AN/APR-1 & APR-4 Radar/Comm Surveillance Receiver

AN/APR-1 Radar/Communications surveillance receiver. (aka APR-1 etc.)
Thumbnail: Imperial Japanese Navy Type 13 Air Search Radar aboard a destroyer. One of many typical APR-1 targets.

On the night of 9 February 1945 the APR-1 receiver aboard the USS Batfish, SS-310 detected the 158 mc radar emissions of the IJN submarine RO-115. In conjunction with its own SJ Radar, the Batfish subsequently torpedoed and sank the RO-115. (Reference 84)

Below is the APR-1 (SPR-1) installation aboard the USS Pampanito SS-383.

SPR-1 / SPA-1 Receiver aboard USS Pampanito SS-383. Photo courtesy San Francisco Maritime National Park Association. Used with permission

The APR-1 was first fielded in 1942, and known to be used aboard US Navy submarines seeking out enemy radar emissions in the Pacific. In shipboard service it was designated the AN/SPR-1.

With nearly identical functioning, the APR-4 came somewhat later, an adaptation built by Crosley used by the Army Air Forces. Those flew on B-17, B-24 and B-29 aircraft in the European and Pacific theaters.

 

These search receivers were designed and built (quickly) to cover known German and Japanese air defense, fighter control and shipboard radar systems. They did this along with a Panadapter that allowed the system to search for, detect emissions, determine their frequency, pulse repetition rate and also any antenna rotation rates.

In addition to its well-trained operator wearing headphones, the AN/SPA-1 Pulse Analyzer (see above) connected to the APR-1 Video output characterized the signals. All important data for closing with, or for employing Jammers to combat them.

Below with the TN-17 Tuner from the AN/APR-4 system plugged in. The normally used Tuning Unit in this configuration is the TU-2/APR-1 versus what I have here; they are compatible.

AN/APR-1 Surveillance Receiver with TN-17 Tuner

Above – monitoring the local airport Tower VHF frequency of 118.1 mc. This airport also simulcasts their VHF tower frequency on UHF for any military aircraft so I can copy that on 239.25 mc as well.

The APR-1 is very similar to the more widely known APR-4 which had selectable IF bandwidth and an IF Gain step attenuator versus the variable gain control of the APR-1. The 2 sets are functionally equivalent although the internal layout of the two main chassis are quite different. With appropriate tuning units they covered from 40 to 3300 mc.

An unusual feature is that the internal power supply operates from 80 or 115 VAC, from 60 to 2600 cps. Odd for an aircraft set but appropriate for shipboard installations. Absent the correct connector, I built this “Suicide Cord” from some molex females and heat-shrink tubing for testing the set for now. The scan motor is 28 VDC.

The TN-17 Tuner covers 74-320 mc and includes 2 type 955 acorn tubes for a mixer and local oscillator function. Although it does not have an RF amplifier stage it receives VHF AM transmissions from aircraft and works quite well with a good antenna. It will also slope-detect FM voice signals but with its broad IF amplifier filter skirts the recovered audio level is low. Not an issue for its intended purpose.

I also have a TU-18 which covers 300- 1000mc. That one also uses a 955 Acorn for a local oscillator but it has a 1N21 crystal mixer. Below is the TU-17 schematic. The TU-2 is essentially identical.

Shipboard antennas included a “Stub” type monopole for the TU-2 coverage of 40-320 mc; “Cone” types for the higher frequencies. The systems included a coaxial switch for selecting appropriate antennas, wave traps and filters. Aircraft and shipboard antennas were different, I would assume pressure-rate antennas and feedlines were unique to submarine installations.

TU-17/APR-4 Tuning Unit

Below: The 78 year-old TU-17 is a Thing of Beauty. The TU and the receiver fired right up – no failed parts. This TU was Moisture Fungus Proofed (MFP) in June 1945 (a month after VE Day) as we prepared for the invasion of Japan. Fortunately, that didn’t have to happen. After WWII the APR-1 reportedly flew on TBM-3S ASW aircraft for awhile.

TU-17 Acorn Tubes

The 955 local oscillator is on the left, it is very loosely coupled to the mixer circuitry by proximity inside the stout casting. The mixer on the right is a 955 with its grid and plate tied together, thus forming a non-linear diode. The IF frequency is 30 mc; the LO being 30 mc above the received signal frequency with this tuner. The specified IF bandwith is 2 mc. The LO and mixer tuning capacitors are tied together through a gear/cam follower system.

Imagine the engineering that went into just the shape of that roller cam follower. Fine tuning the LO frequency to the antenna circuit involved a set of 9 screws accessible from the front panel. These made fine adjustments to the LO tuning capacitor rotational position cam.

“Real radios have Motors – and Springs!”

TU-17 Tuning capacitor drive mechanism

The APR-1’s were made by Galvin (which became Motorola). They were very capable superhet receivers for radio and radar signals. The TN-17 has a motorized frequency tuning mechanism with selectable “start and stop” scan settings. Which I am sure the “Spook” operators appreciated! It works great and is fun to watch.

The main chassis is in good condition however the “HET” (HETerodyne – BFO) function is non operative right now. This set includes all the original allen wrenches, special tuning tools and some spare fuses. Note the tube clamps.

AN/APR-1 Receiver Chassis

I am currently researching the design of the overall APR-1 main chassis which included the power supply, IF amplifiers and signal processing circuits. This article is a work in progress.