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RECEIVE OUTFIT ACAS, RAN PATTERN 16303 - RECEIVER
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LOW POWER MF-HF RADIO COMMUNICATION EQUIPMENT TYPE A618/ACAS

The RAN Type A618/ACAS was a general purpose ship-borne installation of transmitters and receiver. It consisted of major units:

TOP TO BOTTOM - MF TRANSMITTER, HF TRANSMITTER, RECEIVER, 
WITH POWER SUPPLY ON RIGHT.

As I’m concentrating on receivers at the moment the following covers only the Receive Outfit ACAS.

The receiver was made by TCA (Telecommunications Company of Australia, a Philips subsidiary) in 1961. As can be seen from the photo the cabinets allowed the other equipment to stack on top of the receiver, with the power supply separate.

It is a double conversion superhet which covers 40 Kc/s to 30 Mc/s in nine bands, the tuned circuits for which are in a rotatable turret mechanism. There were four degress of selectivity, the narrowest two being achieved by crystal filters. There were two RF stages, two IF stages and three audio stages, including a speech compressor. It had facilities for receiving CW, MCW and voice.

The weight of the receiver was 79 Pds. (35.9 Kg) and dimensions were 17 ¾” Wide x 15 ¼” Deep x 12 7/8” High (451mm W x 387mm D x 327mm H). That did not include the normally fitted shock mounts.

Sensitivity was good (which only means the manuals often give different ways of expressing the figures and it gets confusing trying to compare them). Signal to noise ratio was 10 dB up to 4 Mc/s and rose to 25 dB on the highest band.

An important aspect was the spec. for extraneous RF radiation which was less than 0.1 mv per metre at a distance of 1 nautical mile.

The selectivity was determined by the filters and crystals as follows:

 It had a compliment of 16 valves and employed a second mixer for double conversion on bands 3, 4 8 and 9. On the other bands this valve acted as an additional IF amplifier.

The frequency coverage is:

BAND	FREQUENCY	IF
1	37 to 85 Kc/s	455 Kc/s
2	80 to 176 Kc/s	455 Kc/s
3	160 to 375 Kc/s	1500 Kc/s and 455 Kc/s
4	340 to 800 Kc/s	1500 Kc/s and 455 Kc/s
5	730 to 1710 Kc/s	455 Kc/s
6	1.54 to 3.62 Mc/s	455 Kc/s
7	3.3 to 7.8 Mc/s	455 Kc/s
8	7.0 to 16.6 Mc/s	1500 Kc/s and 455 Kc/s
9	14.8 to 33.2 Mc/s	1500 Kc/s and 455 Kc/s

In addition to the variable local oscillator, 4 crystal locked positions could be selected.

The audio output had separate headphone and speaker jacks and was capable of 2.5 volts into 100 ohms to the headphone jack and 33 volts to the speaker (line output) socket.

The receiver obtained its power from the Power Supply Unit and power consumption was approximately 65 watts. The voltages from the Power Supply were 6.3v ac for filaments, 250v dc for HT and -45v dc bias voltage.

The aerial input could accept aerials with a capacitance of 200 to 600 pF on the 6 lower bands and was designed for 75 W input on the higher bands. The set had an aerial trimmer for fine tuning. The aerial cable was normally routed to one or other of the transmitters which included a high speed antenna change over relay. The receiver was fitted with a muting relay to mute the audio when a transmitter was operating for break-in keying or "listening through".

The valve line up was:
                            V1       6AM5              CV4014                      1^st RF Amplifier
                            V2       6BA6               CV4009                      2^nd RF Amplifier
                            V3       ECH81            CV2128                      1^st Mixer and X1 Oscillator
                            V4       6AM5              CV4014                      VF Oscillator
                            V5       ECH81            CV2128                       IF Amplifier
                            V6       6BA6               CV4009                      2^nd Mixer and Oscillator, or IF Amplifier
                            V7       6AB6               CV2526                      AGC Amplifier
                            V8       6BA6               CV4009                      IF Amplifier
                            V9       6BA6               CV4009                      Beat Frequency Oscillator
                            V10     6AL5               CV4007                      Diode 1 – Signal Rectifier
                                                                                                    Diode 2 – AGC Rectifier
                            V11     6AL5               CV4007                      Diode 1 – Noise Limiter
                                                                                                   Diode 2 – AGC Peak Limiter
                            V12     OA2                CV4020                      Voltage Stabiliser
                            V13     6BA6               CV4009                     1^st AF Amplifier
                            V14     6AV6               CV2526                     Audio Compressor
                            V15     6AM5              CV4014                      2^nd AF Amplifier
                            V16     6AQ5              CV4019                      Audio Output
                            MR1    OA85              CV448                        AGC Clamp Diode

RECEIVER OUTFIT ACAS

The front panel controls, starting top left, are set out below. Most are self explanatory:

DIAL SCALE. This is a rotating drum similar to Collins receivers, with a pointer that moves along it. The scale is calibrated directly in frequency.

AERIAL. Accepts capacitive input on lower bands and 75 W on the higher frequencies.

AERIAL TUNING. A small trimmer capacitor.

H.T. ON – OFF When OFF the H.T. is transferred to a load resistor to prevent the H.T. voltage at the Power Supply from Rising too high.

BANDSWITCH.         Nine positions and rotates a turret with the relevant coils, like the B40/B41 receivers.

SIGNAL STRENGTH METER calibrated in dB above 1 mv, but for convenience only and meant as an indicator and tuning aid, not an accurate level meter.

LOUDSPEAKER with PHONES underneath. Both can be used simultaneously. The Speaker impedance was 500 W and headphones 100 W.

LOWER. For servicing the set could be slid out on runners and then tilted upwards and locked in place to get access to the underneath. Pressing the two LOWER buttons (there’s one on each side of the front panel) allowed it to rotate back to horizontal.

METHOD OF SERVICING CHASSIS

CROSS MOD. This varies the bias on the 1^st RF amplifier to reduce gain

AGC OFF-ON

B.F.O. OFF-ON

BANDWIDTH WIDE – MEDIUM – XTAL NARROW – XTAL VERY NARROW

NOISE LIM. OFF – ON

COMPRESSION OFF – ON This acted to reduce audio level fluctuations when the AGC was switched off.

LOCAL OSC. 1 – 2 – 3 – 4 – VAR.

RF GAIN. This control adjusts the bias on the 2^nd RF Amplifier and the 1^st and 2^nd IF Amplifiers.

LOCK A pushbutton dial lock.

TUNING

RELEASE Releases the chassis so that it can be pulled forward on runners and side rails. Before freeing the receiver from the case the operator had to unscrew 6 “difficult access” screws around the edges of the front panel.

B.F.O. TUNE  - 1.2 Kc/s - + 1.2 Kc/s

AF GAIN

POWER INPUT A multi-pin connector with connections for H.T., Filaments, Bias voltages, Mute Relay, Speaker and Headphones.

EARTH

As well as providing all the power, the Power Supply Unit acted as the common interconnection point for the two transmitters and the receiver. It could be remote from the other equipment and could control their functions. Eg. In remote operation it switched the sets to CW, MCW or voice and had provision for a microphone and morse key.

POWER SUPPLY UNIT

The receiver was very well built and robust as is needed on board ships with big guns. The provision of slides and tilt mechanism reflects the need to service it in situ.

I believe the RAN Corvettes that have been preserved in Australia, (eg. the Diamantina) have the A618/ACAS system on board.

References.
ROYAL AUSTRALIAN NAVY ABR 5043, Instruction Manual for LOW POWER MF-HF RADIO COMMUNICATION EQUIPMENT TYPE A618/ACAS. 1961.

Please note, this article is copyrighted.

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