Picked up a Rough KX-100 Tube Amp - Will need some guidance

Discussion in 'Fisher' started by Tim D, Apr 5, 2017.

  1. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Make sure you plug in ONLY A PHONO. NOTHING ELSE! especially an IPHONE you'll overload the circuit like crazy, besides sounding like the south end of a North bound Brahma bull, you'll end up with an IBRICK!
     
  2. Tim D

    Tim D AK Subscriber Subscriber

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    BAD NEWS

    I was measuring voltages and I'm getting zero volts on the plate (pin 9) of V10 one of the right side output tubes. The plate (pin 9) of V9 is reading 423V. I believe this means that half of the primary winding on the right side OT is open circuit. Dang I didn't need that!

    All of the other voltages are low, but I hate to draw conclusions until that OT is replaced and it is drawing the right current from the power supply. I'm running the variac at about 118VAC.

    Checked the cathode currents across the 10 ohm resistors and here they are. They confirm that V10 isn't running as a result of the bad OT.

    V10 = 0.09V <---- Not working!
    V9 = 0.47V
    V8 = 0.48V
    V7 = 0.36V <--- Will look into this later. Might be a weak tube.

    I'll start the search for a replacement OT. Parts list identifies the two OT's as two separate part no's. Strange.
    T2 Transformer, output, CH B T849-116-6
    T3 Transformer, output, CH A T849-116-5

    Are these interchangable? Which one is the right side? A? Or B
     
  3. Tim D

    Tim D AK Subscriber Subscriber

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    Took the audio investigation a little further this evening before finding the issues in the previous post. I put on the tube shields and worked on grounding the turntable and it sounds much better. It certainly takes a while to warm up with those low voltages. The hum is mostly gone now and the music sounds much better. I think I'll get that OT replaced and then start through the unit again measuring voltages.

    I'm going to shelve this unit for now because I don't want to run it in this partially damaged condition. I'll post on BT for WTB for the OT.
     
  4. fred soop

    fred soop Super Member

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    If the OT is bad you have nothing to lose by opening it. The problem could be just a bad connection where the external lead is connected to the wire from the winding. This could be at the outer end or at the center tap. This drawing also shows the copper shield which will not be on the OT.

    [​IMG]
     
  5. Tim D

    Tim D AK Subscriber Subscriber

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    Excellent information. Now I know what I'm doing tomorrow evening. Thanks Fred!
     
  6. rufleruf

    rufleruf AK Subscriber Subscriber

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    Tim,
    Shoot - that's no good. Amazing that it works as it does in the state it's in.
     
  7. Tim D

    Tim D AK Subscriber Subscriber

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    Yeah Matt. I'm amazed too. Some of the behavior adds up though. It didn't quite sound right. Not terrible, but not great either. It was also a bit short on power. I had to push it a bit to get some volume out of it. It just wasn't musical either. Given that one tube is not functioning on the right and on the left one tube has low current that would certainly be understandable.

    Tonight I'm pulling the bad transformer and starting the surgery. I'll let you know what I find.
     
  8. rufleruf

    rufleruf AK Subscriber Subscriber

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  9. Tim D

    Tim D AK Subscriber Subscriber

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    And Thanks!

    Read through the thread. That is hard core transformer re-winding. Wow. Not sure I want to make that kind of investment for one bad transformer though. You really have to be detail oriented to get the right layering, right number of turns, right taps in the correct winding number, etc. Getting all the kit and materials together for the first one has to be a pretty big deal.
     
  10. Tim D

    Tim D AK Subscriber Subscriber

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    Ok. Opened her up. Not seeing a smoking gu..... transformer. Still by checking with the VOM it is open circuited down inside.

    I had to cut and peel back the outer (heavy) paper wrap. By visual inspection I found only a hair thin wire from the blue lead down into the windings. Not sure if that fine of wire is typical, but it was unbroken for the portion that I can see. If this is the gauge of wire used in the windings, it must have thousands of turns in the primary. Of course these windings would be taking current in the range of milliamps. The cathode measurements I took yesterday are under 50ma, so maybe normal max would be under 100ma. That isn't very much current. Maybe the designer though that this fine fine wire was enough.

    IMG_2745.JPG IMG_2744.JPG
     
  11. fred soop

    fred soop Super Member

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    Thin wire is typical for windings having high voltage. At a typical value of 200 circular mils per amp, a 100 mA winding could use as small as #26 AWG wire which has an OD of 0.016. Did you also check that wire where it runs down the outside of the coils before going in to the actual winding? I once had success with a power transformer by cutting down to the actual winding and tapping the wire, but it has to be the outer layer.
     
  12. Tim D

    Tim D AK Subscriber Subscriber

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    Yeah I tugged on that fine wire as much as I dared. It was attached down into the windings. I'll examine the center-tap to verify that I can't see any burnt wires broken there too. Chances are though that this is broken deep inside.

    Question to anyone. Why does the right and left side OT's have different part no's? They aren't mirrored in any way are they? I can't believe that it makes a difference if I replace it with a left side OT, right?
     
  13. Tim D

    Tim D AK Subscriber Subscriber

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    MORE NEWS! <Good news>

    Did some more cutting and inspecting and found the break on the center-tap. Gosh is it small. I had to remove the cellaphane and some more paper between the incoming lead for the center-tap and the coil. Had to move those hair-width wires. One was solidly connected. One was broken. Now I have to figure out how to do the micro surgery to solder onto the short end sticking out of the coil. There is only about 1 or 2mm of wire sticking out - if that much. Here are pics.

    First pic has my multimeter probe there for scale and to point it out. You can see the two ends circled in red. The unbroken lead to the good end is circled in blue. Notice the scale here. My finger is there with one of the broken ends circled in red. I don't have big fingers.

    So, any suggestions on how to do the micro surgery? How do I get the lacquer off the short end so that it can take solder?

    Can I just use solder and no other mechanical connection?

    IMG_2748.JPG IMG_2747.JPG
     
  14. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    The left and right side output's should be identical. The different part #'s are for FISHER's accounting Dept to keep thing's straight for the bean counters who can't count to 10 on their fingers because of two thumbs. (Toons,......Gets 'em every time!!)
     
  15. fred soop

    fred soop Super Member

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    Removing lacquer: scrape very carefully with a good Exacto knife. I'd put something on the opposite side while scraping to limit flexing of the wire.

    Once you have exposed copper on both pieces, just solder in a section of wire, maybe a strand or two from a larger size wire. If you can make any sort of mechanical connection before soldering, great. Otherwise, a good tack solder job will have to do. The main thing is to not break off that short piece of wire, though sometimes it's possible to pull a bit more out from the coil.

    Doesn't have to be neat or insulated as long as it is a good enough connection to withstand moving back into position. It will be covered when the insulation is folded back into position.

    I goofed on the wire size in the previous post. #26 could actually handle 1 amp with its 254 circular mils. So, this wire size could go as far as #34 or whatever the limit is for actually handling the wire.
     
  16. fred soop

    fred soop Super Member

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    The 600-T driver transformers have different numbers because the one for the left channel has much longer leads to reach the far side of the unit. Don't know if that could be a factor on the tube units.
     
  17. fred soop

    fred soop Super Member

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    Addendum: You probably already know this, but to minimize any flexing of the stub wire, the new piece used for the splice can be coiled around the stub. Doesn't have to be tight as that could lead to breakage. But a loose coil around the stub can then be filled with solder and will be more solid than a simple lap joint.
     
  18. Tim D

    Tim D AK Subscriber Subscriber

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    Well, I found the stub too small to coil it around the tiny 1mm that was there. What I did though was to use paint stripper and then soldering flux to clean the connection on that 1mm. That seemed to clean it pretty well. I tinned it and took 4 strands from some 18awg speaker wire and soldered it on to the stub. I tied the strands back to the center-tap and was able to at least wrap that around the center-tap connection. After that I used some liquid electrical tape and started putting it back together after letting it dry for an hour.

    Here's some pics. I've got the coil all wrapped up. I'm going to repaint the bells before final assembly though.


    IMG_2753.JPG IMG_2752.JPG IMG_2751.JPG IMG_2750.JPG IMG_2749.JPG
     
  19. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    You didn't mention checking your work with a meter afterwards. Hopefully it metered out ok. Looks good from a standpoint of work, one note tho. I would have added another layer of insulation(tape) between the bare wire and that yellow one just for good measure. It probably won't make any difference but it won't hurt. Leave it be for now. close it up and check it again with the meter. If everything checks out, power it up on low voltage, then bring it up in stages checking for shorts and opens. If it gets to full voltages go ahead and use it but keep a close eye on it for a while.

    BTW. PKG arrived. Thanks

    Larry
     
  20. Tim D

    Tim D AK Subscriber Subscriber

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    Hi Larry,

    Yep. The newly connected half of the primary now metered out at 160 ohm DCR which is in the neighborhood of the 149 ohms of the other half of that same primary winding. Not much I can do with the 10 ohm variance. That is a feature of the internals that I can't change. That's under 10% diff.

    My hope is that the solder job will hold. I overlapped the new wire with the old in parallel, so there should be reasonable surface contact - I hope.
     

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