Discussion in 'Fisher' started by dcgillespie, May 31, 2017.
Talk about lousy Rhythm.
In my early days of model railroad throttles and low financial ability, my creations were tack soldered similar to the above. The "rotary switch" was an alligator clip moved to different terminals on a terminal strip. The biggest innovation was the power transistor heat sink. I figured that dropping it in a glass of water would guarantee that the case temperature would be under 100˚ C.
So, a coworker was interested in having me possibly build one for him. I took this contraption to his place, pulled it out of a grocery bag, carefully arranged the tack soldered mess so nothing was touching. So far, so good. Then I asked for a glass of water. He said, "Oh, you don't have to drink water. We have pop." I said, "It's not for me." I think I lost him when I dropped the transistor into the glass of water.
Many years later, I learned that Bob Carver used a coffee can for a heat sink and took his unprecedented 350 watt amplifier to a McIntosh clinic. As they ramped up the power, the lights eventually went off. After that was fixed, everyone went silent as O'Brien was calling out the numbers. 50 watts .... 75 watts .... 100 watts .... .... .... up to 350 watts. What Carver wanted from this was the performance graph with the McIntosh name on it. That enabled him to convince the bank to give him a loan to start building and selling amplifiers.
Heathkit made a 1kw RF dummy load that was a 50w resistor in a 1 gallon can of oil. I've also heard stories of amateur radio guys taking the metal 6L6 tubes and doing basically the same thing in order to squeeze more power out of them without a meltdown.
Overclocker's take a complete PC motherboard and either sink it in Mineral Oil, or water cool them with elaborate systems of piping pumps, heat exchangers, etc.
NJ -- Yes I did! It has figured greatly into the MPX-100 project I'll be posting about soon.
When I was a teenage back in the 60s, I was building Fender Showman knockoff amps for our band, and when I would test them, all I had was a 4Ω 50 watt resistor -- but the amps were capable of nearly 100 watts on a good day (high AC line voltage). I would regularly drop the resistor in a glass of cool water (completely covering it), which absolutely took care of the dissipation issue with the resistor. I still have the resistor, which is no worse for the wear!
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..........
As a first step in addressing the problem, the alignment of the TA's repaired IF strip was rechecked, and found to be exactly as it had been aligned to previously, which was dead on the money. So something else had to be at play.
Switching to the High Level input on the MPX-100, then switching of the Stereo Beacon lamp became virtually normal. A recheck of the noise detector and amplifier circuits in the MPX-100 (which shuts off the Stereo Beacon to prevent noisy reception) showed it was working just fine. Further investigation found that the 6BL8 tube was weak, but its replacement only cemented the Beacon's on status. It was ultimately found that -- as I've mentioned before -- this unit was designed back in the early 60's, when FM stations were few and far between on the radio dial, let alone any of those offering FM Stereo MPX service. In today's major markets however, FM stations are basically banging into each other shoulder to shoulder all the way up and down the dial -- at my location, anyway. With most of these stations running near 100% modulation nearly 100% of the time, the Stereo Beacon circuit in the MPX-100 simply gets overloaded, so the Beacon stays on. Using the Hi Level input resolves this problem. It also aids in another performance aspect of the MPX-100 as well.
Unlike the sub-chassis units that followed, the MPX-100 uses sharp, active HF roll-off circuits comprised only of R/C components to filter out any residual 38 kHz sub-carrier elements remaining in the audio output. While this is effective enough, it is not as effective as the sharp notch type filters that the sub-chassis designs use. However, by adjusting the output level controls on the MPX-100 for unity gain operation based on the gain displayed from the Hi Level input, this results in a level control position that provides a further reduction in residual 38 kHz elements in the output, to aid in this effort. So using the HI Level input on the MPX-100 provides a two fold performance advantage, both improving Stereo Beacon operation (at least in the Atlanta market -- ymmv), and improving 38 kHz attenuation performance.
For the MPX-100 then, that only leaves the setting of the Separation Control. Frankly, while the range of the control could easily accommodate both a directly driven/ KM-60 over the air setting (basically the same setting for both scenarios), and that of best TA-800 performance, it was nowhere near a setting of 4 on the separation control dial. Of course, if the push-on knob has been removed and reinstalled in a different location at some point, who knows if that setting is relevant anymore for this unit. Normally, a discrepancy like this would bug me. But knowing how sensitive such settings are to a number of different factors, how generic the instructions are by Fisher to set it, and the fact that the unit now operates as optimally as any of Fisher's sub-chassis adapters -- both from the generator, and using the same optimum setting, from a KM-60 tuner -- the discrepancy is of little concern.
At this point then, the MPX-100's Stereo Beacon triggers nicely, and the unit delivers very nice stereo sound with the TA-800 in the listening room when the separation control is adjusted optimally for it. For me however, the difference in performance noted between that of the KM-60 and the TA-800 was something I still wanted to chase down, which became yet one more rabbit hole to pursue. Next time.
BELOW: This MPX-100 now operates quite nicely with the TA-800, looking very nice sitting along side it!
View attachment 963045
I spy a MC240? and a LK72 hiding behind all that Fisher Goodness
I've used the old dollar bill between the contacts trick for relays and switches of that type before. Usually works reasonably well as long as you can get it in there. Sometimes you just have to cut smaller strips of paper or actually tear the armature out for a polish though.
I tried the old dollar bill trick, and it did work on 3 of the contacts, but the other three were suborned as a mule. After about 20 minutes, I said hack this, and removed the armature........
Looks like I have some more cleaning to do. This 20001 series model has the relay also. Unfortunately I can't tell which transformer is which. So I'll be sending it down to Dave after some work, (including a new filter cap, an 1800ohm 5w R, and cleaning of relay points). I noticed that mine works better on high level and the separation increases at about 7 of 10. But it doesn't sound overdriven or overloaded like with a separation knob on a WX or MPX-65. It just gets louder. I haven't tried the MPX-100 with the SG-165 yet. Still have to figure it all out (how to use it).
Fisher "Wide Band Multiplex" Equipment
More than just a slick marketing ploy; it really represents a true change in performance specification. And while I was pretty certain where the change was made to achieve this new specification, I still went down this last rabbit hole by conducted some experiments to be absolutely certain.
The ability for good multiplexing lives and dies on response characteristics -- both in frequency, and phase. The two are quite similar, yet different, but both have a significant effect on how effectively FM Stereo MPX actually performs. Quite simply, if all required frequencies are not passed with the same amount of delay or amplitude response, then stereo separation will suffer in decoding FM Stereo MPX broadcasts.
In an FM tuner, audio first appears at the output of the Discriminator stage -- most often taking the form of a Ratio Detector in any Fisher equipment produced after the mid 50's or so. But with FM Stereo MPX signals, the frequencies required to properly reproduce these types of signals reach as high as 53 kHz, with modulation frequencies reaching as high as 75 kHz on either side of the 10.7 mHz IF center frequency. To ensure that the required band-pass through the IF strip is available to adequately handle these signals without deterioration, most IF transformers used in FM MPX equipment have a bandpass of around 260 kHz or so. This allows proper passage of an FM MPX signal without alteration, without being so wide as to upset the selectivity of the receiver: If it were much wider, then multiple stations just .2 mHz apart could start finding their way through the strip at the same time. The point of this is that when Fisher changed to its Wide Band design, such a change could potentially affect the design of every IF transformer used in the new sets with builtin MPX to achieve the increased response, versus those that came before.
To punctuate this even further: The early Fisher "stereo" gear (as in AM/FM Stereo), or advertised as "stereo ready" gear in the early years of stereo (as in, it has a MPX output jack), was all produced with Fisher's money placed on the Crosby multiplexing system being approved -- which was a system that had somewhat less stringent response requirements for an FM tuner's IF strip, than that which the approved GE/Zenith system requires. This basically means that all the pre-built in MPX stereo tuners and receivers that had facilities for future MPX service potentially have a more restricted band-pass through the IF strip -- a band-pass that had to be widened for proper multiplex performance with the GE/Zenith MPX system. Ergo, virtually all of the Fisher stand alone models that were produced with a builtin MPX sub-chassis from the factory have the words "Wide Band Multiplex" added to their face plate or dial glass. To find out just how much of a factor the response issue is, I serviced and checked the alignment of my recent 202T purchase, and pressed it into service early to perform a series of experiments. But first, some comparisons were made.
The 202T was perfect for gathering the information I needed, since it was pre-GE/Zenith stereo ready to the point of needing only to remove a blank plate, and dropping in a plug and go MPX-20 Crosby based decoder sub-chassis, and there ya go. Since the 202T uses the same FM RF front end as the KM-60 MPX tuner I was using earlier, and both the 202T and KM-60 employ a four tube IF strip, it made it rather easy to compare the IF transformers used between the two models. That returned the following information:
1. The RF output IF transformer used between both models is part number ZZ662-117, so there was no difference there.
2. The 1st IF stage output transformer carried part number ZZ629-142 in the 202T, but ZZ2987Y in the KM-60. The Y in the KM-60 part number is not relevant to this comparison.
3. The 2nd IF stage output transformer carries ZZ50210-2 in both units, with the kit tuner again showing a Y on the end of the part number. So there was no difference here, either.
4. The 3ed IF stage outputs into the limiter coil in both models, with each coil carrying a different part number. But this is largely due to the fact that the 202's coil includes voltage doubling diodes for driving the FM indicator eye tube in that model. In the KM-60, the eye tube is always driven by the output of the MPX sub-chassis, so the limiter coil does not include any diodes. However, all of this is a moot point, since limiter coils are not transformers in the signal path, and so they have little if any effect on response of the IF circuits -- just on the amplitude of the overall IF signal. As a result, differences here are not relevant to changes Fisher made to achieve wide band performance.
5. The 5th transformer in each model is the ratio detector transformer, and as suspected, they carry different part numbers, with the 202T employing part number ZZ592-170, and the KM-60, part number ZZ50210-9(Y).
So the two significant differences between my standard bandwidth 202T, and the Wide Band KM-60, are the 1st IF output transformer, and the Ratio Detector transformer. To establish a base, the MPX-100 was first connected to the 202T, with best possible over the air separation performance shown here:
Again, this will still produce very listenable stereo, but as can be seen, is certainly off the mark of what the KM-60 could produce with the same MPX-100 adapter.
At this point then, the original first IF output transformer (ZZ629-142) was removed from the 202T, and a ZZ2987 transformer removed from a junker 4 tuber 400 receiver installed and aligned. By any number of approaches, there was absolutely no change in the 202T's performance as a result of this change: Sensitivity remained the same, selectivity remained the same, eye tube operation was identical, and stereo separation performance showed no change at all, either. This focused the hunt then on the ratio detector transformer, which was my first suspicion overall, since it's characteristics determine so much of a tuner's overall response characteristic. But the IF transformers residing earlier in the IF strip had to first be eliminated, if only to determine how much of the wide band response generated was in fact due to the change in ratio detector transformers.
The original first IF output transformer was reinstalled into the 202T, and then the original ZZ592-170 ratio detector transformer removed, and a ZZ50210-9 ratio transformer (from the same junker 400) installed, and the complete IF strip aligned again. This produced the following over the air separation results, with no particular effort to fine tune the separation adjustment:
The difference is clear and obvious. Also, the separation control setting of the MPX-100 when connected to the "Wide Band 202T", now closely agrees with that control's setting when the MX-100 is directly driven by the MPX generator as well. So the wide band performance of the GE/Zenith based Fisher tuners and receivers is virtually if not completely all achieved by changing out this ratio detector transformer:
For this one now installed:
Again, the letters after the last digit apparently have no bearing on the actual part itself. I know that's not true of Fisher's output transformers. But in the case of these transformers, the letters do not seem to indicate a different component electrically.
Ultimately then, for any piece using the ZZ592-170 Ratio Detector transformer -- which virtually all pre-wide band units did -- using at least the MPX-100 multiplex decoder with it will certainly produce very usable results, but not the very best results possible. If you only listen to an early unit in mono anyway, or have no plans to add an outboard decoder, then the original transformer is just fine. For many if not most folks, even using the original transformer with an outboard MPX-100 will still produce very enjoyable results. On the other hand, if you're looking for the nth degree in Fisher FM Stereo MPX performance from the earlier non-wide band units, then a change in the Ratio Detector transformer to the later part will complete that journey.
Bringing this home to Rob's TA-800, like my 202T, the TA (both of them in fact) are pre-wide band units, employing the older ZZ592-170 ratio detector transformer. As a result, the best over the air separation performance it would achieve is nearly identical to the pic shown of that performance from my 202T before it was modified.
So, yet another potential modification project now to throw into the mix for those with older equipment who are so inclined.
The trick there becomes sourcing the appropriate ratio detector transformer. Short of parts donors, are there any other viable sources? I'm sure someone else was using an electrically identical IF trafo, but of course with a different internal part number. Possibly also there is an available Miller part that would widen the net beyond scavenging from junker post-1961 or so Fisher receivers. If the donors happened to include some of the console tuners, that might prove to be a better donor source since those pieces show up relatively often and don't command a big price. The ones that relied on an amplifier for power and had the sideways dial aren't exactly all that desirable to most.
Gadget -- You're exactly right. I only offered a small portion of all the units I researched to save time and space, but it was clear that from the start of the very first Fisher GE/Zenith based units through to the last of the last vacuum tube units produced, and from the lowliest Fisher portable to the grandest pooh-bahs of the lineup during that time, that the ZZ50210-9 Ratio Detector transformer was their go-to transformer to get that job done. Therefore, basically any of the vacuum tube FM Stereo MPX units that Fisher produced could cough up an appropriate transformer for conversion.
Since I had planned on installing a WX sub-chassis in my 202T and doing the necessary modifications to allow the eye tube to react to the stereo sub-carrier signal strength in stereo mode anyway, I plan on leaving the new Ratio Detector transformer now installed permanently in place. From a practical use standpoint, other than allowing for maximum stereo separation to be achieved, I can detect no actual operating difference in the unit (as in ease of tuning, noise rejection, etc.) versus when it had the original transformer installed.
As for the general replacement items from Miller and the like, my experience has been that while those parts have typically been just fine for general replacement work in strictly mono units, when it comes to tuners intended for MPX operation and/or their multiplex sections, the replacement parts often do not provide the same end result MPX performance as that of the original part. Much like the case at hand with Rob's TA, their performance is or usually is nice enough for normal use. But none the less, it is still not of the performance that the proper part can produce.
I tried a Miller 1465 ratio detector in my 202-R when one of the slugs in its original ratio detector seemed to be stuck. The 1465 was disappointing; I could not get it to align quite right and I ended up putting the original ratio detector back in and, after figuring out that I had made one of those DUH! errors in mixing up the upper and lower slug, aligned again and it is now as good as it gets and Microtune works perfectly.
I agree with Dave that the Miller ratio detector would be OK - and just OK - as a general replacement in a mono, non-Microtune tuner or receiver.
I had had thoughts of finding a 202-T or 100-T and trying a WX or MPX-65 inside, but they are getting a bit too pricey for me. I for one would be most interested in seeing how your 202-T turns out.
Guess I'll have to rob the 65 Ambassador tuner parts carcass, for a zz50210-9 and drop it into the 101-R, then align it. The FUN never stops.
one of these days I really need to get proper alignment gear. I'm curious what mine are doing. I have a few mono setups with MPX converters and some definitely perform better than others. The Fisher WX unit paired with the TA-600 does much better than when I mate it with my Bogen FM-50, and far better than when paired with the Realistic mono tuner. The Pilot FA-590 is so-so, especially with the Pilot MPX-100. I've aligned them as best I can with a VTVM and a THD meter to get the output as clean as possible, but it still lacks something. Never touched the alignment on the WX since I have nothing that will set it up properly.
It's exactly 7 weeks later to the day from when the TA was dropped off, and now both the TA and the MPX-100 have been picked up and are on their way back to NC. Rob's probably one of the younger Fisher devotees, but what he lacks in age, he makes up in passion for his gear! I'm sorry to see it go!!
Dave; I'm looking at the two transformers ZZ592-170 on the 101-R and the zz50210-9 on a '65 59-T Futura/Ambassador. I think I can trace out all the leads, but the one side of the Upper coil is pin 5 on one and pin 6 on the other. Is that a problem? I think I should make the change on the new transformer to pin 6? Is there a specific order in the pin layout such as clockwise 1 thru 6, anti clockwise 1 thru 6 or 1-3-5 on one side and 2-4-6 on the other? Also note that Pins 1 & 4 are reversed due to the diodes internal to the transformer. Should I reverse the leads to pins 1 & 4?
101-R Ratio Detector zz592-170
'65 Futura/Ambassador 59-T Ratio Detector zz50210-9
Larry -- Since you're making the change, I'm glad you spotted the differences.
In changing out the transformer on my 202T, the net effect of the different wiring between the two transformers amounted to simply installing the electrolytic cap (C93 in the 101-R) in reverse from how it was originally installed.
In the older units, the 1K resistor (R76) connected to the anode of CR2 at terminal 4. In the new transformer, the same 1K resistor now connects to the cathode of the diode appearing at terminal 4. So again, you connect the transformer leads up as they were before -- but you reverse the connections of C93, since the diodes are reversed between the two transformers as well, and that's it.
Good luck with it!
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