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Help Saving 400-CX SA-100 and 100-R Tuner

Discussion in 'Fisher' started by moondogtn, Oct 10, 2017.

  1. moondogtn

    moondogtn AK Subscriber Subscriber

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    I have been trying to get a break to get back to this. I have put a parts list together for the EFB but had a few questions. For resistors, will metal film be good? What about power ratings for those not specified? 1/4 watt or 1/2 watt? Here is the current list....I have included extras just because and I know PDF isn't the greatest way to share this. BTW, there are some metal oxide resistors that I am ordering for something else, disregard those. If there is a better way to do this, let me know or I can try to list parts links individually. Thanks

    Bill
     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Moon -- Unless otherwise marked on the EFB™ schematic, .5W resistors are fine, as are the use of MF pieces. The 1M 1W resistor should be a close tolerance piece.

    Dave
     
  3. moondogtn

    moondogtn AK Subscriber Subscriber

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    Dave, thank you. I ordered parts plus extras. I hope to get this done next week providing I don't drop a tube or burn something up. I will need to reread the various threads on the EFB install but IIRC, I simply install(or replace) according to schematic and then redo adjustments in same order?

    Bill
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Yep -- but also be sure you have installed individual cathode resistors and Screen Stability resistors in the process if not already done.

    Dave
     
  5. moondogtn

    moondogtn AK Subscriber Subscriber

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    I haven’t and didn’t know I needed them, must have missed that step? I will have to place another order once I figure out what I need.

    Edited...I can’t believe I didn’t catch that. I will still need to determine what it all means so will likely need some guidance.

    2. Remove the damping control circuits. This is an antiquated feature, and its removal will allow two terminals on each output strip to be used as individual output tube current measurement points, as the original design only allowed for measurement of the total current draw of each channel.

    3. Install individual 10 ohm cathode resistors to allow for current measurements of each output tube at the new terminals created above.

    4. Install individual 100 ohm screen stability resistors. This will allow for the use of a screen supply source of much lower impedance without any concerns of output tube arcing.

    5. Remove the old screen bleeder-regulator circuits.

    6. Install the screen voltage control portion of EFB II

    7. Install the bias supply regulator portion of EFB II.

    I may have some of these values available. This seems pretty complicated to a newbie on tubes since I don’t know what most of this means. Time to do some research. Lol

    Bill
     
    Last edited: Oct 11, 2018
  6. notdigital

    notdigital AK Subscriber Subscriber

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    Moon: If it'll help, I've attached a pic that shows most of the EFB and other mods mentioned above. DSCN0329.JPG
     

     

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  7. moondogtn

    moondogtn AK Subscriber Subscriber

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    Notdigital, I appreciate the pic. I just need to dig into a bit as the terminology has me thrown off. I did look at yours and Dave’s threads. I see what was meant by some of the items. Let me get some better questions together to clarify. Thanks again.

    Bill
     
  8. notdigital

    notdigital AK Subscriber Subscriber

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    No worries. Just a little while back reading latin was easier than reading some of these tech related posts. After pouring over and studying many related restoration posts, my brain developed a library of terms that was sufficient to get me through. Nothing deep but just enough of the basics to address the task at hand. You'll be happy when it all starts to click.

    Just so you know, the pic above is virtually a snapshot of the work Dave did in his thread. Referencing your post #125, what you do not see in the pic is #2 &3. Unabashedly, I copied heyraz' way of providing for cathode current test points which I thought was a slick way of implementing the modification.
     
    Last edited: Nov 9, 2018
  9. moondogtn

    moondogtn AK Subscriber Subscriber

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    I read some more. It’s starting to make sense. I will use some pics with markup to ensure I get it correct prior to applying power.
    Just to make sure I get the correct parts. These two values (10 and 100) should be 1/4 watt? Thanks again.

    Bill
     
    Last edited: Oct 14, 2018
  10. notdigital

    notdigital AK Subscriber Subscriber

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    I use 1/4 watt metal film devices for the cathode or "test point" resistors. Same for the screen stability resistors. I want either or both to let go if an over-current situation develops.

    For sure the cathode resistors should be 1%, very closely matched components.
     
  11. moondogtn

    moondogtn AK Subscriber Subscriber

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    Not, from what I read, that was reason to use 1/4 watt but I wanted to make sure. Also, I plan to use metal film 1% but can also match them too with my meter. Thank you for clarification.

    Bill
     

     

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  12. moondogtn

    moondogtn AK Subscriber Subscriber

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    Ok, so I have been reading and rereading. I want to see if I have this correct. I have attached a couple of pics that I marked up for reference. This is the part that I missed...Steps 2, 3, and 4 of post 125.

    In first pic, I will need to remove the resistors (R59 and R60, .47 ohm 5 watt) from the unit. These are marked with blue X.
    I want to move one side of each capacitor(C14 and C21)from ground lug to ground on respective speaker ground terminal. This is shown with pink arrow.
    I can then install the 10 ohm cathode resistors to their respective terminals denoted by green lines.
    I also remove R49 and R50 also marked with blue X, correct?

    Second pic shows where I will be adding screen stability resistors by moving wiring from pin 9 to pin 8. 100 ohm resistors to be installed between pin 8 and 9 on output tubes V2, V3, V5, V6.

    It will be nasty weather all week long here so perfect opportunity to maybe finish this. Thanks for the guidance. I hope this will start coming together soon.

    Bill
     

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  13. notdigital

    notdigital AK Subscriber Subscriber

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    Well on your way!!!!!!
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    You got it!

    Dave
     
  15. moondogtn

    moondogtn AK Subscriber Subscriber

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    Thanks gentlemen. I don't see any need to be in a rush at this point, lol. So, just trying to proceed with caution and hope to gain some understanding at some point.



    Just to clarify notdigital, FROM above post 133, Second pic shows where I will be adding screen stability resistors by moving wiring from pin 9 to pin 8. 100 ohm resistors to be installed between pin 8 and 9 on output tubes V2, V3, V5, V6. I'm at work but if I recall, the resistor bridges pin 8&9. Both tubes in that channel are daisy chained to eventually connect to EFB.

    I will first need to remove the existing wire that currently runs from C1D to pin 9 on V2/V3 as well as the existing wire from C1C to V5/V6.
    Then, if I am looking at schematic correctly, the wiring from C1D will daisy chain to pin 8 on each channel. I believe its C1D to V2/V3 which are daisy chained for channel A and then C1C to V5/V6 are daisy chained for channel B.
    So, I will run new wire for this, correct?

    You mentioned to make sure terminal is properly connected to cathode(Pin 3?) from R5 and R8 respectively, correct?

    I am sure this will take a bit for me to complete. I will continue working, reading, and asking questions. Thank you.

    Bill
     
  16. notdigital

    notdigital AK Subscriber Subscriber

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    Yes, you would need to "remove" the existing wiring to make the new connections. I put it in quotes because, after analyzing the wire layout, you'll see that some portion of the wire run is the same. What you'll find out is that additional wiring would be needed to make the run complete in the modification. I like to preserve the original look as much as possible so I try not to disturb existing wiring if at all possible. I also try to keep the same color scheme, if possible.You'll find that the run of wiring for the left and right channels will be different so you have to make the adjustment in terms of length.

    And yes, pin 3. Remember, the original biasing scheme provided for test points in pairs. You are now separating those pairs so as to test the tubes individually. If I recall, the wiring originally connected was yellow. You'll have to add more wire to make the 4 connections needed.

    I'm sure I've got a pic somewhere to illustrate the connections. I'll try to find it and post it.
     

     

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  17. moondogtn

    moondogtn AK Subscriber Subscriber

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    Notdigital, I do see that some of the wiring is ran to same place so remove and replace seems wrong to say. I assumed lengths may have been a factor in that decision. I plan to leave existing wiring in place and use where possible.
    I was trying to determine how each tube was going to be biased rather that each channel. If I am correct, it appears to be separated by moving the smaller yellow wire to the terminal next to the original after removing the resistors(R59/R60). I will need to determine which tubes go to what to make it consistent/logical.

    Dave, I hope you don't mind me borrowing your picture to show the completed example.

    First pic is original config. I don't have a better one at the moment. Second pic shows Dave's completed example.

    Bill
     

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  18. notdigital

    notdigital AK Subscriber Subscriber

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    There are 3 basic adjustments to be made when all the mods have been installed: AC Balance, DC Balance, and BIAS. The biasing pots are on the top of the unit. With the mods installed you still bias the individual channels (not the individual tubes.) However, you can TEST each individual tube with the modification. Once the balancing adjustments have been made, then with Dave's EFB installed, the optimal setting should be .24vdc. Originally, I recall the original spec was .33 or .36. I forget but one of the advantages of the modification is that the tube can operate at a lower operating point with no discernible distortion. I believe it's all laid out in Dave's original SA-100 thread.
     
  19. moondogtn

    moondogtn AK Subscriber Subscriber

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    I was interrupted with life, school, and family stuff and am trying to remember where I was....I did say that wrong regarding biasing each tube.

    So, I began by removing R49, R50 and R59, R60. I moved the leg of C14, C21 to the ground lug. I planned to add the 10 ohm cathode resistors.

    I stopped because I think I am not following on a few things......

    Was I mistaken in post 137? Do I move the smaller yellow wires over to the next terminal(common terminals)? If so, where do those wires originate? I want to clarify before moving on.


    Were you able to find any pics?

    I will definitely be rereading everything I thought I had sorted out. Thanks for being patient.
    Edit: I am looking at some pics and notice R19, R36 and associated wiring are removed as well. When does that take place. As I said, I will be doing some more reading.

    Bill
     
    Last edited: Nov 10, 2018
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Moon -- Some pointers to maybe help you out with the speaker terminal strip wiring:

    1. R59, and R60 are both removed.

    2. Capacitors C14 and C21 are re-grounded at the speaker Ground terminal so that they won't be grounded at what becomes the point where the output tube high current ground point is located. More on this later.

    3. The wire from the X terminals back to V1 and V4 are removed. This leaves no leads connected to the Com or X speaker T-strip terminals in either channel.

    4. All the wiring is removed from pin 3 of each output tube socket.

    5. Four new leads are installed -- one from each output tube socket pin 3 up to one of the now free Com or X terminals. You want the wires from the output tubes in a given channel to go to the speaker terminal strip for that channel. As to which tube goes to which terminal within a given terminal strip is your choice. The output tubes are of stagger mounted, so in my own unit, the tubes that are staggered towards the OPT end of the chassis go to the Com terminals, since they are closest to that end of the chassis as well.

    6. From each of the Com and X terminals, a precision 10Ω resistor is connected to the freed up chassis ground terminal that C14 and C21 used to be grounded to.

    7. All of the wiring from the original 3 terminal test point T-Strip is then removed.

    That covers the terminal output terminal strip wiring. At this point, if this is all you have done, then the amplifier will now operate as normal, but you will be able to monitor and now adjust the appropriate bias controls so that all of the output tubes each draw the same amount of current. For the stock design, this is 30 ma per tube, or .30 vdc between each Com and X terminal, and chassis ground.

    Please understand that the pics from the thread on this unit were not of my own unit, but one I was temporarily sent to do just the developmental work on. Therefore, it is not my unit, nor necessarily fully restored, either. I have included pics in this post of the real unit I now own, which is a different unit from that in the pics you referenced. The modification is executed somewhat differently in my own unit, because I allowed for the addition of a choke in the B+ power supply. But for your purposes in this thread here, they are electrically basically identical. Relative to the info just given in this post, you will see that C14 and C21 remain effectively grounded at the same point that they originally were in my unit, but now done so via the ground/mount lug of a new T-strip solder-mounted onto the old chassis ground terminal. The isolated terminal on the T-strip is the new, isolated ground terminal for the four 10Ω resistors, with the ground connection for this terminal made via a lead run back to a can cap ground lug -- this eliminating any high level ground current from flowing through the chassis.

    I hope none of this confuses you any further, but clarifies how the speaker terminal strips should be wired.

    Dave

    BELOW: Closeup of the speaker terminal strip wiring in my own SA-100.
    SAM_2592.JPG

    BELOW: Closeup of the modified bias supply, and the EFB™ Screen Grid Regulator installation on and near the side wall next to the choke.
    SAM_2593.JPG

    BELOW: Closeup of the complete EFB installation. Note that there's plenty of space to execute the installation without any look of clutter or being crammed in. And, without all the heat producing power resistors of the stock design, the unit runs notably cooler now, producing more power, with less distortion, and longer tube life.
    SAM_2597.JPG

    BELOW: Full under-hood shot.
    SAM_2596.JPG
     

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