The Morrow CM-1 receiver - A relic of the cold war's effect on amateur radio

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  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 then 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 was that some of the AM broadcast stations 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.

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 the 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.

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.

It's difficult to know these days how well the average amateur radio operator adhered to this rule, and for those that did, doing so certainly added a bit of complexity and awkwardness to their operating style.  A number of article appeared in the amateur radio magazines at the time describing how to "automate" the detection of the local radio station going off the air - typically by monitoring the AGC voltage of an inexpensive radio and sounding a buzzer or turning on a light.

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) - and they 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 filter capacitors are still in excellent shape - likely a result of the receiver having spent the past six-plus decades in a cool, dry basement.

Figure 4:
Schematic of the Morrow CM-1.  Despite the somewhat unusual tube line-up, it functions
the same as a typical "All American Five" superheterodyne receiver of the era, complete
with a 455 kHz IF.  The main difference is that it has provisions for connecting to an external
alarm or light based on the presence/absence of a signal using the (optional) 6C4 tube and the relay.
Click on the image for a larger version.

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 panel of the chassis 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 series 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.

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 two extra sockets.  A bit of "Google-Fu" found the manual online and I learned why:   This receiver could optionally be fitted with a relay in one of the sockets (this also required 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?

The reason raison d'etre for this device ceased to be when, in 1963, 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 overall is another matter altogether!

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 version.

The usefulness and practicality of this receiver is probably on par with most other vintage radios that one might collect and own:  They are not likely to be "everyday drivers" and more likely to sit on a shelf with other old radios - but this one, at least, has a particularly interesting bit of cold-war history behind its existence.

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 This page stolen from ka7oei.blogspot.com

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A solid state replacement for an old radio's "vibrator" (Wards Airline 62-345)

Figure 1:
The front of the Wards Airline 62-345 with its rather
distinctive "telephone dial" tuning dial.
It's powered up and running from 12 volts!
Click on the image for a larger version.

Quite some time ago - a bit more than a decade - a friend of mine came to me with an old "Farm" radio - a Wards Airline 62-345.  This radio - from the 1930s - was designed to run from a 6 volt positive ground battery system  such as that which one might find in tractors and cars of that vintage.

How high voltage was made from low voltage DC in the 30's

As the technology of the time dictated, this radio has what's called a "vibrator" inside - essentially a glorified buzzer - that is used as a voltage chopper along with a transformer to convert the 6 volts from the battery to the 130-150 volts needed for the plates of the tubes within.  Not only did this vibrator do the chopping for the high voltage, but it also performed the duty of synchronously rectifying the AC waveform from the transformer as the pulses from it would naturally be in sync with the motion of the moving reed, briefly connecting the output of the transformer to the input of the high voltage DC supply when the voltage waveform from it was at the correct polarity.

These devices, as you would expect, don't have a particularly long lifetime as they are constantly buzzing, making and breaking electrical contact and causing a small bit of arcing - something that will inevitably wear them out.  Even if the contacts were in good shape, the many decades of time that have passed will surely cause these contacts to become oxidized - particularly since these devices are in rubber-sealed cans (to minimize noise and vibration) and the out-gassing of these materials is likely of no help in their preservation.

Figure 2:
The chassis of the radio.  The vibrator is in its original
can in the far right corner.
Click on the image for a larger version.

Such was the case with this radio.  Often, the judicious application of percussive repair (e.g. whacking with a screwdriver) can get them going and if the contacts are just oxidized, they will often clean themselves and work again - at least for a while.  In this case, no amount of whacking seemed to result in reliable operation, so a modern, solid-state approach was needed.

The solid-state replacement

As mentioned earlier, the job of the vibrator was to produce a chopped DC waveform, apply it to a transformer for "upping" the voltage and then use a separate set of contacts to perform synchronous rectification - and our solid-state replacement would need to do just that.  That last part - rectification - was easy:  Just two, modern diodes would do the job - but chopping the DC would require a bit more circuitry.

The owner of this radio also had a few other things in mind:  He changed it from 6 volts, positive ground to 12 volts, negative ground so that it could be readily operated from this more-common power scheme.  The change to 12 volt filaments required a bit of work, but since all of the tubes were indirectly heated, the filament supply could be rearranged - but some tubes had to be changed to accommodate different filament voltages and currents as follows:

• Oscillator and detector:  This was originally a 6D8 (6.3v @ 150mA) and it was replaced with a 6A8 (6.3V @ 300mA).  Other than filament current, these tubes are more or less the same.
  

• IF Amplifier:   The original 6S7 (6.3v @ 150mA) was retained.
  

• 2nd Detector/AVC/1st Audio:  The original 6T7 (6.3V @ 150mA) was retained.
  

• AF Output:  The original 1F5 (2.0v @ 150mA) was replaced with a 6K6 (6.3v @ 400mA).  The latter is a pentode, requiring a bit of rewiring and rebiasing to replace the original triode.

• Magic Eye tube:   The original 6N5 (6.3v @ 150mA) was replaced with a 6E5 (6.3v @ 300ma) - which is also more sensitive than the 6N5, giving a bit more deflection.

The 6T7 (150mA), 6A8 (300mA) and the #47 dial lamp (6.3v @ 150mA) are wired in parallel on the low side with one end of the filament grounded while the 6K6 (400mA), 6S7 (150mA) and 6E5 (300mA) are wired in parallel on the high side with one end of the filament connected to +12 volts.  You might notice a current imbalance here (600mA on the low side with 850mA on the high side) but this is taken care of with the addition of 30 ohms of resistance between the midpoint of the filament string and ground to sink about 200mA, getting us "close enough".

He also did some additional rebiasing and other minor modifications - particularly for the rewiring of the AF Output from the original 1F5 to a 6K6 as he swapped a triode for a pentode - which was then  wired as a triode.  The total current consumption of the radio at 13 volts is 1.6 amps - a bit more than half of that being the filament and pilot lamp circuits meaning that about 10 watts of power is being used/converted by the vibrator supply and consumed by the idle current of the audio output and other tubes.

The "chopper" circuit

The other issue with the 6 to 12 volt conversion is that of the primary of the high voltage transformer:  This transformer is center-tapped with that connection going to the "hot" side of the battery (which was originally at -6 volts) - but what this really means is that there's about 12 volts from end-to-end on the transformer at any instant.  We can deal with this difference simply by driving the transformer differently:  Rather than having the center tap "hot" with the DC voltage and alternatively grounding one end or the other as the vibrator did we can simply disconnect the transformer's center tap altogether and alternately apply 12 volts to either end, reversing the connection electronically to preserve the original voltage ratio between primary and secondary.

This feat is done using an "H" bridge - an array of four transistors that will do just what we need when driven properly:  Apply 12 volts to one side and ground the other - or flip that around, reversing the polarity.

Consider the schematic below:

Figure 3:
Solid state equivalent of a vibrator supply.  This version uses an "H" bridge, suitable for
the conversion of a 6 volt radio to 12 volt operation as detailed in the text.
Click on the diagram for a larger version.

This diagram shows a fairly simple circuit.  For the oscillator we are using the venerable CD4011 quad CMOS NAND gate with the first two sections wired to produce a square wave with a frequency somewhere in the 90-150 Hz region - the precise value not being at all critical.  The other two sections (U1c and U1d) take the square wave and produce two versions, inverted from each other.

Figure 4:
The top (component side) of the circuit.  This is built on a
piece of phenolic prototype board.
Click on the image for a larger version.

The section of interest is the "H" bridge consisting of transistors Q1 through Q4 wired as two sets of complimentary-pair Darlington transistors.   Here's how it works:

• Let us say that the output of U1c is high.  This causes the output of U1d to be low as it's wired as a logic inverter.
• The output of U1c being high will cause the top transistor (Q1 - a PNP Darlington) to be turned OFF, but at the same time the bottom transistor of this pair, Q2, will be turned ON, causing the connection marked "PIN 1" to be grounded.
• At the output of U1d - being low - we see that the bottom of this pair of transistors, Q4, is turned OFF, but the top transistor Q3 is turned ON causing V+ (12 volts) to appear at the connection marked "PIN 5".
• In this way, the low-voltage primary of the transformer has 12 volts across it.
• A moment later - because of the oscillator - the output of U1c goes low:  This turns off Q2 and turns on Q1 - and since this also causes the output of U1d to go high this, in turn, turns off Q4 and turns on Q3.  All of this causes "PIN 5" to now be grounded and "PIN 1" to be connected to V+ - thus applying the full 12 volts to the transformer in reverse polarity.

Also shown are D1 and D2, the solid-state replacements for the synchronous rectifier of the original vibrator.  While this could be a pair of high-voltage diodes (>=400 volts) we simply used half of a full-wave bridge rectifier from a junked AC-powered switching supply.  Finally, resistor R3 and capacitor C2 form a filter to keep switching noise and high-voltage spikes out of the power supply of U1 to prevent its destruction - a sensible precaution!

Now some of you might be concerned about "shoot through" - the phenomenon when both the "upper" transistors (Q1, Q3) might be on - if only for an instant - at the same time as the "lower" transistors (Q2, Q4) as the switching is done.  While this may happen to a small extent, it has negligible effect - particularly at the low switching frequency where this effect would constitute a very minuscule percentage of the switching period:  This circuit is efficient enough that no heat sinking is required on transistors Q1-Q4 and they get only barely warm at all.  Were I to build it again I might consider ways to minimize shoot-through, but this would come at the expense of simplicity which, itself, is a virtue - and since this circuit works just fine, would probably be not worth the effort.

Figure 5:
The bottom (wired side) of the circuit with flying leads
connecting to the original base socket.
Click on the image for a larger version.

These days one might consider building this same type of circuit using MOSFETs instead of Darlington transistors (e.g. P-channel for Q1 and Q3, N-channel for Q2 and Q4) and this should work fine - but the Darlington transistors were on hand at the time that this circuit was built and very easily driven by U1 - and the bipolar transistors are - at least in this case - arguably more rugged than the MOSFETs would be - particularly since there was no need to include a "snubber" network to suppress switching transients that might occur.  It's also worth noting that while standard MOSFET transistors would work fine for a 12 volt supply, you'd have to be sure to select "low gate threshold" devices to work efficiently at 6 volts or lower - something that would not really be an issue with the bipolar Darling transistors shown here.

This circuit is simple enough that it was wired onto a piece of phenolic prototyping board, snapped down to a size that will nicely fit into the original can that housed the vibrator.  To complete the construction, the top of the can - which was originally removed by careful filing and prying - was glued into its base using "shoe goo" - a rubber adhesive - keeping the board protected, but also allowing it to be easily disassembled in the future should modification/repair be necessary.

To be sure, the Internet is lousy with this same sort of circuit, but this version has worked very well.

What about the center tap version of the solid state vibrator?

You might ask yourself "what if we don't want to rewire a 6 volt radio to 12 volts?"  As noted previously, the boost transformer in the radio had its center tap connected to the "hot" side - which, in this case, would have been the negative terminal (because many vehicles had 6 volt, positive grounds at the time).  This circuit could be easily modified for that as you'd need only "half" an "H" bridge and the resistors driving the transistors would be changed to a lower value - perhaps 2.2k.  Depending on whether the it was positive-ground or negative ground, or whether the center-tap was grounded or "hot" - this would dictate whether you needed the PNP or NPN halves of the H-bridge.

(If you have a specific need, feel free to contact me by leaving a comment.)

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This page stolen from ka7oei.blogspot.com

 

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