AN/GRC-109 / RS-1 Transmitter with VFO


Of course my favorite military Black Radio. The GRC-109 and its immediate predecessor the CIA’s RS-1.

I was sitting here thinking about the cost of adding additional FT-243 crystals to my existing, pretty large collection when I remembered my old Heathkit VFO. When I upgraded from Novice to General Class license back in 1967, I built the HG-10 VFO kit to use with my Hallicrafters HT-40 transmitter to move up into the General bands with variable frequency tuning. It worked, but it would not drive the HT-40 to full output. Why not try it with the GRC-109 transmitter? This is really a no-brainer but it is the first time I have tried it. A variable frequency transmitter GRC-109 set.

I know: Blasphemy!, OMG!, Get the rope! Torches and Pitch Forks!, A CB-er!, Harrumpf!, Treason!, Bricks through the shack window! etc.

Well, might be fun to try. Seems the transmitter crystal oscillator could accept a VFO signal and then behave as an intermediate amplifier with a tuned plate circuit. The HT-40 did essentially the same thing but it has a front panel switch for VFO or Crystal control to reconfigure the triode drive circuit. In looking at the RT-3 transmitter circuit (an electron-coupled Pierce 6AC7 pentode), the Crystal is connected between the oscillator control grid and  the screen grid via a feedback capacitor C1. Hmmm.

I could plug the VFO into the crystal socket using a defunct FT-243 holder as the plug (just as I used it with the HT-40). Connect the VFO coax signal to the crystal socket connector to the control grid (the grid has a 47K grid leak resistor to ground that would also serve as the VFO load). The VFO signal shield plugs into the other socket terminal but requires an external jumper to ground and this ground also grounds the cold end of the screen feedback capacitor C1. C1 then acts as an additional screen bypass capacitor keeping the screen at RF ground. A tuned-plate amplifier! No modifications to the VFO or the transmitter.

VFO-Crystal socket adapter plug

VFO-Crystal socket adapter plug

So I lashed it up on an RT-3 transmitter, the transmitter from the CIA’s RS-1 set. It is essentially the same transmitter as the T-784/GRC-109 but without the code-burst keyer connector. The oscillator circuits are the same. Simple. Just plug the VFO connector into the transmitter crystal socket as shown but also run a short jumper from the VFO coax shield pin to the adjacent (“upper”) ground post on the transmitter. When operating like this, the VFO develops 4 volts P-P at the crystal socket on 7050 KC. (This “field expedient” should also work with the CIA’s RS-6 set transmitter, the RT-6).

With this high impedance, complex load at the crystal socket, the capacitance of the interconnecting coax cable is part of a voltage divider and it shunts some of the signal to ground. Keep it short. I am using RG-188A/U (had some on hand), it is 29 pf/foot. The effect is pronounced at 21 mc. The original Heathkit provided RG-62/U (13.5 pf per foot) a low capacitance cable. Heads up.

HG-10 VFO Connection to transmitter

HG-10 VFO Connection to transmitter

Short Bus wire ground jumper shown. Also shown is the wiring to the External Key terminals.

HG-10 VFO driving the RT-3 Transmitter

HG-10 VFO driving the RT-3 Transmitter

Above: The system lashup as used for on-air testing on 40 meters. Leaving the transmitter keyed on and then keying the VFO results is a slight chirp, not unexpected. It works best to leave the VFO running and then just key the transmitter normally. In this case with the Signal Corps J-38 key connected to the external key terminals on the transmitter. Go from transmit to receive by simply rotating the VFO Function switch from OPR to STBY; shuts off the VFO.
(I was using a separate receiver with a separate antenna for these tests)

It works. Really well! I am powering the VFO with an external regulated supply (VR-150 then further Zener-regulated down to 102V B+) and AC on the VFO 6CH8 oscillator/cathode follower filaments. The transmitter antenna for these tests was the 80 foot long aluminum rain gutter up on the roof (and it’s raining today, finally!).
My water-cooled antenna…..

RT-3 Transmitter output with VFO control: 7 mc Tests

RT-3 Transmitter output with VFO control: 7 mc Tests

Above: The transmitter 7 mc output signal looks like a nice clean sine wave on the 75 mc bandwidth scope, the same shape as with regular crystal control; 12 watts output on 7 mc in each case. The VFO also drives the transmitter quite well on 80 and 20 meters just like a crystal (even with the 6AC7 doubling the 7 mc crystal or VFO signal to 20 meters).

With a crystal (tripling), TX power falls off about 6 db on 21 mc as expected. The TX-VFO setup described here produces 10 watts output on 80 meters, 12 watts on 40 meters, 14 watts on 20 meters but only 2 watts on 15 meters – due to the VFO coax cable primarily. Still making some comparison measurements. (The 80 meter power measurement was not normalized to VFO drive voltage which is somewhat higher there.) On 80 meters with this setup the VFO signal is 7 volts P-P at the crystal socket.

On the air: Several CW contacts on 7 mc report clean, T9 tone, no chirp, hum (or PLL phase noise garbage). Stability after a 30 minute VFO warmup is very good, drift not noticeable during a 15 minute QSO. This should also work on the 10 mc, 30 meter band by realigning the “6 meters” VFO band components to move that circuit down to 5.05 mc. Further experiments in order but so far so good.

Would you really want to routinely operate a GRC-109/RS-1 set like this? Well the VFO was not designed to be submerged in a swamp as part of a clandestine equipment cache – but it would work around the ham shack if needed.

Low power? Running the stock GRC-109 transmitter with crystal control on 7050 I was able to work CA to France (5674 mikes) with its 12 watt output to a dipole. In very casual operating I have also worked 30 states with it. Low power CW works!

And this is WAY more fun and satisfying than a Ricebox…..

For more than you probably ever wanted to know about the GRC-109, take a look here: