Dynaco FMX-3
A $29.95 kit that turned a mono tuner into a stereo one — and kept a whole generation of Dynaco owners from buying a new tuner.
Overview
The FMX-3 is not a tuner. That distinction matters more than anything else about this little module, because nearly every description of it gets muddled on that point. The FMX-3 is a stereomatic multiplex integrator — the decoder board that takes the multiplexed stereo signal a mono FM tuner already receives and splits it back into two channels. Dynaco sold it as a kit for $29.95 and as a semi-kit for $39.95, and the whole reason it exists is that Dynaco built the mono FM-1 with a socket inside waiting for it. According to the company's own advertising in the period press, the $79.95 mono FM-1 "can be converted to stereo at any time by adding the $29.95 FMX-3 multiplex module."
That sentence is the entire product in miniature. Dynaco's pitch, repeated across full-page ads in High Fidelity, Audio, and HiFI-Stereo through 1962 and 1969, was that the company introduced new products only when they filled a real need and never rendered previous models obsolete. The FMX-3 was the proof of concept: a component that made an existing tuner better instead of making its owner feel foolish. High Fidelity (September 1962, p.98) carried the module in its own right in the Dynaco lineup, listing the "FMX-3 stereomatic multiplex integrator kit — $29.95; semikit — $39.95," alongside the wired FM-3/A tuner at $169.95 that had the multiplex built in.
The performance claim attached to the module is worth repeating carefully. Dynaco's published figure was better than 30 dB of separation on stereo reception using the FMX-3, paired with automatic transition to stereo and a visual indicator the company called the Stereocator. Thirty decibels of separation is a modest number by later standards, but it was the honest one for a tube-era multiplex decoder, and Dynaco made a point of advertising honest numbers — the same ad copy openly conceded that its 4-microvolt IHFM sensitivity spec "appears somewhat archaic when practically all competing tuners imply greater sensitivity in their advertising."
What the FMX-3 is not is a plug-and-play accessory for any tuner. It was designed to sit inside the FM-1, and Dynaco's literature is explicit that the FM-1 included "provision for internal insertion of the FMX-3." The semi-kit at $39.95 existed for buyers who wanted the hard part done; the full kit at $29.95 assumed you could solder and align.
The FMX-3's place in the Dynaco story is as a bridge. The FM-3 stereo tuner, introduced in 1963, had the multiplex built in and sold for $99.95 as a kit or $154.95 factory assembled. The FMX-3 was the path for the owner who already had an FM-1 and wasn't ready to replace it. Dynaco's 1969 advertising made the longevity argument bluntly — the FM-3 was still, after six years, described as the most popular of all stereo tuners, and the mono FM-1 after nine years still outsold all other mono tuners combined. The FMX-3 is the reason a nine-year-old mono tuner could stay in the rack instead of going to the closet.
For the collector, the FMX-3 is a curiosity with a real function. It's small, it's cheap when you find one, and it only makes sense if you own an FM-1 — which is exactly the point. This is a module that exists to serve another piece of gear, and judging it on its own is missing the design intent. What it demonstrates is a philosophy: build the tuner so it can be upgraded, sell the upgrade for less than a third of the price of a new tuner, and let the customer keep listening. Whether that was generosity or the smartest kind of customer retention is a question the ads never quite answer, and probably didn't need to.
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