Just bought a KX-200 - WOO HOO !

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

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Right off the top of my head. C1 installation is the problem. Bend the OUTER tabs upright, and twist them about 90deg. They CANNOT touch the chassis. This CAN is 1/2 of the voltage doubler and must be isolated from the chassis. The "GROUND LEAD" on C1 is actually an interconnect between C1 Negative and C2B Positive which is the ORANGE wire on the tab @ 9oclock. Disconnect and remove the BLACK LEAD at 6 oclock and bend the tabs up and twist. The Tabs bent over and the black "Ground Lead" are causing the short.
     

     

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  2. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    I don't understand, Larry. The old cans had tabs that were touching the chassis. Why didn't they cause a short? I will remove the black wire and try again as that's the only difference. Except for C1, all the tabs HAVE TO go through the slots in the chassis. Oh, maybe you mean only C1 cannot have its tabs touching. I'll try that.

    UPDATE: You were right, Larry ! I removed the C1 ground wire and bent the C1 tabs so they don't touch the chassis and it works again !! THANK YOU !!!
     
    Last edited: Jan 26, 2019
  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    That's what I meant FOR C1 ONLY! When a Can is on the insulator, the tabs don't touch the chassis, isolating it. With the ground wire connected to the tab @ 6oclock, you've effectively shorted out the Voltage doubler. Remove the "GROUND LEAD" from the tab @ 6oclock. Leave the Orange wire on the 9oclock tab as is. This one is correct. Thorne 1.jpg

    As for C3. Look at the label. The sections are TOP TO BOTTOM C-B-A if the voltages are descending. (the Values should line up with what's in the manual. And the designators are on each line.) The values are the same but the voltages are decreasing as you go down the line. The manual has C3B and C3C backward labeled on the schematic. You need 2 resistors for between sections. R72 (BTWN C & B), and R73 (BTWN B & A). R74 taps off "A" and goes to the POWER LAMP(NEON).
     
  4. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Wow, Larry, AND you solved my post 76 issue with the resistors being mismatched from the schematic. After I reread your post several times, I realized you meant C1 tabs. Thanks for taking the time. Thorne

    PS - I gotta say that the new cans seem to have improved the bass response if that makes any sense. I love that deep KX-200 bass. (I do have the bass pumped up in the EQ settings when I play music on my phone).
     
    Last edited: Jan 26, 2019
  5. Linehand

    Linehand AK Subscriber Subscriber

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    Nice, glad you're back in business. If you're streaming from phone or com, check out jbradio2 (do a search). Commercial free hi-res, plays a very wide range of good music.

    It's really amazing how much detail and depth these amps reveal. Music that I would never listen to otherwise becomes palatable (similar to comment you made earlier)
     
  6. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    I'm having troubles with the bias meter. I'm always having to bang the back end of a screwdriver on the chassis near the meter to get the needle to move and then for the bias settings, I'm always all the way at one end of the pot turn. I already dropped the meter resistor from 390 to 240 ohms. Can I go lower to get more pot movement at both ends? Or is there something else I can check to see why the bias settings are always at the extreme? I haven't measured the 7591 pin voltages since I replaced cans C1, C2 and C4. C3 still to go. Last time I measured I was at I think 78% of max wattage.
     

     

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  7. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    HELP. My KX-200 has developed a tick which is heard through the speakers. About every second the amp makes a ticking sound whose frequency is around 200Hz I'd guess. The volume control makes it louder and the loudness switch makes it boomier. The treble and bass controls have no effect on the sound. And it makes the sound on all settings of the INPUT SELECTOR. Last night it was working fine and this morning, the tick. I gently tapped the tubes to see if that would affect the sound but no. I tapped around on the chassis and a few exposed components, no effect. From my description can anyone venture a guess as to what it might be? I think the fact that it's fairly regular every second might provide a clue, maybe a bad capacitor where the tick is the sound of charging and discharging? The last mods I made were to replace the C3 can cap (3 x 40uF caps) and 2 caps (.022uF) on the loudness switch. In both mods the voltage was increased but not the capacitance. Another clue is that the ticking doesn't start until the tubes are warmed up. Sometimes the ticking gets faster.
     
    Last edited: Jan 29, 2019
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    22,205
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    Start by Re-Checking ALL of your Soldering from the last session for cracked, cold joints, and solder bridges, or mis located leads (especially the loudness switch). Swap the 12ax7's from the phono section to the driver/phase inverter. Clean all tube sockets Again.
     
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  9. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Once again, Larry... right on ! It was a solder joint on C3 from last night that somehow came loose and was barely touching.

    I was disappointed because the sound was significantly worse which made me think replacing C3 was a major mistake. But it was that loose lead. Phew.
     
  10. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    I wanted to see how the built-in bias and balance adjustment pots and meter operate compared to measuring the those parameters manually. Even with the 240 ohm change to the meter resistor, I could never get the bias pots to adjust correctly according to the meter. The meter pointer always fell short of the mark. Using the meter, I got these measurements:

    pin3 on all 4 tubes = 389 vdc
    pin 4 on all 4 tubes = 349 vdc
    pin5 V7 = .358 vdc
    pin5 V8 = .371
    pin5 V9 = .312 <--- !!!!
    pin5 V10 = .343
    pin6 V7 = -13.31 vdc
    pin6 V8 = -12.98
    pin6 V9 = -14.52
    pin6 V10 = -13.22
    Note pin6 V7 & V8 differ greatly from pin6 V9 & V10. V7 above is pushing 13.9 watts or 73% of max. V9 above is pushing 12.1 watts or 64% of max. Should I be concerned at the discrepancy?

    I opened up the unit, used a DMM and adjusted the balance and bias for each pair of tubes as best I could. These are the measurements:
    pin5 V7 = .363 vdc
    pin5 V8 = .358
    pin5 V9 = .365
    pin5 V10 = .315 <--- !!!!
    pin6 V7 = -13.18 vdc
    pin6 V8 = -13.21
    pin6 V9 = -13.64
    pin6 V10 = -13.61
    I got pin6 V7 & V8 a little closer to pin6 V9 & V10. V10 above is pushing 12.3 watts or 64% of max. The other 3 are close to each other, around 74% of max.

    As I was adjusting the balance and bias pots, it occurred to me that I don't know what tube parameters I'm adjusting with the pots. I don't know that I can trust the meter. I assume the Balance pots are adjusting something between each pair (L and R) of tubes. I watched the pin 6 values change as I adjusted the bias and balance pots. I assume the goal should be that each pair of tubes are balanced and that the 2 pairs of tubes are balanced. Since I've got a Fixed Bias system, what values are the most important to achieve?

    Thorne
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    When you balance the pair, connect the meter on pin 5 (tube side of the cathode resistor) on both tubes to read the voltage between them. You adjust the BALANCE POT to 0.00VDC. Once they are balanced, then leave the Red probe on one of the tubes pin 5, with the black on CHASSIS GROUND. Set for mv as determined by Ohms Law to approx 75% of maximum dissipation for the tube.

    With the tubes balanced you need to do the bias to just one as the other should read the same. Check and rebalance as necessary. Don't worry about Pin 6. You are concerned with the load on PIN 5. You want this equal across each pair. The voltage on Pin 6 will be different for each tube at this point. Again don't worry about it as the CURRENT (load) on Pin 5 is what's important. (Think of it like unequal sized hoses. To get the same gpm (load) you need to vary the pressure (voltage). The smaller hose will have a higher pressure than the larger hose for the same gpm up to the smaller hose's limit). Both will flow the same GPM, but the smaller hose will push the water further than the larger hose due to pressure difference.

    Once you get the load equalized, then bring up the bias voltage to approximate 75% of Maximum Disspation. The controls WILL BE interactive. So adjusting the Bias controls multiple times will be necessary until adjustments make no or minimal change in the other channel.
     
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  12. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
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    Thanks, Larry. What I'm finding is:

    1. Setting the difference to 0.00vdc between pins 5 on tubes V9 and V10 (the B channel) and then moving the black lead (that was on pin 5 of V9) to chassis ground (leaving the red lead on pin 5 of V10) and adjusting the BIAS control, does NOT guarantee that the pin 5 value for both tubes V9 and V10 will be the same. In fact they are not the same. In this instance I'm getting 327 mV on V9 and 351 mV on V10. Goal is 342 mV on both tubes as that's 70% of max wattage (19w).

    2. I need more range for my BIAS pots because I was not able to get more than 327 mV on tube V9 and I need to get to 342 mV. The B channel BIAS pot was as high as it can go.

    3. Putting the leads on pins 5 on V7 and V8 (the A channel), I was unable to get a static reading. The values continuously changed between -0.8 and > 0. Finally after 1 minute, the values settled down. Wasn't expecting to have to wait for a minute. I suppose I didn't have a good connection, but geez.

    Once settled, I was able to get both pin 5 values on V7 and V8 around 342 mV. And to my surprise, once I got V7 and V8 optimized, when I went back to do V9 and V10, I was able to get them optimized on the first try ! Did my getting V7 and V8 optimized make it easier to get V9 and V10 optimized?

    Thorne

    Larry - I re-read your post and you said "adjusting the Bias controls multiple times will be necessary until adjustments make no or minimal change in the other channel." That answers my question about one channel's adjustments affecting the other channel.

    UPDATE... I made a final pass on all pin 5 values. pin 5 of V7 is only 304 mV. What changed???

    PSS - I tried again and this time I was able to get everything close enough. I guess it really does require measuring over and over. What made it challenging is that the pin 5 value on one tube, V7, kept changing when I moved the lead around on pin 5. I'd move it a little, it would read 304 mV. I move the lead around again, this time it measured 336 mV. I bet when I put it back together and read the on-board meter, it won't tell me I've got both A and B channels optimized. (yup, meter is telling me bias and balance is off. Maybe the 240 ohm meter resistor is not good enough?)
     
    Last edited: Feb 1, 2019
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Tighten the tube sockets on the output's. Sounds like an intermittent in the socket leaves. Which is one possibility as to why the voltages change so much. I'd wait on Dave for the Resistor change/values.
     
  14. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
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    Funny you should mention tightening tube sockets, Larry. I went through all 7591 sockets and cleaned them just 2 days ago. Tightening makes me nervous as it seems to require a skill I have not yet mastered. I've read all about it, but still I'm not very good at it. Keep trying I guess.
     
  15. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Take a small Jewelers Screwdriver Lay the flat between the socket leaf and the edge of the hole with the flat parallel to the edge of the hole. Then twist 1/8 to 1/4 turn and hold for about 5-7 seconds. Do this with each leaf (usually 2 per pin). This will preload the leaves so they grip the pin. Easy enough and it'll last for a good while.
     
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  16. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Larry - I have some new porcelain tube sockets. Is it easy to replace them? I have a partly broken socket on my Hickok tube tester and intend to replace it at some point. The reason I'm stalling is that it looks like the socket is secured to the chassis with rivets. I'm not tool-equipped to replace rivets.
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Take the new socket down to Lowes, Home Depot, or the local Mom & Pop Hardware store.. Get a couple of Machine Screws, star washers, and enough nuts to "DOUBLE NUT" the screws. Probably 4-40 or 6-40 size. 1/2" to 5/8" long should do it. If the chassis is screwed to the wood, unscrew it, drill out the rivets, remove the socket and replace with the new one.

    Make a drawing of the old socket (pins 1-9, whatever) and note components on each pin. Take a few pictures also. Then swap it out.
     
  18. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
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    Drilling through the chassis...yikes. All those components in the way. I'll save that task for when the socket is about to cause the end of the world!
     

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