AN/GRC-9 Radio Set

Updated 12MAR2016:

US Marines of a Tactical Air Control Party with the First Marine Division in combat at the Chosin Reservoir operating a GRC-9 transmitter-receiver.  GN-58 hand cranked generator, cold weather, possibly watching Marine Corps Corsairs or Panthers rolling in on target:  NK or ChiCom troops danger-close.  Hopefully it got REAL HOT for the Reds in short order. Field day of the highest order, not fun.  But I bet it did the job… Note the weapon cleaning rod likely being used for a ground rod.   The Marine holding the T-17 is likely a pilot from the supporting Marine fighter squadron. The Marines found that this type of assignment produced the best air-ground, close air support coordination possible.   Thanks guys.    Official USMC Photograph.

GRC-9 with USMC Tactical Air Control Party - Korea

GRC-9 with USMC Tactical Air Control Party – Korea

Later on……“With voice transmissions ruled out by distances, communications specialists must rely on continuous wave radio operation, using the International Morse Code.  They must be able to receive and transmit at the rate of 18 words per minute, perform maintenance up to Third Echelon and even build a working set out of parts at hand.  Other team members must be able to work at least five words per minute and know how to set up and operate a radio.  Workhorse of the Special Forces is still the dependable Angry Nine, officially the AN/GRC-9.”
Special Warfare U.S. Army, Chief of Information, U.S. Army 1962.

GRC-9 with Army Special Forces, 1964

Left:  The “GRC-9″ at work with U.S. Army Special Forces and Vietnamese irregular forces.  Note the taller G-43 generator had replaced the GN-58 in this instance*. Official Photograph, U.S. Army, August 8, 1964.

The above image is from a Signal Corps film following a Special Forces patrol in Vietnam. The filmer or the person that entered the description in the National Archives file mis-identified the radio as the GRC-109. Incidentally, this 1964 photo is the only one I have found of US personnel using a GRC-9 in Vietnam. Well into obsolescence by then, the GRC-9’s were used by some US advisor personnel, especially early in our involvement. Shown here, likely training and supporting an irregular forces patrol. However, the GRC-9 was widely used by the South Vietnamese government Civil Guard and Self-Defense Corps in the early 1960’s and also by ARVN units as well. The Viet Cong captured many of these radios and used them themselves.

The ChiCom Type 102E set is a repackaged, Chinese copy of the GRC-9 with a few changes. The North Vietnamese Army and VC used the Type 102E extensively in their assault on the south.

The AN/GRC-9 was a late 1940’s development as an incremental improvement and replacement for the SCR-694/BC-1306 which itself was a replacement for the SCR-284. Due to a lack of any actual evidence to the contrary, it is widely believed that the GRC-9 did not see service in WWII as the war ended before it was ready for issue, or even available. There is anecdotal evidence that a Contract date of 1945 was reported but Contract dates and subsequent production and fielding can be quite different. As WWII drew down many late war contracts were cancelled but we don’t know much about this particular contract or any subsequent production run.

The GRC-9 did see widespread service during the Korean war and well into the 1960’s by US forces. It was also widely used by our European NATO allies, probably well into the 1970’s, notably by the French, German, Dutch and others. Germany and France also built copies and made system improvements for their applications, notably the development of an external 90 watt power amplifier. They also saw considerable service in Africa, Indonesia and many other areas around the world.

Its principal tactical use was in relatively short range networks that were beyond the range of the emerging FM voice sets coming into play at the time. With a planning range for CW of 30 miles ground/20 miles vehicular and 10 miles voice (TM 11-263 et al) it could meet that need. It could of course work over much further distances as the situation and conditions warranted but that was probably not its intended role.

Above: US Marines with 2and ANGLICO equipped with the GRC-9, GN-58 and a PRC-8/9/10. Almeria, Spain. Circa 1958. Photo: SGT Ed Nunn USMC

In Civilian Clothes:

There are a lot of GRC-9’s in the hands of European, South American and US Hams where they still enjoy a enthusiastic following. They are versatile, rugged and dependable and are readily put into Ham service without the dreaded “conversion”.

Below is my “GRC-9″ set up on the Pacific beach at Spooner Cove near San Luis Obispo CA during less stressful times.  (Actually, it is a GRC-87 – do you know the difference?*) We were running “Coastwatcher Ops” and transmitting Bikini Reports back to a TCS set at Camp San Luis Obispo acting as Net Control.  This was during the Military Radio Collectors Group meeting in May 2010.

(*The RT-77/GRC-9 receiver-transmitter when deployed with the G-43 generator, LS-7A (or LS-203) speaker and the ME-61/GRC field strength meter is defined as the AN/GRC-87 “system”. (TM11-5820-453-10, 10MAY1963 refers) )  If you’re keeping score.

AN/GRC-87 on the Beach. N6CC

The distance from here to KG6UTS at Camp SLO NECOS was 9.2 Miles over hilly, rocky terrain.  From here to N6IHU was 3.3  miles and N6IHU to Camp SLO was 5.8 miles.   N6IHU was running another GRC-9 in a portable configuration however he was using the DY-88 Dynamotor power supply.  Under the conditions at that time (appx 1000 hours local) the signal to noise ratio of received signals was 2 to 3 times better on the whip than the AT-101 wire which surprised us.  It was running approximately northwest, broadside to Camp SLO, averaging about 6 feet off the sand.  Comms with the 15′ whip antenna were Q5.  I didn’t note the FoF2 critical freq at that time to see if the low wire would work in NVIS mode under those conditions.  Apparently it was lower than our 3885 freq and/or the D Layer absorbtion was very high then.  We ran the standard ground radial wire array, G-43 hand cranked generator and a battery for the receiver to save the “power supply”.  The TCS was running a 30 foot inverted L and N6IHU was running the standard whip antenna.  This setup would have been a very workable tactical circuit. authors Photo:

Coastwatcher Ops Pacific: GRC-87 N6CC

Below is a typical mountain top campsite to cover all circuits. There is the GRC-109, GRC-9, PRC-47 and the mobile GRC-9 all on-line. However, looks like the flash caught the Midwatch off guard. Zzzzzzzz.  Good thing the Sergeant wasn’t around!

Below is a shot of our Perimeter Defense Force checking out the Company Command Post.  Note the GRC-9 AC Power Supply built in the .50 Cal Ammo Box on the ground.  Provides all voltages from 120 VAC.

GRC-9 in the Tactical Operations Center, MVCC/MRCG Camp Delta 4-13

Above: The GRC-9 was operational at the Military Vehicle Collectors of California April 2013 Rally at Camp Delta. A Battalion Communications Center capable of handling comms relay between any possible Time Warp from 1944 through the 1970’s. We worked lots of stations on 40 meters CW with the GRC-9 and its’ simple whip antenna. Always a hit with the visitors.

GRC-9 Night Ops

Above: Later that night…..

Don’t let this happen to you! Do your PMS and then double-check everything!

QSL Card; USS Pampanito SS-383 NJ6VT

Above: QSL Card from the USS Pampanito SS-383, normally moored in San Francisco near the Golden Gate bridge. I worked their ham station, NJ6VT aboard the sub on CW with my GRC-9 from our Forward Operating Base in the Sierra Nevada mountains. Her wartime Navy call sign was NJVT.

Below is a shot of the GRC-9 in the field using the .50 caliber AC power supply. “You have a strange rushing sound in your Transmit audio”….Yup…. This is the setup I used to work the USS Pampanito submarine on CW during a “Museum Ships On The Air” weekend. Better than 150 miles on 40 meters during the day.

Below is another shot while sending Ditties from FOB (Forward Operating Base) Margarita….The little AC power supply in the .50 Cal ammo can makes it convenient to work in camp. Especially when you bring the generator to run the Blender anyway!

Below: Would you believe a solar-powered GRC-9? Been there, done that. Here we are set up in the mountains west of Lake Tahoe at OP COMMANDO, monitoring kayak traffic 2000 feet below us on the North Fork of the American River. Panel charged the deep cycle battery which powered the sine wave inverter that powered the ammo box power supply which powered the GRC-9. Inefficient as hell but it worked!

Below is another shot of that same position – we also had a PRC-47  going on CW and LSB as well. There is the wreckage of a C-46 Commando somewhere within a mile of this location.  Very dense forest in here.  It’s on our list for a search expedition. This spot will make for a very good base camp.

GRC-9 Mobile setup

Above: Mounting my other GRC-9 in the back of the old Bronco makes it pretty convenient while camping. I can check into the Saturday night West Coast Military Radio Collectors Group net on 3985 KC AM, even from my sleeping bag if it gets cold. I usually run 80/40 meter coax-fed dipoles but sometimes just a quarter wavelength wire or the MP-57/MS vehicle-mounted whip depending upon how long I plan to camp in one place. If the racoons wake me up at 0345 I can check the propagation worldwide without getting up.

The Bronco is pretty cramped with camping gear so I had little room for a huge DY-88 dynamotor power supply. So I built a small transistorized power supply that runs off the vehicle 12 Volt battery; with the little supply sitting under the passenger seat. It works well and is more efficient than the dyno but I miss the Dyno Whine during Transmit !

GRC-9 at work in the Commo Bunker

Above: The GRC-9 at work down in the Commo Bunker sending out the nightly SITREP at Zero Dark Thirty.
(BTW, it’s an old military phrase that has been around since clocks were invented and the troops had to get up early. Then the Hollyweird crowd discovered it and was so enthralled with it they named a movie after it…. Same with “Hurt Locker” but I digress…Those people need to get out more often – yeah, right.)
A note on the GRC-9 homebrew power supplies discussed above: I have gotten many requests for the design of the AC and DC power supplies that I have built.

As we know, the GRC-9 requires a number of different voltages from the power supply. 580, 108 Reg, 6.3, 1.5 ……I was able to build supplies around junk box parts I had on hand, but the design is entirely dependent upon the kind of transformers you can get – it is central to any design you could make. It completely defines the circuit specifics you will need. The AC transformer I used in the Ammo Box supply is from an old oscilloscope PS,  that “TV” transformer is a Triad R-50BC – long obsolete. It had lots of different voltage windings. The DC supply is built around a transformer from an old Heathkit mobile transceiver DC supply that was scrapped – and it also had a lot of winding voltages I could work with and adapt. Especially the all-important primary and feedback windings needed for a 12 VDC transistor power oscillator.

GRC-9 Ammo Box AC Power Supply   N6CC

GRC-9 Ammo Box AC Power Supply N6CC

Above: The GRC-9 set up on the workbench along with the “Ammo Box” AC power Supply. A small, simple, compact solution to powering the GRC-9 from AC sources at home or in the bush via a generator.

Without either of the AC or DC oscillator transformers my design would not be useful to anyone trying to replicate them. That said, however, the designs are otherwise straight forward and follow the ideas in any of the older ARRL handbooks for providing those voltages. I used adjustable LM-350 voltage regulators for the +108 and 1.5 volt supplies. I chose to regulate the receiver filament voltage to protect the tubes and I also regulate the receiver B+ in the supply. Consequently, the VR105 (OC3) tube in the GRC-9 does not “fire” with externally regulated +108V applied and this works well.

Unfortunately no one makes suitable (affordable) HV power supply transformers any more in the “transistor age”. Especially transformers for DC-DC HV converters. You must work with what you can find – and that took some digging….
And that was part of the fun……

GRC-109/GRC-9 (Receiver)  DC-DC Power Supply (N6CC)

GRC-109/GRC-9 (Receiver) DC-DC Power Supply (N6CC)

Above: The GRC-109 DC-DC Power supply built into an excess GRC-109 transmitter box. This supply powers the GRC-109 transmitter and receiver from 12 VDC. Since both the GRC-109 receiver and the GRC-9 receiver can both be powered from the BA-48 battery (the common 4 contact connector), this supply also powers the GRC-9 receiver directly as shown above. It uses the standard CD-1119 GRC-9 Battery Cord as seen here. A future project is to include a 6.3 VDC and a +580 VDC GRC-9 transmitter supply inside this box as well. One box will do it all. I just have to find a big DY-88/105 power supply chassis connector.

Look here for further details on that DC-DC power supply:
Problem areas:  Both of my GRC-9’s have been very reliable, they have all the original tubes and capacitors and I have not needed to make any* repairs on them despite 20+ years of frequent use, both mobile and portable. That’s after their military careers. The only item needing attention is the bias battery used to keep the receiver 3Q4 pentode AF power amplifier tube properly biased. (The receiver will work without a battery installed, see below.) I remember recalling that my receivers “sounded” better with the battery installed – and hence the effort to supply that voltage – and the designers thought so as well.

UPDATE: I need some DATA! The Sylvania tube data for the 3Q4 states that to run the tube as a Class A audio Amplifier, the grid bias needs to be – 4.5 volts and that the audio input not exceed +/- 4.5 volts AC. (It doesn’t want you to drive the grid POSITIVE – the definition of Class A.) The data sheet states that the tube biased Class A will exhibit 7% distortion. Seems like a lot to me but in keeping the circuit simple and recognizing that its use was not intended to be “Hi Fi”, 7% was deemed to be acceptable.

So I did an experiment. I set up the receiver to hear its crystal calibrator at 2.00 Mc, driving a 600 ohm speaker. (The receiver audio impedance switch was set to 250 ohms for this test and that alone should produce some distortion by itself). This produced a beat note in the receiver while set to CW. I looked at the output with a scope, both with and without the bias battery. Although I don’t have an audio distortion analyzer, at low audio levels I could see no difference on the scope, the sine wave was very clean in each test. I was a bit surprised. At high audio levels the stage was driven into some distortion and removing the bias made that worse-looking. I also listened very carefully to bias – no bias condition and I don’t think I can hear any difference at “usual levels” although my hearing has taken a beating over the years. (An Audiophool would probably go apoplectic however…)

GRC-9 Receiver bias battery performance comparison test

GRC-9 Receiver bias battery performance comparison test

Above: Fun on the work bench.

GRC-9 Receiver AF amplifier bias battery socket

GRC-9 Receiver AF amplifier bias battery socket

Above: The socket and clamp for the BA-1293 bias battery in an RT-77A. The RT-77 used a hard-wired battery. (The larger diameter contact is grounded.)

I had previously built replacement bias batteries for that application; I used three 1.5 volt AAAA cells from an ordinary 9 volt Duracell battery. Left in series via the spot-welded stainless steel jumper straps, they make a good replacement battery of -4.5 volts. A bit of heat-shrink tubing or tape, and a salvaged battery connector, it fits in the original battery holder.  That battery will last for years – the shelf life of the cells. Improvise, adapt, overcome.

GRC-9 Receiver Bias Battery source

GRC-9 Receiver Bias Battery source

Improvised -4.5 volt bias battery installed in GRC-9 Receiver

Improvised -4.5 volt bias battery installed in GRC-9 Receiver

Above: The jury-rigged bias battery as installed in the socket using a defunct BA-1293 battery base and pins (I didn’t have a complete battery with the plastic case). A bit of black electrical tape and heat-shrink tubing holds it together. Works fine but won’t pass inspection….

“If it’s stupid – but works – it isn’t stupid”
Murphy’s Laws of Combat

It may not sound any better in actual practice but the tube will draw its rated plate current and have minimal extra distortion when biased properly into linear, Class A operation. (Why would you operate any tube with a filament, plate or screen voltage widely different from the design voltages and expect good performance?)

Next I looked at the receiver B+ current draw, with and without bias to see if I could detect any change due to the bias. Removing the bias increases the B+ current draw by about 100-200 microamps from an overall 10 milliamps quiescent draw at low output levels. Negligible difference. Probably more of an increase when the volume is turned up – I didn’t test for that.

No surprises here, it’s all about Class A operation. The 3Q4 pentode stage delivers not less than 7% distortion at low output levels when it is biased correctly. When the Volume is turned up sufficiently to drive the grid positive (with or without the compensating bias) you will get even more distortion. I’m going with the original engineering design – I use the battery.
Gremlins Hideaway: One of my GRC-9 receivers had an annoying, intermittent receiver “cut-out” problem that occurred while turning the tuning knob. The receiver would just quit, sometimes preceded by a scratching noise while the knob was being turned. I assumed it was debris between the tuning capacitor plates causing that (high impedance) stator circuit to short to the grounded rotor plates. I tried several times* to blow the plates clean with a micro duster – that seemed to help – but not permanently – sometimes the problem re-appeared at a different knob position. On the right track.

It started doing it again! I pulled the receiver and removed the shielding covers and blew it out again. Kinda worked. Upon CLOSE examination with a high-magnification eye loupe, I saw a few tiny fibers protruding off the rotor plate but they were not positioned to touch the stator. Ah Ha !! Tin Whiskers. They were about 2-3 mm long, and looked to be about the diameter of spider web silk – if you can get the light just-right to even see them. Maybe a couple of microns in diameter.  If they are here, they are probably elsewhere in the capacitor assembly. They are apparently strong and stiff enough to withstand the micro duster gas blast in some areas that were hard to get to.

Tin whiskers are a well known problem in the electronics business.  They have greatly compromised all kinds of electronics since the environmental-cases started banning lead from tin-lead solder; and apparently even before that. This is the “RoHs” initiative started by the EU and then adopted by the US and others in a knee-jerk reaction.  “A planet completely free of any and all risks.”  But just a little bit of lead alloyed in  significantly inhibits the formation of these tin crystal “whiskers”. Tin-lead alloy solder does not have this problem; lead-free solder and native tin plating DOES. Some really interesting physics going on here….(If you have done any soldering with the lead-free stuff, you know it does not work very well as a solder, future whiskers notwithstanding).

My radio was made well before that decree started causing short-circuit problems, but the plates of my variable capacitor are apparently tin plated. A thorough “cleaning” with a slice of a business card slid inside each rotor-stator plate gap knocked off any other whiskers that were causing the problem with this set. Back in operation, no more receiver drop-outs. (By the way, the problem section of C7 was the local oscillator tuning – the LO stopped at certain rotor positions, thus killing the receiver).

Take a look at the tin whiskers on this variable capacitor frame. (Obviously not a GRC-9 capacitor – but you get the idea!)  Holy crap! (Photo: NASA)







According to NASA’s website on Tin Whiskers, at least 10 satellites have been lost and up to 7 nuclear power plants have been partially shut down due to tin whiskers shorting out critical circuits. Is the “cure” worse than the problem? Look at that photo and extrapolate – you be the judge.