X-100-B left channel tremolo

Discussion in 'Fisher' started by rufleruf, Nov 5, 2017.

  1. larryderouin

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

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    Could very well be. Only real way to be sure is to test it with a meter.
     
  2. rufleruf

    rufleruf AK Subscriber Subscriber

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    Thanks for carrying on the discussion in my silence here. I started a new job today, so I went to bed early last night. I'll try and get through the tests tonight, and check voltages. The left channel worked fine when I tested this after purchase. I think I might have started a thread about it.

    Took a few months off messing with old stereos to focus on other things, now I'm trying to work through some old projects and do a purge to get things back to manageable.
     
  3. Tim D

    Tim D AK Subscriber Subscriber

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    I defer to Dave and Larry. I am behind the curve as I didn't know there was a 5 band code for resistors and in fact there is a 6 band code too. I stand corrected.
     
  4. rufleruf

    rufleruf AK Subscriber Subscriber

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    Dave and all ya',

    I didn't have time to desolder etc, so I did two tests that I think achieves the same end.

    Back story: I made a sound probe to work on old synthesizers, just an rca cable that has one end cut off and an alligator clip installed as ground and capacitor installed as sound probe. I run this into a powered computer speaker and with it I can check the sound along the signal path. a very unsophisticated oscilloscope stand in I suppose.

    So the tests:

    1. With a sound source (my iphone) going in the aux jacks on the X-100-B I turned the volume all the way down (on the x-100- B), and using the sound probe I found normal audio at the volume pot inputs on both channels. 2. Without audio going into any input on the X-100-B , I used my phone and the sound probe to inject sound at the input to the volume pot, and the sound coming out of the left channel was effected by the 'tremolo'.

    I may be able to sneak off later and check voltages... Hopefully maybe?
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OK. So the problem has been localized to the Left Channel power amp.

    Question: Is the driver tube at V5 (for Left Channel) a tube of vintage manufacture (American, British, or German)? Sometimes, modern production tubes from Russia don't like the low current levels that Fisher operates the phase inverter at -- which leads to the second question: Are the noose resistors R55 and R56 still in place?

    Last question: Will the Left Channel produce plenty of power and get plenty loud as a result? Or is it very restricted and limited in the power it can deliver compared to the Right Channel?

    Dave
     
  6. rufleruf

    rufleruf AK Subscriber Subscriber

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    I swapped in all known good vintage 12ax7s and checked the sound before I did the test. It had the tremolo effect. I left them in for the test.

    I'll check the resistors when I check the voltage.

    Answering last question, they seem to be equally loud, the left channel just has the tremolo effect.
     
  7. rufleruf

    rufleruf AK Subscriber Subscriber

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    Okay - ding, no voltage at pin 9. There it is.

    Put in the other OPT from the parts kx100 and YES - it works!

    Meaningless maybe, but nice blue glow in the earlier 'no glow' tube.

    Learning by the day
     
    Last edited: Nov 6, 2017
    sberger likes this.
  8. Tim D

    Tim D AK Subscriber Subscriber

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    Wow Matt. Good work.

    Interesting that you had two bad trannies. Sounds like this one was only partially bad. That's similar to what happened on my KX-100. See post 122 of my thread here where I found the same zero volt on pin 9.

    http://audiokarma.org/forums/index....ube-amp-will-need-some-guidance.765247/page-7

    The primary of the trannie is center-tapped. One side of mine was open thus leaving the tube on that leg without any output while the other was still connected and could kinda continue working. With the loss of current draw on that one tube the whole voltage situation across the entire amp was out of whack. It didn't take long after I started checking voltages to track it down. I did not get a tremolo effect though. All I got was reduced and lower quality output in that channel. I'm guessing your latest trannie failed similarly. I was able to repair mine with some surgery. I was lucky that the break was on the wire just outside of the first windings. I just had to take the shields off, peal back the paper on the trannie, and find it. The wire is super fine like angel hair though and difficult to work with. I only had about 1 or 2 mm of stub to work with too. If yours failed in the same way you might be lucky like I was. Just saying you could try to salvage it as a spare as these go for about a hunert. I started my surgery at post 130 of the same thread.

    So let us know how it works after you check it out a bit more....
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    So you've actually had two bad transformers then, both the original and the first one from your parts unit?

    Tim, was the wire just broken or did it show signs of corrosion or melting? Just wondering if its an over-current condition, environmental damange, or if its just a mechanical failure from thin wires that may not be supported amazingly well.
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Unfortunately, the KX-100 and X-100B OPTs seem to be historically prone to failure -- particularly unfortunate because they are a very good transformer otherwise. Do check the tube that was in the socket that was missing plate voltage. Operating the tube with just screen voltage applied is extremely hard on that grid, as it is then trying to pass all the current flow of the tube by itself which can damage the grid.

    Congrats on finding the culprit!

    Dave
     
  11. rufleruf

    rufleruf AK Subscriber Subscriber

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    I'll check that tube. Any ideas what would render these units output transformers prone to failure as they age?

    I have 4 or 5 bad ones.
     
    Last edited: Nov 7, 2017
  12. Tim D

    Tim D AK Subscriber Subscriber

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    Hi gadget73,

    When I found my trannie failure it almost looked like the wire was snipped. It didn't appear burnt, but I have to believe that it probably separated while in use. I am guessing that it had a weak spot and the current got a little high and it let go. It wasn't like the whole trannie looked like it was overheated and I didn't see any burn marks. Could it have been mechanical failure? Well, maybe but it isn't a moving part so I would lean toward it being an electrical failure. There's a pic of the stub where it separated in my thread if you care to look.

    Tim
     
  13. Tim D

    Tim D AK Subscriber Subscriber

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    Matt,

    Take a look at my thread and see how huge my finger looks next to the wire. It's like an elephant standing next to a garden hose. The primary wire is very thin. I can believe that any unusual current would cause a failure. If they'd just have upgraded the size of the wire one gauge that would have made a difference - my best guess. (The trannies would have been somewhat bigger and more costly though.)
     
  14. Patrice B

    Patrice B AK Subscriber Subscriber

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    Moisture or overheat I would guess. BTW, the replacement opt you sent me for my X-100-2 is going well.
     
  15. larryderouin

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

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    As thin as the primary is, exposire to atmospheric elements for extended periods in a basement could have corroded the wire and caused a fuse like failure when B+ was injected into the transformer. The corrosion could be visible or not. Also, Tim's theory of the wires being stretched to tightly is a viable issue. Put both together and it's a recipe for high incidents of failure in these. I'd suggest a visual inspection of the transformers on the X-100 family (incl the KX-100) and a variac power up test with the end bells OFF before re-installation on the unit.
     

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