The AT-3 transmitter was designed and built by RCA Canada, (Montreal)
for the Royal Canadian Air Force. This was on the direction of Air
Vice Marshall (AVM) Ralph McBurney who was the RCAF's Director of Signals
when WWII started. The Canadian Army used the same transmitter but
designated it as the C-33 set. CAP (Canadian Air Publication) No. 63 identifies
it as RCAF stores number 10D/1272. RCA also lists the AT-3 as the
TE-176 and also as C-33 in the manual dated June 1942. Under model TE-176,
the transmitter is part number is #MI-22106 while the remote control
is #MI-22038-B. Can anyone explain the purpose of the 'TE' designator that
RCA used on their equipment manufactured in Montreal? Contact : <jerry.proc@sympatico.ca>
The AT-3 was painted blue while the C33 was painted brown.
David Lawrence relates some AT-3 history. "The contracting procedure was very efficient. At the start of the war, AVM McBurney invited representatives into his office from RCA Canada, Canadian Marconi and Northern Electric Canada. He told them that the RCAF needed three wireless sets - a general purpose one for aircraft use, a small one for fighters and a big ground station transmitter. After one hour's discussion he said, "RCA, you can build the ground station (this became the AT-3); Marconi - you can build the fighter set and Northern, you can build the general purpose set (which became the AT1/AR2). Any questions? No! Get out of my office and get to work." This story was told to me by AVM McBurney while we stood in front of an AT1/AR2 and an AT-3 during the opening of the Military Communications and Electronics Museum in Kingston, Ontario.
SPECIFICATIONS
* Frequency range: 2.5 to 20 MHz.
* Frequency Control: Master Oscillator or 1 of 2 crystal controlled
channels.
* Power : 300 watts out.
* Modes: CW, MCW or VOICE
* Keying: Telegraphic keying to 60 words per minute, either CW or MCW.
The radio frequency generating
oscillators are keyed for CW operation. On MCW only, the
modulation, and not the carrier, is keyed.
* MCW: The carrier is modulated by a 1,000 cycle per second tone
* Voice Performance: Over the frequency range of 400 to 3000 cycles,
the overall distortion does not exceed
7.5% for 85% modulation.
* Remote Control: Permits operation of transmitter up to 10 miles distance.
* Power Input: 110 VAC , 60 Hz,
1.75 kw maximum draw. Operation at 220 VAC 25 Hz requires
a frequency changer #10D/1274.
* Dimensions: AT3 Transmitter is approximately 68 inches high, 32 inches
wide and 22 inches deep.
Remote Control is approximately 9-3/8 inches high, 12-5/8 inches wide and
11-3/4 inches deep.
* Weight: 630 pounds for the AT3; Remote Control is 35 pounds;
Frequency Changer is 625 pounds.
BLOCK DIAGRAM
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| The operating frequency and mode was selected by using the telephone style dial on the transmitter. The 802 (transmitting pentode) and 805 (high µ transmitting triode) tube types, introduced in 1935 and 1936 respectively, could operate up to 30 MHz. (Diagram courtesy RCA) |
CONTROLS and INDICATORS
(a) The following controls, necessary for routine tuning and operation of the transmitter, are located on the .front panel.
Master Oscillator-Crystal Oscillator switch for A channel
OSCillator TUNING control for A channel
I.P.A. TUNING control for A channel
I.P.A. BANDS switch for A channel
P.A. TUNING control for A channel
P.A. BANDS switch for A channel
ANTenna COUPLING control for A channel
ANTenna LOADING control for A channel
Master Oscillator-Crystal Oscillator switch for B channel
OSCillator TUNING control for B channel
I.P.A. TUNING control for B channel
I.P.A. BANDS switch for B channel
P.A. TUNING control for B channel
P.A. BANDS switch for B channel
ANTenna COUPLING control for B channel
ANTenna LOADING control for B channel
Master OFF-ON switch
OPERATE-TUNE switch
TELEPHONE-TELEGRAPH switch
LOCAL-REMOTE switch
Dialing OFF-ON switch
PLATE-ON-STANDBY switch
Impulsing Dial
Switch for test meter
Located just below each tuning control is a friction lock to prevent
accidental movement of a control once it has been set. All switches are
provided with positive positioning detents.
(b) The following meters are located on the front panel to facilitate tuning and to provide a check on performance.
ANTenna CURRENT for A channel
ANTenna CURRENT for B channel
P.A. PLATE CURRENT
I.P.A. PLATE CURRENT
MODulator CURRENT (cathode)
P.A. PLATE VOLTS
MODulator FILament VOLTS
LINE VOLTS
A test meter that may be switched to read
REMOTE LINE current
AUDio OSCillator plate current
V.O.C. Key Relay current
FIRST AUDIO plate current
DRIVER CATHODE current
MODulator GRID current
I.P.A. GRID current
P.A. GRID current
OSCILLATOR plate current
(c) The following controls when once set do not ordinarily require further adjustment and are located inside the transmitter:
MICrophone VOLTS (R707)
AUDIO GAIN (R505) ,
Audio OSCillator OUTPUT (R50l)
P.A. PLATE VOLTS adjustment potentiometer (R6l0)
(d) The following controls and meter are located on the panel of the Type AT3 Remote Control:
OFF-ON switch
TELEPHONE-TELEGRAPH switch
Impulsing dial
LINE current meter
(e) The OSCillator OUTPUT control R90l and MIC. OUTPUT control R904 are located inside the Remote Control unit.
TUBE COMPLEMENT
The vacuum tubes used for the different circuits in the AT3 transmitter are:
R.F. Oscillators 2 x 802
Intermediate Power Amplifier 1 x 807
Power Amplifier 2 x 813
Audio Amplifier 1 x 6SD7
Modulator Driver 1 x 807
Modulator 2 x 805
Bias Voltage Rectifier 1 x 5Z3
Intermediate Voltage Rectifier 2 x 866
High Voltage Rectifier 2 x 872
Audio Oscillator 1 x 76
V.O.C. Control 1 x 6F8G
(b) The type AT3 Remote Control uses:
Audio Oscillator 1 x lH4G
TRANSMITTER
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REMOTE CONTROL
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Earle Smith VE6NM remembers one of the AT-3 quirks. "The IPA and PA
coils had a habit of getting gummed up to the point where the sliding pickup
rollers would jump off the coils when tuning. You were supposed to
shut down the transmitter, take off the side panels so the safety interlocks
would properly open, short the coils to ground for safety's sake, and put
the sliding roller back on the coil."
Material Contributors:
1) Ray White <r.p.white(at)sympatico.ca>
2) David Lawrence VA3ORP <va3orp(at)kos.net>
3) Andre Guibert <aguibert(at)sympatico.ca>
4) Earle Smith - VE6NM <t16ru672(at)telusplanet.net>
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