Fisher MPX-100

Fisher's universal multiplex adaptor promised that stereo FM would be a pleasure, not a problem. The Stereo Beacon meant you never had to guess.

Overview

The FCC approved FM multiplex stereo broadcasting in 1961. Every hi-fi owner then had the same problem. Their existing tuner could pull in the mono sum signal, but it couldn't decode the L-R sidebands up at 38 kc. The Fisher MPX-100 was Long Island City's answer. Fisher billed it as a universal stereophonic multiplex adaptor, built to work with any tuner or receiver that used either a ratio detector or a discriminator to demodulate the FM signal - not just Fisher's own gear. That universality was the whole pitch. The manual is confident about it, too: the company claimed that years of FM tuner design had made the MPX-100 "the best performing and most convenient multiplex adaptor in existence."

The headline feature was the Fisher Stereo Beacon. A front-panel lamp lit automatically when you tuned to a multiplex broadcast and went dark when the station went back to mono. No switching, and no squinting at a signal-strength meter to work out whether you had stereo or just a strong mono signal. Fisher's period advertising leaned hard on this point, contrasting it with "other brands" where the owner had to figure it out. The automatic switching system went a step further. It sensed whether the incoming program was stereo or mono and put the adaptor into the correct mode by itself, so nothing needed touching on the adaptor when a station changed format mid-broadcast.

A single front-panel switch gave you two separate noise filters. One worked on the subcarrier alone and kept full frequency response. The other applied a steep roll-off above 8 kc, for multiplex signals under difficult reception conditions. Feedthrough connections were provided for every tuner type, and they worked even with the MPX-100 switched off. That's handy, since the adaptor doesn't have to be powered up just to pass your tuner's signal through to the amplifier.

The measured performance was respectable for the era. Frequency response covered the full transmitted signal from 20 to 15,000 cps within 2 db. Stereo separation at 1 kc was specified as better than 35 db. Harmonic distortion at 2 volts output and 1 kc was less than 0.5%, and hum and noise below 2 volts output measured better than 70 db down. Sensitivity required a minimum 19 kc input of 15 millivolts, with maximum allowable input of 2 volts on the low-level jack and 8 volts on the high-level jack. Gain was 16 db, input impedance a very high 15 megohms, and output impedance 3000 ohms. Power consumption was 25 watts from 105-120 volts at 50/60 cycles.

The circuit was serious engineering for a box this size. A 19,000 cps Hartley oscillator synchronized the subcarrier, with a double-tuned plate circuit filtering out the 38,000 cps carrier needed to detect the L-R sidebands. A band-pass filter fed only the L-R sidebands to the product-detector tube. A sharp series-tuned filter at the detector output stripped the 38 kc carrier, then passed the demodulated L-R audio into one arm of a resistive matrixing network. The other arm received an R-L signal of equal amplitude through a wide-band phase inverter. A 75 µs de-emphasis network across both sections of the balanced matrix equalized response and cleaned up any carrier residue. The manual is candid about why tolerances matter: just 3 degrees of phase difference and ±0.3 db of amplitude variation between the main channel and the subcarrier envelope will degrade separation to no better than 29.7 db, the figure the FCC itself asked stations to maintain. Close-tolerance parts and careful alignment were needed to keep separation up.

Installation was where the "universal" claim had to hold up, and Fisher supplied a detailed guide. For ratio detector tuners, the MPX output went to the low-level input, with the rear-panel Separation control set at 3 for unbalanced detectors and 4 for balanced ones. Fisher tuners and receivers each got their own recipe. The 600 and 800 series, the 500-S, the 100-T, the 202-T, the 101-R, and the rest all had specific cabling and switching instructions. Even older tuners without a multiplex output jack - the FM-80, 50-R, 70-RT, and early 80-R and 80-T units - could be adapted. You added a standard RETMA phono jack near the 6AL5 tube socket, wired through a .01 mfd, 250V capacitor to the appropriate pin, then connected it via the supplied 3-foot cable to the high-level jack.

Only three controls were needed in total: the Separation control and the two Output Level controls, all on the rear panel. The manual says this was possible only through careful design and precision parts. An adaptor that had to work with a huge range of tuners would normally demand far more fiddling. Once set during installation, the rear controls could be left alone.

The MPX-100 was priced at $89.50. Fisher's own advertising was frank about the number. The company had originally expected to charge $129.50, but the volume of Fisher tuners and receivers already in the field made large-quantity production possible and the lower price a reality. The same copy carried an unconditional guarantee that the adaptor would outperform any competing unit regardless of price. Fisher also offered the 40-U Custom Cabinet in oiled walnut specifically for the MPX-100. The front panel dimensions - a 4½ inch by 4-7/16 inch cutout, with panel height and overhang matching the current Fisher receiver and tuner line - meant it could drop into a custom installation alongside the rest of a Fisher stack.

Period coverage treated the MPX-100 as the natural companion to Fisher's tuner line. High Fidelity (June 1961, p.23) announced the adaptor alongside the FCC's multiplex approval. High Fidelity (September 1961, p.19) ran the "Multiplex Is A PLEASURE, Not A Problem" campaign that made the Stereo Beacon the centerpiece of the pitch. HiFi-Stereo (August 1961, p.7) put it under a "Best for ALL Radio Reception" banner covering stereo, FM, multiplex stereo, FM-AM, and monophonic FM and AM. The category was brand new, and the MPX-100 arrived within months of the FCC decision.

One caveat comes straight from the manual: the stereo subcarrier is inherently noisier than the main carrier. Fisher said so plainly, and advised owners chasing weak or distant multiplex stations to consider a higher-gain antenna or to relocate the existing one. This was not a defect of the MPX-100. It was a fact of the transmission scheme. The two noise filters existed because Fisher knew the subcarrier would be the weak link.

For collectors today, the MPX-100 is one of the more interesting Fisher accessories. It bridges the mono tuners people already owned and the stereo broadcasts that were just beginning. It's a small chassis, it needs a tuner to be useful, and its value is tied to the Fisher tuner or receiver it was bought to pair with. The Stereo Beacon is the detail that dates it most precisely to 1961 and the first months of multiplex.