SO NOW YOU HAVE A PRC-47

by
Dennis Starks; Collector/Historian
Midwest Military Communications Museum



PART I, SOME HISTORY


The PRC-47 as we all know, is a fine radio. I'll begin with the little bit of historical information I now. As early as 1960 the PRC-47 was in existence, & as far as I know there was never a government contract with Collins to developed the radio. Collins, always a step ahead of the government, I suspect already had the radio either on the drawing board, or completed when the government decided they needed such a piece of equipment. This was indeed the case with the now famous 618T series SSB aircraft transceivers(another story). Of the over 200 various SSB radios tested in Vietnam during our tenure there, the PRC-47 was one of the very few accepted,& adopted as standard issue.

No other government contractor is ever known to have produced the set, as with it's sister radios, the PRC-41,& PRC-38 developed around the same time. The first & heaviest users of the PRC-47 were the US Navy/Marine Corps, though we know it was also used by all the other branches of the military,it is only their use that has survived in printed documents & personal accounts.

The PRC-47 saw extensive use as a simi fixed,& mobile station with US Navy SEABEES, dditionally it is known they were mounted in rivereen patrol craft, jeeps, & helicopters. In the case of the vehicular installations, no such installation equipment has ever been found, & is very highly sought after by me & other members of this group. Additional ancillary equipment was provided to also allow TTY operation in these capacities.

The only known group to actually use the set as a manpack radio were the US Marines FAC's also known as Pathfinders. Here, it was combined with the PRC-10*, the MAY* or MAW*, & the EE-8* field telephones. *Replacement of these types began in late 1965,& was not completed until 1969. This with the PRC-25, PRC-41, & TA-312 respectively. Though we know this, other later model equipment was in widespread use by other military units, these dates are correct for the Marine Pathfinders. The MAY is a post WW-II VHF AM transceiver that would have been used where operations with South Korean or Australian aircraft were expected. The MAW is it's UHF counterpart. Though the PRC-74 was in use by this time, the PRC-47 was retained because of it's higher power, thus greater range.

The job of these FAC(forward air controller) units for the most part was to go out & look for something that aircraft could shoot at, then coordinate their air strike from a supposedly safe hidden location. It should be noted that while these poor bastards were carrying all this very heavy commo equipment, they were still being issued the M-14 rifle!

Early problems with the radio were few. The only two complaints from front line units were #1, the set's limited operational time on batteries in high power. #2, the radios cabinet was not completely water tight. In the case of #1, it seems that whether military or civilian, an operator fears wearing out a high/low power switch, so they just leave in high, even though it is seldom needed. A gas powered generator was eventually built by Homelite designed around one of their chainsaw engines, that put out 100vac/400cps. I've only ever encountered two of these little generators, their owners didn't know the difference between 60 & 400cps thus thought them to be very valuable. Not me! #2, it was reported that if the radio was set up for operation on marshy ground, water could leach through the bottom cabinet screws. 1/2 cup of water inside of an operating radio could distroy it. It is not known whether the late screws with rubber "O" rings fixed this problem or not.

As already noted, the PRC-47 was eventually used in some capacity by all branches of the US military, we just don't know exactly how. In approx 1988, a military equipment dealer in Florida, reported the sale of several radio set's to a South American CIA operative. This operative was later killed & the radios never paid for. In the mid 1980's the Government began liquidation of their vast stocks of these radios,(the good old days before crushing ,burning,& use as tank obstacle courses). Early prices ranged from $400 up.

PART II, PREPARATION

Preparation;


Before you start messing with your PRC-47 you'll need to gather a few things. Below is a list of what's needed.

#1,H-33 handset or M-29 mic

#2,LS-166 speaker(insure that it still has a U-77 connector, the government converted many of these for use with later model equipment & changed the connector to a U-229)

#3,A good 24vdc,20 amp power source.

#4,CX-8394,24vdc power cable.

#5,CB type 3/8"x 24 thread antenna mount with SO-239,1/2" hole mount type. Fuse clip such as is used with the larger buss fuses or similar.

#6,Depo maint manual.(should be on it's way to you now)

#7, 90 weight gear lube, & bearing grease(gun grease)

Lubrication;

The first thing you need do is lubricate the set. As with all equipment of Collins origin, there is a god awful monkey motion gear train that connects the front panel controls to all their associated gut's. These need be hosed down with your favorite spray lubricant, & worked back a forth some. The turns counting dials on the antenna tuner must be taken apart, cleaned of old grease & re-greased using a concoction of 90w & gun grease. Gun grease is to thick itself & will cause trouble later. So we thin it with 90w gear lube, stinks like hell but works good. This mixture also works very well when rebuilding such things as electric drills etc. Lubricating the radio before starting is important as the gears a somewhat fragile & will strip out, this is particularly true of the antenna tuning controls.

Making connections;

If you have obtained a CX-8394 dc power cable, your half way there. If not & you don't wish to pay the current price of one & would rather wait for a bargain(usually what I do),there are several things that you can do. First, the CX-8393 AC power cable can be used but must be taken apart & re-wired, sometimes they are potted but don't let that detour you. One advantage to this cable is the much longer wire, sometimes well over 20ft & as long as 100 ft. This was to allow you to use the radio far removed from the noisy generator it was most likely connected to. If the cable is over 15ft however, it will need be shortened because the gauge of wire it uses is not sufficient to handle the current required for DC High power operation at a length of over 10ft.

For portable, battery power ops, if your financially secure you can obtain the original battery/box(BB-451), cable(CX-8395), & adapter(MX-4430). I do not recommend this avenue for several reasons, but in short, the silver zinc battery is very expensive do to it's silver content, & it takes a special electrolyte. The battery case minus the cells can sometimes be found, but it's best just used as a display item, this because you cannot fit enough batteries in it of sufficient amp/hour ratting to operate the radio(in high power). Most portable user's(including myself) opt for a 50 cal ammo can & LARGE gell cells.

If you wish to get started before you have the proper power cable, it's not to hard to make connections to the power connector using a scavenged 7 or 9 pin miniature tube socket. You simply take it apart & use the individual pin connections. This is what I had to do in the early days of PRC-47 availability, before it was possible to get any accessories. This system also works very well with even more obscure, or oddball connectors such as pre-war types that can no longer be found. Never make Tack-Solder connections to you radio's various connectors, you can always find some size of old tube or vibrator socket that can be taken apart & used.

For use with a 50 ohm feed line, you can try to find the original antenna adapter(don't have the number handy),but this is not a very common adapter,& it terminates in a "N" connection. Better to obtain a CB radio type 3/8 x 24 antenna mount. These are used extensively with mirror mount antennas or via a 1/2 " hole, etc. Be sure to get the one that uses an SO-239 type connection. You simply take the SO-239 part of this mount, chuck it in a vice & run a 3/8 x 20(or is it 18?) die over it for the proper thread. Place the (un-shoulder) nylon insulator back on the connector & screw it into the antenna receptacle on the PRC-47. Then a ground wire is soldered to one of the larger Buss type fuse holders or similar clip. This is clipped over the PL-259 once it's connected to your new adapter. This all closely approximates the original "N" type adapter. A ground strap can also be soldered to an exposed part of the SO-239 adapter, but you run the risk of melting it's internal nylon insulation. A handy ground terminal is located next to the large porcelain antenna insulator on your radio.

The next installment will cover some simple mods & tips of operation, but if you can't wait follow these two demands. #1 you must have enough DC power source available to operate the radio! Stay away from high power operation until your will practiced in low power tuning. #2 follow the tune up procedures in the manual exactly! Those instructions in the lid are only a condensed version & do not cover 50 ohm operation. DO NOT key the radio for more than a few seconds at a time while tuning because you'll either fry the tube or the germanium power oscillator transistors in the power supply, they are fragile anyway. If you connect DC to the radio & have an high current drain for no reason, these transistors are probably already bad.

Places to get accessories;

Fair Radio Sales, has most all the standard accessories. 419-223-2196 Mil-Comm Exchange, may have some acc avail, email; ckuzel@aol.com Tartan Electronics in Arizona at one time had a lot of acc. I do not have any contact info, but they once advertised extensively in Nuts & Volts. Some members of the group may have extras, or sources, if so I'll post these with the next installment.

SOME HINTS & MODS

Disclaimer;

Normally I am apposed opposed to any sort of modification to an original piece of military radio equipment. But in the case of the PRC-47 which is fairly common at present, I take exception for several reasons.

#1, It is always preferable to have an operational & in use radio. This demonstrates to all the abilities of a radio far better than printed matter & a static display.

#2,there is a thrill that cannot be described when you tell some ass hole on the air with his $1500 Icom or assorted other shit of oriental origin, & his 2kw amp, that your operating a 30plus year old radio at 20 watts on a 15ft whip.

#3,we will not make any modifications to this radio that are not easily reversible, will alter it's appearance, or original design parameters.

#4, when you key this thing up on the air, people will for some reason, know that it's a Collins!

 There is no better way to preserve the heritage of this equipment than to have it on the air & let people see & hear it in operation, your personal satisfaction will be immense, I guarantee it!

Getting Started;


You should now have the needed accessories to get your PRC-47 fired up, & checked out. All gears trains have been cleaned & or lubricated, & a preliminary check indicates all is operational.

Dummy Trouble shooting;

If at first things don't seem to be working to well; Insure that while in receive the antenna load controls are in the ball park for the frequency in use, their settings will affect receiver sensitivity. Slightly loosen the hold down screws on each module, & wiggle them around to insure good contact on their mateing connectors. If you experienced a high current drain, or fire flying when you connected DC power, & your using an original power cable, the power oscillator transistors in the power supply are most likely shorted. This is very common, mine have been replaced by me twice. Fear not, they are common types, & easily replaced. They are located on the chassis just behind the front panel, not on the power supply module. If replacement is needed, replace them both with identical types even if only one is bad. I've been told several times by Kevin, at Mil-Comm, that he uses silicon types instead of the original germanium to permanently fix this short coming, but I have never tried it.

Audio Output Help;

Now that everything is operational , you'll notice that, as with most portable military equipment, the audio output is a little anemic, especially if used with both an H-33 & LS -166 at the same time. However with this radio we have lucked out, & this problem is easily fixed. The front panel volume control is in fact an attenuator between the audio output stages & the load. The actual volume control is a screwdriver adjustment(R-54) on the AM-3506(A8A1) Audio Freq amp module. It is best adjusted by tuning in a good signal such as WWV, with the front panel volume control turned to minimum, adjust R-54 until you can just hear the signal, then back off slightly. This radio is capable of window shattering audio output.

LSB Operation;

The best, & simplest method to make this radio work on LSB is to just change the mechanical filter in the Amplifier-Modulator module AM-3507(A8A2). The needed filters can be found, without to much trouble. In the recent past Fair Radio has had suitable filters twice at a price of $25.00(but not at this time). My first LSB PRC-47 filter was robbed from a URT-23 while I was still in the service. The second came from a board at Fair radio that contained both SB filters for $25.00. The last also came from Fair, this only a couple of years ago. This board was advertised as a Magnavox filter board with a USB filter on it, also $25.00. However, USB or LSB is not determined by the filter, rather it is the mixing scheme & whether or not high or low side mixer frequency injection is used. In this case while the filter on this board was correct for USB operation on that radio, it was suited for LSB use on the PRC-47. Interestingly Fair had these boards for several years, once I found a need for a second LSB PRC-47 & ordered some, I spread the word that these filters would work for LSB ops. Fair sold out of them a couple days later.

About two years ago, a source in California offered LSB conversion kits for about $45.00, these where advertised in Nuts & Volts. Two types were available at that time. One was a filter change method, the other was a mixer freq change(describe later). Alternately, both USB,& LSB filters can be robbed from 618T modules & several others. Also one of our group has a few filters left that will fill the bill(see our last two group want/trade post).

The second method, though I don't care for it, will allow selectable SB operation with the PRC. It entails the substitution of your own 500kc reference freq to the balanced modulator. No mods to the original 500kc reference osc where required (which is strictly TABOO!). This method used the multiple positions of the on/off/light dimmer switch to effect sideband selection. Here the panel lights were wired for their brightest, then the switch connections re-utilized. This system has one major drawback, while USB ops are unaffected, the panel frequency display is offset by 3kc while in the LSB mode, which will require some brain thought each time a frequency is dialed in. This all was originally detailed in Electric Radio, the author at that time offered a kit of all needed parts. Don't bother asking me, I don't remember which issue, or the name of the author, & I'm not a subscriber to the overpriced rag.

Lastly, it is possible to incorporate selectable SB using two filters & the dimmer switch for selection as in method #2. A second filter can be cram fit into the AM-3507 module, then selected via subminiature relays robbed from other equipment. This is the system that mine presently uses.

Fine Tuning;

As a mater of preference you can add a fine frequency tune to the radio without making any significant modifications, & using the sets existing design. Personally I believe as did the designer, that everybody on the air should be on frequency, but we live in the real world & they aren't. Also my radio was most often used in round tables with operators running AM, & rudely, seldom on freq, thus an extremely annoying heterodyne noise was present.

Your control will be located in the hole now occupied by the AC fuse holder. You remove it, insulate & secure inside the radio with whatever method you wish. The heart of this mod, is as said before, using the original design of the radio. I well not go into fine detail, you will need do some experimentation yourself. I'll just tell you how & where it's done, & what to lookout for.

Located on the underside of the main chassis is relay K-6. It's purpose is to switch between two different source voltages that are supplied to varactor diodes in the translator module. These varactors, are used to maintain frequency tracking throughout the transceivers range. To this end, a tracking voltage is Normally supplied by discriminators. Should frequency deviation progress past a point, the relay will throw providing a fixed(fail safe) voltage to these varactors thus keeping the set in operation though possibly off freq.

In our application, we simply energize the relay & provide our own variable voltage rather than the fixed one. So what we'll need is a well regulated variable voltage, with limited range to sub for the fixed voltage. The control(pot) for this variable voltage is located on the front panel. It is desirable that this control also have a switch so that normal 1kc operation can be obtained by switching voltage on & off of Relay K-6.

There are several things you must consider while undergoing your experimentation.

#1,You must have a well regulated voltage source. My first attempt at this conversion simply used a resistive padder & the original voltages found in the radio. This resulted in FM'ing on transmit, & a severe warbling while receiving strong signals at high volume. This was caused by voltage variants in the primary power supply with transient current drains. Thus you must use your own regulated voltage derived from the primary 24vdc, this by virtue of a precisian variable voltage regulator IC.

#2,Your front panel control must be provided with limited travel. i.e. an internal trimmer should be used to limit the amount of voltage the front panel control can vari. If this refinement is not include, the radio will actually operate many KC's from the displayed frequency(too much travel).

 This condition can be so acute that operation half way down in the 160mtr band is possible while the radio is set at 2mc. Frequency stability will however suffer & you'll have no idea what frequency you are operating on. Best by far to limit the range of this control to 1kc at most.

Good Luck!
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The preceding was originally published as a series of articles in the "Military Collector Group Post", an international email magazine dedicated to the preservation of history and the equipment that made it.  Unlimited circulation of this material is authorized so long as the proper credits to the original authors, and publisher or this group are included. For more information concerning this group, the use of our material, or membership contact Dennis Starks at,
military-radio-guy@juno.com.
A list of selected articles of interest to members can be seen at:
http://www.softcom.net/users/buzz/backmail.html

Dennis Starks; Collector/Historian
Midwest Military Communications Museum

*************************************************************

PRC-47 Germanium Versus Silicone; Nick Broline

I put silicon replacements in for Q1 and Q2(replaced with MJ4502s), but they don't oscillate properly. Assume that they aren't biased properly but don't know how to fix this.

Steve H.

----------------------------------------------------------------------------

Steve,

What you are suffering from, and correctly concluded, is the difference in leakage current and bias turn-on threshold between the germanium and silicon power devices. Fortunately, the means to determine the correct bias is most probably done using experimental means rather than analytical.

What I would suggest doing is the following:

1) Apply power to the inverter and measure the collector currents (like, for the center-tap of the power transformer). (Mind you, I do not have a schematic of the '47, so I'm making a lot of assumptions.)

2) Let us assume that the current drain is very low....say less than 0.1 A....I suspect that it will be zero.

3) Locate the resistor that connects the base winding with the same voltage as the collector supply (minus). Slowly decrease this value by inserting larger values in parallel with it. Note that if you place a resistor that is about 10 times the resistor's value, that will decrease the total resistance by about 10%. At some point the inverter will start, and will continue to run, probably no matter what you do to the resistor. You can use a resistor that is much lower wattage than the one in the circuit, as it will be supplying a small portion of the load.

4) The resistor that you are looking for is the one that will allow the inverter to start reliably every time, but not a lot less resistance than that.

5) Things to look out for................

 In general: The larger of the resistors in the base bias circuit supplies the turn-on bias. Once the supply is oscillating, it is not terribly critical. The lower resistor limits the base drive. The larger the switcher gain, the larger it can be.

Don't short the collector supply to the bases unless you just like replacing transistors!

Maybe this'l help??????

73 Nick Broline W5FUA

SO NOW YOU HAVE A PRC-47 - another copy of this article.

Back to Brooke's PRC-47, Military Electronics Information, PRC-68, Home page.