector the sets I have worked on can easily produce 100 watts in High power mode when aligned properly but the 20 watts setting usually does the job.

PRC-47 deployed with whip, legs and ground radials

The above setup with the big batteries will keep your FOB operational for a LONG time.

DC Power female connector wiring:

+28 volts on Pin H, – goes to Pin E (ground).  Three jumpers are also required.  They go between Pins A&B, C&D and J&K.  The 11 contact cable female connector is a Burndy MS3126F18-11S.

BT

Speaking of batteries, a note on transmitter power:  The radio can output either 20 or 100 watts via the front panel XMTR PWR power selector switch.  The difference between them is 10*LOG(100/20) = 6.99 = 7 db.  In your typical ham radio receiver, 6 db = 1 “S” unit on the signal strength meter.

So that’s what the guy at the distant receiver perceives as you increase from 20 to 100 watts output, a little over one “S” unit.  He may not even notice the difference.  If you were JUST in his noise level, it might make the difference – but not much.  Your batteries will appreciate the significantly lighter load however.
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So back to the forest; LZ Lamb Chop, August 2013. We had the PRC-47 going on USB and a GRC-109 running CW contacts on a 4 day camp trip to the Sierras. A rather informal Tactical Operations Center (and barracks). Shadows getting long.

PRC-47 at LZ Lamb Chop

Here is the initial setup. The PRC-47 was running on and off for 4 days powered by 2 deep cycle 12 volt batteries in series. With a receiver draw of only 600 ma, this kept us going without a need to recharge. We operated the ’47 on 60 meters to a dipole mounted about 15 feet up in the trees (plus 6800 feet of mountain below us). Good comms with our like-minded buddies back home. Mostly operated at the 20 watt power setting. We eventually had to relocate into a shady area – the high altitude sun was pretty intense here…

PRC-47 with whip antenna

Above: The PRC-47 being set up with the issued whip antenna. Here setting up a single ground radial rather than the issued ground radials, it worked well as an expedient. However the circuits we had planned to our friends back home necessitated either a low dipole or an inverted “L” to enable NVIS operation at the times and freqs we were using. Otherwise the whip works quite well over much longer paths.

Our pre-arranged Nets were with stations between 125 and 190 miles and they were very solid during the day using the low horizontal wire antennas.  He also worked a local station in Truckee CA, maybe 25 miles from here on 60 meters.

We also tried an Inverted L antenna that my buddy brought back from Vietnam. It was captured from the Viet Cong by the U.S. Marines of the First Marine Division. It was formerly owned by enemy units operating in the “Rocket Belt” around Da Nang, South Vietnam in 1968. See the posting on the GRC-109 elsewhere on this website for more details on this antenna. Below, it is connected to the PRC-47 with its spade-lug connector.

Viet Cong antenna connected to PRC-47

Wiring up the PRC-47 with the Viet Cong wire antenna.

Captured Viet Cong wire HF antenna

Above: The Viet Cong antenna. The insulator at the end as well as the one here at the vertical-horizontal wire junction were broken. Note the use of enameled magnet wire to seize the loops at the insulator eye. The insulation was pretty much falling off – but it worked.

We also used it on the GRC-109 running CW on 7050 KC. I am pretty well convinced that it is the issued antenna for the ChiCom Type 102E, a copy of the US GRC-9 HF field set. These inverted “L” antennas were provided with the NIB Type 102E sets that were imported by Red Star Radio several years ago.

Although the insulators were broken and much of the insulation had dried and cracked, it tuned and worked pretty much as any Inverted L would.
However, no chirpy, 5 character cipher groups using cut numbers were heard. They had faded out long ago.

AR
NNNN

For some more info on PRC-47 field ops, take a look here: PRC-47 Field Ops