Remote Control by Tone Set - A History

By William F. Ruck, MRHS Maintenance 

Part II of a three part series...

After Newsletter No. 17 hit the streets with the first installment of Bill Ruck's compelling history of transmitter remote control by tone set Bill wrote to say it's actually a three part series.  So we have an additional episode of tone set passion and drama to look forward to, torn from the flaming pages of obscure technical manuals.

When we last left Bill in episode one he had brought us up to the tube version of audio frequency shift keyers.  Let's listen now as we hear Bill say...

The first part of this series explained why communications is best with separate transmitter and receiver locations and how the transmitters could be remotely controlled.  This article goes into detail on the tone sets used by KPH and now KSM.

Background - 

In 1959 when RCA built new Building 2A in Bolinas and equipped it with new transmitters they also built a new control room.  The new control room was equipped with the latest state-of-the-art equipment including RCA Model 901 and 902 Frequency Shift Tone Transmitters and Receivers.  These were clearly advanced for the day as they were solid state - germanium transistors.

It is curious to note that the RCA Model 901 F/S Tone Transmitter and the RCA Model 902 F/S Tone Receiver were actually manufactured by Tele-Signal Corporation in Hicksville, New York.  This was not uncommon for RCA.  Their corporate philosophy was, in summary, "If it is not sold by RCA you don't need it."  In many instances specialty items such as these tone sets and test equipment was manufactured by others under contract and was delivered in RCA standard colors with an RCA logo on the front panel.  (The traditional round RCA logo is commonly known as a "meatball".)

Rack of 902 receiving tone sets (and one 901 set) in the control room at Bolinas

The Radio Corporation of America Model 901 Frequency Shift Tone Transmitter and Model 902 Frequency Shift Receiver is a completely self-contained unit, including power supply and monitoring facilities.  It is mounted on a 5-1/4" by 4" panel and has a depth of approximately 10-3/4".  This structure permits mounting of up to four such transmitters or receivers in the space of one standard 5-1/4" x 19" panel.  Such groups can be stacked with other groups of such equipment.  Up to 48 channels can be readily mounted into one standard relay rack.  In such multi-channel systems all transmitters and receivers are alike: plug-in type frequency determining networks and channel filters determine the channel frequency, frequency shift, and channel speed assigned to each unit.

Although all of the 901 F/S Tone Transmitters and 902 T/S Tone Receivers have identical circuitry, the bandpass filters are designed for a specific frequency and are not interchangeable.  There are plans for center frequencies with standard networks.  One plan is shown below.

Table of frequency determining networks

The packaging of the units has been made with regard for ease of maintenance.  All small components are mounted on one single "card", on the reverse side of which the wiring is printerized.  This "card" is readily tilted out of the unit, giving complete access to all parts.  Test points are available directly on the front panel and permit monitoring and alignment of the unit without removing it from the rack.  Frequency adjustments, such as center frequency, mark and space frequency adjustments, though an integral part of the plug-in network, are accessible from the front panel.

Tone set circuit board showing germanium transistors

For the point-to-point service RCA equipped the San Francisco office with type 901 F/S Tone Transmitters which connected via their dedicated telephone circuits to Type 902 F/S Tone Receivers in the Building 2A control room Rack 2.  There the low level keying was patched to banks of Type 901 F/S Tone Transmitters.  The audio output of those f/s transmitters was used to modulate the radio transmitters in Building 2A and Building 2 used in the point-to-point service.

For KPH, the maritime service, Type 901 F/S Tone Transmitters were installed at Point Reyes and fed the dedicated telephone lines to Bolinas where a bank of Type 902 F/S Receivers in Rack 3 decoded the audio and keyed KPH transmitters in Building 1 and Building 2.

When the point-to-point RCA Global Service was discontinued after the satellite transition the 901 and 902 tone sets used by KPH continued to serve the station as the newer point-to-point transmitters in Building 2A were converted to Morse operation and connected to the 902 f/s receiving tone sets from Point Reyes.  Today the same 901 and 902 tone sets continue in service to key the KSM transmitters in the same way as they have been used for more than 40 years.

These tone sets have demonstrated excellent reliability.  About the only problems that we have with them is replacement of power supply electrolytic capacitors.  Anyone using vintage equipment recognizes that these capacitors do not last forever and whenever we find one tone set to be erratic the first thing we do is to replace the old power supply capacitors.  We know from bitter experience that old C/D and Mallory capacitors with paper tube outer jackets are long past their expected life. 

Other than that every once in a while we have to swap out erratic tone sets and align them.  RCA left us with a test jig that allows the 901 transmitter and 902 receiver to be connected back-to-back.  In short order they are realigned and ready to be put back into service.

Jig for test and alignment of 901 and 902 tone sets and for the 909 switch (seen in background, behind the jig)

We did learn during the restoration of the teletype order wire that 901/902 alignment is much more critical in the teletype service than in the Morse service.  We've learned how to quickly align them with a 50% duty cycle square wave at teletype speed.

RCA Model 901 F/S Tone Transmitter System Description -

Refer to Figure 4, F/S Tone Transmitter Block Diagram.

A series tuned circuit, consisting of C1 and L is driven by two states of impedance stepdown converters.  At resonance the voltage across L is 90 degrees with respect to the voltage across the tuned circuit.  The voltage across L drives a phase splitter which in turn feeds a series RC circuit consisting of C2 and R1 as long as the Keying Transistor is cut off.  Varying R1 permits changing the phase angle from nearly zero to almost 180 degrees with out any changes of amplitude.

With R1 adjusted to a resistance equal to the reactance of C2 (at center frequency of the tank circuit) the voltage at the junction of C2 and R1 is 90 degrees and the phase fed back to the impedance converter is correct for regeneration, causing the circuit to oscillate at center frequency.

Increasing R1 causes the phase angle to differ from 90 degrees and the circuit will oscillate at a lower frequency of which the phases will again be correct for regeneration.  Application of sufficient current to the Keying Transistor to saturate the base will effectively shunt R2 across R1, lowering the resistance which in turn causes the circuit to oscillate at a higher frequency.

At correct adjustments of R1 and R2, the Mark and Space frequencies will result from equidistant phase departure from 90 degrees at the tank circuit.

The output is coupled out through a stage of amplification and a bandpass filter.

The circuitry of the 901 F/S Tone Transmitter is well though out because the same unit can be used in different keying schemes.  The units are all the same and the different modes are determined by how the rack connector is wired.  The 901 F/S Tone Transmitter can be wired as follows:

1.  Contact Keying.  Used at KPH (now KSM) for Morse.

2.  Current Keying with external 60 mA line battery.  Used for teletype.

3.  Current Keying with self contained line battery using Line Battery Model 914 accessory.

4.  C.W. Tone Keying with C.W. Tone Demodulator 904 accessory.

5.  Voltage Keying into 2,200 Ohms.

6.  Voltage Keying into 100,000 Ohms using Voltage Keying Adaptor Model 905 accessory.

RCA Model 902 F/S Tone Receiver System Description - 

Refer to Figure 3.  F/S Tone Receiver Block Diagram

The incoming frequency shift signal is fed through a bandpass filter and a pre-amplifier to a push-pull limiter.  The square waves from this limiter are fed to the polar detector.  Simultaneously, through a resistive divider accomplishing both attenuation and impedance stepdown, the square waves are fed to a series tuned circuit tuned to center frequency.

The voltage across L is fed through a limiter to the polar detector.

The polar detector is essentially a phase discriminator, the voltage output of which depends on the relative phase of the square waves coming from the push-pull limiter and those from the limiter after the tuned circuit.

At center frequency the tuned circuit produces a 90 degree phase shift.  At mark and space frequency phase shifts of less, and more than 90 degrees respectively occur at the tuned circuit.  The output of the polar detector feeds a simple low pass filter.  A voltage sensitive trigger driven from the low pass filter is adjusted to give unity weight to the output.

Like the 901 transmitter, all of the 902 receivers are identical except for the band pass filter.

The 902 receiver can be connected in the following ways:

1.  Current keying into a standard 65 mA 2000 Ohm loop using a Model 909 Transistor Switch accessory.

2.  Low level keying output (-7 volts into 12K or larger)

3.  C.W. Tone Output with C.W. Tone Keyer Model 906 accessory

4.  Standard Relay Output with Transistor-Relay Model 907 accessory

5.  Current keying into a single printer or loads not exceeding 250 Ohms with a Transistor-Keyer Model 908 accessory.

Most of this report was directly quoted from flaming pages of the RCA Model 901 and Model 902 manuals.

The last part of this series will explain how an individual operator at RS could select which transmitter to key.