THE "DANCING EYE TUBE" ISSUE:
Sorry about delay regarding this modification. I've just had too many irons and not enough fire lately! But it seems that there is interest in this issue after all. In the 500C and 800C receivers and 100-B, 200-B, and R-200 tuners, this issue also manifests itself as a low sensitivity (or high threshold) for triggering the unit into FM Stereo mode automatically. As a result, most pics you see of one of these units operating with the Stereo Beacon lamp illuminated will almost always show the selector switch in the FM Stereo Filter position -- as that position creates a forced stereo reception mode, turning on the Stereo Beacon lamp at all times. That is, all stations will pass their signal through the MPX section whether they contain the identifying 19 kHz pilot signal or not. Even if no station is tuned in, the lamp will be lit. Theoretically, this should not be a problem but in reality it is, as purely mono stations (i.e. talk radio, etc) will typically display a slight distortion if listening to them through the MPX circuitry (i.e., in stereo mode). This in fact is the reason for providing the ability to switch an FM Stereo MPX tuner back and forth between stereo and mono modes -- either manually as done with the 400, or automatically as done in the Stereo Beacon models. In FM Mono mode, the audio signal is taken directly from the Ratio Detector stage, bypassing the MPX circuitry. That way, any unbalanced signals from the free running 38 kHz oscillator on the MPX sub-chassis can't find their way into any purely mono FM audio, and cause the distortion otherwise to be generated.
OK. So the problem at hand then is, if you're going to provide some sort of FM Stereo MPX indicator, what should the threshold be for the unit to trip modes into or indicate stereo reception? And why have any threshold at all? Well first, there is the issue just discussed above. There needs to be some preset threshold level so that the unit can have a knowledge of when to indicate or automatically trigger -- or not, just the same way a thermostat in your home triggers the heat or AC systems. But the threshold itself (i.e., where the thermostat is set) can be a matter of discussion.
As designed, the threshold was set rather high. To understand why, you need to understand the realities of the day: It's 1963. FM Stereo has just been approved by the FCC. Of the two or three FM stations that existed in an average size city, maybe only one or two were stereo, and at that, not all the time. The crazy crowded FM band of today with stations belching out 100% modulation 99.9% of the time at seemingly every .2 mHz interval on the dial could not have even been imagined, let alone realized. And, the programming was almost guaranteed to be classical or light jazz, and that was IT. Importantly, processing of the audio signal was minimal. So what's the difference between these two scenarios? NOISE! Not the music (although much of it is), but quite literally, noise, as in noise from the 38 kHz AM sub-carrier that comprises the stereo information sub-channel within the main FM carrier signal. If reception was not strong, and there were numerous quiet spots in the classical music format, then any noise was going to be very apparent -- and we can't have that when showcasing the brand new FM Stereo MPX baby now can we! What to do? Set the stereo switching threshold very high, and get the word out every way possible about how much better your antenna must be to receive the new FM MPX Stereo signals properly. That way, when the Stereo Beacon is triggered, its guaranteed to be a huge selling point and sound great.
In the ensuing nearly 55 years however, things have changed: The Effective Radiated Power of FM stations is typically far greater than those of the early pioneering stereo stations. Signal processing has elevated average modulation levels much higher. Recording processes have improved, and tastes in music have shifted. The bottom line, is that usable FM MPX Stereo signals are available today at weaker carrier levels than was the norm in the early 60s. Against this, the old threshold standard that Fisher used is obsolete. Today, that threshold can be lowered for more dependable Stereo Beacon switching, and less tango with the 400's eye tube.
So where exactly is the threshold for stereo switching established in your favorite Fisher MPX unit? To keep the discussion simple, we'll limit it to the MPX-65, and its close cousin, the WX chassis. There are some other close sub-chassis kin as well, but this will cover the vast majority of pieces out there.
In the classic Fisher multiplex circuit (nearly standardized and available with the schematic in the service manual of every unit identified above), there is hidden away in the circuitry of V100 a noise detector circuit, comprised of L101, CR100, R204, and C207. The Cliff notes version of how it operates is that Z-100 is part of a tuned circuit that is peaked at 19 kHz, allowing maximum transfer of the 19 kHz pilot signal that is part of the composite signal appearing at plate of the first section of V100 (pin 1), into the grid of the second section of V100 (pin 7). Any noise components of the signal at pin 1 are rejected by the tuned circuit of Z100, and therefore appear across L101. CR100 rectifies any voltage appearing across L101, with its polarity set to produce a negative DC voltage at its output. This voltage is filtered by R 204 and C207, and applied to the pin 7 grid via R203. As designed then, the more noise voltage that is developed at the output of CR100, the greater the negative voltage is that is applied to the grid at pin 7. This voltage can easily reach levels great enough to "cut off" (or nearly so) the second section of V100, which then greatly limits the triggering signal sent to V101, and the indicator output. Hello dancing eye tube, and Stereo Beacon that rarely triggers.
The answer to limiting this noise detector action is really rather simple, and consists of adding a single quarter watt resistor to the circuit, based on the tuner that the MPX chassis is being used with. For all units, the resistor is installed across R204 (i.e., from the anode of CR100 to ground), with its value roughly based on the number of IF limiter tubes used in the tuner section. Therefore:
1. All receivers should have a 47K resistor installed.
2. FM-100B and 200B tuners should have a 68K resistor installed.
The values indicated are purely of my own determination for my own use. I have installed these values in my 400 and 800C receivers, and 100-B, 200-B, and R-200 tuners, and they all work fine. Of course, they are based on properly aligned and operating units. Using higher values raises the threshold, while lower the values will reduce the threshold even lower -- but watch out that you don't go so low as to allow false indicator triggering. The values chosen steer safely clear of that issue in my units, and generally allow the lowest practical threshold to be had in normal use. The bottom line is that now in FM Automatic, instead of the Stereo Beacon indicating when the old stereo threshold has been met, it now simply indicates reception of a stereo station, and I can then tell the unit if/when I chose NOT to listen in stereo by turning the selector switch to mono FM. That approach just works better for me, as I rarely if ever turn my units to FM mono. They just remain in FM Automatic virtually all of the time now, with the Beacon triggering off and on between stations, and between mono and stereo stations as you would expect. On my 400, the eye tube is now well behaved on all but the very weakest (i.e. unusable) stereo stations.
I think you'll find that this modification greatly improves the operation -- and your enjoyment of your Fisher FM MPX Stereo units.
Good luck with it!
Dave