Mosley CM-1 Receiver


Mosley CM-1 Receiver

Selling for about $175, the CM-1 faced formidable competition from its better known National (NC-270) and Hallicrafters (SX-140) peers. Performance-wise, however, the CM-1 outgunned every radio in its price class. Undoubtedly, the homely styling of the radio contributed to its poor sales.

In 1961, following two-years of development, the Mosley Electronics Corporation introduced to the ham community its first radio, the CM-1 amateur band receiver.

The company's high expectations for its new product were soon dashed, however, and after a year of disappointing sales, the company abandoned the effort and never again strayed from its main business of making antennas.

However, the few hams who took a chance and purchased a CM-1 were rewarded for their courage with a very fine radio. In an era of me-too designs, when competing offerings differed mostly by their front panel layouts, the CM-1's circuit was a breed apart.

Description: The CM-1 covers the 80m-10m amateur bands plus WWV in seven 650 kHz segments, with a 12" dial scale. The receiver features an S-meter, noise limiter, variable BFO, and both AM and product detectors. Selectivity is rated at 2.5 kHz (-6db), stability at better than 500 Hz (after a brief warmup), and sensitivity at better than 0.5 microvolt for 10 db signal-to-noise ratio.

In-and-of-themselves, these features are not particularly unusual. However,the unique qualities of the CM-1 really become evident when one looks under the hood (below, right).

Surprisingly, the CM-1 packs all of its features into only a 5-tube design. Even more surprisingly, all five tubes, from the mixers, to the VFO, to the 5 watt audio stage, are 6AW8As!

And the CM-1 is as cleverly engineered as it is eccentric. In fact, one has to admire the engineers for the sheer economy of their design; there isn't a superfluous lockwasher or resistor to be found in the radio. In fact, the design seems so mature that it is easy to believe the company's claims that an enormous effort went into its conception.

Interior View of the CM-1

The CM-1 is built on a plated steel chassis, with a large central VFO enclosure. Considerable design effort was expended in minimizing the radio's parts count. Even the panel light (left, above) does double duty, illuminating both the dial scale and the S-meter. Thanks to its 5-tube design, the radio draws only 35 watts.

Mosley CM-1 Block Diagram

CM-1 Underchassis View

The underchassis of the CM-1 is uncluttered and the point-to-point wiring is competent, but not elegant. Five crystals mounted on a small circuit board (below) are used to mix the higher bands down to 80 meters.

Concertor Crystal Board

Circuit Features: As can be seen from the above block diagram, the CM-1 is basically a 4-tube 80 meter single conversion receiver, preceded by a single tube, crystal-controlled converter for the higher frequencies. Selectivity is provided by four 455 kHz IF transformers. There are two stages of preselectivity in front of the crystal converter stage.

Note that the CM-1 has no RF amplifier, which was a pronounced departure from the practice of the day. Instead, all of the system gain is provided by the triode mixers and IF amplifiers.

The five 6AW8As are dual purpose tubes, having a triode section and a pentode section. Although the use of identical tubes parallels the practice used for some military receivers, where parts availability was a design consideration, one cannot help but wonder why the CM-1's designers felt obliged to follow the same constraints? (Did Mosley find a boxcar somewhere full of 6AW8As?)

Operating Impressions: The CM-1 is a surprisingly good performer. With its clunky styling, it looks almost like an inexpensive hobbiest kit, but when powered up it becomes immediately evident that it is a serious receiver. In fact, I'd say the CM-1 has much the same feel as the more costly Drake 2B, but with smoother tuning (thanks to anti-backlash gearing),

The CM-1's audio fidelity is excellent, and the frequency stability is outststanding. After a slight warmup, the radio will stay put on an SSB net for hours. Banging on the cabinet has no effect on the stability.

CM-1 Dial Scale

The small print and multiple scales make the frequency dial of the CM-1 hard to read. This kind of ergonomic shortcoming detracts from the overall quality of the radio's design.

The radio is not without shortcomings, however. At night, one hears strong 80 meter signals break through on the higher frequency bands. This is the price paid for the economy of the conversion scheme; the radio is, after all, an 80 meter receiver with a crystal converter. And the company's claims of single-signal SSB reception are a bit of a stretch. The selectivity is only fair -- about what one would expect for 455 kHz IF transformers. All in all, however, this little radio is a sleeper. Don't miss the chance to check one out if the opportunity arises. It's not a match for your Collins 75A-4, but it delivers an amazing amount of bang for the buck. And that, after all, is what good engineering is all about.

Visitors since Jan. 6, 2001