While most of the attention these days is focused on our restoration of ship-to-shore station KPH (now operating under the call KSM) it's worth keeping in mind that the original Marconi long wave transmit and receive stations and the later RCA shortwave transmit and receive stations were not concerned with ships at all. They were engaged in the business of trans-oceanic point-to-point communications with the cities of the Pacific rim. This was a truly massive operation that ended in 1973 when the last shortwave circuit - to Tahiti - closed down.
In past numbers of the Newsletter we have documented the restoration of transmitter No. 298 at Bolinas by a highly skilled team of current and former HP engineers. No. 298 began life as an independent side band point-to-point transmitter.
Now comes Mr. Steve Pazar, a member of the HP transmitter restoration team, to tell us of the companion dual diversity receiver, the R-3.
Steve, please take it away...
While restoring the RCA T-3 transmitter (MHRS newsletter #10), we noticed that a companion receiver was mentioned in the manual. As it turns out the T-3 represents only half of the full story. The other half is the R-3. At this point it is not certain that any R-3 receivers were used at KPH but it is likely that they were. After all, RCA owned and operated this site for several decades. The only hint is an R-3 manual in the archives and a single module in the store room at BL. More research is needed to definitively determine if they were used or not.
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| Receiver R-3 as shown in the manual. Note the step and the handle above on the divider between the racks for reaching those top level controls. |
Having owned and restored a number of SW and ham receivers over the years, I was curious about the R-3. The following is a brief overview of the details of design and construction of this amazing receiver. This isn't something you're likely to find in anyone's ham shack!
We apologize that the block diagrams are not as easy to read as we would like but they're included for information purposes.
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Block diagram 4-2.
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| Block diagram 4-3 |
RF Amplifier:
This receiver utilizes 4 bands (2.8 - 5.0 MC, 5.0 - 9.0 MC, 9.0 - 16.0 MC and 16.0 - 28.0 MC). Each band in each receiver includes 3 tuned RF amplifiers (four tracking permiablity tuned circuits each) a mixer, and a cathode follower buffer. Selecting the band turns on the filaments for the pair of amps needed.
The HF Oscillator unit provides a signal that is 1 MC higher than the channel frequency to each mixer. AGC from various sources control the gain of the RF amps. A separate antenna input connector is provided for each band and each receiver. There is a single knob for each RF Amplifier. An internal view of one of these shows the lead screw and rack for the tuning slugs. Two of these units are used, one for the two low frequency bands and another for the two high frequency bands. A total of 36 tubes are used in these.
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| RF amplifier tuning rack. |
HF Oscillator:
This is the common VFO for both receivers, and it is a PLL source. Harmonics from a 1 MC ovenized crystal oscillator (the 6th through the 31st) are used along with a Collins PTO to generate the signal used to derive an output that is 1 MC about the channel frequency. The channel frequency is selected via two front panel knobs. The output of the HF Oscillator is buffered and connected to the mixers in the RF Amplifiers. 14 transistor and 2 tubes used. Its interesting to note there were two versions of this unit, the first using only tubes.
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| R-3 HF oscillator. |
1 MC IF:
This unit performs 5 functions; Two, 3 stage 1 MC IF amplifiers, 1 MC to 100 KC IF conversion, 100 KC xtal reference oscillator, dual channel sideband plus carrier AGC and AGC switching, 16 tubes used.
900/1100 KC Oscillator & AFC:
This unit provides either a 900 KC or 1100 KC signal to the second converter in the previous unit. This allows selection or upper or lower sideband. A comparative AFC is also included which utilizes the 100 KC reference. AFC is accomplished with a motor driven capacitor. This shifts the 900/1100 KC oscillators to track the incoming signal. A squelch circuit is also included. 12 tubes used.
Carrier Conditioner:
This unit performs three functions; It extracts the 100 KC carrier from the 100 KC IF to provide carrier AGC, provides the voltage for squelching the the AFC function in the previous unit under weak signal conditions and and provides a constant level carrier for use in the demodulator and AFC circuits. 11 tubes used.
Roofing and Sideband Filter:
This is a fully transitorized unit. Three 100 KC IF band pass filters are used per receiver. Four bandwidths are available (12, 9, 6.2 and 4 KC). Upper and lower sideband filter (these are seperate) are also included. It is unclear from the manual if these are Collins mechanical or crystal lattice filters. 14 transistors used.
Demodulator:
This is another fully transistorized unit that contains the demodulators and audio amplifiers for both receivers. Two are required for ISB operation. The demodulator consists of two balanced demodulators, one per receiver. The reference signal is selectable between the 100 KC reference oscillator and the constant level carrier output from the Carrier Conditioner unit. Two low level audio amps are also on board. 14 transistors used.
Diversity Phone Combiner:
This unit takes the audio from each receiver and using 3 filters per channel breaks it down into 3 bands (high, medium and low). 6 filter all together. The audio signal that passes through the low frequency filters is rectified and compared. The resultant analog signal (not digital) changes the gain on two post filter amps, favoring the channel with the largest signal. The same process occurs for the high and medium audio frequencies. The output from all 6 amps are combined giving a single audio channel. 41 transistors.
There are also a number of optional modules available for processing signals; Frequency Shift Tone Receiver, Dual Diversity Comparator (different than the Diversity Phone Combiner), and a CW tone keyer. There is also a power supply unit and control panel for the entire receiver.
Construction of the modules is very similar to the T-3 with all chassis and panels made out of heavy gauge aluminum. One nice feature of the R-3 is not only do the modules slide out for servicing, but the chassis can tilt on their slides for easy access.
As you can see, this is a very capable and flexable receiver system that incorporated many new (for the late 1950's) technologies.
I would like to thank Sheldon Daitch for sending a copy of his manual of the RCA R-3 which was the source for this article. He also provided the picture of R-3's in use at the VOA site where he was an engineer. According to Sheldon, the VOA used a lot of these receiver in their facilities; 8 at Greenville, NC, as many as 25 at Kavala, 10 at Rhodes and Banguio and unknown number at Bangkok, Munich, Tangiers, Sri Lanka and Liberia.
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| R-3 receivers at VOA Greenville, NC site. |
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| RF amplifier data plate. |
If anyone has any additional information about the R-3, where it was used or if any still exist, I would certainly like to hear from you. A long term goal of mine is to try to collect enough modules to cobble together as much of an R-3 as possible, and building what cannot be found if needed. So far only have one of the RF Amplifier modules.
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| One of the operating positions for Steve's amateur station, W6SSP. |
