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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    On the can's or HER KNIFE????? You better hope she's not as good as my wife with a 12" cast iron Frying pan i she finds out. She throw's strikes with that thing at 50 paces, and GOD help the poor SOB (Usually me) on the other end of it's flight path! It's got a trajectory like a German 88. Flat and Fast!!!!
     
  2. rufleruf

    rufleruf AK Subscriber Subscriber

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    Tim,

    it's neat to see the parts I sent getting used. Good job on the cans. Someone needs to make a tool that works as good as the kitchen knife, but doesn't require the removal of the cans. That's the part that makes me procrastinate.

    Matt
     
  3. Tim D

    Tim D Well-Known Member

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    Wow Larry. You're married to a tough one. Mine would just make me buy her a new one if she thought it'd been damaged. In reality though I'm the one that sharpens those knifes, so no harm - no foul.
     
  4. Tim D

    Tim D Well-Known Member

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    580
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    Matt,

    Thanks for that part. Yep, it's coming together. I've got a cap coming so that I can finish C3, but the other two are ready to re-install.
     
  5. Tim D

    Tim D Well-Known Member

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    580
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    I put in the in-rush limiter (one) last night. I also have the 100 ohm screen grid resistors on order to help protect the output tubes from arc'ing. Also ordered new rectifiers. Essentially the power supply will have been rebuilt when done.

    I'll be measuring the various resistors in the power supply before I reinstall the caps just to make sure they are still in tolerance.

    I'm still cogitating on what to do about the bias on the control grid. I may go with the X-100-B type which adjusts the bias for all the output together. That's pretty simple to implement and will only amount to a 5K pot and a couple of resistors.

    Oh yeah. I also ordered a 0.01uF cap for the power supply. I found a difference in the X-100-B where there was this cap between the rectifiers to ground. The consensus is that this is to filter transients from the line and from the switching of the rectifiers. It is a cheap way to cut down on noise and the cost is negligible. If anyone wants to know which cap this is I can provide a pic of the X-100-B diagram with it circled. Also see post #44 item 1 for this and some other X-100-B diffs from the KX-100.

    EDIT: Forgot to add that I've ordered replacement 0.047uF coupling caps as well.
     
    Last edited: May 3, 2017
  6. Tim D

    Tim D Well-Known Member

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    Here's a pic of C1 and C3 assembled and ready for re-installation, and C2 with one of the two caps I need to finish it.

    IMG_2717.JPG
     
  7. larryderouin

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

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    Matt; Ask Santa for one of those new fangled oscillating does everything but wipe yer butt and bring you beers(although it can probably be goaded into bringing you them if you gave it one or 2) saw's and a couple of Fine tooth metal blades. Cuts thru the can caps in a matter of seconds and they leave a gluable surface.
     
  8. Tim D

    Tim D Well-Known Member

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    580
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    Making progress. Getting really close to bringing it up on the variac. Here's what is completed.

    - Cleaned the chassis as best i could and removed all the corrosion that I could. The Krud Kutter was instrumental in removing the corrosion, but I had to mask it off to preserve the stenciling. It's a mixed bag as to the results, but it is after all a survivor. I managed to get all of the corrosion off the original RCA jacks and speaker terminals which was a surprise. That whole exercise was hard work and time consuming. I certainly couldn't afford to have paid a technician to do that.
    - Cleaned the faceplate. Some of it was cleaned with brasso and some with glass cleaner. The stenciling is in pretty good shape and the faceplate looks good for starting out as a basket case mess.
    - Replaced a knob and some knob brights with thanks to another AK member. Cleaned them with soap and water and shined them with Brasso.
    - Replaced two switches with thanks to that other AK member.
    - Recapped the whole power supply. Ended up stuffing two of the original caps, but I got a replacement for the cardboard covered C1 thank to that other AK member.
    - Recapped all six of the 0.047 coupling caps - four of which were for the output stage.
    - Added in the 10 ohm cathode resistors so I can measure the cathode current.
    - Added in the 100 ohm screen grid resistors on all 4 output tubes to protect the output tubes from arc'ing that can be caused by the lower Power Supply impedance.
    - Replaced both rectifiers.
    - Managed to scrounge a full set of tubes that all test good. Some from other units in the queue and some from the big auction site.
    - Added in a transient cap across one of the rectifiers to ground. This cap isn't part of the kit version, but is in the factory version. i added it in because the cost was minimal and the piece of mind was worth it.
    - Added in an inrush limiter to the primary side of the power supply per general recommendations.
    - I've put in a 3A slow blow fuse and a new cap thanks to that other AK member.

    I'm going to postpone painting the trannies until I am sure they are ok. No sense painting something that might be bad. I've put the faceplate and knobs back on. Next I'll wax the chassis and put in the tubes. After that it will be time to bring it up on very low voltage to make sure there are no shorts. I'll get a series of small fast blow fuses for the initial power up on the variac. I think I can get an assortment at the local auto parts store. If everything goes well there is a chance that I might have this powered up over the next few days.

    I have not touched the DC bias circuits for the control grid yet. I'll take the initial measurements as I bring it up and see how matched the whole system looks and see what kind of current I get at the 10 ohm cathode resistors.

    Here are some pics. The little baggie contains some of the parts removed. Not sure why I have them in the bag. I have a habit of keeping that old junk around for a while and then later convince myself it is trash and get rid of it.


    IMG_2725.JPG IMG_2724.JPG IMG_2723.JPG IMG_2722.JPG
     
  9. rufleruf

    rufleruf AK Subscriber Subscriber

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    Looking good! I have 2 KX100's and both have the Canadian sticker yours has. Huh.
     
  10. rufleruf

    rufleruf AK Subscriber Subscriber

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    Tim,

    I used the polish pictured below on a really crappy looking KX200 and the chassis shines - I don't think it was as oxidized as yours, but it was quite rough. It will take the lettering off if you're too aggressive, but as can be seen on my KX200, it didn't take much if any with a good polish. I don't usually polish the chassis if they're nice, but this KX was pretty rough, and I didn't think I had much to lose.

    [​IMG]

    [​IMG]
     
  11. larryderouin

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

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    Tim;
    Auto parts store FUSES are a BIG NO-NO
    . They are usually rated 32volts. You should go to Home Depot or Lowe's and get the fuses which carry a 125 or 250V rating.
     
  12. Tim D

    Tim D Well-Known Member

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    580
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    Got it. Thanks Larry. Chances are they will only have larger values anyway.
     
  13. larryderouin

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

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    Tim;
    The 2 H.D. and 2 Lowes near me (within 5 miles) all have down to 1a and up to 10a AGC and slo blo's. Buss or littlefuse. 5packs in the metal flat pack.

    Check your PM's.

    Larry
     
  14. Tim D

    Tim D Well-Known Member

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    Found some at HD. I'm running on a 1.5A fasts blow. Brought it up today and watched for the magic smoke. Took my time. It took a couple of hours to get it all the way up. I even ran some music through my turntable through it. Sounds not so good. Weak and a little buzzy/hummy. One channel extra weak. Now the real fun begins. I powered it down for now and I'll have to start thinking about analyzing what's going on. At least we have power and no smoke.
     
  15. rufleruf

    rufleruf AK Subscriber Subscriber

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    Did you wiggle the RCA jacks?

    When I powered up my KX200 the first time, the bias and balance were off by a bit and it was really not great sounding. Bringing those in really improved the sound.
     
  16. Tim D

    Tim D Well-Known Member

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    580
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    Yeah. Wiggled the jacks quite a bit. Reset the tubes. Twisted the knobs. Gonna try the aux input and rotate in some other tubes. I'll check the voltages and bias. Failing that I'll really start digging in with the o-scope and a signal generator.
     
  17. larryderouin

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

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    Did you plug in the phono plugs into MAG 1 or MAG 2? Mag 2 and CER are tied together apparently so don't use MAG2 and CER at the same time.

    It will sound tinny and weak if you plug a turntable direct into AUX unless the cartridge is a ceramic. THAT will work directly but a magnetic will need a phono preamp to run in AUX.

    Make sure the Selector is set to the same input the cables are in. Then set the mono /.Stereo switch to Stereo or Reverse. Try both Mag 1 and MAG 2. There is a TAPE/phono EQ switch that needs to be set to phono. Exercise it 20-30 times 1st. Set it for phono EQ. Then change it to Tape EQ and note the difference. The mids are a bit more forceful, along with an enhanced bass. High's aren't boosted much in TAPE. But I like it better than phono on my X101-B.
     
    Last edited: May 8, 2017
  18. Tim D

    Tim D Well-Known Member

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    Larry,

    Started with Phone Lo and moved to Phone Hi. Flipped the tape switch. Funny thing was that halfway through toggling the tape switch I suddently got more/screechy output. There is a possibility that I've got a bad cap in the preamp or some such. Next I want to try to bypass the phono/tape head preamp and put a different source in through the Aux just to try to see if the problems are with the phono preamp. These are easy things to try that will provide some info. I've got some extra 12AX7's that I can rotate through the low signal portions of the amp to see if maybe I've got a bad tube.
     
  19. larryderouin

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

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    My X-101-B does the same screech when moving the switch. Even put in a new switch. No joy.
     
  20. Tim D

    Tim D Well-Known Member

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    Location:
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    NEWS!!

    It works through the Aux inputs using my iphone just fine. That tell us a lot. Been playing Pandora through it for the past hour and sounds pretty good. Not as good as my low power Marantz, but respectable none the less. It's not super loud but it's filling the house with music.

    Hopefully the problem really is in the phono pre-amp and it's not just an interaction/interference with other parts of the amp. I think I can narrow this down now that I have a local area to look.

    I'm also sure that I'm going to have to do a lot of digging to check the voltages and tolerances throughout. I believe that the heater circuit for those small signal tubes that are connected through the control grid network of the output tubes has low voltage. I measured not much more than 35V and the spec/diagram shows it should be 42V. It did take a while for it to warm up. It's gratifying to know though that the core is good and the trannies are all good. That's a good place to start.
     
    Last edited: May 9, 2017

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