Last tests with Collins R-390A
17 JAN 2021
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Every one knows the legend Collins R-390A /URR as well R-389
There is more story named R-725/URR
reproduction
Arvin
Industries, Inc. - R-725/URR
Here is the story, from Henry Rogers at the Western Radio Museum here: http://www.radioblvd.com/MilitaryCommunicationsGear.htm
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The R-725/URR is a 1967 Electronic Assistance Corporation-built R-390A receiver that was modified (in 1967) by Arvin Industries, Inc. for the USAF to use in semi-portable radio direction finding systems. Each R-725 receiver had the following modifications installed. First, the standard R-390A IF module was replaced with a new manufacture Series 500 IF module built by Arvin Industries or Servo Electronics. The Series 500 IF module was essentially a R-390 IF module (six IF amplifiers with no mechanical filters) that had minor updates to coax connectors to allow the Series 500 IF module to be installed with no modifications to the R-390A circuitry. However, further design development for the R-725 modification turned up a 60hz modulation problem that required additional modifications. A small chassis is mounted in the main frame space directly in front of the power supply module. This chassis has a 25vac transformer, two resistors and a connector-harness. This was a "hum-bucking" transformer that basically disconnected the VFO tube, the BFO tube and the ballast tube and powered the tube heaters with a "floating" 25vac (not referenced to chassis) and then used the resistive divider connected to B+ to "swamp" the AC with DC. The result was these tube heaters and ballast tube series string operated on +25vdc. To further protect the PTO from 60hz hum pickup, the entire PTO case had a grounded ferrous metal shield installed. The final modification was to the IF Output connector. The larger Series 500 IF module prevented connecting the IF Output cable to the back connector due to lack of clearance. A special "low profile" right-angle coax fitting was installed that allowed the IF Output to be available at the back panel. The contract number for the R-725/URR was DAAB05-67-C-2338 with a total number of receivers modified being less than 300.
The Non-Secret R-725 Story - The purpose of the R-725 mods was for compatibility with military portable direction finders that used four vertical antennae per installation along with three receivers. The DF system used went back to the Bellini-Tosi type of DF set-up that used two crossed loop antennae with a rotating loop inside to create a radio-goniometer. Bellini and Tosi had discovered that crossed loop antennae would "re-radiate" the signal they were receiving within the small field inside the antenna's space. The "re-radiated" signal retained all of the directional properties of the original signal and could be measured for varying signal intensity dependent on direction. The crossed loop antenna size didn't affect the frequency of operation allowing for reduction in the size of DF loops on LW. The original Bellini-Tosi system dated from around 1900 and the system was sold to the Marconi Company around 1907. By the early twenties, vacuum tube amplifiers were being added to increase performance capabilities of the DF antennae systems. The most common B-T DF systems used the crossed loops but some larger systems used the four-square vertical antenna system. This system was developed by Adcock during WWI and because the connections to and from the four square verticals were underground it didn't respond to skywave propagation and allowed ground wave DFing over long distances. The B-T DF and Adcock systems continued to evolve and improve with the systems being used throughout WWII. During WWII, oscilloscope displays began to be used for direction indications. After WWII, larger DF systems continued to be developed up to the mammoth "elephant cage" antennae ("Wullenweber" was the actual name) that were over a thousand feet in diameter and consisted of several "rings" of circular antennae all working to provide accurate DFing over great distances and wide frequency spans. By the 1990s, most of these large arrays were becoming obsolete and nowadays most have been dismantled.
The mechanical filters used in the R-390A resulted in signal path phase shifts that caused errors to show up in the DFing electronics. When used with the four square antennas, the low frequency modulation added via the radio-goniometer interacted with the mechanical filters creating the error. Early versions of this DF set-up had used R-390 receivers and the radio-goniometer was located quite a distance from the receivers to reduce any interference. In the 1960s, the USAF wanted to reduce the size of the entire DF system so it could be towed around on a trailered hut. This meant the radio-goniometer had to be in the same room as the receivers. This was going to require some protection to certain receiver circuits. The R-390 had been out of production for several years, so the solution was to design the new portable system to use modified R-390A receivers that could be easily purchased. Arvin Industries was the main contractor with Servo also doing some rework. The modified receivers would have the Series 500 IF module, essentially a R-390 IF module that was slightly updated to not require any rework to the R-390A receiver it was installed into. That eliminated the mechanical filter phase shift problem. Additionally, with the close proximity to the radio-goniometer, a 60hz hum appeared on the PTO tube filament and that also interfered with the LF modulation of the DF system. A special "hum bucker" chassis was added to the receiver that essentially operated the VFO tube, the BFO tube and the 3TF7 Ballast tube on +25vdc. Also, a grounded ferrous metal shield was added to the PTO housing to prevent hum "pick up." Arvin bought new R-390A receivers in 1967 direct from Electronic Assistance Corporation and the modifications were installed at Arvin. When complete, the receiver was tagged as "R-725/URR." The tags will generally show Arvin Industries as the contractor but sometimes Servo Electronics will be encountered. Arvin ink-stamped a serial number on each Series 500 IF module and when that module was installed into the receiver that same serial number was stamped onto the front panel data plate.
The Secret Project - Was there another purpose that was the "real" reason that the R-725 was created? According to an article that appeared in Electric Radio in January 2006 by Chuck Teeters, there was a "top secret" purpose for the R-725 and the receiver "mods" were primarily created for that "secret" project. The R-725 was a product that resulted from the Cold War jamming that was common between the USA and the USSR. In the mid-to-late 1960s, the NSA, the USAF and the Signal Corps were developing a new system called "Tropicom" that was an upgrade to the antennas and transmitters to improve HF communications for the military. The Tropicom upgrades also included the incorporation of the "F9c" anti-jamming/crypto system. The F9c system used a spread spectrum transmission of digital noise and signal that ran through a digital encrypo-key generator that had 144 stages of looped-feedback that also fed through phase modulators to maintain proper phase relationships of the signal and noise. When recombined at the receive end the signal to noise extracted the signal and left the noise and any jamming attempts far below the signal level. Since the system used spread spectrum, the signal couldn't be detected without the proper combination of equipment and decryption and that left any jamming attempts at just "blind" shots. However, when the F9c was used with a R-390A on the receive end, the phase changes in the mechanical filters interfered with the recombination process and the system didn't work. When used with R-390s with a standard IF amplifier circuit, the F9c system worked fine.
Since the R-390 receivers dated from the early-1950s, there were only a limited supply of those receivers still available and those that were available needed constant maintenance. The ultimate solution was to have new R-390A receivers with newly-built R-390 IF modules installed available for the Tropicom system.
In order to keep the F9c project "secret," the actual use of the R-725 couldn't be known to those outside the Tropicom project. Since there really was the Adcock DF system upgrades that really did need a non-mechanical filter type R-390A, the R-725 was directed to be built for the DF purpose only. However, those running the F9c project had the R-725 order quantity doubled and half of the R-725 receivers were procured for F9c use while the other half went to the DF systems. The secret classification stayed on with the F9c system and it was used for quite a long period with many upgrades over the years. So, even though half of the R-725 receivers were used in direction finders, the other half had a "secret life" used in the anti-jamming/crypto communications world of the NSA, the USAF and the Signal Corps.
Performance - The R-725/URR is very much like listening to a R-390 receiver. The modifications to the VFO-BFO heaters using the "hum-bucker" are not audible. The big change is the Series 500 IF module. With six IF amplifiers, the R-725 has plenty of gain. So much, that most strong signals will push the Carrier Level meter to 70db or 80db and then if the receiver is tuned off of the signal, the meter drops to 20db or less. I have the IF gain reduced by 40%. Audio sounds slightly different than the R-390A with mechanical filters but still there is lots of selectivity and QRM is not a problem. The R-725 is basically like having an R-390 without all of the maintenance headaches.
Scroll down...
The R-725/URR is a 1967 Electronic Assistance Corporation-built R-390A receiver that was modified (in 1967) by Arvin Industries, Inc. for the USAF to use in semi-portable radio direction finding systems. Each R-725 receiver had the following modifications installed. First, the standard R-390A IF module was replaced with a new manufacture Series 500 IF module built by Arvin Industries or Servo Electronics. The Series 500 IF module was essentially a R-390 IF module (six IF amplifiers with no mechanical filters) that had minor updates to coax connectors to allow the Series 500 IF module to be installed with no modifications to the R-390A circuitry. However, further design development for the R-725 modification turned up a 60hz modulation problem that required additional modifications. A small chassis is mounted in the main frame space directly in front of the power supply module. This chassis has a 25vac transformer, two resistors and a connector-harness. This was a "hum-bucking" transformer that basically disconnected the VFO tube, the BFO tube and the ballast tube and powered the tube heaters with a "floating" 25vac (not referenced to chassis) and then used the resistive divider connected to B+ to "swamp" the AC with DC. The result was these tube heaters and ballast tube series string operated on +25vdc. To further protect the PTO from 60hz hum pickup, the entire PTO case had a grounded ferrous metal shield installed. The final modification was to the IF Output connector. The larger Series 500 IF module prevented connecting the IF Output cable to the back connector due to lack of clearance. A special "low profile" right-angle coax fitting was installed that allowed the IF Output to be available at the back panel. The contract number for the R-725/URR was DAAB05-67-C-2338 with a total number of receivers modified being less than 300.
The Non-Secret R-725 Story - The purpose of the R-725 mods was for compatibility with military portable direction finders that used four vertical antennae per installation along with three receivers. The DF system used went back to the Bellini-Tosi type of DF set-up that used two crossed loop antennae with a rotating loop inside to create a radio-goniometer. Bellini and Tosi had discovered that crossed loop antennae would "re-radiate" the signal they were receiving within the small field inside the antenna's space. The "re-radiated" signal retained all of the directional properties of the original signal and could be measured for varying signal intensity dependent on direction. The crossed loop antenna size didn't affect the frequency of operation allowing for reduction in the size of DF loops on LW. The original Bellini-Tosi system dated from around 1900 and the system was sold to the Marconi Company around 1907. By the early twenties, vacuum tube amplifiers were being added to increase performance capabilities of the DF antennae systems. The most common B-T DF systems used the crossed loops but some larger systems used the four-square vertical antenna system. This system was developed by Adcock during WWI and because the connections to and from the four square verticals were underground it didn't respond to skywave propagation and allowed ground wave DFing over long distances. The B-T DF and Adcock systems continued to evolve and improve with the systems being used throughout WWII. During WWII, oscilloscope displays began to be used for direction indications. After WWII, larger DF systems continued to be developed up to the mammoth "elephant cage" antennae ("Wullenweber" was the actual name) that were over a thousand feet in diameter and consisted of several "rings" of circular antennae all working to provide accurate DFing over great distances and wide frequency spans. By the 1990s, most of these large arrays were becoming obsolete and nowadays most have been dismantled.
The mechanical filters used in the R-390A resulted in signal path phase shifts that caused errors to show up in the DFing electronics. When used with the four square antennas, the low frequency modulation added via the radio-goniometer interacted with the mechanical filters creating the error. Early versions of this DF set-up had used R-390 receivers and the radio-goniometer was located quite a distance from the receivers to reduce any interference. In the 1960s, the USAF wanted to reduce the size of the entire DF system so it could be towed around on a trailered hut. This meant the radio-goniometer had to be in the same room as the receivers. This was going to require some protection to certain receiver circuits. The R-390 had been out of production for several years, so the solution was to design the new portable system to use modified R-390A receivers that could be easily purchased. Arvin Industries was the main contractor with Servo also doing some rework. The modified receivers would have the Series 500 IF module, essentially a R-390 IF module that was slightly updated to not require any rework to the R-390A receiver it was installed into. That eliminated the mechanical filter phase shift problem. Additionally, with the close proximity to the radio-goniometer, a 60hz hum appeared on the PTO tube filament and that also interfered with the LF modulation of the DF system. A special "hum bucker" chassis was added to the receiver that essentially operated the VFO tube, the BFO tube and the 3TF7 Ballast tube on +25vdc. Also, a grounded ferrous metal shield was added to the PTO housing to prevent hum "pick up." Arvin bought new R-390A receivers in 1967 direct from Electronic Assistance Corporation and the modifications were installed at Arvin. When complete, the receiver was tagged as "R-725/URR." The tags will generally show Arvin Industries as the contractor but sometimes Servo Electronics will be encountered. Arvin ink-stamped a serial number on each Series 500 IF module and when that module was installed into the receiver that same serial number was stamped onto the front panel data plate.
The Secret Project - Was there another purpose that was the "real" reason that the R-725 was created? According to an article that appeared in Electric Radio in January 2006 by Chuck Teeters, there was a "top secret" purpose for the R-725 and the receiver "mods" were primarily created for that "secret" project. The R-725 was a product that resulted from the Cold War jamming that was common between the USA and the USSR. In the mid-to-late 1960s, the NSA, the USAF and the Signal Corps were developing a new system called "Tropicom" that was an upgrade to the antennas and transmitters to improve HF communications for the military. The Tropicom upgrades also included the incorporation of the "F9c" anti-jamming/crypto system. The F9c system used a spread spectrum transmission of digital noise and signal that ran through a digital encrypo-key generator that had 144 stages of looped-feedback that also fed through phase modulators to maintain proper phase relationships of the signal and noise. When recombined at the receive end the signal to noise extracted the signal and left the noise and any jamming attempts far below the signal level. Since the system used spread spectrum, the signal couldn't be detected without the proper combination of equipment and decryption and that left any jamming attempts at just "blind" shots. However, when the F9c was used with a R-390A on the receive end, the phase changes in the mechanical filters interfered with the recombination process and the system didn't work. When used with R-390s with a standard IF amplifier circuit, the F9c system worked fine.
Since the R-390 receivers dated from the early-1950s, there were only a limited supply of those receivers still available and those that were available needed constant maintenance. The ultimate solution was to have new R-390A receivers with newly-built R-390 IF modules installed available for the Tropicom system.
In order to keep the F9c project "secret," the actual use of the R-725 couldn't be known to those outside the Tropicom project. Since there really was the Adcock DF system upgrades that really did need a non-mechanical filter type R-390A, the R-725 was directed to be built for the DF purpose only. However, those running the F9c project had the R-725 order quantity doubled and half of the R-725 receivers were procured for F9c use while the other half went to the DF systems. The secret classification stayed on with the F9c system and it was used for quite a long period with many upgrades over the years. So, even though half of the R-725 receivers were used in direction finders, the other half had a "secret life" used in the anti-jamming/crypto communications world of the NSA, the USAF and the Signal Corps.
Performance - The R-725/URR is very much like listening to a R-390 receiver. The modifications to the VFO-BFO heaters using the "hum-bucker" are not audible. The big change is the Series 500 IF module. With six IF amplifiers, the R-725 has plenty of gain. So much, that most strong signals will push the Carrier Level meter to 70db or 80db and then if the receiver is tuned off of the signal, the meter drops to 20db or less. I have the IF gain reduced by 40%. Audio sounds slightly different than the R-390A with mechanical filters but still there is lots of selectivity and QRM is not a problem. The R-725 is basically like having an R-390 without all of the maintenance headaches.
