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SR-209
HF/VHF/UHF/SHF surveillance receiver

SR-209 is a modular solid-state HF/VHF/UHF/SHF surveillance receiver, developed around 1965 by Astro Communication Laboratory (ACL) in Gaithersburg (Maryland, USA). It covers a frequency range of 2 MHz to 12 GHz 1 spread over several frequency bands, each requiring a specific plug-in unit. The SR-209 was one of the standard receivers of the US Central Intelligence Agency (CIA).

The receiver is housed in a 19" 2U rackmount enclosure and offers space for two same-size plug-units; one at either side of the fixed IF/AF unit at the centre. Depending on the application, it can accomodate two tuning heads, or a signal display unit (SDU) plus a single tuning head [B].

The image on the right shows a typical SR-209 that is equipped for reception of the CIA bugs of the 1960s and 70s, which operated in the 380 MHz frequency band. At the left is a green SDU-102AP
CRT
display unit. At the right is the SH-203P-1 tuning head that covers 250—500 MHz.
  
SR-209 surveillance receiver

The receiver is suitable for the reception of
AM
,
FM
and
CW
signals, and also for pulse-modulated signals, which were generated by some of the CIA's covert listening devices (bugs) of the era. The IF2 is at 21.4 MHz 2 which is also used to display the IF passband on the signal display unit (SDU).

Up to three different
IF
strips can be installed – plus a spare – to allow for different bandwidths. With the receiver shown here, 300 kHz, 500 kHz, 2000 kHz and 3000 kHz
IF
strips are present. In the Swiss Army, the SR-209 was known as E-648 [3]. Within the CIA it was known as SRR-23 [6].

  1. The SR-209 was initially advertised as a 2-7000 MHz receiver [B], but a plug-in for the 7-12 GHz band was latter added to range of available tuning heads [C], making it effectively a 2-12000 MHz receiver.
  2. When using the SH-100-series tuning heads, the IF2 is at 455 kHz.

SR-209 surveillance receiver SR-209 front panel (cover removed) IF-strip IF-212-3000 SH-203P-1 tuning head Signal Display Unit (SDU) Front panel Rear panel FM signal on the SDU
A
×
A
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SR-209 surveillance receiver
A
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SR-209 front panel (cover removed)
A
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IF-strip IF-212-3000
A
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SH-203P-1 tuning head
A
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Signal Display Unit (SDU)
A
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Front panel
A
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Rear panel
A
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FM signal on the SDU

Controls
The SR-209 has a modular construction. It is housed in a 2U 19" rackmount case that consists of three major parts: a mainframe – which contains the power supply unit (PSU), the Intermediate Frequency sections (IF) and the audio circuits (AF) – at the centre, and two expansion bays at the left and right sides respecively. Each of the bays can hold a tuning head – or RF front-end – but the one at the left is also suitable for a Signal Display Unit (SDU) – or frequency spectrum display.


The diagram above gives an overview of the front panel, which has an SDU-102AP Signal Display Unit at the left, and an SH-203P-1 tuning head at the right. In this configuration, the receiver – which has previously been owned by the CIA – is suitable for the 250 — 500 MHz band, which covers the 380 MHz band in which many of the CIA's covert listening devices (bugs) operated.


All connections – with the exception of the headphones socket – are located at the rear, as shown in the diagram above. As each expansion bay can take a tuner, there are two N-type sockets for connection of the antennas. At the left is the 115/230V AC mains power input, which requires a rather special type of plug. The desired local voltage is selected with two toggle switches at the right. Once configured, they are protected by a metal cross bar, to prevent accidental change.

Main frame with SDU and tuning head installed Main frame - front panel Tuning head front panel Signal display unit SDU-102AP AM signal on the SDU FM signal on the SDU Tuning head with dial light switched on IF-strip selector (with one inlay present)
B
×
B
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Main frame with SDU and tuning head installed
B
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Main frame - front panel
B
3 / 8
Tuning head front panel
B
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Signal display unit SDU-102AP
B
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AM signal on the SDU
B
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FM signal on the SDU
B
7 / 8
Tuning head with dial light switched on
B
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IF-strip selector (with one inlay present)

Parts
Parts
Base unit (main frame) Plug-in tuning head Signal display unit (SDU)
SDU
Intermediate Frequency (IF) pluggable PCBs
IF
AC mains power cable
AC
Headphones Operating instructions and technical manuals
Base unit
The enclosure can be seen as the main frame. It contains a mains power supply unit (PSU), three power stabilizing circuits, up to three different IF strips (plus a spare one) and the AF circuits, like AFC, squelch and audio (headphones) amplifier.

It also holds the control panel at the centre. This section is identical for all variants of the SR-209, although the IF strips can be selected at will. The base unit has two expansion bays, each of which can accomodate two different tuners, or a single tuner and a Signal Display Unit (SDU).

  
Main frame with two empty slots

Tuner
The receiver is suitable for frequencies between 2 MHz and 12 GHz, divided over 14 bands, each requiring a dedicated tuning head, or tuner. The main frame can accomodate two such tuners.

The image on the right shows the SH-203P-1 tuner, which covers the 250 — 500 MHz band, in which many CIA bugs operated at the time.

 Overview of available tuning heads

  
Tuning Head SH-203P-1 (250-500 MHz)

Signal Display Unit   SDU
For surveillance and countermeasures jobs, it can be useful to have a real-time visual display of a section of the radio spectrum. In such cases a Signal Display Unit, or SDU, could be installed in the leftmost expansion bay. Two such SDU's were available: one for the 2 — 45 MHz range, and one for the 45 — 12000 MHz range.

The SDU is built around a classic green phosphor cathode ray tube (CRT), and displays RF signals between DC and 3 MHz in the IF passband. The image on the right shows the SDU-102AP.

  
Signal Display Unit (SDU)

Intermediate Frequency plug-in   IF
The receiver can be configured for up to three different
IF
bandwidths, that are selected with a rotary switch at the front panel. Each bandwidth requires a dedicated IF-strip to be installed in the IF-slots at the centre part of the mainframe.

Four IF-slots are present, of which only the left­most three are wired to the selector. The right­most slot can hold a spare IF-strip. The image on the right shows three installed IF-strips (300, 500 and 2000 kHz), with a spare one (3000 kHz) extracted from its unwired slot, laying on top.

 Overview of available IF-strips

  
Spare IF board removed

Mains power cable
The SR-209 has a rather exotic AC mains power input socket at its rear panel, which requires a special type of plug. It is suitable for 115V and 230V AC, but the two voltage selection switches at the rear panel must be set to the correct voltage, prior to connecting the mains cable.

The image on the right shows the original power plug with integrated bajonet fitting. For safety reasons, the original rubber cable (which had become brittle over the years) has been swapped for a new one, and the US-style power plug has been swapped for a European continental one.

  
Mains power cord

Headphones
Audio is provided at the 6.3 mm jack socket at the front, at a level that is suitable for a pair of 150Ω - 600Ω headphones. When using high-impedance headphones, a 150 ohm shunt resistor should be connected in parallel.

A separate 600Ω balanced output is available at the rear for connection to an external amplifier.
  
Headphones

Manuals
Original documentation of the SR-209 is very hard to find. Luckily, our receiver came with most of the original manuals intact, which are available for download below. So far, we have only recovered the manuals of the base unit, one of the tuning heads and two of the IF-strips.

We are still looking for the manual of the Signal Display Unit SDU-102AP. Please help us to expand this page, by providing that manual.

 Get the manuals
  
Manuals

Mains power plug (receiver-side) IF-strip IF-212-3000 Tuning Head SH-203P-1 (250-500 MHz) SH-203P-1 tuning head Tuning head - rear panel Adjustable coils Signal Display Unit (SDU) Signal display unit - rear panel
C
×
C
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Mains power plug (receiver-side)
C
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IF-strip IF-212-3000
C
3 / 8
Tuning Head SH-203P-1 (250-500 MHz)
C
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SH-203P-1 tuning head
C
5 / 8
Tuning head - rear panel
C
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Adjustable coils
C
7 / 8
Signal Display Unit (SDU)
C
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Signal display unit - rear panel

Block diagram
The operation of the SR-209 is explained in the simplified block diagram below. At the top is a typical tuning head (tuner) for UHF reception. It has a free running Variable Frequency Oscillator (VFO) that operates 60 MHz above the desired frequency (Fc), resulting in a 60 MHz IF1 signal. The IF1 signal is mixed with 81.4 MHz from a Voltage Controlled Oscillator (VCO), which results in a 21.4 MHz IF2 signal, that is fed to one of (up to three) IF-strips in the receiver's mainframe.


Note that different IF-strips are available, for different frequency ranges and for different band­widths. The block diagram above shows typical IF-211, IF-212 and IF-112 strips. Each IF-strip produces AM and FM signals, that are fed to the Automatic Gain Control (AGC), the (optional) Automatic Frequency Control (AFC), the Carrier Operated Relay (COR, or squelch), the video amplifier (VA) and the audio amplifier (AA). The signal strength indicator is driven by a signal from the AGC, whilst the tuning indicator shows the discriminator signal from the video amplifer.



Interior
The mainframe is housed in a strong 19" 2U rackmount enclosure. The interior can be accessed by removing the top and bottom covers, each of which is held in place by two knurled bolts at the rear. After loosening these bolts, the cover panels can be slid towards the rear and removed.

Inside the case is an aluminium construction that holds the parts that are common to all SR-209 models. At the rear is a heavy mains transformer with three stabilizing circuits on plug-in cards: one for +24V, one for +12V and one for -12V.

Towards the front are five long plug-in cards, and four smaller ones. From left to right are up to three IF-strips, plus a 4th spare one, followed by the AGC board. The smaller boards hold the (optional) AFC unit, the COR (squelch), the video amplifier (VA) and the audio amplifier (AA). The latter delivers the output to the headphones.
  
Power stabilizers

The audio signal is also available on a terminal block at the rear. COR, the carrier-operated relay, allows the audio to be switched off when no carrier is detected. A set of free contacts from this relay is also available on the terminal block at the rear. The green lamp is lit when COR is active.

Depending on the selected frequency segment and the requirements of the user, up to three IF-strips can be fitted inside the mainframe. The image on the right shows a typical IF-212-3000 IF-strip, that offers a bandwidth of 3000 kHz. It is likely that this strip was used for the reception of pulse-modulated listening devices (bugs).

An unwired slot in the base unit, allows a spare (4th) IF-strip to be stored inside the receiver. It can be swapped with an active one, in just a few minutes. For this reason, the text in the windows above the rotary selector can also be swapped.
  
IF-strip IF-212-3000

Each IF-strip directly produces an AM and an FM signal that are used for several purposes. The AM detector produces the signal for the AGC, whilst the FM discriminator drives the AFC. Both circuits remain active, even when the alternative modulation type is selected by the operator.

Considering its age, the SR-209 is extremely well built, using only first class components and gold-plated contacts where necessary. Most circuits are built with transistors from brands like Motorola and General Electric (GE), but the AFC board holds a very early implementation of a modular circuit block, or integrated circuit (IC).

It is an operational amplifier (OpAmp), supplied by Zeltex Inc. in Concord (California, USA) and is visible in the image on the right as a large black block at the centre. Inside this block, which was made by Philbrick, is a potted hybrid circuit. 1
  
AFC board

At the bottom of the mainframe is the wiring between the sockets of the board, the AC mains transformer and three large capacitors that are part of the power supply unit (PSU). All wiring has teflon insulation, and shielded coaxial teflon wiring is used for the RF, IF, video and audio signals.

The mainframe has two expansion bays – one at the left and one at the right – that can take two types of plug-in units: a tuning head (tuner) or a Signal Display Unit (SDU, or panoramic display). This means that a typical SR-209 receiver can hold two tuners, or an SDU and a single tuner.

At least 15 different tuning heads were offered by ACL, each with its own frequency range. In addition, at least four different swept tuning heads were available for electronic tuning and automated search. Depending on the frequency, each tuning head was constructed differently.
  
Local oscillator and first mixer

The image above shows the local oscillator of the SH-203P tuner (250-500 MHz). All HF tuning heads in the SH-100-series (2-45 MHz) produce an IF output signal at 455 kHz, whilst all VHF, UHF and SHF tuning heads of the SH-200-series (20-12000 MHz) produce an 21.4 MHz IF signal.

For this reason, there are two different SDUs: one for 455 kHz and one that is suitable for 21.4 MHz signals. Inside the SDU is a narrow cathode ray tube (CRT) with a green phosphor screen, much like the ones that were present in old oscilloscopes. The image on the right shows the CRT, plus one of the driving circuit boards.

Behind the CRT (close to its connections) is the HT-section with a transformer and a cascade circuit. The remaining circuits are located at the bottom of the unit. The CRT can be adjusted with four recessed multi-turn potentiometers.
  
Cathode Ray Tube (CRT)

The SDU-102AP allows part of the IF pass band – adjustable from DC to 3 MHz – centred around 21.4 MHz, to be displayed on the CRT screen. A crystal-based marker can be enabled, to help find the centre frequency on the screen. In the same vein, the SDU-100P (needed with the 2-45 MHz HF tuners) has a sweep width of 0-10 kHz or 0-50 kHz centred around 455 kHz [B].

  1. The Zeltex ZEL-1 is listed in the service manual [C] as the Philbrick PP65AU [a]. It was probably supplied to Zeltex as an OEM part. George A. Philbrick Researches (GAP/R) in Dedham (Massachusetts, USA) was one of the pioneers of the Operational Amplifier [5].

Main frame with two empty slots Main frame - bottom view Power stabilizers IF and AF sections Spare IF board removed IF-strip detail IF-strip detail Card-to-socket contacts
+24V stabilizer board 12V stabilizer board IF-strip IF-212-3000 AGC board AFC board COR board (squelch) Video amplifier Audio amplifier
Tuning head interior First amplifier Second amplifier and filters Adjustable coils RF filter and first RF amplifier Ganged tuning coils Local oscillator and first mixer 60 to 21.4 MHz converter
SDU interior Cathode Ray Tube (CRT) SDU bottom view SDU side view SDU detail SDU detail Screen adjustments HT circuit
D
×
D
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Main frame with two empty slots
D
2 / 32
Main frame - bottom view
D
3 / 32
Power stabilizers
D
4 / 32
IF and AF sections
D
5 / 32
Spare IF board removed
D
6 / 32
IF-strip detail
D
7 / 32
IF-strip detail
D
8 / 32
Card-to-socket contacts
D
9 / 32
+24V stabilizer board
D
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12V stabilizer board
D
11 / 32
IF-strip IF-212-3000
D
12 / 32
AGC board
D
13 / 32
AFC board
D
14 / 32
COR board (squelch)
D
15 / 32
Video amplifier
D
16 / 32
Audio amplifier
D
17 / 32
Tuning head interior
D
18 / 32
First amplifier
D
19 / 32
Second amplifier and filters
D
20 / 32
Adjustable coils
D
21 / 32
RF filter and first RF amplifier
D
22 / 32
Ganged tuning coils
D
23 / 32
Local oscillator and first mixer
D
24 / 32
60 to 21.4 MHz converter
D
25 / 32
SDU interior
D
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Cathode Ray Tube (CRT)
D
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SDU bottom view
D
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SDU side view
D
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SDU detail
D
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SDU detail
D
31 / 32
Screen adjustments
D
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HT circuit

Restoration
The SR-209 featured on this page was manufactured by ACL in August 1973, and has previously been owned by the US Central Intelligence Agency (CIA). It was used throughout the 1970s and 1980s for testing covert listening devices (bugs) that operated in the UHF band around 380 MHz.

Once the receiver was decommissioned – some time after the transition had been made to bugs operating at 1500 MHz – the receiver was put on a shelf under humid conditions, where it stayed for the next 30 years, until it was donated to Crypto Museum. Although the exterior had collected quite a bit of dust, the interior was surprisingly clean and the components looked like they were made yesterday. After reconditioning the electrolytic PSU capacitors, the receiver was switched on and worked immediately. Even the frequency calibration was still spot-on after all these years.

The following (minor) problems were observed:

  • Mains cable brittle (unsafe)
  • Meter glass broken
  • Strong 50 Hz hum when COR (squelch) is active
  • Power lamp broken
Fortunately, the original power cable (with the exotic plug) was present, but the rubber mantle had become brittle and showed signs of disintegration. For safety reasons it was replaced by a modern neoprene power cable with a suitable wall-socket plug for use in continental Europe.

The next thing to be fixed, was the transparent cover over the signal strength meter, which had come off. As the meter is mounted very close to the upper edge of the case, it is likely that it got damaged when it was moved from one shelf to another. Luckily, the parts were still present, and were easily glued back in place with Cyanolit™.

The problem of the 50 Hz hum was more serious and it took a while to find its cause. Whenever the SR-209 was not receiving a carrier, and the Carrier Operated Relay (COR) – or squelch – fell off, a 50 Hz hum was heard in the headphones.
  
Resistor mounted between contacts 5 and 6

Strangely, the hum only started about 1 second after the COR relay had turned off. Initially it was thought that it might be caused by a faulty bridge rectifier in one of the power rails, but after checking the DC power lines for any AC components (ripple), this theory had to be discarded.

Next, the audio amplifier input was checked, and it was observed that the hum appeared there as well – about 1 second after the COR had turned off – although the supply to the audio pre-amplifier had been fully cut-off at that stage.

Long story short: it was caused by the 600Ω transformer at the output of the power amplifier. Apparently, the open winding for connection to an external amplifier, picked up hum from the mains network and fed it back into the amplifier. It was fixed by mounting a 1KΩ resistor accross contacts 5 and 6 of the TB1 terminal block.
  
Pilot lamp

The last problem – the broken power lamp – was fixed more easily than initially anticipated. It is shown in the image above and is in fact a 28V filament lamp, mounted inside a pre-fabricated enclosure. Luckily, these pilot lamps are still available from the original manufacturer Dialight [b].

Mains power cord Mains power plug (receiver-side) Pilot lamp Green pilot lamp Terminal block at the rear Resistor mounted between contacts 5 and 6
E
×
E
1 / 6
Mains power cord
E
2 / 6
Mains power plug (receiver-side)
E
3 / 6
Pilot lamp
E
4 / 6
Green pilot lamp
E
5 / 6
Terminal block at the rear
E
6 / 6
Resistor mounted between contacts 5 and 6

Versions
  • SR-209
    First incarnation of the SR-209 receiver
  • SR-209-6
    Special version with large field strength indicator
  • SR-209 C
    Last known incarnation of the SR-209 receiver
Specifications
  • Modulation
    AM, FM, CW, Pulse
  • Impedance
    50 Ω unbalanced
  • Audio
    100 mW into 600 Ω external speaker
  • Video out
    5V pp max. into 93Ω
  • Video impedance
    93Ω, BNC
  • Display output
    21.4 MHz
  • Meters
    Tuning, Signal strength
  • Power
    115V AC (option: 230V AC), 50-400 Hz
  • Dimensions
    19" rackmount
  • Weight
    30 pounds
RF tuning heads
  • SH-102 P
    2-6 MHz
  • SH-103 P
    6-20 MHz
  • SH-104 P
    20-45 MHz

  • SH-200 P
    20-45 MHz
  • SH-270 P
    20-70 MHz
  • SH-201 P
    30-100 MHz
  • SH-271 P
    55-260 MHz
  • SH-202 P
    90-300 MHz
  • SH-272 P
    225-400 MHz
  • SH-203 P
    250-500 MHz
  • SH-204 P
    490-1000 MHz
  • SH-205 P
    990-2000 MHz
  • SH-206 P
    1990-4000 MHz
  • SH-207 P-1
    4-7 GHz
  • SH-208 P-1
    7-12 GHz
Swept tuning heads
  • ESH-201 P
    A: 30-55 MHz
    B: 55-100 MHz
  • ESH-202 P
    A: 100-190 MHz
    B: 190-300 MHz
  • ESH-203 P
    300-500 MHz
  • ESH-204 P
    A: 500-740 MHz
    B: 740-1000 MHz
IF units for series 100 tuning heads
  • IF-112-01
    1 kHz
  • IF-112-05
    5 kHz
  • IF-112-10
    10 kHz
IF units for series 200 tuning heads
  • IF-220-10
    10 kHz
  • IF-220-20
    20 kHz
  • IF-211-60
    60 kHz
  • IF-211-75
    75 kHz
  • IF-211-100
    100 kHz
  • IF-211-150
    150 kHz
  • IF-212-300
    300 kHz
  • IF-212-500
    500 kHz
  • IF-212-1000
    1000 kHz
  • IF-212-2000
    2000 kHz
  • IF-212-3000
    3000 kHz
  • IF-212-4000
    4000 kHz
  • IF-212-8000
    8000 kHz
Options
  • SDU-100P
    Signal Display Unit, for use with series 100 tuners → spec
  • SDU-102AP
    Signal Display Unit, for use with series 200 tuners
    spec
  • FRO-201P
    Frequency readout plug-in 2—300 MHz
  • FRO-203P
    Frequency readout plug-in 2—500 MHz
  • ESH-200
    Sweep (scan) option)
  • FRO-201
    External frequency readout unit
  • BP-201P
    Rechargeable NiCd battery for mobile use
SDU-100 P
  • CRT size
    1 x 3 inches (25 x 76 mm)
  • Sweep width
    0-10 kHz and 0-50 kHz (continuously adjustable)
  • Resolution
    400 Hz and 2 kHz
  • Sweep rate
    4 Hz
  • Intermediate
    IF1: 80 kHz, IF2: 15 kHz
  • Response
    ±1.5dB (amplitude response)
  • Sensitivity
    10µV @ 1" deflection
  • Marker
    455 kHz (crystal based)
  • Linearity
    5%
  • Power
    ±12V DC (provided by SR-209)
SDU-102 AP
  • CRT size
    1 x 3 inches (25 x 76 mm)
  • Sweep width
    DC to 3 MHz (continuously adjustable)
  • Resolution
    10 kHz
  • Sweep rate
    20 Hz
  • Intermediate
    IF1: 4.3 MHz, IF2: 455 kHz
  • Response
    ±1.5dB (amplitude response)
  • Sensitivity
    10µV @ 1" deflection
  • Marker
    21.4 MHz (crystal based)
  • Linearity
    5%
  • Power
    ±12V DC (provided by SR-209)
Connections
Output connector TB1
Six contacts are available on a terminal block at the right side of the rear panel. The first three contacts are operated by a relay inside the COR (squelch) unit. The situation is shown when no carrier is received. The 4th contact provides ground and the last two contacts (5 and 6) provide a balanced 600Ω audio output, which can be fed to an external amplifier or telephone line.

  1. COR relay normally open
  2. COR relay common
  3. COR relay normally closed
  4. Ground
  5. Audio out 600Ω (balanced) 1
  6. Audio out 600Ω (balanced) 1
  1. Note that a 1KΩ resistor should be connected between points 5 and 6 when the output is unused, to avoid hum on the headphones output. The resistor effectively loads the output transformer.

Glossary
AFC   Automatic Frequency Control
AGC   Automatic Gain Control
BFO   Beat Frequency Oscillator
Optional oscillator that feeds a signal to the AM detector, to make CW signals (morse) audible. Note that not all IF-strips have a BFO.
COR   Carrier-Operated Relay
Noise cancelling when no signal is present. Also known as squelch.
LO   Local Oscillator
Generally speaking this is the first oscillator in a receiver that directly controls the reception frequency. In this case, it is a free-running variable frequency oscillator (VFO).
SDU   Signal Display Unit
Also known as spectrum display, or panoramic display.
VCO   Voltage Controlled Oscillator
Oscillator of which the output frequency is determined by a DC voltage.
VFO   Variable Frequency Oscillator
Documentation
  1. ACL catalog
    Date unknown. 1

  2. The SR-209 Receiving System, Brochure
    June 1971. Retrieved from NASA CR-130232, January 1973.

  3. VHF/UHF Receiver Type SR-209, Manual
    CM-303046. ACL, date unknown.

  4. Plug-in tuning head SH-203P-1/AFC (250-500 MHz), Manual
    CM-303046. ACL, date unknown.

  5. IF Amplifiers IF-212-300, Manual
    CM-303046. ACL, date unknown.

  6. IF Amplifiers IF-212-500, Manual
    CM-303046. ACL, date unknown.

  7. SDU-102AP & SDU-103AP, Manual
    ACL, date unknown. 1
  1. Document kindly provided by Terry O'Laughlin [1].

Datasheets
  1. Philbrick, PP65AU datasheets 1
    December 1964 — November 1965.

  2. Dialco 507-series pilot lamps, datasheet
    Dialight Corporation, date unknown.
  1. Documents obtained from Joe Soesa at Philbrick Archive [5].

References
  1. Terry O'Laughlin, Astro Communication Labs (ACL)
    Retrieved February 2019.

  2. Ian O'Toole, SR-209 VHF/UHF Receiver
    28 April 2003, updated 11 December 2009.

  3. Martin Bösch, E-648 — ACL-SR 209
    21 April 2012.

  4. RX-Control, ACL SR-209 Receiver
    11 April 2015.

  5. Joe Soesa, collection of documents related to PP65AU OpAmp
    Website: philbrickarchive.org. Retrieved March 2019.

  6. NRP to CIA, order for Government Furnished Equipment (GFE)
    28 February 1985.
Further information
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Crypto Museum. Created: Tuesday 26 February 2019. Last changed: Wednesday, 20 March 2019 - 08:11 CET.
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