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

    Messages:
    486
    Location:
    Pleasanton, TX
    Dave, I appreciate the thorough explanation and the pictures for a visual. I looked further after posting questions yesterday and assumed some of what you explained. Still, I wanted to be certain. I was able to get a few items checked off.
    Items 1, 2, and 4 above are complete. 3 is partially done. I was interrupted and had to go to work. I will continue and update soon!

    Thank you again for the clarification!

    Bill.
     

     

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  2. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    The terminal strip section is complete. I also removed R19, R36(10ohm) from three terminal test point and all associated wiring.
    Not sure if i mentioned it but, I removed R49, R50.

    I plan to add 100 ohm screen stability resistors tomorrow.
    Then, maybe, I can get to work on EFB! Any pearls of wisdom are welcome!
    Thanks for getting me through that!

    Bill
     
  3. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    I got the screen stability resistors added. I was able to use existing/leftover wire.

    I am now looking at building and adding the EFB, but
    I will need to place another order.

    Terminal strips(what is correct nomenclature?) https://www.mouser.com/ProductDetai...=sGAEpiMZZMvz8LftK4jerkUaD0IcaHDhHpkXpfvzp08=

    .47uF cap, 600V(I must have removed this as I know I had it in my previous cart!)

    Insulating kit https://www.mouser.com/ProductDetail/Keystone-Electronics/4724?qs=sGAEpiMZZMvIGAkyPb0MlMiVJqkddDiy

    I will review what I have once again and see if I need anything else. Apparently, I was not ready to do this. It will get done...eventually.
    Thanks

    Bill
     

    Attached Files:

  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,847
    Location:
    Ball Ground, GA
    Bill -- Your choice of T-strip is perfect. Also, if you switch to using an STF10NK50Z type Mosfet, you can dispense with the insulating kit since this is type of device is plastic -- you simply use some thermal paste and bolt the sucker right to the chassis, with the plastic case providing all the necessary insulation. With the built in protections this device also has, you can dispense with the 12 volt Zener diode as well.

    Dave
     
  5. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    Dave, I can take whatever route you suggest. I already have the MOSFET (FQPF9N50C) and 12V zener.

    But, I could order STF10NK50Z from digikey along with those other few items. I was trying to place one order if possible.

    The closest MOSFET match that I can find at mouser is
    https://www.mouser.com/ProductDetai...GAEpiMZZMshyDBzk1/Wi5%2bqLKYs9z/zl0g8DiPvReI=

    Let me know what you think. Cost is negligible so either way is fine.
    I am so looking forward to getting this amp finished.

    Bill
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,847
    Location:
    Ball Ground, GA
    I don't think that's a plastic device, and so would gain you no ease in construction over what you currently have -- therefore, I'd stick with what you have.

    Dave
     

     

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  7. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
  8. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    I got parts in last week but was unable to get anything done due to holiday activities. I was able to work some last night and wanted to update the thread with progress.

    I removed R45, R46, R47, and R48.

    I also removed C2, M1, R51, and 56. I have new parts in place but nothing soldered in just yet. I also prefabbed and installed the EFB Bias supply regulator. I have not soldered the two wires...One to the emitter from R52, R54 and the jumper from the collector connecting positive side(anode) of diode M1.

    I hope to continue tonight but had questions regarding mosfet.

    I did order the STF10NK50Z from digikey as I liked the plastic housing option and omitting the zener.

    I had originally ordered the FQPF9N50C from mouser. I just looked closer and your schematic actually uses FQP9N50C. I must have looked wrong initially or something. Upon looking at data sheets last night, it appeared the housing for the FQPF9N50C was plastic. I opened up the package and they are! I wanted to mention this as this would maybe help someone else. I don’t know how or why I picked that one to order but specs look the same at first glance?

    I attached a few pics just to show progress.

    Thanks for all the help....trying to stay motivated. It seems like I have to work in spurts and then remember where I was, lol.
     

    Attached Files:

  9. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    I was able to get the other components installed but not soldered. I left the .47 cap out for the pictures as well as to solder prior to installing. Does this all appear to be correct? Based on the schematic and the pictures, I think this is it. The .47 cap will go from positive side of 50uf cap(ground) to the Gate of the mosfet. I ended up using the latter suggestion, Dave. Using the STF10NK50Z allowed me to omit the zener diode as well as insulating kit. I still used the kit minus the mica insulator.

    Bill
     

    Attached Files:

    Last edited: Nov 29, 2018
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,847
    Location:
    Ball Ground, GA
    Looks good Bill -- Hard to tell from the pics, but make sure that the 270K is a 1W or better device, and the 1500Ω is a 5W or better device.

    There are two "improvements" that have been implemented to this type of EFB™ topology, that do not so much arise out of need when installed in the SA-100, but are more of a standardized improvement to the basic circuit to help make it as universal as possible for the wide variety of installations and applications it has seen. The improvements include:

    1. Installing a 1000Ω .25W resistor in series with the Gate lead (installed right at the device between the device and the extended lead soldered to it) to act as a gate stopper. In the SA-100 application, this is really not needed. But for installations where the Mosfet is remotely mounted some distance from the circuit it connects to, the gate stopper resistor ensures stable operation without possibility of oscillation.

    2. Adding a conventional silicon diode from the Base of the inverter transistor (where the 1Meg resistor connects to), to ground, with the cathode (banded) end of the diode grounded. This diode prevents the base of the inverter transistor from being pulled positive (to any significant value) at shut down when the negative bias supply powering the EFB circuits (in full or partially) also powers the heaters of small signal tubes. In this case, the negative supply will be discharged very quickly at turn off -- faster than that of the B+ supply -- allowing the Base of this transistor to be elevated positively with respect to ground. The diode prevents this from happening during this time, but is normally reverse biased during operation, making its presence otherwise invisible during normal operation. In the SA-100, the bias supply does not power any small signal tube heaters, so the problem is quite remote in this application. But for applications where the bias supply shares double duty, its installation will ensure the problem never becomes an issue.

    To date, there have been no known component failures regarding these two concerns before these improvements were implemented. They are simply proactive measures made to ensure the basic topology is as bulletproof as possible with regards to installation and application details.

    Dave
     
  11. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    Dave, the 270k is a 1 watt but I can upgrade any of the resistors if recommended. The 1500? I only have a 15000 1/2 watt the collectors of the KSA1013 transistors. It sounds like I may be missing something? I may not have been using the most up to date info. I was using the schematic found in the improving Fisher SA-100 EFBII thread.

    Interesting changes noted above. I will investigate adding if I do this to any other equipment. First I need to finish this!

    Bill
     

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,847
    Location:
    Ball Ground, GA
    The resistor I'm referring to is the 1500Ω Drain resistor for the Mosfet. It is specified as being a 5W device, but the resistor you have installed in that location appears to be much smaller in wattage rating.

    Dave
     
  13. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    Dave, I apologize but I don’t see this resistor. I attached the schematic I am using. It only shows the 470 ohm 2 watt from drain on this, correct? I am obviously missing something or there is a later revision of this schematic?
    I also added a pic of how I currently have it hooked up. I labeled for reference in case I am looking at something wrong.


    Bill
     

    Attached Files:

    Last edited: Nov 30, 2018
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,847
    Location:
    Ball Ground, GA
    Bill -- It is I who needs to apologize. In looking at my original schematic for the EFB™ modification, I originally in fact did have a 470Ω, 2 Watt resistor at the Drain terminal of the Mosfet. At some point, I came along and changed it to a 1500Ω 5 Watt resistor, and failed to note when the change was made, or update the published schematic.

    This all came about no doubt because unlike my other releases of EFB modification/applications that were developed on my own pieces of equipment, the EFB modification for the SA-100 was originally developed on another AKer's SA-100 as a developmental piece only to prove out the concept of applying a new form of EFB via Screen Grid and Control Grid Regulators to an existing as designed fixed bias, pentode based output stage, in what I was then collectively calling EFB II at the time. Up to that point, the only EFB topology I had fully proven out was that originally developed for the small Dynaco amplifiers, that uses a 3-T regulator in the cathode circuit of the output tubes to provide both fixed bias operation, and EFB action in one simple circuit. But other designs like the SA-100 needed a different approach of applying EFB to gain all that the modification has to offer in the way of benefits. The original SA-100 project then was really an effort to prove out the alternate approach of applying the concept.

    Then some time later, I got my own SA-100, and tweaked the circuit in the process not only with the two modifications I mentioned earlier, but also by changing to a plastic Mosfet that no longer needed the external 12 volt Zener diode for protection, and increasing the value of the 470Ω 2W resistor, to 1500Ω at 5 Watts. The change in Mosfet was purely to facilitate easier mounting of the device, while the purpose of the resistor change doesn't impact performance of the circuit at all but rather, allows the Mosfet to operate incrementally cooler, while providing even more protection for the device in case one of the output tubes should go south. With the 470Ω resistor, the device is dissipating 1.26 Watts under quiescent conditions, and 3.68 Watts under sustained full power output in both channels. With the 1500Ω resistor installed, dissipation under quiescent conditions for the device changes very little down to 1.12 watts, but under conditions of full sustained power output in both channels, dissipates just 2.0 watts, so the revised resistor value represents a worthwhile improvement -- but only relates to enhancing the durability of an already durable design. Therefore, you can make the change to the updated value as you see fit. Again, I apologize for the confusion all of this caused. I will try to make some time soon to go back and update the original thread with an updated schematic.

    I hope this helps!

    Dave
     
  15. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    Dave, that helps immensely! I did notice it on the clone pictures you posted to assist me but assumed it was unique to that build.

    I can add a 1500 ohm 5 watt resistor to the cart I already have ready with my Bozak crossover parts.

    One other question. Once I get this amp all soldered together, what will be the correct alignment procedure?
    Again, I certainly appreciate you taking the time to review and then, patiently,?explain in entirety. I hope to better understand this stuff at some point.

    Bill
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,847
    Location:
    Ball Ground, GA
    That's the simple part!

    1. Set the DC Bias and Balance controls in the middle of their range.

    2. Set your DC voltmeter on a 2 volt setting and connect it across the two 10Ω cathode resistors (TPs) for Channel A. Polarity of the leads is unimportant.

    3. Turn the amplifier on and adjust the Channel A DC Balance for 0.00 volts. The reading will fluctuate somewhat, which is not unusual. Adjust for an average of 0.00 volts if that is the case.

    4. Now remove the Black voltmeter lead and connect it to the chassis. Adjust the Channel A Bias control for a reading of 0.24 vdc.

    5. Repeat this procedure for Channel B.

    6. The controls are interactive, so you will need to go through the entire process a few times to really zero in on the desired readings for both channels.

    7. Use the Sticky for the procedure I developed for setting the AC Balance controls. The procedure may need to be modified slightly (different pin numbers etc.) for the SA-100, but the process will be the same.

    That does it!

    Dave
     

     

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  17. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    Dave, Success!!...I think.
    I finally got the parts installed and made some progress. I was able to make bias and DC balance adjustments but the
    AC adjustment I want to make sure I understand.
    I found the following info in post #25 of this thread.
    http://audiokarma.org/forums/index....0-power-supply-bypass-capacitor.523067/page-2
    This is what I used.

    I listened a bit just through the junky speakers I had hooked up for alignment.
    Hopefully, I will have time this weekend to hook up some real speakers.

    I noticed something odd on the preamp. The balance doesn’t seem to work properly. I can’t remember if it was doing this before. I may not have checked that while listening before as I only listened for a short time. It’s not a huge deal as it usually stays centered but just noticed it. May need to open it up again to see what’s going on with that control.

    Thanks for getting me to where I am now.

    Bill
     
  18. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    I hope everyone had a nice Christmas holiday break. I know I have been slacking in pic department so here are a couple of shots. I cleaned up and shot the cage to make it look better.
    I have replacement jewels that are supposedly original. They are close. I will install those this week.

    Critiques are fine....it sounds great.
    Regarding the balance issue. It must be something with Aux 1? It work properly on Aux 2. I also haven’t tested phono either. It will all get done....

    Bill
     

    Attached Files:

  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,847
    Location:
    Ball Ground, GA
    I believe your version of the CX has individual Aux 1 level controls -- check to see that they are either full up, or balanced if set midway.

    Your gold paint on the SA-100's cover looks great! What did you use?

    Dave
     
  20. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    486
    Location:
    Pleasanton, TX
    Dave, good news, I rechecked the levels. I backed them off from max and now all is well on balance.

    The cover isn’t perfection may actually take the time to do it over. It is much better than it was though.
    I used Rustoleum, metallic finish. I took a picture of the can since there are so many types.

    Bill
     

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