Fisher X-1000 restore hold my hand

Discussion in 'Fisher' started by Kingbleu, Oct 27, 2015.

  1. Kingbleu

    Kingbleu Active Member

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    Should I be using a different method for measuring bias voltage because now that I have done the modification the bias has raised a lot in both channels.

    Before the bias was .088 and .097

    Now the lowest I can set the bias is 1.53 and 1.30
     
  2. larryderouin

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

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    where are those extra leads from the screen pwr going????? Each side(channel) has a separate Screen Supply. C-56C runs the screen for V11 & V12, while C-57A runs the screen for V13 and V14. Trace both or all leads coming off each pair of tube screen connections and list where they ALL terminate.

    Pin 8 on the pair has a straight wire with a junction in the middle with a 10ohm resistor (R133 or 134) going eventually to ground. replace the wire from pin 8 to pin 8 with 2 seriesed 10 ohm resistors. At the junction of the 2 resistors, connect the 10 ohm resistor (R133-134). Then connect the free ends of the resistors to pin 8 on each tube. This is shown on the drawings I posted on post #115. DON'T get ahead of us, as it causes problems with us trying to figure out what you're doing and having to redo mistakes, and can cause damage to the unit.

    Suggest you read the thread on the X-1000 that Dave did (see post 13 this thread) and pick and pull posts that pertain to this, then post the links to those posts. OR READ the thread and WAIT FOR DAVE for final analysis.
     
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  3. Kingbleu

    Kingbleu Active Member

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    133
    Location:
    Southern California
    That is correct the lead from screen power pin 4 on V12 terminates at C-56

    The lead from screen power pin 4 on V14 terminates at C-57
     
  4. Kingbleu

    Kingbleu Active Member

    Messages:
    133
    Location:
    Southern California
    That is correct pin 8 has a straight wire from pin 8 V11 to pin 8 v12. Then R133 is connected to pin 8 on V12 and pin 1 (ground) V12. The junction in the middle is throwing me off because I can see it in the schematics but it looks like I just stated in real life

    What I did is exactly what you are describing as illustrated in the picture of post #120

    The two 10 ohm resisters are connected in series with one end connected to pin 8 V11 and the other end connected to pin 8 V12. The junction is then connected with one leg of R133 and the other leg of R133 is connected to pin 1 V12 / ground.

    I think I'm good here unless Im not understanding something.
     
  5. Kingbleu

    Kingbleu Active Member

    Messages:
    133
    Location:
    Southern California
    I think I figured out the new way of measuring the bias. The reason my readings were so high is because I was measuring the bias with one probe on ground and one probe on pin 8, which is the way you do it with a stock fisher.

    However after the modification if I measure the bias with one probe on ground and one probe at the junction of R33-R34 and the 10 ohm resistors I get accurate readings.

    How would I measure the DC balance now that I have done slight modification work in the output stage area, that include adding separate cathode sampling resistors, and Screen Stability resistors to each tube?

    Before the modification I was told one way to measure DC balance was to use a sensitive pair of headphones and with the volume at zero adjust the balance for an equal amount of hum in each speaker.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,841
    Location:
    Ball Ground, GA
    King -- Normally, you would remove the original 10Ω resistor, so that each pin 8 has its own dedicated 10Ω resistor straight to ground. There is no need to retain the original resistor when the individual cathode resistors are added.
    Alternately, you can leave the original resistors in, but as you found out, accurate total current draw is only indicated when taking readings directly across the original resistor. Accurate individual tube current draw is only indicated when taking readings directly across the individual 10Ω resistors. Removing the original resistor when installing individual resistors eliminates all this nonsense, so that you then bias each tube for .43 vdc between pin 8 and ground.

    If your Fisher will not bias properly either way, then the bias supply will need to be modified to supply more bias voltage than the stock design allows for. I have devised a way to do that, but I would make sure your measurements are accurate first to determine if that is actually needed or not for the particular tubes you are using.

    Dave
     
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  7. Kingbleu

    Kingbleu Active Member

    Messages:
    133
    Location:
    Southern California
    I just removed the original 10 ohm resistors.

    I just biased each channel to 0.43 vdc but I had to play with the dc balance to get both channels at 0.43 vdc

    Thanks Dave
    Thanks Larry
    Thanks Audiokarma
     
  8. Kingbleu

    Kingbleu Active Member

    Messages:
    133
    Location:
    Southern California
    :banana: Im just trying to delete post #128 but it wont let me. If you saw the video before I took it down ignore it. It was just the way the song was mixed in the studio. :eek:
     
    Last edited: Oct 6, 2017

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