The 51H-3...
R-105A/ARR-15 HF receiver...is to my knowledge the first remotely-tuned receiver made by Collins.  Several previous Collins transmitters used Autotune units, but this was the first receiver to use one of the mechanical positioning units.  The unit in the photo is actually a military R-105A/ARR-15, but is pretty much the same as the more rare commercial unit.

  Incidentally, the electrical difference between the R-105 and the R-105A is that the older unlettered version used a crystal rectifier as a detector, while the newer "A" version used 1/2 of a 12SL7 -  apparently a superior device for that critical application.  More trivia -  the PTOs used in the ARR-15 are the 70E-2 (2-3 MC main HF oscillator) and the 70E-3 (450-550 KC variable BFO).  I knew you'd ask that -  the 70E-1 is used in the ARC-2...

  What you see above is the radio clamped in a shockmount.  The front panel measures 10 3/8" wide by 7 7/8" high.  The radio is 21 9/16" deep (including knobs & handles) and weighs 39 1/2 lbs.  Controls include a combination ON/OFF switch & circuit breaker, a volume control, a ten position 'CHANNEL' selector switch, a 'BAND' switch, a 'TUNING' knob, a 'BFO / CALIBRATE' knob, and a 'CW / MCW-CAL' switch.

  The radio was intended to be used as a fixed-tuned receiver and operated on one of the ten possible preset frequencies.  It does, however, work fine as a tunable receiver -  just select a channel, wait for the Autotune apparatus to cycle completely, then unlock both the 'BAND' and 'LOCK' locking wingnuts and operate normally.  Note the prominent warning on the front panel:

NOTIFY
PILOT
BEFORE
UNLOCKING
AUTOTUNE

  If you neglect to tighten the 'BAND' and 'LOCK' wingnuts before selecting a new channel (or before the pilot selects a new one on his remote control box), you'll generally lose whatever frequency info was stored in both the current channel and the newly selected channel -  probably not catastrophic to most boatanchor users, but could cause trouble in the original heavy bomber application...

  Considering that the channel frequency memory in this old beast is analog in nature and totally mechanical, it is really incredible just how repeatable the tuning scheme is.  I got these measurements from the pictured unit -  it was built on a 1952 contract and has a 1959 overhaul tag.  The 3 MC frequency is the output of the PTO at the high end of it's range.
 -  Initially, I tuned the radio to get the 3000.041 KC PTO reading.
 -  The 'After lockdown' reading indicates how much the PTO frequency changes due only to locking down the Autotune unit with the 'LOCK' wingnut.
 -  Each of the 'rechannel' readings indicate how the PTO frequency changes after a rechannel.
Measurement Frequency Delta frequency Delta PPM 20 meter Delta
Initial setting 3000.041 - - -
After lockdown 3000.073 32 CPS 10.6 PPM 151 CPS
First rechannel 3000.226 185 CPS 61.6 PPM 877 CPS
Second rechannel 3000.253 212 CPS 70.6 PPM 1006 CPS
Third rechannel 3000.247 206 CPS 68.6 PPM 977 CPS
Fourth rechannel 3000.252 211 CPS 70.3 PPM 1001 CPS
Fifth rechannel 3000.253 212 CPS 70.6 PPM 1006 CPS
Sixth rechannel 3000.255 214 CPS 71.3 PPM 1016 CPS
Seventh rechannel 3000.245 204 CPS 67.9 PPM 967 CPS
Eighth rechannel 3000.247 206 CPS 68.6 PPM 977 CPS
Ninth rechannel 3000.244 203 CPS 67.6 PPM 963 CPS
Tenth rechannel 3000.239 198 CPS 65.9 PPM 940 CPS
  Not too bad for an old bucket of bolts...

  "One KC per division on all bands" -  remember that old S-Line advertising line from the early '60s?  This critter had it back in 1946.  The trick is the well-calibrated tunable IF section and an accurate crystal calibrator.  In setting up an Autotune channel on a particular frequency, you move the variable IF to effectively offset the tuning dial such that the 100 KC calibrator signal is received on the desired channel frequency, then lock down the Autotune units and return the IF strip to it's center frequency.  (Note that whenever you move the variable IF off it's detented center frequency, the crystal calibrator is turned on and connected to the RF amp in place of the antenna -  the variable IF is used only for presetting a channel frequency.)

  Lets say you want channel 3 set up on 3885 KC -  the tuning procedure is like this:
 -  lock down both the 'BAND' and 'LOCK' wingnuts
 -  select channel 3 and wait for the Autotune to settle down
 -  unlock the 'BAND' and 'LOCK' wingnuts
 -  turn the BAND knob so that the 3.5-5.5 MC band window is displayed
 -  tune the TUNING knob to around 3885 (yeah, you'll have to interpolate between 3.8 & 3.9.
        You don't have to be exactly right at this point -  just get it pretty close)
 -  rotate the BFO-CALIBRATE knob so that 85 is displayed
 -  turn the CW / MCW-CAL switch to CAL
 -  you'll hear a beat note -  the 3900 KC calibrator pip beating against the BFO.  Use
        the TUNING knob to zero-beat it
 -  lock down both the 'BAND' and 'LOCK' wingnuts
 -  be sure to rotate the BFO-CALIBRATE  knob back to the detented position at 00 KC
 -  essentially the CW / MCW - CAL switch is a CW / AM switch now -  set it accordingly

  Of course this 'fine-tuning' procedure can be used without ever assigning a channel number -  just don't lock down the 'BAND' and 'LOCK' wingnuts and don't ever move the CHANNEL switch...

  I have an aversion to 'War Crimes' and generally try to stay away from modifications, but a nice addition to this radio might be a 6 KC mechanical filter somewhere in the IF section.  In order to preserve the tuning scheme, you'd have to set it up such that when the BFO - CALIBRATE control is off the '00 KC' detent, the entire variable IF setup is still in play -  the mechanical filter would be used only when you're at the fixed '00 KC' position.  If you try this, please let me know how it works.

  Around 12/20/98,  Cliff  "Buzz"  Mueller saw this page and wrote the following:

  Checked out the ARR-15 page and it brought back memories of my days in the Navy, as a flight communicator in P2V anti-submarine patrol bombers. Normal patrol was 8-10 hours but sometimes we pushed our fuel load to 12 hours. My radio gear consisted of 2 - ARR-15 and an ART-13 that was used on CW almost exclusively. Our radios had switches mounted near them marked "LOCAL - REMOTE" to shut off the pilot's control box so he couldn't cycle it on you.  Used one ARR-15 with the ART-13 and the other ARR-15 was set on WWV for the navigators use.
...
  I've attached a picture showing me in the radio compartment with the ARR-15 and ART13 some what visible in the background. The other pic is of a P2V.
...
  I was on active duty with VP-24 "The Batmen" at Norfolk, VA from 7-60 to 7-62.  I was then in the Naval Air Reserve until 7-68. While in the reserve in Oct/Nov 1962 I was activated for the Cuban missle crisis for 30 days of ASW (Anti-Submarine Warfare) and shiping patrols. During that time we flew in P2V-7's which were equipped with ARC-38A (618-S), ARC-94 (618-T) and ARR-41 radios along with a crypto device and Mite teletype. Quite an upgrade from the ARR-15 & ART-13.
...
  I'd be happy to have my name on your web page. It is Cliff Mueller and I was a ATN2 (Avionics Technican Navigation/Communications) Petty Officer Second Class while on active duty. I attached a picture of our squadron patch as it's unique.

73
Buzz

P2V5 'Neptune' on patrol

Buzz at work in a Neptune

Squadron patch of VP-24, 'The Batmen', of Norfolk, VA


  This last photo is very large so I didn't include it in this page - it's a good pic of the radio position in a Neptune. Shot includes an ART-13 (or similar), a pair of ARR-15s, an LM-14, and much else. It's 1495x1229 pixels in B&W, so it'll take some time...       (click to view)

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