AN/VRC-10 / RT-68 Transmitter-Receiver

UPDATED 12JUNE16

Some fun with an RT-68 VHF FM Transceiver set up with its PP-112/GR matching power supply: The basic VRC-10 pair… I don’t have a shock mount just yet.
Or a Jeep…..

GRC-8 / RT-68 Transceiver N6CC

Documentation:
Refer to Tech Manuals TM11-289, (19 March 1951) and TM11-289/TO16-35RT66-5 (3 December 1953) among others.

The AN/VRC-10 FM radio system and many related ones are based upon the RT-68 transmitter receiver shown here. These radios were designed by the U.S in the late 1940’s and saw service late in Korea and many other theaters in the 1950’s and 1960’s. This one has a 1951 Contract date. They were phased out in the mid – 1960’s by the AN/VRC-12/RT-524 systems. Reference (20). As part of a versatile system, it provided FM voice communications with a power output of up to 15 watts, 2 watts on low power. Planning range between similar units was approximately 15 miles for stationary vehicle operation, 10 miles between vehicles in motion.

This “Old Family” radio runs wide band FM with an effective Carrier-Operated squelch. The receiver is 85 KC wide at the 6 db down points; hence the 100 KC channel spacing. The transmitter deviation is probably +/- 15 KC. The receiver sounds nice and clear with punchy, room filling audio when listening to modern FM transmitters operating in the Land Mobile bands. If those signals are pretty weak (and can’t fill the pass band anyway), it sounds pretty noisy as expected. Working with my VRC-7 / RT-70, PRC-6, PRC-10 and PRC-25 it sounds great – both ways.

John Rambo even used one (Rambo 27 I think) in the headquarters of the evil Lieutenant Colonel Padovsky in the POW camp. Rambo switched freq’s on the RT-68 and – even with no power supply connected or even present – ahem – informed the treacherous Murdock that “I’m coming to get YOU”.  Murdock somehow heard him.  Impressive!

This radio operates on 170 discrete channels in the 38 – 53.9 MC range, designated for Infantry units. In those days there were 3 discrete frequency bands for ground forces; combined arms communications interoperability came later as technology improved.  Armor operated in the 20 – 27.9 MC range (AN/VRC-8) and Artillery was in the 27 – 38.9 MC range (AN/VRC-9) with their RT-66 and RT-67 respectively. The middle “artillery” range overlapped with the other two by 9 100kc spaced channels to permit communications between combat arms – except the Infantry could not communicate directly with Armor. Great. But this was driven by the technology available at the time and was a big improvement over the pioneering WWII tactical FM sets.

These radios were part of an overall U.S. Army communications system and were interoperable with the man-pack PRC-8, PRC-9 and PRC-10 respectively. It was also backwardly interoperable with the WWII SCR-300 / BC-1000 Walkie Talkies. The AN/VRC-10 was further interoperable with the PRC-6 Infantry portable on the high end of the VHF band where short antennas were more efficient (and more importantly, harder to see). It could also communicate with tactical air assets using the AN/ARC-44 sets in those aircraft.

This series also permitted a field telephone interface so a field telephone operator could communicate by a remotely located radio at a favorable terrain location. Since radios are frequent artillery magnets, this was a good idea. See the Technical Manual, Reference (19) for further details.

I obtained mine at a Ham swap meet “condition unknown” but it looked to be in NOS condition – a good find for 20 bucks. Upon opening it up at home, I found that it had been dropped hard sometime in its first 50 years of life, rendering it NFG (Not Functioning Good). Further investigation revealed a slightly bent chassis frame but the main tuning gang capacitor assembly in the RF chassis had suffered a broken ceramic rotor shaft – an unrepairable show stopper. A REALLY bad show stopper. No rotation, shorted plates, unknown short circuit damage etc. These radios are very rugged – they are the radio equivalent of the Main Battle Tank. However, F = MA. Every time.

After obtaining the RT-68, I obtained a 24 volt PP-112A/GR power supply at another swap meet and an interconnect CX-1211/U “dogbone” cable to power the radio. These two units along with the shock mount, control box, antennas and audio devices was known as the AN/VRC-10. With a combined weight of 73 pounds just for the RT and Power Supply, this is a true Boat Anchor – or install one in your Prius to keep from getting stuck in the snow.

Apparently the Army operator, organizational and depot level repair guys thought my RT-68 was beyond repair, so it was probably “excessed” as Beyond Economical Repair or relegated to “storage” for repair if they ever got really desperate for a replacement radio. Hence its NOS condition – it was clearly never installed or used anywhere. The radio is practically built around this variable capacitor and the radio must be completely disassembled to get to the capacitor – and then about 20 inaccessible solder connections had to be opened to be able to remove the part. That after you first removed a bunch of unrelated mechanical gears, couplings, shields and other access interferences. So that’s the job I inherited.

RT-68 6-Section main tuning capacitor Ass’y

Above is the RF chassis after removal from the front panel and main chassis – then separated from the IF chassis. The 6 gang main tuning capacitor is shown here, missing the gear driving the broken shaft of C1.

RT-68 main tuning capacitor – broken ceramic rotor shaft

Above shows the broken (white) ceramic shaft that the rotor plates are connected to. When the shaft broke, all the ball bearings supporting the gear shaft evaporated to parts unknown, Can’t get the ceramic out, can’t insert new ball bearings, can’t imagine an adhesive that would work in this instance.  Beautifully machined brass and stainless steel parts, especially the antenna trimmer cam system seen here.  Much more eye-appeal than varactor diodes!

Fair Radio came through with a parts chassis, sans tubes, crystals, meter, cabinet etc – but it contained a usable C1 capacitor assembly. Lots of troubleshooting time, circuit study, test gear setup, then surgery. Two long days later, the capacitor had been mechanically swapped out. Two more long days of alignment fun and old Serial Number 41444 was back up to specs. Fortunately the PP-112A/GR power supply fired right up with the 3 different vibrators humming right along. Now I have to find an MT-299 shock mount and a few other bits for the complete VRC-10 system.

AN/VRC-10 FM Radio Set N6CC

Until I find a reasonable MT-299 mount, I decided to build a field expedient “mount” to keep the radio and power supply mechanically connected. This is primarily to protect the Dog Bone interconnect cable assembly while moving the chassis around.

I took a piece of 3/4 inch plywood and mounted five pieces of 3/4×3/4 inch, 1/8 inch thick angle stock to the plywood base. The chassis skids of the RT and Power Supply fit tightly between them and they take the chassis weight. I took some 1/8 inch diameter steel rod and used it to slide through the angle rails and the steel roll-pins in the chassis skids. These 2 long pins serve to capture the 2 chassis to the “mount” but don’t bear any weight. I threaded the ends of the rods and tapped holes in the end-plate as shown in the photos. This keeps the steel pins in place. Works great. Next step is to install four Barry Mount vibration isolators under the plywood.

Homebrew VRC-10 Radio Set Mount N6CC

Above:  The plywood base with four aluminum angle rails and end-plate to accept the RT-68 (foreground) and power supply skids.

Steel rod threaded into mount rails

Above:  Detail on threaded steel pins as they attach to the mount.

RT-68 mounting assembly – Steel Roll Pins

Note the Roll-Pin in the Power Supply chassis mount skid. These were originally used in the MT-299 as clamp anchors.  These Roll-Pins are steel, slightly bigger than 1/8 inches I.D. Perfect fit for the steel rods to attach the chassis to the mount. I use the same technique in my VRC-7/TR-70 in the Bronco. (Can’t afford one of those shock mounts either!)

RT-68 sitting on mount

Detail.  Note looped end of steel pin protruding from angle aluminum.  Just unscrew and pull out to remove chassis from mount.

MVCC Battalion Comm Center Tent

Here’s the VRC-10 on its first field deployment: The Battalion Comm Center at Camp Delta, Military Vehicle Collectors of California rally and camp out. The VRC-10 covered the VHF tactical circuit feeding a “jungle antenna” wire ground plane suspended from the trees.  Very effective combination!
In typical vehicle service the antenna system would include the AB-15 base and one each MS-117 and AB-24 mast sections to approximate a quarter wave whip.

VRC-10 in the Tactical VHF Position

Here flanked by the PRC-10 and PRC-25 FM radios.
“White Rook this is Raven. BEADWINDOW – you are on the wrong circuit! Out”

VRC-10 Radio Wire Integration

VRC-10 Radio Wire Integration

Above: Meanwhile, down in the Commo Bunker: The VRC-10 remotely controlled via the GRA-6 Radio Wire Integration system. The transmit/receive remote control can be located up to 2 miles away when connected by WD-1A/TT infantry field telephone wire. This system works great.

For more information about this set and the VRC-7 controlled by field telephone systems, take a look here: AN/GRA-6 Radio wire integration