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Fisher kx 100b hum...maybe..I guess

Discussion in 'Fisher' started by MikeTest, Jul 22, 2018.

  1. MikeTest

    MikeTest New Member

    Messages:
    7
    Location:
    Kansas
    I realize volumes have been written and careers spent on susceptibility and emissions but ask your indulgence as I demonstrate my inexperience. My interest has recently turned to tube amplifiers with a seemingly fully functional kx 100b as my first victim. The only issue I know of is a slight 60 hz hum at the speakers (both channels) that gets louder and softer with the volume control. I am not calibrated to what's normal but don't think it excessive and figure I would chase it in the future with recap and the usual stuff (although I have scoped the power supply and it looks good to me....besides I thought power supply noise would show up at 120 hz...but I digress).

    My intention was to study tube theory in parallel with taking measurements on the amp to gain some practical experience. I realize it is seldom a good idea to just poke around on stuff, especially stuff that can kill you so I am careful and methodical.

    I did not get too far. I started at the phono preamp and put a scope probe one resistor away from the gate of 12AX7 (R9 for those that want to follow along). Looked normal to me i.e. looked like the input which I had at about 10mvac from signal generator. Tone volume responded accompained with the aforementioned underlying 60hz.

    This is where things went off the rails. Not wanting to continually change the scope setting yet wanting to know dc as well as ac I connected a Fluke 77 to the same place as the scope probe. When I did so the 60hz got a lot worse (both thru the speakers and on scope).

    So I go into disconnect and turn stuff off mode. In one of the iterations I noticed it was not the meter that caused the noise but the presence of a meter lead even without the meter. Result was the same with a single jumper cable like it acts like an antenna picking up 60 hz. But from what source ? There is nothing on any amp in or outs. The amp is off and not even plugged into ac. I shut off ac power to my garage/shop. There is nothing running on ac within 30 feet. Yet, when I clip a jumper on there is 60hz (with sufficient power to have an effect) showing up in the amp. You may ask how do you know if you have everything (including the scope) turned off ? Which is a good point and if this post weren't already ridiculously long I would go into the dozens of other observation I made in coming to this belief (not to be confused with a stone cold fact).

    I would be happy to clarify or provide more information if anyone cares to weigh in.
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,678
    Location:
    Ball Ground, GA
    Hi Mike -- Welcome the The AK Fisher Forum! Nice to have you here.

    Since you are (apparently) new to vacuum tube circuits, there is one thing you will need to learn about them, which is very important to your question: Compared to SS designs, vacuum tubes operate at very high impedance levels -- particularly at the tube's grid (the equivalent of a mosfet's gate). Therefore, without proper shielding, just touching your finger to the grid terminal, or adding a length of wire to the grid terminal will cause it to pick up all manner of stray 60 Hz noise that is all around us -- even if just in the wiring in the walls of our homes. Your observation then about placing a scope or meter probe on a grid terminal -- particularly that of the phono preamp tube where gain is extremely high (over 5000 from input to output with the volume control at max) -- is very normal.

    Fisher amplifiers are normally very quiet with the volume control at minimum, and even advanced, typically only display a hissing sound -- as long as the bottom plate and any other shielding panels are in place. Power supply noise will in fact show up as 120 Hz noise, so if you are getting 60 Hz noise, look for lead placement issues (lead dressing), a missing power switch shield cap, or even a missing AC line bypass cap (if used). Also, try reversing the AC plug and see if that helps. 60 Hz hum is always the result of AC power wiring inducing noise into sensitive circuits where it should not be. Again, since vacuum tubes operate at high impedance levels, it is easy for sensitive leads to pick up noise if the build is sloppy or not properly executed.

    I hope this helps!

    Dave
     
    Last edited: Jul 23, 2018
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  3. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,245
    Location:
    Melbourne, FL
    MikeTest, welcome to AK!

    I'll step out of the way. The doctor is here. Carry on Dave.:thumbsup:
     
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,583
    Location:
    Southern NJ
    Also if you have any tube shields missing, re-install them. For testing it would be worth making up some shorting plugs for the input. Its just an RCA with the center lead tied to the shell. it helps make sure you're not picking up noise from the open input jack to screw with any testing.
     
  5. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,245
    Location:
    Melbourne, FL
    Most importantly BE CAREFUL! There are some very high voltages inside the unit. If you don't know how to be safe around high voltages, enlist someone that does know.

    Hum can come from external source inputs too. Does it happen on all input selections? Have you unplugged all the input cables and does it still have the hum? Have you tried flipping the power plug at the socket? (The stock power cord came non-polarized). Do you know the status of the power supply caps? Have those caps recently been checked/replaced? I haven't used the shorting plugs myself and my KX-100 is silent without them and doesn't exhibit hum, but you could give the shorting plugs a try. Having said that if it is happening on all sources with all the sources cables unplugged then you probably have something going on internal. You might consider cleaning all the tube sockets and make sure they are all making good connection. Do an inspection of the underside wiring and look for damaged parts. You might clean all the slide switches and the selector switch. I used Deoxit to clean the contacts.

    I ended up going through my KX-100 very thoroughly. I went so far as to replace all Power Supply (PS) caps, the rectifiers, coupling caps and power resistors. I also checked all of the resistors and found a few that were out of spec and replaced them. I also verified all the voltages throughout (against the available schematics) and fixed some problems along the way. Keep in mind these were built as kits originally and there may be some mistakes in the build. I found a couple of mistakes in mine and some later hacks that I had to undo. Mine isn't 100% stock anymore and I have a few of the more popular mods on mine. It's a nice unit and well worth restoring to good operation. Mine is in my main rotation now.

    Good luck. The community is here to help, but Dave is the respected Doctor here. His expertise is one of the more entertaining things to watch and to learn from. (But he's not the only one to learn from. There are others that aren't life long technical, but they will share their experiences and help guide you almost as well as Dave can teach. While I'm not a total novice, I am in the category of just sharing what I've learned. I'm not the Doctor that Dave is.)
     
  6. MikeTest

    MikeTest New Member

    Messages:
    7
    Location:
    Kansas
    View attachment 1242177
    My unit is the Stratakit kx100 for which I have an assembly manual but don't have (haven't found yet) a schematic. The schematic I am using is for a x-100-b, which I thought was the same as the kit. However, your comment about AC line bypass caps got me looking in this area. As far as I can tell there are no AC bypass caps at all on my unit nor is there any mention of them in the assembly instructions. On the other hand the x-100-b shows C33 (schematic below). I am not sure that is considered an AC bypass cap but my amp does not have it. In any event I wonder if it would be wise to add AC bypass to my unit just on the principal (in my past life I was trained to never use the S word).
    upload_2018-7-23_17-0-30.png
     

     

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  7. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,245
    Location:
    Melbourne, FL
    That is one of a couple of differences between the X100B and the KX-100. The C33 I think is for switching noise. I added one to my KX-100, but after the fact I was thinking that I should have done more research to get a cap that is designed to fail open rather than fail shorted. Maybe Dave might comment.

    Here is a quote from one of my own posts on my KX-100 thread about the differences.
    =====
    Discovered differences between the KX-100 and X-100-B.

    Just as a side note. I've been digging through the diagrams and parts lists for the KX-100 and the X-100-B. I have diagrams for two different versions of the X-100-B but I can't find any electrical differences in the two versions of the X-100-B. Here are the differences between the KX-100 and X-100-B that I've found so far.

    1. The X-100-B has a single cap added to the power supply where the secondary of the power transformer joins the two diodes. This is listed as C33 which doesn't exist on the KX-100 and it runs from that junction to ground. It's listed as a "Molded 0.01 uF, 20%, 600V". I'm wondering if it is some kind of filter for high frequency (RF maybe?) that might come in on the line? Useful at all? [EDIT: Found a thread that talks about this extra cap from dcgillespie. It's in post #51 1st paragraph of this thread: http://audiokarma.org/forums/index.php?threads/fisher-kx-100-x-100-b-restoration.572641/page-3]

    2. Values for C23 and 24 were changed from 100 pF (KX-100) to 180 pF (X-100-B). These are in between the two halves of each of V5 (and V6). I.e. Between the Plate of the first half and the grid of the second half. These caps go to ground. Same reasoning? Is this a filter to reduce RF interference? Is this useful at all?

    3. The rest are relating to the difference in the bias circuits with the KX-100 (fixed) and X-100-B (adjustable - with a single adjustment for the entire output stage).
    a. R69 (in the X-100-B) has been moved to where R70 was located (in KX-100).
    b. R70 (in the X-100-B) has been added to a new location and is now a 5K pot.
    c. R76 has been added in the X-100-B in the location where R69 was in the KX-100.

    That's all I've found so far and it might be complete as I've gone through the entire parts lists of all 3 versions. (Assuming that there were no other part relocation's excepting for what I've already identified in the bias circuit for the output stage.)
     
  8. MikeTest

    MikeTest New Member

    Messages:
    7
    Location:
    Kansas
    Thanks Tim D. Sorry for re-ploughing the same field. I realize now how much history their is in the forum I need to catch up on before I ask rookie questions.
     
  9. fred soop

    fred soop Super Member

    Messages:
    1,975
    There is much discussion about XY rated capacitors (the death cap) on the AC line, but actually, all AC applications (transformer secondaries) should use XY capacitors as well. This would be especially true for tube equipment with the high voltages, not so critical with 40 volt SS supplies.
     
  10. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,245
    Location:
    Melbourne, FL
    Yep Fred. That’s what I was talking about. XY. I’m going to look up my order and see what I bought for mine. Thanks.
     
  11. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,245
    Location:
    Melbourne, FL
    MikeTest,

    BTW, there is no letter on the KX-100 model number.
     

     

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  12. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,245
    Location:
    Melbourne, FL
    MikeTest,

    Just saw a digital copy of the KX-100 assembly manual on the big auction site. I have no idea of the digital quality or whether it has the schematic. You can ask the seller questions though if you are interested.
     
  13. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,245
    Location:
    Melbourne, FL
    MikeTest,

    Just saw an original copy of the KX-100 assembly manual on the big auction site. If you are interested you will want to ask if it includes the big schematic diagram. I don't see it in the pics. The prices looks good if it has that diagram. There are other diagrams that normally come with it that are helpful, but not absolutely necessary that were for the original assembly.
     

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