The Fisher FM-100 is a 1959 model FM tuner using ten tubes and including a provision for an add-on multiplex decoder. Without a multiplex decoder, the unit functions as a monaural FM tuner. As with all Fisher equipment of the time, the FM-100 is a high quality tuner and was priced accordingly when new. Avery Fisher was not shy about mentioning names of prominent people who owned Fisher equipment in his brochures – movie and TV stars, recording artists, politicians, and other prominent names in society.

I had purchased this tuner a few years ago and restored it to working order at that time. It was easy to recap as it only uses five brown “dog turd” paper capacitors plus the AC line death cap, a three-section electrolytic capacitor and a couple individual electrolytics. I was impressed with its performance as soon as I finished restoring it. It has an RF amplifier plus four stages of IF amplification including one limiter, so it is capable of excellent performance – and indeed provides it. The variable muting control comes in handy if you want to hear silence when tuning between stations.

Being an early stereo-capable tuner, though, it includes some features which were rendered obsolete once the FCC settled on an FM stereo standard in April 1961. Fisher designed this tuner for use with the Crosby multiplex system, as Fisher was betting that the Crosby system would become the standard. It did not. The FCC picked what was then known as the GE-Zenith multiplex system instead. This is the standard which continues in use today for FM Stereo.

One of the features Fisher included, but is now useless, is a Channel Separation control. Apparently, Fisher intended the user to be able to adjust channel separation at will with the Crosby multiplex decoder which was planned for this tuner. In GE-Zenith type multiplex decoders, the channel separation adjustment is made once and then left alone.

The early Fisher stereo-capable tuners can be used with multiplex decoders designed for the GE-Zenith standard – but compromises must be made if you wish to incorporate this into your early Fisher tuner.

In this post, I am going to describe to you how I used a solid-state multiplex decoder with my FM-100 tuner to make it truly “Stereophonic.”

Top view of the Fisher FM-100 tuner.

Let us look at the top of my FM-100’s chassis. Notice at lower right in the photo above that there is a removable metal plate. I will follow the instructions printed on this plate – TO INSTALL MULTIPLEX ADAPTOR REMOVE THIS PLATE AND DUMMY PLUG. The “dummy plug” refers to the nine-pin plug which is plugged into the nine-pin socket labeled MPX CONNECTOR SEE MANUAL. As I do not have an extra 9-pin plug, I will have to reuse this “dummy” plug with my new solid-state multiplex decoder. More on that later.

Close-up of the removable metal plate, where the new multiplex decoder will be mounted.

Remove two screws and voila – a nice opening for a multiplex decoder.

I’ll need to make some changes to the FM-100’s circuitry to ensure a multiplex decoder will work properly and take full advantage of the benefits of FM Stereo. The first thing that will need to be done is to replace the original Fisher ZZ592-170 ratio detector transformer (Z5) with a Fisher Wide-Band ZZ50210-9 ratio detector transformer. The reason this needs to be done is because the original ratio detector transformer can only pass a bandwidth of 150 kHz. This is barely adequate for FM Stereo reception when using a solid-state multiplex decoder as I am going to do here, and really inadequate when using one of the Fisher tube-type multiplex decoders. The Wide-Band ratio detector transformer provides a bandwidth of around 260 kHz, which results in much better FM Stereo performance. These transformers may be obtained from Fisher tube-type parts chassis made after late 1961.

Please note that the diodes in the Fisher Wide-Band ratio detector transformers are connected in reverse of the older, narrow band ZZ592-170 ratio detector transformers. This requires some additional changes to the tuner’s circuitry. Resistors R62 and R65 must be switched (I suggest you just replace them with new resistors), and electrolytic capacitor C66 must be connected in reverse compared to original. See below for needed changes.

Changes in the ratio detector circuit in the FM-100.

I had previously made these changes when I restored my FM-100, so it was ready as I had initially planned to install a modified Fisher MPX-65 tube type multiplex decoder. Unfortunately, none of the MPX-65 chassis I had on hand would work properly.

Some other changes need to be made in the function switch and right channel audio output circuitry. First, we need to remove one ground connection from S1, Section 1, Front. This change will ensure we can switch the tuner between FM Mono and FM Stereo, and that the tuner will only go into mono mode when the function switch is in the FM position. The Main Channel and MPX Channel positions will not have any effect since these positions were designed for use with the Crosby FM Stereo system. When we are finished, placing the function switch in the Main Channel or MPX Channel positions will have the same effect as having the function switch in the MPX Stereo position.

Also, so that we have audio output to the right channel outputs (Channel B) at all times – whether the tuner is set to mono or stereo – we need to make changes to S2, Section 2, Rear so that the right channel audio is connected direct to the Channel B audio output jacks. There is a blue wire which runs from the junction of R29, R33 & C44 and terminal 10 of S2, Section 2, Rear. There is another blue wire which runs from the Channel B audio output jacks to terminal 8 of S2, Section 2, Rear. You can either run a new blue wire from the junction of R29, R33 & C44, or you can just remove the blue wire ends from the switch and splice them together. I chose the latter. See below for these changes.

Audio and switching circuitry which needs to be modified in the FM-100.

I should mention the EXT jacks. These were designed for use with a separate FM or AM tuner, and will no longer be used.

The new solid-state decoder I will be using can be found on the FM MPX website. Be aware, however, that these boards are no longer being sold. It uses an LM4500 integrated circuit which is no longer available, and the designer ran out of LM4500 chips which is why he stopped selling these boards. In fact, I purchased two of the last boards he had, with a project like this in mind.

I finally got around to building one of the decoder boards by purchasing the needed parts to install on the board from Mouser Electronics. The suggestion on the FM MPX site of using a voltage doubler to run the decoder from the 6.3 volt filament line is a good one, since the FM-100 includes a dedicated 6.3 volt filament line just for the intended add-on Crosby multiplex decoder. I built the voltage doubler on a small, separate perfboard. You can easily find voltage doubler circuits online. The designer of the FM MPX board recommends one shown at upper left of the image here, which is the one that I used. It uses two diodes and two 1000 uF electrolytics.

Both the voltage doubler and MPX decoder board were mounted on a piece of aluminum, intended for use as the bottom of a project box. These are available from Mouser Electronics and are very close in size to the blank metal plate which was originally installed on the FM-100 chassis.

MPX decoder and voltage doubler boards mounted on the aluminum plate, bottom view. The LM4500 IC still needs to be installed here.

As mentioned earlier, I did not have an extra nine-pin plug, so I used the tuner’s original dummy plug for the new multiplex decoder. First, I removed the jumper wire between pins 3 and 9, and then proceeded to hook up the needed wires as shown below.

Hookup for the nine-pin plug for use with the new multiplex decoder.

Notice how the nine-pin connector provides connections for the 6.3 volt filament line which will be fed to the voltage doubler circuit, which in turn provides power to the multiplex decoder board. It also connects left and right channel audio into the tuner. Pin 1 of the connector connects to the GND for MONO point on the MPX decoder board, which is the mono-stereo switch for the board. S1, Section 1, Front (as modified above) will now connect the GND for MONO point to ground when the function switch is in the FM position, making the tuner go into mono mode. But in any other position of the function switch, the GND for MONO point is ungrounded, allowing FM Stereo reception.

All of the changes made to this FM-100 tuner are reversible if it is ever desired to restore the unit back to its original design.

As outlined on the FM MPX website, initial setup of the decoder is simple. You will need to have an LED connected to the board, even temporarily if you plan to connect the LED to the front panel as I intended to do. For testing and adjustment purposes, I merely placed an LED in the proper holes, with the cathode in the + hole, without soldering. The solder holes are plated through which makes this easy.

The VCO Adjust potentiometer (P1 on the board) will have to be adjusted before the decoder will work properly. Fortunately, the adjustment can be made without test equipment. Tune in a strong FM station broadcasting in stereo. Turn the adjustment screw of P1 fully counterclockwise. Now slowly turn the adjustment screw of P1 clockwise until the LED lights up. Note the position of the screw. Continue turning the P1 adjustment screw clockwise until the LED goes out. Note the new position of the screw. Now slowly turn the adjustment screw counterclockwise until you reach a point halfway between where the LED first lit up and where it went back out. P1 is now properly adjusted. Full instructions for adjustments may be found here. I did not perform the stereo separation adjustment (P2) as the separation sounded good to me as it was, with P2 set at its halfway point.

An LED is temporarily placed in the MPX decoder board. Note that it is illuminated. The board is working!

I soldered a long pair of wires to the LED terminals, as I intended to make a permanent installation of an LED on the front panel of the tuner where it could be easily seen. Once I was finished with the 19 kHz adjustment (P1), I removed the temporary LED and installed the entire unit on the FM-100’s chassis.

Here’s where things get interesting. I ran the wire pair inside the chassis until it reached under the tuning eye tube (EM84/6FG6), just to the left of the tuning shaft. At this point I drilled a hole in the chassis, added a rubber grommet, and ran the wire pair up to the top of the chassis. I removed the tuning eye tube and also removed the plastic bezel which fits into the front panel from the rear. Using another tuning eye bezel salvaged from a junked Fisher console tuner unit, I carefully drilled a hole just large enough for a red 2mm LED bulb to fit into. I did not want to use a large LED, nor did I want to drill a hole into the original bezel. I put the original bezel away for safe keeping and glued the LED into the salvaged bezel. If you do not have a spare bezel, I would suggest casting a replica from plastic resin and then drilling it – don’t modify your original part in case you ever want to restore it back to original appearance.

Modified tuning eye bezel with added LED

Please do not drill a hole for an LED into the front panel itself. This would permanently damage the appearance of the unit and we do not want to do that. A plastic tuning eye bezel can be repaired, or a replacement could be cast from new plastic and then painted to match the original. A metal front panel cannot be easily repaired or replaced.

When the glue dried, I connected the wires to it and carefully soldered them in place, covering the joints with heat shrink tubing afterward to insulate them. I installed the modified tuning eye bezel and reconnected the tuning eye.

Now that everything was hooked up, it was time to try out the modified tuner.

I hooked the tuner up to my workbench receiver by running audio cables from the tuner’s Channel A & Channel B outputs to the receiver’s AUX Input jacks. I turned on the receiver, placing it in the AUX position, and turned on the tuner, placing the function switch in the MPX STEREO position. I tuned in a strong FM Stereo station. The tuning eye indicated best tuning position and the LED was on. It works! In LIVING STEREO!

When tuned off station, the LED is off…

…and when tuned into an FM Stereo station, the LED is on.

It was great to be able to listen to FM Stereo from a 1959 model tuner which was built before there was a definitive FM Multiplex Stereo standard. And until I installed this solid-state multiplex decoder, this tuner only functioned as a mono tuner. Now, it truly is “Stereophonic”…just as the front plate indicates at upper right.

This FM-100 is now truly “Stereophonic”!