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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    Good job. I wouldn't worry about the difference between output's. The diff could be wire length as Fred noted in a post.
     
  2. Tim D

    Tim D AK Subscriber Subscriber

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    Larry,

    I concur it is the length of winding wire. The differences are between the two halfs of the same transformers primary. Total primary DCR is 305 ohms.
     
  3. rufleruf

    rufleruf AK Subscriber Subscriber

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    Great find Tim. I am now inspired to dig in to the couple of dead transformers I have around.

    How does it sound?
     
  4. RS Steve

    RS Steve Tube Junkie Subscriber

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    Nice work, hope it solves all your problems, nice to see the old iron saved if possible. :thumbsup:
     
  5. Tim D

    Tim D AK Subscriber Subscriber

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    No sound yet Matt. I put a coat of paint on the core and the bells. Waiting for it to dry before I reassemble. I'm using gloss black engine paint that I picked up from Wally World. I figure it can stand the modest heat that these OT's generate. Hope to re-assemble it tomorrow morning and give it a try. I'll start measuring the voltages and currents for real then.
     
  6. larryderouin

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

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    If mama will allow it, bake 'em in the oven @ 200*F for a few hours (upside down). Kill Two Gremlins in one transformer. Bake off any moisture inside, and Bake the paint HARD! Just make sure the vent fan is running on HIGH!
     
  7. Tim D

    Tim D AK Subscriber Subscriber

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    I suppose I could do that tomorrow while the wife is at work. What mama doesn't know.....
     
  8. fred soop

    fred soop Super Member

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    Glad you were able to salvage the transformer.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Super job Tim! Glad you were able to save it!

    Dave
     
  10. fred soop

    fred soop Super Member

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    Was actually Dave that first mentioned wire length difference a while back. I had never thought about it before.
     
  11. Tim D

    Tim D AK Subscriber Subscriber

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    Well, I have the OT re-assembled and re-installed.

    The voltages are definitely better, but not quite where the diagram says it should be - maybe down by 10%. I'll be recording the voltages and posting them. I might need some recommendations on next steps for the voltages. Also, the turntable hum is back and the left channel is tinny (very low on bass) when run on phono. I'm going to flip the plug on the amp and see if that helps with the hum, but I probably have to go through the phono preamp and see if there is a problem tube or some other issue. When I run it on Pandora through the Aux inputs it sounds normal/good now. I do think that one or two of the 7868's I bought is weak though and I'll be looking at that. I'll probably rotate the output tubes and see if the currents/voltages move around with bad tubes.
     
    Last edited: Nov 7, 2017
  12. Tim D

    Tim D AK Subscriber Subscriber

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    Here are some pics. Can you guess which of the OT's is the one that was worked on?
    IMG_2758.JPG IMG_2757.JPG IMG_2756.JPG
     
  13. Tim D

    Tim D AK Subscriber Subscriber

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    So, I found some time tonight to play it again. Before powering it up I flipped the plug on it. Suddenly it is working right. I wonder if the plug polarity can have that big an impact? In looking at the schematic the primary of the PT is not grounded anywhere, so I'm unsure why that has an impact. Could I have an accidental ground on the primary side? I can certainly trace it out. I don't think so, but if anyone thinks that is a concern I'll certainly do that.

    A little while later a big hum hit and by wiggling the cables into the magnetic/Lo phono inputs it went away. I am going to have to check those connections as they seem a little undependable.

    I find that the unit is running reasonably cool. After an hour none of the trannies is noticeably hot. In fact they are only luke warm. That is in part because I'm running the variac at about 118VAC.

    I found the same thing as Larry that the equalization sounds better in the "Tape" setting rather than the "Phono" setting. I'll run it in both settings and we'll see how it turns out.

    On to finer tuning now then. Here's what is in the queue to do for this unit in no particular order.

    - Finish painting the trannies. Gotta make them all match now.
    - Wax the chassis and faceplate.
    - Go through the voltages in detail. I need to start thinking about what is the right operating ranges for this system.
    - Start rotating the tubes to determine which ones really are weak.
    - Start looking for a cabinet. (Long term and no hurry)
    - Determine if I need to upgrade the bias for the control grid to the X-100-B style or the IBAM discussed in other X-100-B/KX-100 threads. (Larry, I know you think this doesn't apply, but why are those other AK threads not being challenged?)
    - Read up on the other mods out there. I see where there is mention of separate heater circuits and such.
    - Determine if I need to build a bucking transformer to keep the input voltage low for longevity. I want to free up the variac for the next two Fishers I need to get working on.

    Note in all this I've been running this through a 2A fast blow fuse. The unit was spec'd to have a 3.2A slow blow. If noone raises a concern I'm going to leave it this way as it gives me peace of mind.
     
  14. fred soop

    fred soop Super Member

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    It would be quick to check for a ground on the primary side but also check the location of the death cap. There is also capacitance between transformer windings and reversing the plug would put the opposite end of the winding at ground potential. This can also sometimes be observed when measuring leakage.

    The standard leakage test is to disconnect all external connections (speakers are isolated from any outside ground connection, so can be left connected), then connect a 1500 ohm resistor and 0.15 µF capacitor in parallel between the chassis and earth ground. Measure voltage across the RC combination. Leakage should be no more than 0.35 volts for any combination of plug orientation and power on or off.

    As an additional experiment, disconnect the RC network and measure voltage chassis to earth with a digital meter (high input impedance). You may find close to 100 V for some combinations. This is a very low current path from capacitive coupling in the unit and is the cause for the light tingling that one may feel when lightly touching the unit.
     
  15. Tim D

    Tim D AK Subscriber Subscriber

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    Thanks Fred. That's a pretty easy check. I'll stop and get the parts from the local store. So I should measure from chassis to the house ground in a 3-prong outlet?
     
  16. larryderouin

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

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    With regards to the cathode bias mods. I tend to leave it as is. I only have one FISHER that is Cathode biased (guess which one!) and it's easier for me to leave the bias as is as the tubes are matched already. I have enough troubles keeping the fixed bias straight in my head.

    You run the risk of blowing thru fuses @ 2A although you've been lucky so far. Up to you what you eventually choose, but if it were my amp, I'd go up to a 3A.
     
  17. fred soop

    fred soop Super Member

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    House ground at the third prong is fine. BUT, if you have an old house with replaced receptacles, that ground could be questionable. I worked on a project many years ago where the grounds on 2 adjacent receptacles (2 duplexes, 4 outlets) measured 63 volts. That explained why we were blowing chips right and left as there were power supplies connected to both sides feeding equipment that was interconnected.
     

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