Eico MX-99 (1961–1962, possibly 1963)

A little metal box that let mono FM tuners hear in stereo two years before stereo FM was even legal.

Overview

The MX-99 is a multiplex adaptor - the box you bolted to the back of your mono FM tuner so it could pull two channels out of a broadcast that, officially, wasn't supposed to have two channels yet. Eico built it in the United States and sold it two ways: as a kit for $39.95, or wired for $64.95. For a hobbyist in 1961, that was the price of a decent turntable cartridge. It bought entry into an experiment that wouldn't become a legal broadcast standard until the FCC adopted one.

The interesting part of the MX-99 is the timing, not the sound. Eico's own advertising for the unit ran from late 1961 into early 1962, and the copy leaned hard on the fact that multiplex stereo was arriving, not that it had arrived. The front panel script reads "Eico MX99. For FCC approved system," which is a odd thing to silkscreen onto a product that shipped before the FCC had approved anything. Eico was betting that the Zenith-GE multiplex system would win, and built the hardware to decode it in advance.

It is not a receiver, not an amplifier, and not a tuner. It is a decoder that sits between a tuner with a multiplex output and your preamp. Without a compatible tuner, the MX-99 is a very nicely finished paperweight.

Key Features

Zero Phase Shift, Filterless Detection

Eico's own literature describes the MX-99 as employing "an original method of zero phase shift, filterless detection of multiplexed signals," and the company filed a patent on it. The claim is that the unit delivers stereo from a stereo broadcast and mono from a mono broadcast automatically, without spurious signals either way. Filterless matters because the subcarrier and pilot tones sitting in an FM multiplex signal can beat against a tape recorder's bias oscillator and produce audible garbage on a recording. That was a real problem for anyone taping off the air in 1962. Eico avoided the phase-distorting filters entirely rather than try to filter around them.

Designed for the Zenith-GE Multiplex System

The MX-99 was built for the Zenith-GE multiplex system, the same scheme that became the basis for the FCC standard. Eico guessed right, and a decoder built for the experimental system turned out to be broadly compatible with what broadcasters eventually adopted.

Tuner Compatibility

Eico specified the MX-99 for its own FM equipment - the HFT90, HFT92, and ST96 tuners - and also for "any other component quality, wide band FM tuner provided with a multiplex output." That last clause is the important one. The MX-99 will decode for a tuner that isn't an Eico, provided the tuner has a multiplex output to feed it. Collectors note the MX-99 turning up paired with McIntosh, Fisher, and Scott tuners, which is exactly the use Eico was advertising.

RCA Outputs and Composite Input

Signal in is a composite FM signal via the 19 kHz subcarrier. Signal out is left and right audio on RCA phono jacks. Rated frequency response is 30 Hz to 15 kHz, a figure the fact sheet flags as typical for the era rather than a measured result, so treat it as nominal. The chassis measures approximately 7" × 5" × 2.5" in a metal enclosure with an aluminum front panel. It is a small component by the standards of 1961 hi-fi, and it slots into a rack without demanding a shelf of its own.

Tube Complement

Here the documentation gets messy. A 1964 Eico catalog lists the MX-99 as using 2× 12AU7, 1× 12AT7, 1× 6AU6, 1× 6D10, 1× 6X4, and six diodes. An owner's post lists 6X4, two 12AU7, 6AU6, and 12AT7, missing the 6D10 and the diode count. The catalog is the more authoritative document, so the fuller complement is the one to trust. What's not in dispute is that the MX-99 is a tube unit. That contradicts a separate claim in the source material that it was a passive, unpowered design with "no tubes or transistors to worry about." The passive description does not survive contact with the Eico catalog. Buyers should assume tubes.

Historical Context

Eico's advertising for the MX-99 ran in Audio (November 1961, p.15), the same months the FCC was finalizing the stereo FM standard. Production is generally placed at 1961-62, with one source extending it to 1963. That's a short run, and it suggests the MX-99 was a transitional piece: once stereo FM tuners with built-in decoders arrived, a standalone adaptor had no reason to exist.

Before the standard took hold, stereo FM was an experiment run by a handful of stations, and the MX-99 was built for listeners who wanted in on it. It required a tuner with a multiplex output, which in the early 1960s meant high-end or modified gear. The MX-99 was never a mass-market accessory.

Collectibility & Value

Complete MX-99 units, particularly with original packaging and manuals, reportedly trade in the $200 - $400 range through auction and specialty vintage audio dealers. Kit versions with unused components may bring more, since an unbuilt kit is a different category of object entirely. A restored example has been listed in the European market at €380. An eBay listing for a unit described as a stereo tube MPX multiplex decoder sat at $99.95. Owner and service manual photocopies circulate for around $24.99, and Photofact coverage exists. A decoder with no documentation is a hard restoration.

Restoration is generally considered straightforward for anyone with basic soldering skills, and replacement parts are readily available. The usual suspects apply. Coupling capacitors degrade over time and cause signal loss or channel imbalance; modern film capacitors bring performance back. The carbon composition resistors in the matrix network drift in value, which shows up as degraded stereo balance and separation. Testing and swapping in 1% tolerance metal film resistors is the standard fix. The RCA jacks and internal wiring oxidize and loosen, and contact cleaner plus a re-soldered joint resolves most dropouts. One owner has demonstrated filter capacitor sections on a working unit holding up well, which is the exception rather than the rule at this age.

One caveat has nothing to do with condition: because the MX-99 predates the finalized standard, it may not decode modern stereo broadcasts correctly. The unit's audio quality also depends heavily on the tuner feeding it and the preamp downstream. The MX-99 adds no amplification of its own to flatter a weak source.

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