Just bought a KX-200 - WOO HOO !

Discussion in 'Fisher' started by thornev, Jan 13, 2019.

  1. thornev

    thornev Well-Known Member

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    765
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    Now testing the PHONO inputs, "MAG-2" on this kx-200. I can't get it to sound as full as with signals played through AUX-1. And when I turn the volume to max with no signal source (turntable cables unplugged), I'm getting a low-frequency click every 1/2 second. And then sporadically I get some noise that lasts about 1 second and that sounds like the alien in the movie "Annihilation". And the alien is coming out much louder from the right channel. And then there's the requisite 120Hz hum in both channels.

    I unplugged everything, only speakers are plugged in and I still get the noise when on MAG-2. I get it on MAG-1 also. TUNER is dead quiet as are AUX-1 and AUX-2. So this is noise in the 2 MAG circuits.

    Then I tried different tubes, Genalex B759 gold pins. With those I get white noise out of both channels. Weird. Trying different tubes, now they all produce white noise, louder in one channel. Tried some old old RCA tubes and... alien is back !
     
    Last edited: Jan 20, 2019

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    See if turning the treble control down eliminates the clicking noise and the alien residency with all tubes.

    Dave
     
  3. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
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    "See if turning the treble control down eliminates the clicking noise and the alien residency with all tubes."

    The tone controls have no effect at all on the noise. I think the phono preamp stage is early in the entire circuitry (tubes V1 & V2 of V1-V6). The tone stack (V3 & V4) and High FIlter amps follow the phono preamp stage. Does Fisher deploy Negative Feedback loops? (I think that's more a guitar amp thing) I changed PI tubes (installed an old RCA 12AX7A and a tube with the old "7025" markings) but haven't adjusted either PI. I don't believe that's the problem.

    There's apparently a 47k ohm resistor in the phono preamp circuit. I'll check it. Is it safe to turn the unit on with the preamp tubes removed? Maybe that can tell me something. The service manual states that feedback is used around each stage to reduce internal noise and distortion. Maybe one of my feedback loops is bad? How do I identify them?

    Finally, V5 & V6 are direct-coupled PI and voltage amps and are last in line before the output transformer and tubes.

    Maybe a grounding problem or lack of at some stage?
     
    Last edited: Jan 20, 2019
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    You've got it backwards -- negative feedback loops is a high fidelity/stereo thing. It is little used in guitar amps if at all, and sparingly if done so. That's why guitar amps are so sensitive to tube rolling, whereas hifi equipment is not. But with so many guitar amp techs entering the vintage stereo equipment field, they don't know otherwise.........

    Fisher equipment that is physically laid out as the KX-200 is is sometimes subject to unwanted positive feedback by way of the low level inputs being so close to the output stages -- particularly Channel A. Turning the treble control down minimizes the amount of unwanted feedback if its occurring. Since your symptoms were not affected by turning the treble controls down, then replacing the power supply cap serving the phono preamp stages would be the next thing to check........

    You can operate the amplifier with the phono tubes removed with no problems.

    The KX-200 being a kit, it is always good to verify that the kit was actually built correctly to begin with.

    Dave
     
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  5. Linehand

    Linehand AK Subscriber Subscriber

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    Aside from this issue, how does this compare to your 500c?
     
  6. thornev

    thornev Well-Known Member

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    765
    Location:
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    "replacing the power supply cap serving the phono preamp stages would be the next thing to check........"

    Thanks, Dave. I'll see if I can figure out which cap you mean. I've ordered the Hamfest Hayseed kit so that should cover it. A previous owner soldered a loose electrolytic (C1 I believe) on TOP of the chassis where a can cap used to be. The only thing holding it in place is that it is soldered to 2 resistors (which should be a single 1.8k) UNDER the chassis ! I'm curious to see the kit's "build manual". I think I saw a place that has copies for sale. I bet it explains a lot about what the parts are for and man that would be very helpful to my electronics education.

    "Aside from this issue, how does this compare to your 500c?"

    The 500-C and kx-200 are of course similar. I think the overall sound is better on the kx-200. Deeper bass, more clear highs, and because I'm feeding AR-2Ax, power hungry speakers, I've got the volume on the kx-200 turned up to 1 o'clock so it's really pumping out sound. I like the simplicity of the KX circuitry compared to the 500C. I'm impressed by the fancy, ahead of its time (?) gizmos like STEREO DIMENSION. I use it occasionally; haven't used the CENTER SPEAKER LEVEL.The overall sound is fun to listen to.
     
    Last edited: Jan 20, 2019

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The power supply cap serving the phono preamp stages is one of the caps in the can caps -- which will be replaced when you install the HH caps you've ordered.

    Sounds like you have quite a project on your hands! If you can get an assembly manual, that will no doubt be a tremendous help!

    Dave
     
  8. thornev

    thornev Well-Known Member

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    765
    Location:
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    I cannot find a kx-200 build manual, but I've downloaded a kx-100 manual. Hopefully they are similar. I'll have to research the differences.
     
  9. Tim D

    Tim D AK Subscriber Subscriber

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    Thorne,

    I'm guessing there's gonna be a lot of differences between the KX-100 and KX-200. I believe the KX-200 have whistles and bells that are going to make comparisons difficult. Can you find any diagram for he KX-200?
     
  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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  11. Tim D

    Tim D AK Subscriber Subscriber

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    I find the service manual more useful than the assembly manual.
     

     

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  12. thornev

    thornev Well-Known Member

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    Are MYLAR capacitors the same as FILM capacitors? mouser doesn't list anything called MYLAR so I bought FILM. Mistake? I have orange drop caps and yellow cylindrical (film) caps. Can either replace the MYLAR?

    And is it desirable to replace the .047uF 400V with .047uF 600V ? I figure that given the higher voltage into the house, replacing with higher voltage parts makes sense.

    UPDATE: It seems mylar is a type of film capacitor. Are orange drop better than yellow cylindrical? And is it OK to mix these 2 types?
     
    Last edited: Jan 21, 2019
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Mylar is the DuPont brand name for Polyethylene terephthalate . Film caps are generally either made from polyester or Polyethylene.

    If the coupling caps are 250V and 400V, (and that possibility exists as they are in the receivers for the most part(400, 500/800b/c)) then bump them up to 400V and 600v. You don't need a 600v cap in th 250V posit. If all 400V go to 600V. Yes either can replace the Mylar ERO-FOL's.

    Depends on whether you get the generic yellow axials(polyester) or Illinois MKP axials(polyethylene). AFAIC it's pretty much a wash for either compared to the Orange drops for SQ. Your ears may say differently. However for size and installation convenience, the MKP's and generic yellows have an edge over the Orange drops.

    As for mixing them up, SURE. Mix them by section, not by side. In other words, unless you want to compare the O.D. to the yellow, Don't put O.D. on one channel and a MKP on the other channel in the same place.
     
  14. tube-a-lou

    tube-a-lou Super Member

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    Hi Guy's, Great thread thornev!
     
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  15. thornev

    thornev Well-Known Member

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    765
    Location:
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    I replaced C25, C26 (which are attached between the center tab on the Bias adjustment pots and the center tab on the Balance adjustment pots) that are 50uF 70vdc electrolytics with 100uF 100vdc. Is increasing capacitance in general a bad move? I've read conflicting opinions. I trust this "group".
     
  16. 8xlaxx

    8xlaxx It's all ball bearings nowadays. Subscriber

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    Great thread @thornev
    Hope you find and knock out the gremlins!

    :lurk:
     
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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Nah that won't hurt anything. If anything, Probably add a little more filtering to the bias circuit, that's all.
     
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  18. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
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    Did you know that the LOUDNESS CONTOUR control on the kx-200 decreases its effect (boosting the bass and treble frequencies, aka "the smile curve") as one increases the MASTER VOLUME? (so one doesn't blow out one's speakers at extremely loud volume by accidentally turning on the LOUDNESS) Now THAT is a pretty darn nifty add-on that has to have been unique and innovative in the mid-60's.
     
  19. thornev

    thornev Well-Known Member

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    765
    Location:
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    There are 4 diodes in the kx-200. They're labeled as SD1 - SD4. In the parts list all it says is "silicon diode, high voltage" and "silicon diode, filament & bias". All 4 diodes are connected to the power transformer secondary, red lead with yellow stripe and brown lead and orange lead. None of these descriptions gives me a clue as to what to buy. The print on these little things is too small to read (see schematic in post 19). Can someone help me out? I have some 1N5408 which I believe are for the 500-C power supply. And I have some STTH3R06RL that are rated at 3.0 amp and 600 v. I think I need 1,000 v, but I'm not sure. THanks.
     
    Last edited: Jan 21, 2019
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The volume control in your Fisher receiver works exactly the same way: Compensation is decreased as volume is increased. The Loudness setting is only supposed to be used at low volume settings anyway to account for the human ear's natural roll-off of both low and high frequency response as the absolute sound level decreases.

    The 1N5408 diodes will work just fine in all four diode positions of the KX-200.

    Dave
     
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