The Comcraft CTR-144 FM transceiver - A "multi-mode" solid state 2 Meter transceiver from the early 1970s.

Back in 1972 an ad appeared in the Ham magazines for something that probably seemed amazing at the time:  A solid state two-meter transceiver that covered the entire 2 meter amateur band (144-148 MHz - plus adjacent MARS and CAP frequencies) and did not need crystals!  This was the Comcraft CTR-144.  In advertisements from 1972 this radio cost $389.95 according to an advertisement in the January, 1972 issue of 73 magazine and as $489.95 in an ad in a 1973 73 Magazine advertisement (see Figure 6):  These prices are equivalent to over $3000 in 2026 dollars - for a two meter transceiver!

About Comcraft

Figure 1:
The Comcraft CTR-144 - a VFO solid-state 2 meter
FM/AM transceiver from the early 1970s.  A ham today
would probably be at loss when trying to use it!
The large knob is for tuning the receiver while the "VFO
Tuning" is for the transmitter frequency.  Aside from the
"tune" and "load" controls the rest are self-explanatory.
Click on the image for a larger version.

I haven't been able to find much about the Comcraft Company, other than they was in Goleta, California and that it seemed to exist from at at least 1971 until some time after 1976 when they released one of the earliest synthesized 2 meter transceivers, the CST-50.  I suspect that they may have made products for other markets, but I haven't been able find additional information.

More about the CTR-144

The CTR-144 is a "multi-mode" 2 meter transceiver in the sense that it can operate using both FM and AM - the latter being still present on the band in the early 1970s, but on its way out of common usage.  Compared to radios made since, this is an "odd duck" in the sense that it has separate tuning for the receiver and transmitter, both using VFOs - although it can accommodate three internal and one front-panel crystal for transmit for frequently-used repeaters and those applications that required such (e.g. MARS, CAP).  In other words, it doesn't have a conventional switch for a +/- 600 kHz repeater offset like that of modern radios, but rather you have to tune the transmit and receiver separately to the repeater's input and output frequencies, respectively.  Additionally, it has a built-in power supply which allows it to operate from 120 Volts AC as well as a 12 volt input and provisions for an internal battery pack for "portable" use.

Figure 2:
The power amplifier section of the CTR-144 - solid-state
from the early 70s.  The "tune" and "load" air-variable
capacitors are visible, as is the side-mounted antenna jack.
Click on the image for a larger version.

As with some other radios recently featured on this blog, I acquired this one from a silent key friend of mine - and even into the early 2020s, he'd occasionally use this transceiver on some of the local repeaters that don't require a subaudible tone.  It also has a very wide range "S-meter" which, unlike meters on most modern FM transceivers, would give a repeatable, reliable indication of signal strength over a range of just a microvolt or so to extremely strong. ¹  Being able to operate from a 12 volt vehicle and combined with the S-meter's wide dynamic range also  made it a decent receiver for direction-finding not only with a small directional antenna, but to know when one was getting close to the transmitter itself.

About the transmitter

The radio isn't particularly high-power:  The specifications state that it's finals are rated for 12 watts input power with about 6 watts out. ² Considering the efficiency of early 1970s VHF power transistors, the output power of this transceiver is still in the range of 4-6 watts across the band, depending on frequency and supply voltage - plenty of power for local and moderate-distance repeaters when used with even a modest outdoor antenna.

Figure 3:
Before the mid-late 70s there were no specific FCC rules
for the spectral purity of VHF transmitters, but current rules
still allow transmitters of that vintage to be used despite this.
Were it made today, the CTR-144 would not pass muster.
Click on the image for a larger version.

Out of curiosity I checked this transceiver's spectral purity on my service monitor (see Figure 3) and I wasn't terribly surprised at what I found:  While there were a few minor non-harmonic spurious emissions that were at least 40dB down from the carrier, the second harmonic was around -27 dBc - roughly 4 milliwatts.  While this would not be given a "pass" if the radio was made today (the current rules state that spurious/harmonic signals of a transmitter with 25 watts or less should be -40 dBc) there were no such rules prior to about 1977.  As noted in the current FCC rules, (§ 97.307(e)) a radio from this era is exempt from these rules but, if using a radio such as this causes interference, the onus is still on the amateur to resolve any issues.

If one looks at Figure 1 you'll see that there are two tuning knobs:  The larger one - with the slide rule that covers 144-148 MHz - is for the receiver, but the smaller one, labeled "VFO Tuning" is what sets the frequency of the transmitter and it is completely independent of the receive frequency.  What this means is that if you wish to use a repeater, the receiver is tuned to its output and the transmitter is set to its input frequency which, in the day, may have been a bit of a challenge:  The VFO Tuning knob doesn't have a great deal of tuning resolution on its dial so getting it "on frequency" back in the day probably meant setting the knob as close as possible visually and then getting "talked in" to the proper frequency.  When using this transceiver, my friend would have a frequency counter handy to allow him to dial in the transmit frequency precisely and monitor it to counter drift.

The transmit VFO itself operated from 7 to 9 MHz and this frequency was then mixed with a 65 MHz crystal to yield 72-74 MHz - and this was then doubled to the 144-148 MHz 2 Meter band.  With this scheme the transmit frequency could be reasonably stable as the most drift-prone component, the 6-9 MHz VFO, was operating at a low frequency.

A close inspection of  the front panel  in Figure 1 will reveal - along the right edge of the front panel - both a "Tune" and "Load" control.  For anyone using a radio with vacuum tube finals, controls labeled thusly would be familiar, but as this transceiver has a solid state amplifier these simply provide peaking/matching for final amplifier as indicated on the "S" meter rather than the "dip and load" of the older technology.

Figure 4:
The bottom of the CTR-144 showing the
rather hefty power transformer and, on top,
the doubler stage for the transmit chain.
The modulation transformer can be seen to
the left of the large, blue capacitor.
Click on the image for a larger version.

To produce frequency modulation it is the 7-9 MHz VFO itself that is modulated, but if a crystal is used, phase  ³ modulation is applied to its output prior to being multiplied by 18 to the desired transmit frequency.  For AM, high-level modulation is accomplished by modulating the +12 volt supply for both the final amplifier transistor and its driver via the windings of an autotransformer - a rather "old school" way of doing it, but one that eliminates the need for another, higher-voltage power supply to achieve the desired peak output power. ⁴

About the receiver

While schemes to up-convert a low-frequency VFO for transmitting had been used for a long time, the way this transceiver goes about receiving is a bit more interesting and complex.  While more modern analog receivers would generate a local oscillator near the receive frequency and convert the desired signal to one or more lower-frequency IF (Intermediate Frequency) stages where a single channel would get demodulated, this works more like the receive section of a transverter in that the entire 2 meter band is is down-converted to the range of 14-18 MHz using "high-side" injection (e.g. the local oscillator is above the receive frequency) at 130 MHz from the same 65 MHz crystal as is used for transmit.

Another VFO - the one tuned by the large knob with the receive frequency indicated by the slide rule - tunes across the 14-18 MHz IF to down-convert the received signal down to a fixed IF of 2 MHz where it is filtered and demodulated by a CA3075 integrated circuit.

Using the CTR-144 on the air today

Figure 5:
The front end and receive section of the CTR-144.
The three-section air-variable capacitor tunes the local
that down-converts from the 14-18 MHz first IF to the 2 MHz
second IF where an integrated circuit is used for audio
demodulation.  In the upper-left is the MOSFET front-end
and filtering that give this radio good sensitivity - even by
today's standards.
Click on the image for a larger version.

Using the CTR-144 today is admittedly awkward compared to a modern radio.  The receiver's slide-rule dial will get you "pretty close" to frequency, but unless the signal is actually present, you won't be able to tune it in for certain.  For simplex use, once you have tuned the receiver, there is a "Spot" switch that will inject a weak signal into the receiver that you can use to tune the transmit VFO to the same frequency by listening for and zero-beating it:  In this way the transmit and receive frequencies will be very close to each other.

Repeater operation is another matter. ⁵  Back in the 1970s you might have spent a few dollars to get transmit crystals for your favorite repeaters which would circumvent the problem of not knowing one's exact transmit frequency with the VFO.  But what if you didn't have a transmit crystal for the frequency?  Here, the "Spot" button is less-useful since, unless you happen to tune in someone's signal as they are transmitting to the repeater on its input frequency, there would normally be no signal at all to which you could precisely set its frequency.  When my friend used this radio in later years he had a frequency counter in his ham shack so that he could determine precisely the transmit frequency - a luxury that would have been unthinkable when this radio was first introduced. ⁶

Figure 6:
A Comcraft advertisement from
the April, 1973 issue of
73 Magazine
Click on it for a larger version.

Final comments

• When I first placed this transceiver on my workbench, it worked perfectly (although I haven't tried the built-in AC power supply) aside from two of the #47 dial lamps having burnt out.
• When the sensitivity was checked, it was only slightly deaf, but just a couple of minutes of time with the service monitor and tweaking the RF and IF stages brought it into specs.  It's interesting that while the manual (see link, below) has some "adjustment" information, it doesn't have a fully-detailed alignment procedure for the transceiver, likely because, as is true today, few people would have access to the necessary equipment to accomplish the task.  As it was the manual and schematic informed me as to how it worked so I was able to come up with a procedure "on the fly" to bring the radio back up to snuff.
  
•  When 2 Meter FM first appeared, it had not yet standardized on what we now call "Wide" FM (e.g. +/-5 kHz deviation).  Different channel spacings and modulation widths could be found in different parts of the country, including +/- 15 kHz deviation - particularly on UHF.  The manual for the CTR-144 specifies a deviation of +/-7.5 kHz - yet another "standard"!
• The specified receiver bandwidth is 22 kHz which is somewhat wider than the typical 15-17 kHz for a "wide" (+/-5 kHz) FM receiver.  With this bandwidth - plus the probability of the receiver's frequency not being centered - an adjacent-channel repeater (15 or 20 kHz away, depending on the spacing in your area) is likely to result in received interference.

Comcraft CTR-144 manual

I was fortunate enough to find the manual for this transceiver in my friend's files which has been scanned and may be found at this link:  Comcraft CTR-144 Manual (35 MB, .PDF).  At the time that this blog was posted, this manual was not to be found anywhere else on the Internet.

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Footnotes:

1. On most FM transceivers, the S-meter will start indicating on very weak (and possibly noisy) signals and hit the "peg" (e.g. full-scale) even on a signal that is only modestly strong.  What this means is that for direction-finding purposes, most modern FM transceivers will give a "pegged" S-meter indicating even if you are some distance from the transmitter.  The CTR-144 on the other hand reached full scale only if you happened to be right next to the transmitter while at the same time started indicating on a signal that was almost too weak to copy meaning that it could be used both at a great distance or in the final search for an unknown transmitter. 
2. Until the mid-late 1970s, amateur radio power limits were specified as input power to the final amplifier stage.  This method of transmitter power was likely an artifact of the days before RF power meters were common:  It's trivial to calculate the input power to the final amplifier if one knows the current and voltage, but that isn't the case for trying to measure RF power.
3. "Phase" modulation doesn't change the frequency directly but rather it "wiggles" the phase of the signal in accordance to the applied audio whereas actual "Frequency" modulation does change the frequency of the signal - in this case, by electrically shifting the VFO frequency:  Mathematically, the two are related since, if you could continually change the phase, frequency modulation would be the result.  With "FM" as used in the amateur service, one need only be able to modulate down to 200 Hz or so for voice meaning that with phase modulation, you could "temporarily" change the phase enough to effect a de-facto frequency shift.  As the amount of change from a single phase modulator stage is quite small (perhaps +/- 60 degrees) this is usually applied at a lower frequency - in this case, at the 8 MHz crystal frequency and since this is multiplied by a factor of eighteen, our original +/-60 degrees of phase change becomes (18 * 60) 1080 degrees which is enough to appear as an adequately-modulated "FM" signal.  Mathematically, if one boosts the "highs" of the audio at a rate of 6dB per octave (e.g. doubling the deviation with every doubling of frequency) a signal generated using "FM" is indistinguishable from one that was originally phase modulated.  In fact, it is "PM" (Phase Modulation) that we use on "FM" as amateurs:  The "highs" are boosted (e.g. "emphasized") on transmit as they are the first to get noisy with a weak signal, and on receive they are restored back to their original level (e.g. "de-emphasized") which effectively reduces the amount of noise that is heard and makes weak, noisy signals more "listenable".
   
4. "High-level" modulation typically refers to applying the audio to the output RF stages(s) of a transmitter by increasing/decreasing the supply voltage.  Traditionally, this was done with an audio amplifier driving modulation transformer:  On peaks with 100% modulation the voltage applied to the final amplifier is approximately doubled, yielding the  four times the peak envelope power - but it also allowed the voltage to go to near zero at the other extreme.  On this transceiver, an audio amplifier drives an auto transformer (e.g. one with a single, tapped winding).  With no modulation, the 12 volt power supply for the finals goes through it unchanged, but with audio, the transformer action allows the requisite 24-volt(ish) peaks to be applied to the amplifier stage - all without needing the complexity of a 24 volt power source.  This method was "old" even when this radio was made, but it is simple and effective.
5. When this transceiver was made it was fairly rare for an amateur repeater to use a subaudible tone as doing so would mean that an amateur using it would not only have to have the proper crystals (for most radios of the day) but also a means to produce the needed subaudible tone frequency,  Later, in some areas subaudible tones were introduced when a geographically-adjacent repeater was on the same frequency to prevent users in an overlap area from bringing up both repeaters.  In some cases in amateur radio service, subaudible tones are used owing to occasional interference issues:  This is often considered to be a "band-aid" solution to a problem that is often due to the repeater's receive signal path not being able to adequately reject off-frequency signals (e.g. overload) - but sometimes it might be needed on a "busy" site where other users' gear isn't as well-filtered as it should be.
6. Back in the early 1970s a frequency counter of any sort would be a somewhat rare and expensive piece of test equipment - and this would be particularly true for one that could operate at VHF!  By the late 1970s and early 1980s technology had progressed to the point where such devices were more affordable and available to the average amateur. 

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This page stolen from ka7oei.blogspot.com 

 

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