Fisher 400 low right channel on fm stereo

Discussion in 'Fisher' started by Rapscallion, Sep 10, 2018.

  1. Rapscallion

    Rapscallion New Member

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    Hi everyone.

    Recently picked up a 400, had the basic power supply work done. Sounds great on aux and phono and fm mono. But on fm stereo, the right channel volume is about 1/2 of the left. I’ve tried swapping around the 12ax7 tubes but no difference. Any ideas?

    Thanks in advance.

    Brent
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The problem will almost surely be in MPX-65 MPX sub-chassis. Check the tubes there as well. If not, it will need to be serviced.

    Dave
     
  3. Rapscallion

    Rapscallion New Member

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    Yes, I’ve swapped the tubes around in that section and used deoxit on the tube pins to no avail. I guess I’ll try tightening up the contacts for the tube pins next.
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If you're comfortable working on the unit, then replacing the two output coupling caps would be the next order of business after checking the tube sockets, and also replacing the 1 uF electrolytic composite signal coupling cap. The output coupling caps .047 uF caps associated with the 12AX7 tube that sits off by itself from the other two, and the 1 uF cap is close to and connects to the 67 KC trap coil, which is labeled on top of the chassis.

    I hope this helps!

    Dave
     
  5. Rapscallion

    Rapscallion New Member

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    Thanks Dave

    I’ll take a look at those. I assume those are all polarized caps?
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    No -- only the 1 uF is a polarized electrolytic cap. The coupling caps are mylar caps.

    Dave
     

     

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  7. Rapscallion

    Rapscallion New Member

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    Ah. Ok. Glad I asked.
     
  8. Rapscallion

    Rapscallion New Member

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    Sounds like the output coupling caps are likely suspects, since there are two of them, I assume one for each channel, right?
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Correct.
     
  10. Rapscallion

    Rapscallion New Member

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    Ive seen something about removing the big red filters as well, you think that’s a good idea?
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    NO!!! Where ever that got started from on the interweb has really done the Fisher audio community a real disservice. Those coils form a sharp 38 kHz notch filter to remove remnants of the AM subcarrier used to carry the stereo information channel within the main FM carrier, and also work in conjunction with the de-emphasis components to minimize the 19 kHz pilot tone in the output, while providing the correct de-emphasis response down in the upper portions of the audio bandwidth (up to 15 kHz for FM transmissions). Removing or altering these filters can seriously place your speaker's tweeter elements in danger of burnout without you even knowing it. A tweeter element can typically only withstand a maximum of a watt or two of power applied on a continuous basis, which is perfectly adequate for even large sound systems since so little audio energy exists up in that region. But if speaker efficiency and listening conditions are such that they start getting large doses of 19 kHz energy because the pilot signal is no longer appropriately attenuated (due to the networks being removed), and you're unaware of the potential problem (as many folks are), then you'll wonder what happened to your tweeter elements without ever really knowing why. Fisher vacuum tube receivers can pass their full rated power at 19 kHz, and while they can't at 38 kHz, they can still pass plenty enough power to take out a tweeter at that frequency.

    In my thread on maximizing the performance of Fisher FM MPX tuners and receivers, I give the results of frequency response tests I have run on the stock design of these filters, which show that as designed, they produce very acceptable adherence to the 75 µsec de-emphasis standard, while also providing very good attenuation of both the 19 kHz pilot and 38 kHz sub-carrier frequencies as well. Their design and resulting component values are quite precise when all the necessary variables are accounted for, being optimum as presented for the functions they serve. Therefore, I would encourage you to leave well enough alone as is with these circuits.

    Dave
     

     

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  12. Rapscallion

    Rapscallion New Member

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    Well I’m taking that as a resounding “no” then! Thanks. What voltage should I be getting on these caps? I see 100, 250, or 630 volts in the .047 uf caps. I assume higher is safer, is that right?
     
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    For the mylar caps, a 630 volt rating is fine, with various manufacturers making caps of this value at that voltage rating that are no bigger, and likely smaller than the original pieces. For the electrolytic cap (which can also be a mylar if you chose), you will want a 350 volt rating or better. As an electrolytic piece, a cap of this value (1 uF) at 350 volts is about half the size of the original component. If you chose mylar for this location as well, a 400 volt piece will be required, and will be significantly larger than the original component -- but can still be fit into place with some effort.

    Dave
     
  14. Rapscallion

    Rapscallion New Member

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    Excellent. Off to frys I go! Thanks so much! You are the Fisher whisperer!
     
  15. Rapscallion

    Rapscallion New Member

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    Hey Dave

    Just took off the bottom cover and took a peek. The 1uf looks easy to do. The .047s look like a bear to access! My fingers are not tiny enough! Any pro tips? Looks like a very tiny soldering iron and lots of tweezer skills are necessary!
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Yeah, the coupling caps can be a bit of a bear if you're not comfortable working in tight places like that. One thing that sometimes helps is to remove the screws holding the sub-chassis in place, and pull up on it just enough so that you can better get to things from the side above the main chassis. Another option is to simply remove it. It is connected to the main chassis by 8 color coded leads. If you disconnect the leads at the main chassis end, then you can remove the entire sub-chassis which eases the job considerably.

    Just be sure not to touch the blue separation control, it normally sits in the 5:00 - 7:00 range for most examples. The setting is factory set for optimum channel separation -- which deteriorates rather quickly when moved from the optimum setting.

    Be careful replacing the 1 uF cap -- one lead connects to the 67 kHz trap coil, and the other end to the resistors providing the balanced feed to the diode matrix circuit. Keep your heat as low as possible, and don't pull so hard as to damage the terminal on the trap coil. A snip and tack approach might be best if you are unsure.

    Dave
     

     

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  17. Rapscallion

    Rapscallion New Member

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    Thanks. You’re a little late on the warning for the channel separation dial, but it seems to be optimum around the midrange, so I’ll just try to tweak it once I’ve replaced the caps. I might have a local tech do it, worth the $50 or so to not worry about screwing it up.
     
  18. fred soop

    fred soop Super Member

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    It would seem that the separation control could be set using a scope in XY mode looking at the audio and adjusting for the largest rats nest, in lieu of having a regulation multiplex generator.
     
  19. Rapscallion

    Rapscallion New Member

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    Thanks Fred. That’s definitely above my pay grade, but I’ll pass that info on to my tech, I’m sure it will sound less like Greek to him!
     
  20. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Rascallion. There is a thread around here (probably in the Multiplexer thread) where Fred shows a couple pictures of a Multiplex signal off spec and on spec. It does look like a ball of Steel wool when it's in spec on the scope.
     

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