Motorola CM 540
A very differnet approach to the new 40 channel CB. Photos will be added in depth.
The CM-540 is a unique radio in many ways. Built for Motorola around 1979 by a company in Japan, the CM-540 was one of the CB radios by Motorola to compete in the new 40 channel CB market.
The radio was unable to compete with the Cobra radios made in Hong Kong, and then in Taiwan. Motorola CB radios were short lived, the cost of the design and the fact that Japan was getting out of the CB market ended Motorola’s venture into CB. They were built and felt like a commercial radio with some features not found on any other radio.
The first thing I made a note of was the Microphone design. Although; not found in all of Motorola’s radios, the CM-540 uses a power microphone. The voltage for the power microphone is supplied by a 13.8 volt source from the radio. The microphone also uses a channel selector on the Microphone. The drawback for this design is the Microphone cord. There are plenty of wires in the cord and a special microphone plug is used.
The radio has the ability to change the channels on the radio and has a scan feature. At a quick glance the radio looks like a Ham 2 meter. It is heavy at 5 lbs; it is 2.6” high x 7” wide x 9.5” deep.
The most unique feature about this radio is it has a single conversion receiver of 455 KHz. This is where the design gets complicated. The input frequency is only 455 KHz away from the mixer frequency. In order to eliminate the three other frequencies created by the mixer, a tight filtering design is needed to get a direct Intermediate Frequency of 455 KHz. Most single conversion receivers use 7.8 MHz. This makes the elimination of the unwanted frequencies easier to remove with less expensive filters.
Dual conversion receivers usually will use 7.8 MHz or 10.695 MHz as the first IF, and then mix it down to a second IF of 455 KHz. The CM-540 goes straight to a single conversion IF of 455 KHz. There is a feature on the radio that I have not seen on another CB; an IF gain. It also has the standard RF gain. Between these two features the back ground noise can be eliminated. The noise blanker is also variable.
Another unique adaptation is the elimination of a separate first RF transistor. The input signal is fed straight to the mixer. The mixer is a dual gated J-FET designated Q101 and is a Q3A10. Motorola uses its own nomenclature for numbering common components. This can make finding suitable replacement parts hard to locate and cross reference.
The audio circuit has a built in audio compressor. The transmitter is simple and ahead of its time. The transmit frequency is generated by the PLL circuit and is mixed with a constant crystal controlled 29.485 MHz frequency.
Overall the radio is hard to work on and the schematic by Sams Photo Facts is the best schematic for the CM-540 in print, but it is one of the last of the CB series service manuals published by Sams, and it shows. The Sams Photo Fact number is # 293 pages 27 through 44. You will not find many of these radios as scrap. If you find one that is working, then consider yourself extremely lucky. Overall the radio works very well.
Radios for pictures provided by courtesy of Big Al
Sams photo facts # CB-293 for a service manual.
More information to come.
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