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| Figure 1: The front panel of the Morrow CM-1 CONELRAD monitor. The meter indicates relative signal strength and below it is the on/off/volume control. The dial - with a pair of "CD" markings, is a smooth-tuning reduction drive - which is also very accurate. Shown here, it's on and tuned to a local station. Click on the image for a larger version. |
If you were an amateur (Ham) radio operator in the late 1950s and early 1960s, you probably had a broadcast radio turned on in the background whenever you were on the air - or at least you should have!
The reason for this was spelled out in the FCC rules: Beginning January 1, 1957, Section 12.192 required all amateur radio stations to monitor a broadcast station - at a minimum of ten minute intervals - to see if it transmitting, while they (the amateur) was on the air. If the station had gone off the air, it was required that the amateur determine if this was because of a CONELRAD alert - and if so, immediately cease transmitting.
What was CONELRAD?
CONELRAD, short for "CONtrol of ELectromagnetic RADiation", was a system - part of the U.S. Civil Defense - where, if an imminent attack of Soviet bombers was detected, ALL radio stations - commercial (AM , FM and TV) and amateur - were to go off the air to prevent their signals from being used as directional signals for navigation by the enemy. The exception to this were some of the AM broadcast stations which were to ultimately occupy either 640 or 1240 kHz - but only for a few minutes at a time, the frequencies to be used by several different stations during that period - in a "round-robin" sort of system. The "musical chairs" of transmitters, frequencies and locations was intended to make it difficult for the Soviet bomber to use them for navigation.
If you have seen an AM broadcast receiver intended for the U.S. market from the time period when the CONELRAD system was in existence (from about 1951 to 1963) you may have noted that at 640 and 1240 kHz there are triangular marks - often with the letters "CD" in them (the Civil Defense logo) - on the dial signifying the two CONELRAD frequencies - and these symbols are clearly visible on the tuning dial in Figure 1. The intent of this is that civilians would tune to those frequencies to receive civil defense instructions during the "on" periods of the broadcast station near them.
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| Figure 2: Rear of the CM-1. The terminals connect to normally-open contacts of the relay (if fitted) and the wire antenna can be seen emerging from the chassis. Between the two is a potentiometer for adjusting the S-meter and relay sensitivity. Click on the image for a larger version. |
How it worked
As you might imagine, this system was cumbersome: All stations had to be alerted in some way and "key" stations - perhaps notified via a wire service - would turn their transmitters off-on-off and on again in five second intervals and then transmit a 1 kHz tone for 15 seconds: As transmitters of the day weren't designed to be "short-cycled" in this way, it was hard on the equipment - sometimes leading to failures. There was also the requirement that some transmitters - and their antenna systems - change from their normal frequency to either then 640 kHz or 1240 kHz CONELRAD channel - a further complication - to participate in the on/off "round robin".
Needless to say, this system had several well-publicized false alarms as well as failures when tested over the years that it was extant. Eventually, because of the implementation of ballistic missiles rather than piloted bombers, its reason for existing ceased in 1963.
How it affected amateur radio operators
As noted earlier, if you were an amateur radio operator, you were required to monitor a local broadcast station while you were on the air at least every ten minutes: If the radio station went off the air, the idea was that you were to check for a CONELRAD alert and if it was happening, immediately stop transmitting.
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| Figure 3: Top of CM-1 chassis as seen from its left side. In the fore- ground is the tuning capacitor, IF transformers and amplifier tube and main filter capacitor. The 6C4, if installed, would be plugged in the empty socket just visible behind the tuning capacitor. It's well laid-out - even the audio transformer is at an angle to minimize coupling to the AC power transformer. The antenna wire is lightly coupled to the preselector circuit. Click on the image for a larger version. |
There were also some receivers designed for just this purpose.
The Morrow Radio Mfg. CM-1
The Morrow CM-1 was one such receiver. The receiver pictured was found among the effects of a good friend - and fellow amateur radio operator - that passed away several years ago, sitting dusty on a shelf in his basement. After careful inspection, I was surprised to see that it had no bad capacitors: The components used in its construction - particularly the capacitors - are all good-quality Cornell Dubilier (all are ceramic disk types other than the main filter) - seemed to be a cut above those found in a typical, cheap domestic radio of the time and it worked perfectly the first time it was turned on - and I haven't even bothered checking the tubes: Even the main electrolytic capacitors is still in excellent shape - likely a result of the receiver having spent the past six-plus decades in a cool, dry basement.
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Figure 4: |
A "brief" circuit description (Refer to Figure 4) While many "AA5" receivers use a resonant loop - either a ferrite stick or a coil wound on the back cover for an antenna - the CM-1 uses just a short piece of wire, depicted in the upper-left corner of the diagram connected to C1, one half of the tuning capacitor and inductor "L1". This and the 50-100pF capacitor "lightly" couple a short piece of wire to the resonant circuit. This works, but its effects can be swamped if a long piece of wire (more than a few feet/meters) is used which can spoil image rejection an cause the radio to overload on strong signals. The 6EA6 converter doubles as both the local oscillator - its frequency determined by the other half of the tuning capacitor C2 and L2 - and mixer as it converts and amplifies the received signal to the Intermediate Frequency (IF) of 455 kHz via its plate through transformer T1 which forms a selective band-pass filter. Following T1 is a 6BJ6 which functions as an amplifier at 455 kHz and this goes to T2, another 455 kHz IF transformer, and then to the 6AV6 diode/detector/amplifier tube. The secondary output of transformer T2 applies the 455 kHz signal to a diode section and since its cathode is grounded, it causes that part of the transformer to go negative with the rectified DC voltage as well as the detected audio: The received audio is then coupled via a 0.01uF capacitor and 10 MegOhm potentiometer to the triode section of this tube for amplification where the audio is then coupled to the plate of the 6AS5 - its grid biased slightly negative by the 220 Ohm resistor in the cathode - and amplified. Functioning as a "Class A" single-ended audio amplifier, transformer T3 couples its high-impedance plate to the low-impedance speaker. Referring back to the bottom of the secondary of T2 where the audio is tapped, the negative DC voltage that is also produced there is in proportion to the amount of signal getting to the 6AV6 detector tube and ultimately, the strength of the signal to which the radio is tuned. This voltage is smoothed to remove the 455 kHz IF by a 250pF capacitor and then sent to one of the grids of the 6EA6 converter tube through the antenna-tuning inductor, L1 and also to one of the grids of the 6JB6 IF amplifier via the secondary of T1 and when the voltage gets more negative, its sensitivity is reduced. In this way the "AVC" (Automatic Volume Control) is formed so that both weak and strong stations produce similar amounts of audio. This same AVC voltage is also sent to the (optional) 6C4 tube: If this voltage is very negative - as it would be with a moderately strong signal - this tube is "cut off", but if the signal disappears and the AVC voltage is less negative, it will conduct and thus turn on the (optional) relay. For the signal level meter, a voltage divider using a 15k resistor and 2k potentiometer (seen at the bottom of T1) sets the voltage threshold on one side of the 1 mA meter - the other side of which is connected to the cathode of the 6JB6 IF amplifier tube along with a 1k resistor to ground. There are two mechanisms of action here for driving the meter: If there is more signal into this tube, it conducts more current and the voltage goes up across the 1k resistor, but a higher signal will also result in a more negative AVC voltage which will negatively bias the tube and reduce the current. It's this latter aspect that prevails: A more-negative AVC will reduce the tube's conduction and also the voltage across the 1k resistor and when this voltage is lower than that across the 15k-2k voltage divider, the meter will move up-scale. The final section is the power supply which consists of a transformer to isolate the circuit from the mains. The high voltage is half-wave rectified by a "modern" (for the time) selenium diode and then filtered by one section of a two-part capacitor which is then decoupled by a 750 ohm resistor in series which then goes to another capacitor. In this way, the voltage is reduced somewhat and better-filtered than it would be with just a single stage of smoothing. |
The CM-1 is a compact, tube (valve) type superheterodyne receiver
from this time period that was designed to indicate when a station went
off the air. As can be seen from the photo, it looks more like a piece
of ham gear than the AM broadcast band receiver in that it has a
"nicer" geared (and accurate) tuning dial and a meter than indicates the relative
strength of the signal to which it is tuned.
Internally, he CM-1 itself is mostly an unremarkable receiver: Electrically, it's a variant of the "All American Five" (AA5) superhet that was produced by the millions over about four decades, but with a few interesting differences. Sporting a power transformer, all of the tubes use six volt filaments and with the isolation, it - unlike most later, cheaper versions of the "AA5" - it does not have a "hot" chassis (e.g. one that is connected to one side of the line cord) - but with a solid-state (selenium) rectifier rather than the typical 35W4 and a rather different six volt tube line-up than a typical AA5: A 6BE6 as the converter, 6BJ6 as the IF amplifier, 6AV6 as the detector/amp and interestingly, a 6AS5 (a tube typically used in car radios) for the audio output.. As can just be seen in Figure 5, the 6AS5's "getter" shows discoloration and Figure 3 reveals that there is a slight stain around the audio transformer where some of its wax wicked onto the chassis due to heat, both indicating that this receiver has quite a few "on" hours!
An interesting departure from a typical AA5 is its antenna connection. Most AA5s had either a ferrite loopstick or a large coil of wire wound on the cardboard/masonite/phenolic back cover of the receiver that doubled as an RF pre-selector, but this receiver simply has a piece of wire capactively coupled to the tuning network connected to one of the capacitor's gangs.
According to the manual, only 2-3 feet (50-95cm) of wire is necessary for reception and testing of this CM-1 indicated that this was true - but it also means that one must resist the temptation to connect it to a longer wire: Doing so simply overloads the receiver and wrecks image rejection, particularly since the front-end preselector tuning is overcoupled to the antenna itself, effectively bypassing it altogether.
During testing - with only the 2' (50cm) wire antenna, when placed outside I found that the receiver was capable of picking up even the weaker signals across the broadcast band - but since it is a wire rather than a coil, it's non-directional and is far more prone to pick up local "E-field" noise energy than the typical "H-field" loop which is not only directional, but offers a degree of rejection of such noise. Practically speaking, one could probably add a loop antenna, connecting to the wire and the grounded screw (one of the two used for the relay) on the back panel to improve reception in today's modern electrical environment.
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| Figure 5: The other side of the top of the chassis. In the foreground is the power transformer and next to it is the 6AS5 audio amp. The socket next to the 6AS5 is for the (optional) relay, driven by the (also optional) 6C4. The antenna tuning coil is visible next to the tuning capacitor, almost against the front panel. Click on the image for a larger version. |
When I first removed the receiver from its chassis I thought that it was missing two tubes as evidenced by empty sockets, but I then realized that it had a selenium rectifier accounting for there being only four tubes and immediately wondered why there were extra sockets. A bit of "Google-Fu" found the manual online and I learned why: This receiver could optionally be fitted with a relay (also requiring another tube - a 6C4 to detect the loss of signal and drive the relay) that was intended to operate a sounder, a light or some other means of indicating loss of signal. I find it interesting that this relay and its driver tube were omitted as shipped from the manufacturer, but I suspect that this was a cost-saving measure - and it may have been enough for most amateur radio operators using this receiver to simply glance at the front-panel meter occasionally to see if a signal was still there.
Is it still useful?
Certainly, the reason raison d'etre for this device ceased to be in 1963 when the FCC rule requiring the amateur radio operator to monitor broadcast stations while operating was rescinded and removed from FCC Section 12.192, but since it's rather compact and has a speaker that dominates the top of the case means that it still works as a pretty nice AM broadcast receiver. Whether or not one might think an AM-only receiver is still useful is another matter altogether!
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| Figure 6: Bottom of chassis of the CM-1. High-quality components - including all Cornell Dubilier capacitors - are used throughout. The orange object left of center is the selenium rectifier and the oscillator coil is just right of center. Click on the image for a larger verison. |
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This page stolen from ka7oei.blogspot.com
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