Getting the Most From Fisher FM Stereo MPX Tuners and Receivers

[dcgillespie]

THE 400's STEREO BEAM: CLOSING THE GAP (Pt 2)

1. NON FM FUNCTIONS: As mentioned earlier, early versions of this receiver simply cut off the B+ power to this tube when any non-FM function was selected, so the tube simply went dark. In later versions, Fisher (wisely) left the B+ applied to the tube at all times (to enhance tube life), but had the beams fully open and darken -- with either approach ultimately indicating that FM functions are not engaged. EM84 tubes will not fully retract as discussed.

2. FM MONO: Here the beams of an EM84A tube will each extend in more than half way in from their cut-off position when tuned to any strong station, as in this mode, the eye tube is acting as a conventional tuning indicator, reacting to the strength of the FM carrier signal. Closure is limited in this position, as further closure is reserved for indications of stereo MPX reception when FM STEREO or FM STEREO FILTER is selected. In the latter two positions, a true mono FM station will only cause the beams to just slightly extend in from the their resting (cut-off) position, as in this scenario, they are only reacting to random supersonic noise components that are present in the reception of a mono station. For this and other reasons, mono FM stations should always be tuned and listened to with the receiver set to FM MONO, so that optimum reception can be achieved for that type of transmission.

3. FM STEREO/FM STEREO FILTER: Here the beams of an EM84A tube extend to approach closure with reception of a strong MPX Stereo FM station. In this mode, the eye tube is still acting as a tuning indicator, but now, it is reacting to the strength of the stereo pilot signal, rather than the strength of the FM carrier signal. While it won't happen on every station, there should be good agreement on most stations between the optimum tuning point where the FM carrier signal, and the stereo pilot signal, is maximized. This can be checked by observing the optimum tuning point on an FM stereo MPX signal, when turning the Selector switch between FM Stereo (or Stereo Filter), and FM Mono positions. If the optimum tuning points are notably different in these two modes, then it usually indicates an issue with alignment.

A few pics are included. Since the EM84A's and EM87's full closure or full retraction/darkening are a defined condition, pics of those tubes/scenarios are of little value. Therefore, the following pics are all of a new EM84 tube (the old one was shot) using the modifications and scenarios discussed, so its limitations and the improved operation from the modifications installed can plainly be seen. The receiver used is a client's that had just undergone a full alignment of all RF, IF, and MPX circuits:



ABOVE: With the modifications discussed in place, this is the EM84 tube with the selector set to a non-FM position. Along with the panel lamps, it does serve as a power indicator, but is not the indication Fisher intended for the tube in this scenario.




ABOVE: Here, with the modifications discussed in place, the EM84 is mimicking the same gap closure produced by the EM84A in the stock design when tuned to a strong stereo station. For reference, the gap is 1/8th inch.




ABOVE: The same as pic #2, but now the 1.8M resistor has been removed.




ABOVE: The original EM84 tube removed from the set. The darkened areas define just how wide the gap is when using this tube in the stock design: 5/16th inch between the darkened florescent areas.




ABOVE: The 470K resistor (R26) discussed, residing within the boot cover of the eye tube.


Next time, I'll get back on track with some muting and Automatic FM modifications, and also address the 400's often dancing Stereo Beam.

Dave

 

[hunter00]

have to throw out a huge thx Dave .I had not listened to my Fisher -FM-100c in close to a yrs as the last time I fired it up it sounded enemic and underperformed up to my expectations based on my memory.
Reading this post just now motivated me to put some time into it so I tried it out with the other Fishers in my stack and it sounded very vailed,i pulled it out cleaned all the tube sockets ,all the tube pins ,deoxed the rca's and voila it came back alive!!I'm sure it was the tube sockets needing a refreshment cleaning..
cheers!

hunter

 

[39cross]

Dave, thanks for the wealth of information! I hope you will you have a chance to finish the topics you mentioned back in post #16, FM Automatic switching feature, etc.? That would be cool to learn about too.

 

[dcgillespie]

39 -- I plan to, but life has been quite busy for me this year. And, motivation to finish it has been lower because with so many FM stations broadcasting such a poor audio signal, I didn't know how much general interest there would be in these topics anyway. It was offered because sooo many of the units that come across my bench really have problems in the AM, FM, and MPX circuits -- problems that won't be fixed by cleaning tube sockets, installing new tubes, performing a recap, or even giving it a good alignment -- that I thought it might be a good idea to define just how well the AM and FM Stereo MPX sections SHOULD work, when operating properly as designed. Also, the FM environment has changed radically since this equipment was designed, so there are things that can be done to make them better equipped to deal with that new environment. As a result, it is my contention that many of today's newer Fisher fans who have equipment that has been restored with the traditional, well accepted processes, really don't know just how well the tuner circuits in their units can work, because -- again -- other than how general recapping procedures apply to them, the tuner circuits are so often treated as the stepchild of receiver circuits. So anyway, thanks for the shout out, and I do plan on finishing the thread up -- just as soon as I can!

Dave

 

[buglegirl]

Hiya Dave,

And, motivation to finish it has been lower because with so many FM stations broadcasting such a poor audio signal, I didn't know how much general interest there would be in these topics anyway.

I am very interested and not because I care much about local over the air FM broadcasts. I have had decent luck with building low powered whole house FM transmitters so I can listen to my Computer Stream on my 800C in the other room. Yes I could get a device to do this local to the 800C but I also broadcast to some of my older FM radios and consoles as well.

So getting the most out of the Fisher MPX and Tuner is of great interest to me just for this alone.

Thanks Again For This Thread!!

Frannie

 

[larryderouin]

I'm really interested in this as I have a late 400 on the bench with a Dancer, and my late model is also a dancer. And neither of them give lap dances. Bump and Grind is all they do.

Actually I think it will help those who have to take it to a shop. With the info in hand, the tech may/will be able to get whatever fisher he has on the bench that much better. And for those who can do on the bench, it helps a heck of a lot too, in that they will be able to tweak out that last percentage or two to get it sounding like it actually should on FM.

 

[39cross]

Thanks for the update Dave, looking forward to more info whenever you can find the time. I just picked up a 500c from Audiodon which I will be refurbing, but even in its neglected, stock form the mpx FM sounds very, very nice. I have to give it the nod over my Sherwood S8000-IV. Bringing out the best in the 500C will be a glorious sound I am sure.

 

[streetwise]

Dave would I be right in my calculation for the mono fm Deemphasis being mouser parts in my late model Fisher 400, 1800pf polystyrene cap and a 42.2k 1/4w resistor for 75uf?

 

[dcgillespie]

Yes you would! That puts it within about 1.3% of the ideal 75 uS target, which is outstanding!

Dave

 

[larryderouin]

What cap is he replacing, Dave? I usually just replace the mono De-Emphasis resistor with a 41.2K from 47K and the Stereo's with 16K from 22k or 27k whichever happens to be on the 400 or 800c at the time.

Larry.

 

[streetwise]

Im switching the ceramic C38 to polystyrene as dave suggested earlier in this thread. using 42.2k is close 41.7k would be ideal but isn't an available mouser part. although a vishay 41.7k .1% is available on ebay.

 

[streetwise]

Also Larry or Dave for the 16k on stereo de-emphasis how do you do the math on that one I get the mono de-emphasis. Capacitor x Resistor = .075

 

[larryderouin]

I'm a math idiot if it involves more than basic addition, subtraction, multiplication or Division. Substituting letters for numbers gives me migraines! I read the values somewhere back in the past but don't remember if it was here or somewhere else. Do it the same way as for Mono, then do the change for each channel.

This is for STEREO De-EMPHASIS ONLY.

I get right around 75.2 for 16K resistor and no change to the cap(.047uf). (Stereo De-Emph). It's about 126 with no changes to any parts.

Change both 27K or 22K (I've seen both in the MPX 65, R-222 & R223) to 16K, and replace the Brown dogturd .047uf with the same values. C221 & C222.

 

[larryderouin]

On the MONO the 47K and .0018uf gives you. 84.6.

41.2K &1800pf = 74.16 I prefer to be a little under if I can't get on exactly. In this case it won't make a hill of beans tho. Just get it as close as you dare. Mouser has 41.2K 1% resistors. Then series a 470 ohm 1/4w to it. Total resistance is 41 ,760. ohms. X .0018uf = 75.06us.

 

[streetwise]

duh... Thats right STEREO=2 thanks Larry I wasn't thinking there, I also like the idea of the two resistors in series vs. having to order off ebay to get the 75.06. I have one more question regarding the tuner in the Fisher 400. The previous owner aligned the FM but the dial is off on all channels for some reason if i tune to 88.1 its actually 90.1, 96.1 is 98.1 and so on is there a fix for this with out having an oscilloscope and fm generator?

 

[streetwise]

Ok Larry just brought our mono de-emphasis as close to 75 as can be with mouser parts. 41.2k resistor with 470ohm and 1800pf cap= .075006

 

[larryderouin]

Grab the KM-60 / KM-61 manual. As it's a kit and the tuner is basically the same as the 400, you can use it to align it fairly well. Follow the directions. If you have a late model unit, add another tuning transformer to the mix.

It could be as simple as the needle is off, or he didn't know what he was doing.

1st, tune all the way down to the lower end to the stop, and make sure the pin is on the LOG SCALE ZERO (0). If Not, loosen it from the string, adjust and tighten back up. Lock it at the clamps on the needle assy with a drop of Fingernail polish. Seriously! Then check the station locations on the dial. If still off, pick 2 KNOWN Stations, @ 90 and 106mHz or as close as you can possibly get them. If you have another receiver/tuner, get it within a foot or two of the 400, and set it to the frequency needed, between stations without interference (you want to hear hiss), then tune yours to match. When you get an exact match the hiss will null out on both units as the oscillators are in sync at that point. On the low end locate the oscillator pot and adjust it carefully and slowly while adjusting the needle toward the station or where the 2nd receiver is tuned to. When you have that done, run the 2nd tuner up to 106 or close to it on a clear area. Then tune the 400 to null out the noise. If the needle is off from what the 2nd receiver is showing use the trimmer and the knob to tweak the high end up or down to the indicator on the 2nd receiver, until you get a null. Go back to the low end and make sure it's still set where you left it. Adjust if necessary. Repeat on the high side. Repeat 2 more times, as it'll get better each time. Incorporate the KM-60 instructions into this along with the 400 instructions. Watch out for the Z4 instructions on the Later model units with regard to the tuning eye. The instructions are wrong. http://audiokarma.org/forums/index.php?threads/400-eyetube-adjustment-procedure-wrong.687346/

 

[dcgillespie]

If I may -- please recheck earlier in this thread, where I discuss the fact that when the 400 is set to stop at 0 on the Log Scale, it will normally go well beyond 100 at the top of the log scale. The most accurate dial accuracy alignment will be had when the total travel of the pointer is centered over the entirety of the Log Scale. That ensures the greatest frequency compliance between pointer indication and actual frequency received, over the whole of the FM band.

If the pointer is consistently off by the same amount over the whole band, then either the pointer is not accurately positioned on the string, or the oscillator is well out of adjustment as Larry said. If the oscillator is that far out of adjustment, then there is a good possibility that the entire RF front end is out of alignment as well.

As for the de-emphasis of the FM Stereo MPX signal, I would encourage you to also go back an review the tests I did on the frequency response of the Fisher MPX decoder earlier in this thread as well. Due to the presence of a notch filter in the output of each channel of the decoder, the math is much more complicated, as the notch filter is working in conjunction with a simple R/C filter as well, so calculation is not nearly so easy as it is with the mono network. My tests suggest that Fisher did a very admirable job of threading the response needle in their decoder design: If it is too heavy, the loss of HF information is apparent. If too light, it can allow too many 38 kHz artifacts to get through from demodulation of the stereo sub-channels, which can do everything from really goof up tape recorder bias oscillators, to damaging HF speaker components. I have found that using the original stereo de-emphasis networks as designed produces a very similar aural presentation to that presented by an accurate mono de-emphasis network.

Dave

 

[streetwise]

Grab the KM-60 / KM-61 manual. As it's a kit and the tuner is basically the same as the 400, you can use it to align it fairly well. Follow the directions. If you have a late model unit, add another tuning transformer to the mix.

It could be as simple as the needle is off, or he didn't know what he was doing.

1st, tune all the way down to the lower end to the stop, and make sure the pin is on the LOG SCALE ZERO (0). If Not, loosen it from the string, adjust and tighten back up. Lock it at the clamps on the needle assy with a drop of Fingernail polish. Seriously! Then check the station locations on the dial. If still off, pick 2 KNOWN Stations, @ 90 and 106mHz or as close as you can possibly get them. If you have another receiver/tuner, get it within a foot or two of the 400, and set it to the frequency needed, between stations without interference (you want to hear hiss), then tune yours to match. When you get an exact match the hiss will null out on both units as the oscillators are in sync at that point. On the low end locate the oscillator pot and adjust it carefully and slowly while adjusting the needle toward the station or where the 2nd receiver is tuned to. When you have that done, run the 2nd tuner up to 106 or close to it on a clear area. Then tune the 400 to null out the noise. If the needle is off from what the 2nd receiver is showing use the trimmer and the knob to tweak the high end up or down to the indicator on the 2nd receiver, until you get a null. Go back to the low end and make sure it's still set where you left it. Adjust if necessary. Repeat on the high side. Repeat 2 more times, as it'll get better each time. Incorporate the KM-60 instructions into this along with the 400 instructions. Watch out for the Z4 instructions on the Later model units with regard to the tuning eye. The instructions are wrong. http://audiokarma.org/forums/index.php?threads/400-eyetube-adjustment-procedure-wrong.687346/

Larry Thanks your guide helped. The oscillator was out of alignment Im not going to play with the rest of the tuner section because I can get every channel I can think of pretty well. Im sure there's room for improvement but that'll do for now. as for the other issue I had in another forum about the fm being really funky sounding one of the tubes in the mpx finally went and after replacing the problem is slightly better I might pull the mpx out at some point for an alignment but at that point I should do the whole tuner as well. Dave thanks for your knowlede on the string dial placement I would have set it better if the previous owner didn't glue it in place its only about an 1/16" off from extending below as it goes above. This is a life saver for this unit as our local North Country Public Radio is at 88.1 on the dial I couldn't come anywhere close to getting that or the alternate transmitters around 88.5

 

[Djcoolray]

From my understanding of things and being exposed to a compulsive audiophile that even had a WWII aircraft carrier radio set occupying an entire bedroom. There was allot of competion between the makers of Fisher, Harmon Kardon and Ampex concerning the MPX units and they all claim to be the best. Due to my age I have heard all three continuously for about a decade during the time when cabinets reigned supreme. But the thing I liked about the old cabinets is the improvements that can be made over solid state. Even so, it's hard to say which is better because no one cabinet sounded the same. People back then didn't just plug stuff in and just use it, not like today.