MPX-100 PROJECT
You know, every time I get a project that I think will be a puff piece, it never turns out to be that way anymore -- and so it was with the MPX portion of Rob's TA-800 project as well. Rob got this unit from Larry D, who had already done a great job of recapping all the mylar and small electrolytic caps throughout the unit (leaving only the PS can cap to finish up that effort), but also noted that the unit was still displaying poor separation, as a pic I attached to a previous post confirmed. Replacing the can cap did not impact that issue, and wasn't expected to.
This was my first experience with an MPX-100, so in giving it a once over, it quickly became apparent that these units have a few different features to their design that the standard bearer sub-chassis designs don't have. Since (to my knowledge) there is no alignment procedures published for this unit, and scant little else to go on but a schematic, troubleshooting the separation issue necessarily took the project down a number of rabbit holes that had to be chased out, which took a few days to do. Because the issues found will likely affect other owners who have/use an MPX-100 "version II" unit (my designation) like this one (for serial numbers > 40000, but possibly >20000), I thought it would be good to highlight the main points of interest uncovered with this unit, and with the effort to have it properly decode over the air MPX signals from the TA-800. For the record, the original rendition of the MPX-100 used what is commonly referred to as matrix style design, while the version II units utilized the superior time switching format that Fisher adopted for all of its built-in MPX decoders.
First up however, is some general house keeping issues to be aware of:
1.
Power Supply: The can cap is not the easiest piece to remove due to the tight space in these units, but it can be done with sufficient persistence. The primary consideration here is that when the original selenium bride rectifier is replaced with silicon during routine power supply work, R50 (dropping resistor between the output of the bridge and first filter cap) needs to be increased from 1200Ω @ 1W (original value) to 1800Ω @ 5W. In the original design, this part was seriously under-rated (by nearly 100%) due to not only the DC voltage drop across it, but also the significant AC Ripple current dropped across it as well. A 5 watt device will operate at 65% of rating, allowing for a reasonable amount of de-rating to account for under hood temps, and provide for long life operation.
BELOW: New can cap installed. Space is surely tight to work in.......
BELOW: The dropping resistor coming from the new bridge rectifier has been increased in resistance and wattage to properly adjust operating voltages, and give the resistor a reasonable life expectancy.
2.
Stereo Switching Relay: Like the early FM-100B/200B tuners, this version of the MPX-100 uses an open-frame 3PDT plate relay to handle automatic stereo/mono switching, although unlike those tuners, the contacts do not directly switch the audio signal like they do in the tuners but rather, control the common ground connection of the demodulators (among other things), and of course the Stereo Beacon as well. However,
like those early stereo MPX tuners, cleanliness of the relay contacts is critical. Switching such low level signals does nothing to aid in keeping the contacts clean, so they can easily become compromised over time -- not from the usual pitting or burning but in fact, because there is no arcing at all. Micro-arcing aids in burning off oxidation that forms over time which acts to keep the contacts clean. When there is none, the contacts can then become compromised to such low level signals, requiring more maintenance than if the contacts were handling more current through them. The contacts are coated to help minimize this concern, but 55+ years later, the contacts in relay of this unit were still seriously compromised, causing the initial separation problems. Due to how the relay is mounted in this unit, the only real way to resolve this issue is to disassemble the relay to clean the contacts, since no amount of Deoxit products resolved the contact resistance that the relay in this unit displayed.
But disassembling the relay brings other issues with it, that being re-establishing a proper gap for the NO contacts and the armature gap when the relay is re-assembled and in a resting state. Even if the relay does not require disassembly for cleaning, these gaps should be checked anyway, since the board that the stationary contacts are mounted on is attached to the relay frame with two small screws -- which can become loose over the years with use, and upset the gap setting. Or, if the relay is disassembled and only casually reassembled, it is easy to upset these gaps since the contact board allows for significant adjustment as to location upon reassembly. In this particular unit, besides the dirty contacts, there was hardly any NO contact gap, which only further acted to cause intermittent relay operation. Once the stationary and armature contacts were cleaned, setting the NO contact gap to .007 inch, and armature gap to .024 inch resulted in very dependable and consistent relay operation.
BELOW: The armature has been removed, allowing all the contacts to be to be thoroughly cleaned. Loosening the fixed contact board allows the armature to be easily released from its pivot point.
BELOW: With the contacts cleaned and the armature reinstalled, the fixed contact board is adjusted to achieve the NO contacts and armature gaps noted.
With these two issues addressed, the unit was then set up for alignment. On the bench, the unit now displayed significantly better separation (~20 db), but still notably off from what a typical properly operating Fisher MPX sub-chassis can deliver when directly driven from the MPX generator (~38 db). The later sub-chassis designs are comparatively straight forward in their alignment process, but as mentioned, this unit includes additional elements that complicates the process:
1. The sub-chassis units use a single slug 38 kHz sub-carrier oscillator coil, but this version of the MPX-100 uses a double tuned 38 kHz transformer, which throws a whole new wrinkle into the alignment process.
2. This version of the MPX-100 also includes a sub-carrier balance control inside the chassis as well.
There are other differences, but these are the two that relate to alignment of the unit.
Tackling the 38 kHz transformer first, it became apparent that the top slug (primary winding) primarily controlled the output amplitude of the 38 kHz oscillator, while the bottom secondary slug controlled its frequency/phase response relative to the pilot signal in similar fashion to the single slug coil of the sub-chassis units, which allows a proper 2:1 Lissajous pattern to be established. A first effort then had the top slug peaked for maximum output, and the bottom slug adjusted for a stable 2:1 pattern. This produced a very high degree of separation (~40 db) when driven directly by the multiplex generator (Fisher Model 300). But the acid test is always how a decoder performs when connected to a tuner (a KM-60 was the test tuner), and here, it failed miserably. Tuning was extremely touchy, separation was weak, and the stereo was noisy even on strong lab signals. Back on the bench, when driven directly by the generator, it performed flawlessly again. The KM-60 was just off a fresh alignment, with the tuner and its internal WX sub-chassis decoder displaying excellent performance and over the air separation. Hummmmmm......
Since peaking the primary winding didn't work, various other approaches were tried, including use of a frequency counter to set the free running oscillator frequency at precisely 38.000 kHz, adjusting for equal output from the balanced output winding of the 38 kHz transformer, most stable operation from lowest pilot signal, etc., yet none of these approaches delivered performance typical of Fisher MPX operation. Ultimately, it was found that by adjusting the primary slug so that 2:1 Lissajous was centered between the points where rotation of the secondary slug either way from optimum caused the pattern to unlock did the trick: On the bench, separation was still ~40 db, and connected to the test tuner, the touchy tuning was gone, separation was great on strong or weak signals, and the excessive stereo noise was gone as well.
BELOW: Over the air 1 kHz separation performance of a recently aligned KM-60 tuner used in this project to act as a point of reference. The signal is taken from the tuner's output jacks. In all cases, the sine wave represents an L only signal, while the center line represents the amplitude of the R channel.
BELOW: Same scenario (over the air signal), except that the output is now taken from the MPX-100, driven by the top chassis MPX output jack on the KM-60. Amplitudes are increased to detail separation performance.
Adjusting the sub-carrier balance control is a rather straightforward effort, so with that and the procedure for adjusting the 38 kHz transformer nailed down, an overall alignment procedure was really starting to take shape. With the unit so aligned and the PS and Stereo Switching Relay work all finished up then, the
real acid test could now be performed: Trying out the unit with Rob's TA-800. Performance on the KM-60 was now excellent. But connected to the TA-800, and adjusted for that model as shown in the MPX-100's Owner's manual (Low Level MPX input, Separation Control set to "4"), it was not to be. And, operation of the Stereo Beacon was a concern as well -- it simply remained on. HHHUUUUMMMMM. Continued..........