Getting the Most From Fisher FM Stereo MPX Tuners and Receivers

Discussion in 'Fisher' started by dcgillespie, Mar 17, 2016.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    INTRODUCTION

    I have always had a soft spot for Fisher vacuum tube FM, FM/AM, and FM/AM Stereo MPX tuners and receivers. Of all the pieces of this type of equipment that I have seen over my lifetime or has come across my bench, it is the Fisher gear that never fails to impress. Further, within the Fisher line, I think their FM Stereo MPX gear was their finest hour. This is hardly to knock their lines of integrated, preamp, and power amplifiers (including those sections within their receivers). Rather, it is to highlight the fact that designing and building really good FM Stereo MPX equipment back in the day was no easy feat. If it were, why were so many pieces of this gear from other manufacturers little more than junk by comparison? There were some other good pieces to be sure. But taken on the whole, which includes sensitivity, selectivity, ease of tuning, stereo performance, and not to forget the shear number of various pieces they produced with these qualities, it would be hard to identify any other manufacturer who was so recognized for producing such superb reception equipment.

    Today however, most of this equipment is at least 50 years old, which can create a number of potential issues, ranging from younger folks not fully understanding the equipment, to service issues, to the fact that the FM band is now vastly different from that which existed when this equipment was designed and produced. Add to that the potential indiscriminate use of inappropriate modifications to some pieces, and the result is that many folks may not realize just how well these units can perform when operating properly.

    There is no doubt that FM broadcasting has deteriorated at many stations across the nation, let alone how you may question available programming. There is an Atlanta country station that has their signal processors and compressors wound up tighter than a drum -- so tight that I find it un-listenable. Their modulation level is 100%, 100% of the time. The pumping in and out of the signal is just gross. But there are still a couple of good stations here and that, coupled with the fact that I always want my equipment to operate the best it can (regardless of use time), keeps me ever looking at how to get the best from this equipment.

    In large part, this is best accomplished by making sure that a given piece operates as it was intended to, while meeting factory specifications in the process. However, the usual restoration practice of recapping the electrolytic and film caps and replacing the selenium rectifier that works so well in restoring audio circuits -- and needs to be done in tuner circuits as well -- can leave a number of issues unresolved in tuner and MPX circuits. Also, there are some modifications (or practices) that can be used to greatly extend tube life, and others that will simply improve tuner operation. So, I thought I would gather them all together for a one stop presentation to the Fisher community. A few have been discussed before, but there are some new tricks included as well.

    First however, as a baseline, it will be helpful to establish what a properly operating tuner/receiver acts like:

    A. In major markets you will very likely be able to receive multiple stations per mHz across the dial. For units with Automatic Stereo Switching, a number may not automatically trigger stereo reception unless you live in the city or manipulate the antenna, but the vast majority of them will be very listenable in mono mode. But even in the country where I live, my dial is pretty well packed from one end to the other. My lab is in my basement at the end of the house that is fully underground -- as therefore is my simple dipole antenna as well. Yet even with this restraint, I routinely listen to stations 100 miles away rather easily, in good stereo.

    B. For units with a tuning meter, typical strong stations will produce a signal strength of "4". It typically takes a very strong signal (residing close to the transmitter) to produce readings much over 4 after the unit has reached normal operating temp. For units with eye tubes, strong stations will close the eye to typically less than 1/8 inch gap when the eye is acting as a signal strength indicator -- except for the 400 receiver. When this receiver is set to FM mono mode, strong stations will close the gap such that the width of the gap is basically the same length as either one of the beams coming in from the sides. In MPX stereo, any unit with an eye tube that indicates MPX reception will typically close the beam to less than 1/8 inch on strong stations. Finally, contrary to popular belief, Fisher did not design the eye tubes to close (although some particular tubes will), and in fact, you don't want them to. First, once the beam closes, it ceases to indicate optimum tuning since movement of the beam can no longer be discerned. Secondly, if the beam overlaps, it causes the florescent screen to burn very quickly where it overlaps. The screen on these tubes already wears quickly enough, while overlapping just accentuates it.

    C. The tuning action will be sharp (a defined tuning peak), but not touchy, and capable of distinguishing between two close stations (i.e. 200 kHz apart) easily enough.

    D. Stereo performance will be clear and distortion free (no ringing sound), with very wide separation on material that is so recorded to display it, and non-finicky to tune on stereo stations of good signal strength. For units with automatic stereo switching, the design is such that it takes a fairly strong stereo signal to activate stereo mode -- this to ensure noise free stereo reception. Fisher notes the importance of using a good antenna in the owner's manuals for all of their units to ensure good stereo reception. It is much more important than that required for reception of mono FM stations.

    E. Frequency accuracy will generally be quite accurate at the ends of the dial, but may show some acceptable error (<.3 mHz) in the mid-frequency range depending on model.

    F. Of course, with these units all being built into a steel chassis that is partially coupled to the AC line, they require a reasonable antenna, and will typically receive nothing with no antenna attached if the bottom cover is in place (however, dedicated console tuner/preamp units will typically receive a few stations even with no antenna attached since the antenna connections are located above the chassis on those models). Generally however, a standard dipole will provide very capable reception in most locations.

    If these qualities do not define how your Fisher tuner or receiver operates, then it likely needs attention. Reduced sensitivity, selectivity, tuning action, stereo performance, or excessive dial glass calibration error almost surely indicates that some attention is needed. It is my experience that most units in fact do -- because most people who sell these units at auction or otherwise either know they're not right and are selling them off, or just aren't impressed but have no clue that they aren't operating correctly. Also, there's a lot of these units that have had attempted repairs made through alignment efforts, but were left worse than before the effort was started, because alignment wasn't the problem. Personally, I have an 800C, FM-100B, and FM-200B in my own collection. Two of them were sold with glowing reports of their performance, while the third was understood to need some work. In reality, all had way, way more problems than their previous owners ever realized (or would admit to).

    Beyond the basic restoration efforts, a complete restoration service for tuner/receiver equipment often takes specialized equipment and knowledge that many techs just don't have. If your unit does require more than basic service, make sure your chosen tech has the necessary equipment and knowledge to properly service vacuum tube FM Stereo MPX gear. Otherwise, it can result in an expensive disappointment.

    Next time, we'll start getting into all of this. But for now, this should give you a good idea if your Fisher is operating as well as it should be.

    Dave
     
  2. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    There WILL be a POP QUIZ on ALL subject matter covered at ANYTIME during the course. I got a feeling................this will need to be STICKIED.
     
  3. RS Steve

    RS Steve Tube Junkie Subscriber

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    Someone tuned a few of my Fisher's up,( :bowdown: DAVE) now their radio sections sound better than most of my solid state tuners. :thumbsup::thumbsup:
     
  4. bce22

    bce22 Active Member

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    This is going to be good.

    Unfortunately I don't have a Fisher Tuner at the moment, only a "lowly" Scott 370 tuner. Someday I'll be able to use this thread personally but for now it's simply "school time"!
     
  5. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    A great thread this will be, Dave redid all my Fishers as well, they perform perfectly.
     
  6. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    And the trick is to keep them that way Al. One of these days, ONE of THESE Days.....
     
  7. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Dave "IS" the MAN!!!!
     
  8. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Larry, I bet if "Alice" had a Fisher then Ralph would of been nicer. LOL. Now that Daves taking some well deserved time off from tech duties I will have to hold all new Fisher purchases awhile....sigh Al
     
    Last edited: Mar 17, 2016
  9. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Al; And here I was all set to spend my tax $$ on some gear. Didn't have the $$ 24 hours and the element on the upper oven of our 1974 GE Americana Double oven range went tit's up, needed a new door gasket too. :wtf: The upper elements are 4 times more expensive than the lower elements.:yikes: Then not 3 days later I go down stairs in the basement and I've got 6" of water.:rant::rant::rant: 2 new sump pumps and a new Wet/dry Vac then 3 days vacuuming and cleaning up. And NO $$$$. Good thing Lawn season is coming up. Good thing I'm handy around the house. Labor on the element and gasket would have been close to $200.00:whip:
     
    Last edited: Mar 18, 2016
  10. vendo81

    vendo81 AK Subscriber Subscriber

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    Great information on the wonderful Fisher tuners we own :) Dave is a storehouse of knowledge!
     
    N9JCQ likes this.
  11. bhamham

    bhamham AK Subscriber Subscriber

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    Agree, thanks for taking the time to share it.
     
  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    ESTABLISHED MODIFICATIONS

    Besides basic recapping restoration efforts, there are typically two or three areas where well established modifications are applied to tuners, and/or the tuner portion of stereo receivers:

    1. For stand alone tuners, replace the selenium rectifier pack with a silicon package: This is simple enough, should always be done, but should also always have a 100 Ohm 5W resistor added between the output of the bridge, and the FIRST filter cap. Therefore, the new rectifier package will no longer connect directly to the can cap or discrete caps replacing it, but do so through the new resistor. The resistor almost perfectly mimics the internal drop of the old selenium rectifier pack when it was healthy.

    2. Adding a Current Limiter on the primary of the power transformer. This is typically done to help protect the power switch and tame today's higher AC line voltage. As for switch protection however, even Fisher's most complex tuners draw only a fraction of that which its receivers draw at turn-on, so the need for adding a limiter for this purpose is really minimized. As for today's higher AC line voltages, that will be discussed when Tube Life is addressed.

    3. FM Mono de-emphasis network. Over compensated, this one has always been a bit of a mystery, with the most common excuse offered that Fisher did this to make Stereo sound, sound more impressive. Of course, adherence to EQ standards was nowhere NEAR where it is today. But honestly, Fisher was all over the map with this one -- for no apparent engineering reason -- with NO consistency between various models.

    The standard for de-emphasis in the United States is 75 us, yet Fisher tuners and receivers vary anywhere between 70.5 us (Fisher console dedicated tuner/preamps and 126.9 us (Fisher FM-202B) regarding this standard. The effect of deviating towards greater values is to produce an increasing roll-off of high frequencies -- somewhat in keeping with the typical description of Fisher sound. With deviations of this degree, it really should be corrected.

    The standard Fisher mono FM de-emphasis network consists of a 47K resistor, and a capacitor, ranging anywhere between .0015 uF and .0027 uF, with the varying cap values producing the various de-emphasis values used. The proper de-emphasis value is achieved by any combination of resistance in Ohms and capacitance in uF that has a product of 75. Practical concerns place limits on how low the resistance value can go. However, using a network whose resistance value deviates as much as 25% from Fisher's standard of 47K produces no measurable change in performance. Therefore, either the resistance or the capacitve elements (or both) can be changed (within reason) to achieve the target value.

    As for which component to change, early and lower tier products from Fisher used a ceramic capacitor in this network, whereas later and premium units used high stability polystyrene capacitors in the network. In achieving the target de-emphasis value, it is best to change the ceramic cap where those were used to a proper value high stability cap, while in units with a high stability cap installed, it is best to change the resistor to a more appropriate value.

    4. FM MPX Stereo de-emphasis networks. Oh Boy. This one has received much discussion -- even from yours truly. In an effort to put some closure to the matter (even for myself), I took my fully restored FM-202B, and used it to make appropriate measurements. This tuner has had the original FM Mono de-emphasis value of 126.9 us corrected to within 0.30% of the target 75 us value (original .0027 uF ceramic cap changed to .0016 uF), while the de-emphasis networks in the MPX decoder remain absolutely stock as built, and the decoder being the proper decoder for the FM-200B. With the tuner so modified, a Fisher 300 Multiplex Generator was then used to broadcast various stereo (i.e. applied to both channels) test frequencies into the receiver, with its response measured directly at the output of the FM Mono de-emphasis network, and the L&R outputs of the MPX decoder chassis. Pre-emphasis was not used at the transmitter (eliminating any inner-channel errors that the pre-emphasis networks in the 300 might introduce), and the output of the signal generator was held constant so that a "flat" signal was transmitted at all frequencies. The response data produced then is therefore NOT a basis for how flat the overall response of the transmitter/receiver combination is, but rather, how closely the stereo de-emphasis networks in the decoder track with an ideal 75 us de-emphasis curve, as represented by the FM mono setting. Additionally, because of the specific test frequencies used, the data also can be used to verify the existence of an accurate 6 db/octave slope with a 75 us time constant (as specified by the FCC), for the FM Mono setting.

    Sidebar: A 75 us network has a crossover frequency (-3 db point) of 2,122 Hz. By using this level and frequency as a reference point and then testing succeeding frequencies in octave intervals above the reference frequency, the accuracy of the slope can then be verified. As a practical matter, the reference frequency used for the test was 2,125 Hz, and octaves thereof.

    The results are as follows:



    STANDARD MONO STEREO L STEREO R DIFF (avg)
    1. @ 2.125 kHz: -3.0 db -3.0 db (ref) -3.0 db (ref) -3.0 db (ref) +/-0 db

    2. @ 4.250 kHz: -6.0 db -6.5 db -7.3 db -7.2 db -0.75 db

    3. @ 8.500 kHz: -12.0 db -11.95 db -14.05 db -13.85 db -2.0 db

    4. @ 17.00 kHz: -18.0 db -18.2 db -23.05 db -22.90 db -4.775 db

    EDIT: Boy, that chart sure got jumbled up -- everything is run together! Hopefully you can make sense of it. It didn't look that way when I was making the post. Not sure how I could fix it either.........


    Note the very close tolerance that the results in FM MONO mode produce against the ideal standard. Again, this was with the corrected FM mono de-emphasis network installed in the tuner. As for the response of the stock decoder, Fisher was clearly trying to perform a balancing act. The de-emphasis specified for FM MPX Stereo broadcasts is the same as that for the old FM mono transmissions. They did a pretty fair job of maintaining a reasonable compliance at the output of the decoder up to about 10 kHz (where the vast majority of music resides), but couldn't avoid the large dip in response that the decoder's notch filter creates at 19 kHz. This filter is necessary to prevent the Pilot Signal from appearing in any substantial amount in the stereo output signal. A simple 75 us network (as used for FM Mono signals) does not provide sufficient attenuation, so a notch filter is used. The downside of course is that it also acts to attenuate signals in the upper audio spectrum as well, as the growing difference at 17 kHz shows. For reference purposes, the response at the output of the stock decoder at 19 kHz is down (on average) 32 db, relative to a 1 kHz reference frequency, so you can see the black hole that the notch filter creates at this frequency, sucking in everything around it.

    Personally, after significant listening sessions and analysis of the data, I have elected to keep the stock values for the decoder in place. Between my reduction in HF hearing and the typical material that is broadcast, it is rather hard for me to hear any response difference that I could consistently identify. And, for the sake of your tweeters, keeping the pilot signal minimized in the stereo output signal is important.

    Next time, tube life issues will be discussed, as will some quirks of specific units. After that, a simple but effective enhancement to the FM Automatic Stereo switching feature will be offered.

    Dave
     
    Last edited: Mar 19, 2016
  13. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Thanks Dave, another great post and very informative. Will have to re read this a few times as I am trying to learn. Will be following ....Al
     
  14. buglegirl

    buglegirl In The Direction Of The Singularity Subscriber

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    Hiya Dave,

    Getting the popcorn out :)

    :lurk:

    Frannie
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    ADDENDUM:

    Before I move on, I should note that regarding my previous post, the response measurements made on my FM-200B at the output of the stereo decoder will be slightly better than that measured at the same point on any of the Fisher vacuum tube stereo receivers. The dedicated tuners specified a low plate resistance tube for V-100 and V-102 in the multiplex chassis (12AT7), and used a 22K resistance for each of R222/R223 at the output of the MPX circuit. This version of the MPX decoder design represents (I believe) the original design of these units, as it predates those used in the vacuum tube MPX stereo receivers. When the receivers were released, their decoders changed V-100 and V-102 to a higher plate resistance 12AX7 tube, and increased R222/R223 to 27K each. Both of these moves in theory, will cause some degree of increased HF roll-off. In actual practice however, the effect will almost certainly be so slight as to only be measurable with test equipment, rather than producing any audible change in the listening room. It is important to understand however why the tube change was made, which thanks to Frannie (buglegirl) for donating an unused adapter from her "crap pile" (her words -- gotta love that girl!), will be discussed under tube life below.

    TUBE LIFE

    1. Heater Voltage: The small pentode tubes used in the IF portions of Fisher FM gear is very susceptible to developing conditions of heater/cathode leakage, or outright shorts across those elements, due to breakdown of the heater/cathode insulation. It is highly common to test these tubes and find that a leakage condition exists, even though the tubes often test (otherwise) as GOOD. High heater voltages to these tubes only acts to accelerate the possibility of this condition developing. Therefore, holding the heater voltage down to no more than +103% will really help to minimize this condition from happening. In my own units, a CL-90 in my tuners and a CL-80 in my receivers achieved the desired outcome for my line voltage level. Being aware of the heater voltage applied to the tuner circuits and taking appropriate action will definitely help maximize the life of these tubes.

    2. Tube Function: For 6AU6 tubes that develop significant heater cathode leakage, you can extend their useful life by using them in the IF Limiter Stages of the IF Strip. Tubes with significant heater/cathode leakage that are used in the IF Amplifier stages can introduce noise in the output by way of the AC heater voltage being introduced into the IF signal. It can do this because the cathodes of the IF Amplifier stages operate above ground, while the heater circuits references ground. Therefore, the cathode becomes a point where noise can be introduced. However, in the Limiter stages, the cathodes are always connected directly to ground. Therefore, there can be no noise introduced by way of this element -- even if heater/cathode leakage exists within the tube used. For reference, in units with IF strips consisting of three tubes, the last one will be the Limiter stage. For units with a four tube IF strip, the last two tubes will be Limiter stages. For units with a five tube strip, the last three tubes will be Limiter stages, while in units with a 6 tube IF strip, the last four tubes will be Limiter stages. Using your tubes with heater/cathode leakage that are otherwise good in the limiter positions then will also help to maximize the usefulness of these tubes.

    3. In the past, I've theorized that the reason that the original tubes used at V-100 and V-102 in the original WX MPX sub-chassis (12AT7) were changed to 12AX7 tubes in the MPX-65 (as used in the receiver setting), was due to streamlining Fisher's tube inventory -- getting rid of the 12AT7 in any new designs. That may still have been true, but if so, it was a side benefit -- not the motivating factor, as further testing has shown.

    My original thread on substituting 12AT7 tubes into V-100 and V-102 in Fisher receivers centered on determining the effect such a change had on stereo separation and the signal developed at the stereo indicator output from the MPX circuit. With the limited scope of that testing, it was determined that stereo separation was completely unaffected by the use of either tube at V-102, while changing to a 12AT7 in a 12AX7 based decoder at V-100 would have just a very slight affect on stereo separation (which could easily be corrected by touch up of the stereo separation control). Therefore, either tube could ultimately perform identically with regards to developing a proper stereo signal. However, further testing has shown that other more important changes occur with these tube swaps, in addition to the original facts presented.

    In the tuner setting, the typical B+ voltages applied to the WX MPX chassis points B+1 and B+2 are 125-150 vdc and 165-175 vdc respectively. These are also the typical voltages that my alignment jig provides to a prospective adapter when using it to align a multiplex sub-chassis, as those levels will serve to adequately power any of the Fisher tuner/receiver MPX decoders. The alignment jig is also what was used when the initial tests were made that were published earlier. At these voltage levels, the 12AT7 is the preferred tube in both locations, and indeed is the preferred tube to use overall in these locations, as by all appearances, Fisher's basic time-division decoder design was originally based on these tubes. But the 12AX7 can still provide very acceptable performance in either location, and offers greater flexibilities and economies with regards to the stereo indicator output.

    However, the supply voltage to B+1 in the typical Fisher receiver is more on the order of 300 vdc, or at least double that for the this supply point in the typical tuner setting -- and even more so with today's higher line voltage levels. At this voltage level, it produces significant problems if 12AT7 tubes are used at V-100 and V-102, due to the type of bias arrangement (contact) used for these stages. For starters, it causes the input section of V-100 and both sections of V-102 to dissipate about 1W of power in each section -- much more than these sections need to to do their jobs properly (this power level is enough to actually drive a small speaker). Additionally, the plate load resistors for all three of these tube sections basically get slow cooked, as they are now dissipating more power than they are rated for, which will be detrimental to long term dependability and alignment stability. In addition to these issues, it also causes the B+ dropping resistors R120 and R123 in the receivers (based on the 800C schematic) to operate above their dissipation rating as well, in addition to lowering all the basic operating voltages throughout all the small signal circuits of the receiver. The casual tube rolling exercise through the MPX section of a Fisher receiver then can really put a number of components in distress when the exercise strays outside of rolling with the specified tube (12AX7).

    For a little more, Fisher could have designed the power supply of their receivers to accept the WX decoder (using the 12AT7 tube) as is, but of course, that costs more money than changing out tubes. So that's what Fisher did: they simply chose to change out the two 12AT7s for a tube that would still properly bias itself when using contact bias at the elevated supply voltages that could be economically obtained in the receivers, resulting in the switch to the 12AX7 for the receiver venue.

    So if desired, how difficult would it be to properly adapt the receivers to use 12AT7 tubes at V-100 and V-102? Actually, with today's smaller components, not that hard:

    1. Locate the orange wire that supplies B+1 power to the MPX sub-chassis, and disconnect it where it connects into the receiver power supply (usually at the B+ supply point for the Filter or Tone Control 12AX7 stage closest to the MPX subchassis).

    2. Insert a 15K 2 watt resistor between the original supply point and the orange wire.

    3. Connect 22 uF 500V electrolytic cap between the connection of the 15K resistor/orange wire connection, and ground (negative to ground).

    4. To ice the cake, I also change R123 to a 2.7K 2W resistor. This has nothing to do with the MPX tube change modification, but results in very normal supply voltages at the 350 volt and lower supply points, to help account for today's higher line voltages -- even when a current limiter is installed to help aid that effort as well.

    And that's it. I leave it up to the individual to install the addition B+ dropping network as they best see fit. I have performed this modification to my own 800C, and it works beautifully allowing 12AT7 tubes to be used and now bias properly at the V-100 and V-102 locations. Not only does it allow then for the optimum tube to be used based on the design, but for this application, the 12AT7 is a much heartier tube as well, and so will well outlast the 12AX7s as originally specified for use in the receivers.

    The rest of this post will be split into a second post to accommodate content.

    Dave
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OTHER SCHTUFF (UNIT SPECIFIC)

    1. 500C/800C RECEIVERS:

    A. In my 400 receiver improvement thread, when doing those modifications, I advocated removal of the permanent LF filter built into that unit and most Fisher integrated amplifiers as well, as today, its need is an anachronism. These permanent LF filters are also built into the 500C/800C receivers as well, but should not be removed from these units. Use of the diode matrix circuit to perform the audio stereo/mono switching in these units eliminates the use of a relay and its concerns with these types of signals, but does have a down side: It produces a very strong pulse at its output when it changes state. If the permanent LF filters are removed on the bigger receivers, the LF pulse produced from the matrix board will temporarily overload the Low Filter Amp tube with each change of state, producing a very lazy switching event. The permanent LF filters prevent this from happening, and therefore should not be removed from these models if the FM Automatic switching function is to operate smoothly. Of course, the 400 receiver does not have the FM Automatic function, so this concern does not apply to that receiver.

    2. FM-100B TUNERS:

    a. Late versions of this tuner also used the diode matrix circuit to perform the automatic stereo/mono switching of the audio signal, but did not have adequate attention given to the effects of the LF switching pulse from the matrix board as described above. Therefore, if the rear panel audio level controls are operated near full setting on this version/model, the pulses can create lazy switching events due to overload of V-9, in a similar manner as explained above. In late versions of this tuner then, this can be corrected by lowering the value of C50/C55 to .005 uF. This will still allow for full audio response down to 20 Hz, but introduce enough ultra-low frequency filtering to prevent the overload during switching events.

    b. Because of the physical layout and the design of the AF Amplifier stage (V-9) operating at such high input impedance levels, at least the late versions of this unit are also susceptible to picking up a popping noise in the output when the stereo control relay changes state. Adding traditional dual suppressor diodes to the relay's coil will suppress these noises. One diode should be connected directly across the coil (cathode end to the terminal of the relay coil connected to B+), while the other diode should be connected in series with the lead from the other relay terminal, going back to the Selector Switch (cathode end oriented towards the switch). Finally, adding a .01 uF 400 volt cap across terminals 2&3 at the rear of S-2 (Selector Switch) will contain any switching noise generated when manually switching between forced mono and forced stereo functions.

    3. 400 RECEIVER DIAL ACCURACY:

    For those who do their own alignment touch-up to this receiver, in addition to the alignment error that Larry found in the Fisher Service Manual, also note this as well: Fisher always instructs to first make sure that the dial pointer stops (without forcing it) on the Log Scale at 0 before starting any alignment procedure. This is true for all Fisher tuners and receivers, and in most of these devices, when adjusting the dial pointer to stop at 0, it also stops at 100 at the top of the Log Scale as well.......except in the 400, where like the Energizer Bunny, it just keeps going. This is obviously a mechanical design error that results -- when following the alignment instructions to the letter -- with pointer frequency tracking error in the middle of the band, even though the extremes of the band may be adjusted to track almost perfectly. This can largely be corrected by instead of adjusting the pointer for a 0 Log reading at the low end of the scale, adjust the pointer to extend equally beyond the log scale at BOTH ends of the scale. This will then center the length of the dial plate frequency markings within the travel of the tuning condenser, so that when the high and low portions of the band are calibrated accurately, the middle of the band will remain accurate as well.

    4. 800C AM TUNING METER FIX:

    After I got my own 800C and flushed all the bugs out of it, it still bugged me that the AM tuner meter always showed very limited response -- even to strong signals. Other 800Cs I've come into contact with displayed the same exact symptom, so it was clearly a design issue. I know -- it's AM. But this is a Fisher 800C, and even the AM should work properly. This one seems to be a missed detail from when they gathered together the AM parts and pieces from previous designs to incorporate them into the 800C. The fix is simple enough: Connect a 560K small wattage resistor across C49 (i.e., from the AVC line to ground). This modification will then have typical strong AM stations registering at "4" on the tuning meter, just as the same type stations register with FM listening.


    In the last section, I'll address the FM Automatic switching feature in more detail for all units that include this feature, and also address the muting function for the FM-100B/200B (and FM-1000) tuners as well. Some simple modifications can really improve the effectiveness of these features.

    Dave
     
  17. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Dave; The reason the data above got jumbled is the formatting within the forum program. It allows 2 spaces only without any other keys pressed. So you'd need to add periods, or dashes between columns. Here's what I figured out. If it's wrong please let me know and I'll fix it.

    .......STANDARD....MONO.....STEREO L......STEREO R.....DIFF................(avg)
    1. @ 2.125 kHz:....-3.0 db......-3.0 db (ref)......-3.0 db (ref)....-3.0 db (ref)...+/-0 db

    2. @ 4.250 kHz:....-6.0 db.......-6.5 db............-7.3 db.............-7.2 db...........-0.75 db

    3. @ 8.500 kHz:...-12.0 db......-11.95 db.......-14.05 db.........-13.85 db.....-2.0 db

    4. @ 17.00 kHz:....-18.0 db.......-18.2 db.......-23.05 db.........-22.90 db.....-4.775 db

    Larry
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    That's perfect Larry! Thanks for the explanation!

    Dave
     
  19. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Dave you did a great job on my 800C and the meter on AM works fine with the mod you did. Your info on the 200B makes me appreciate mine even more, thanks for the work on that as well. Glad to see your finally getting some time to work on some of your own projects! Waiting to see pics of your 200B. :) Al
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    THE 400's STEREO BEAM: CLOSING THE GAP (Pt 1)

    I thought I'd throw this one in next, as over the years, there has been much discussion about whether the eye tube on this receiver should or shouldn't ultimately close -- and if not, how far should it close. This topic really got started when EM84 tubes were first used to replace the original EM84A eye tube, and it was noticed that the beams no longer closed as tightly as they used to. With the EM84A basically unavailable anymore, the EM84 was reluctantly used as a replacement. Later, it was noticed that the 6HU6 (EM87) would be a better replacement for the original high sensitivity tube specified. So how close of a replacement is it?

    The EM87 is in fact more sensitive than an EM84, is a direct plug in replacement (by in large), and in my experience, shows even slightly more sensitivity than and EM84A does in the 400 receiver. In a purely unscientific sampling of the half dozen or so EM87s I've worked with, all could easily extend the beams on a strong stereo broadcast in a properly aligned receiver to achieve the same gap closure distance that the specified EM84A will (1/8th inch on average), and often produce a slightly small gap yet. The EM87 does have slightly increased heater current requirements over the EM84 family, but it is insignificantly so in terms of the total current draw of the set, with ultimately the increased heater requirement indicating the EM87 to be a heartier tube, and therefore, longer lasting.

    Importantly, the EM87 does one other thing correctly in replacing the EM84A: In late versions of the 400 receiver, which maintains B+ to the eye tube at all times, the beams of the EM87 can be made to fully retract and darken when non-FM functions are selected, which the lower sensitivity of the EM84 will not allow it to do in the design of the 400. This makes it very easy to immediately spot any late model 400 receiver that has had the eye tube replaced with an EM84: When retracted on non-FM functions, you will still see a portion of the beams protruding in from the edges, since they won't fully retract in 400. With the EM84A, they do, and may do with as well with the EM87 in the stock design. This feature, coupled with its high sensitivity, makes the EM87 the best replacement for the original EM84A by far. It is not a perfect replacement however, so the tube will benefit from a simple circuit modification I've developed to optimize its operation.

    The modification is simple enough: Replace R26 (470K) resistor with a 1M resistor of the same wattage rating as the original. That's it. This resistor is located within the rubber boot covering the backside of the eye tube socket. This modification then allows the beams to fully retract on non-FM functions, and go dark. On strong Stereo MPX stations, the gap will typically close tighter than that of an EM84A, closing to within 1/16th inch on average. If it is desired to adjust the gap -- either to open it to that which an EM84A produces -- OR cause the gap to completely close -- it can be adjusted by adjusting the 1.8M resistor up or down, that's located at the indicator output from the MPX sub-chassis. In early versions, this resistor is located on the sub-chassis itself, and is identified as R206. In later versions, this resistor is located on the receiver chassis, on a T-strip near the MPX sub-chassis. Raising it's value will further close the gap, while reducing it will widen it. Removing the resistor altogether will typically cause the gap of the EM87 to overlap which should be prevented, as over time this will promote a burn mark on the florescent screen at the point of overlap. Therefore, some value of resistance at this location is still desirable when using EM87 tubes.

    For those with the original EM84A, removing the 1.8M resistor typically allows the tube to just close on strong Stereo MPX signals.

    Ultimately, as shown by design evolution, Fisher never intended for the eye beam to completely close (although some combinations of stock receiver and particular EM84A tube will close the beam under strong stereo signal conditions). In fact, Fisher even went so far as to install an added component in at least the 70-something serial number units, to help ensure that the eye tube would NOT close under any condition of operation. This is primarily because once the beams close, their effectiveness as a signal strength indicator is greatly diminished. Since they wanted the Stereo Beam tube to still be able to indicate precise tuning on even the strongest stereo signals, they took an additional step to ensure that the beam would never completely close. This was absolutely accomplished in late-late model serial number units by bridging R22 (a 3.3M resistor) with a 4.7M resistor. This measure ensured then that even the most sensitive EM84A tubes would not completely close under strong stereo signal conditions. The adjustment information mentioned above is based on this resistor being removed if present.

    EM84: For those with EM84s installed who wish to make the best of this tube, a circuit modification I've developed will cause its performance to be more on the order of an EM84A. However, while the modification will cause the gap to close similar to that of an EM84A, the telltale sign of the beams still protruding in on non-FM functions will still be there. Overall however, the modification greatly improves the display appearance of the EM84 tube when used in the 400. Given that the EM84 is the most plentiful (and least expensive) of the three tubes, it makes this approach the

    SIDEBAR: To make the beams of an EM84 react exactly like that of an EM84A (i.e., the ability to fully close, or fully retract and dim) would require at least the addition of an additional stage of DC amplification to make up for the reduced sensitivity of the EM84 tube. This can be done, but gets necessarily complex for many diyers, or requires more modification to the unit than some might wish to perform. Therefore, I've also developed a simplified modification for the EM84, that causes it to achieve the same gap closure as that of the stock design w/EM84A, and more if desired. The modification itself is simple enough to perform:

    1. Locate the 470K resistor (R26) located at the back of the eye tube socket, and disconnect the end of that resistor soldered to pin 6.

    2. Solder a length of wire to the free end of the resistor, using tape or heat shrink tubing to insulate the connection. The wire should be long enough to twist in with the existing twisted leads serving the eye tube that come up from the underside of the chassis, and be able to extend below the chassis with this group of wires, and over to the area of V11.

    3. Underneath the chassis, locate the single terminal T-strip where resistors R85 and R86 connect to from V11.

    4. Connect the new lead from the eye tube to one end of a 330K .5 watt resistor, and the other end of this resistor to the terminal on the T-strip identified in #3 above.

    As before, the gap can be adjusted with the 1.8M resistor previously identified: Removing it will allow the gap of the EM84 to nearly close with this modification. Lowering the value of the 330K resistor from step 4 will in fact then cause the gap to close, but also cause the beams to protrude in from the sides ever so slightly more as well.

    This presents a range of options you can use then depending on your own personal appearance preferences for the Stereo Beam display, and the particular tube and tube type you are using. Also, you have the original typical gap closure distance to use as a guide when making any checks or adjustments. Finally, it might be helpful to some to describe how the Stereo Beam should react in various scenarios, which will be presented along with some pics in the second installment.

    Dave
     

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