Improving the Fisher 400

Discussion in 'Fisher' started by dcgillespie, Mar 29, 2013.

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I got nothing. Other than using Mouser or Digikey (your choice).
     

     

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  2. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Dave,
    Sorry for a bit of a sidetrack. Not so long ago there were rumors floating about , about a universal EFB board in the works for 7591 applications. Am I wrong? Did that prove impractical?
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    NJ -- I did in fact design one of the two universal EFB(tm) circuits -- the one that would be used in a project like the Fisher receiver of this thread. It is basically a board for use where conventional fixed bias already exists, with the board then providing EFB control of both control grid and (as needed) screen grid voltage supplies. I then had the board laid out and had a preliminary run of boards produced. That aspect of the project turned out great, but at that point, it basically stalled as I simply have not had the time to develop a user's manual for it.

    There are a couple of inherent problems in releasing the board even when a user's manual can be developed:

    1. It would necessarily have to be sold to advanced builders and experimenters, as every application is slightly different not only in basic operating parameters, but also in how an appropriate negative bias source is developed. For the EFB control grid regulator to operate properly, the original bias voltage typically has to be doubled so there is a greater level that then can be regulated down to the appropriate level. The board will accommodate a great variety of ways to achieve that for all the various designs out there, but it will take someone with an appropriate knowledge level to be able to then determine what the best approach would be. At this point, providing all the support that would require even for advanced builders would be overwhelming.

    2. . Experience has shown that in the vast majority of installations into existing equipment, the board is not practical, as even though it is quite small for the wide variety of scenarios it can operate in (2 3/8 x 2 5/8) most pieces don't have the room for it. It would likely work well for basic power amplifier units, but integrated amplifiers and receivers almost always end up with the EFB circuits being hard wired into the unit, disappearing into and becoming part of the overall original wiring.

    As a result, while I've got the board that is the toughest to develop, I simply don't have a way to provide the support it would require. I plan to re-visit it all later in the new year after I have had a chance to deal with some immediate family issues.

    Thanks for the interest!

    Dave
     
  4. streetwise

    streetwise Active Member

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    Would anybody be interested in doing this rebuild for me. I don't think I have the tools or enough experience to pull this off. also looking for someone to align the tuner and mpx? let me know thanks

    Paul
     
  5. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    I'll be glad to help you but unfortunately I'm living in Colombia South America. Ask Larry he did it several times in the past to another Akers

    Luis
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I'm not doing outside work any more, due to worsening degenerative disc disease and arthritis problems. It's getting hard enough to do my own stuff. Thanks for the thought tho Luis
     

     

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  7. streetwise

    streetwise Active Member

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    sorry to hear that Larry, anyone else?
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    Doubled from what the existing supply is, or do you mean you need to have double the voltage available vs what the grids actually get? I'm giving some thought on attempting this in a Sherwood 5500, and all I can get for a bias supply in stock form is -32v. The tubes run around -17v. Not sure if that is enough overhead or not. It uses the bias supply to feed some tube heaters, so I don't think I can wire it up into a voltage doubler either.
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Double the supply voltage at the bridge, split it, then drop it down on one side for the heaters, and keep high for the EFB, and bias. Or another Supply circuit strictly for the EFB with the higher voltage.
     
  10. cj2152

    cj2152 New Member

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    Power Mosfet “STF10NK50Z” for the “EFB modification” is Obsolete

    Hi Fellas

    New here, ... I’ve read every thing I could get my hands on about my old Fisher 800C. I installed the Metalbone Restoration kit and the unit sounds real good again. (I’ve had this old thing for about as long as I care to remember :)

    I decided to install all of Dave’s modifications. Just spent yesterday afternoon ordering all the parts from Mouser ... except the the Power Mosfet “STF10NK50Z” for the “EFB modification” it is now Obsolete. Most suppliers don’t have them in stock. The only place I found some was at Digi-key. I ordered a couple just to have an extra on hand.

    The Digi-key part # 497-4654-5-ND

    Just wanted to let you know about this FET, in case you‘re thinking of doing the modification, or if you want to grab one as a replacement for the future.
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    Ended up doing something else. The Sherwood used a full wave center tap setup originally. I changed it to a bridge and it gave me plenty enough volts. The heaters got switched around to a series string so they run on the 60v I get now, no resistors involved. I'll flesh all that out elsewhere once I get it behaving how I want it to.
     

     

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  12. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    A good replacement for this obsolet MOSFET is the STF11N50M2. attached is the data sheet.

    The only difference with the obsolete MOSFET is that its current is less in 1A (8A) and the Vds is 550V. That IMO can works well due to the low current that the Dave's design manages for the EFB.

    @Dave do you have any feedback about the suggested MOSFET??

    Luis.
     

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  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Luis -- Your search produced an excellent substitution -- Thank-you! Besides having adequate voltage, current, and dissipation ratings, the replacement device importantly has all of the required Mosfet protection elements built in internally (as the original piece did) that would otherwise require more external components to accomplish, and is also a plastic device as well (also like the original piece), making for easy mounting considerations. Thanks for your efforts, Luis. I fear however that as time goes on, more and more traditional components will fall by the wayside in favor of SM technology, making these kinds of exercises more and more a part of any ongoing project component list.

    Dave
     
  14. streetwise

    streetwise Active Member

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    Dave -- I am talking to Jim Mcshane about his rebuild kit to start restoring my 400 before taking the dive into your mods. Alot of the values he has for replacement parts appear to be larger values than what is listed on the original and your Rev. 7 schematics. Would these have to be replaced again after completing the rebuild to get proper function out of your mods? My can caps have already been rebuilt for example but don't hold the exact 30-30-30-30 values you have listed.
     

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  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Your can caps are fine.

    In looking at your power supply restoration list, about the only new parts that would not be reused after doing your initial restoration if you installed my mods (particularly as it relates to EFBtm) would be the replacement 220 uF bias cap, and the 1.2K 10 watt resistor -- which could certainly be saved for other Fisher projects. The great majority of parts for the EFB modification represents new components on top of those on your list, whereas the great majority of the components originally replaced are done so because of the basic restoration, and remain even when the modifications are added. In other areas -- like modifying the power amplifier circuits -- the modifications will require replacement of existing components. But they are not components you would be replacing due to the power supply restoration.

    I hope this helps!

    Dave
     
  16. streetwise

    streetwise Active Member

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    Thank you Dave that was very helpful I was just concerned you had certain values set and replacing with higher values would mess with voltages when your mods are in play. I look forward to continuing this project
     

     

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  17. dannyr

    dannyr Active Member

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    hi, I've been meaning to ask if anyone has any ideas why if i plug in and use a eq etc into the tape loop of the tweaked 400 that the preamp's volume drops. I like to sometimes use a parametric to tweak things a bit. Usually i'd just put it between the sources and amp but i'd like to use it with the tuner.
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Hi Danny -- No doubt it is due to the relatively low input impedance of your EQ unit. If you electrically "moved" the Tape Monitor switch in the circuit so that the monitor input reflects the same sensitivity as the Aux input represents, then the drive impedance at the Tape Output jack is the same as that presented by the source selected. Therefore, if you have (say) a CD player connected to the Aux input, it should have no problem driving your EQ unit, with likely then no loss of signal. However, the internal phono preamps and the output of the MPX sub-chassis in the 400 are not so capable, so that your EQ unit would likely load those sources down significantly. If you did not move the Tape Monitor switch, then connecting your EQ unit to the old Tape Output circuit will load ALL inputs down equally, since that signal was derived at a different point in the signal chain.

    Question: Did you install the little buffer stage that (in part) allows conversion of the old Reverb jacks into new Preamp Output/Power Amp Input jacks? If so, and if your EQ unit is capable of supplying a 2 volts RMS output (almost all such units can), then you could remove the jumpers and connect your EQ unit to those jacks. So modified, the output impedance at the Preamp Output jack is just 100 Ohms, which will more than easily drive your EQ unit, and, it will free up your Tape Monitor input for other sources, too.

    I hope this helps!

    Dave
     
  19. dannyr

    dannyr Active Member

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    Hi Dave, thanks yet again for the very informative response. Once again I learned a lot.

    I did add the buffer board though I removed the preamp/poweramp input jack conversion while tracking down an issue that turned out to be something else. Now I have a reason to reinstall it. I still have to do the phono mods. I believe that the EQ's output impedance is 300ohm. It's a Technics SH-9100.
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I couldn't find any info on your unit on the web, but it's the input impedance that is of concern. If you can find out that specification and post it here, then we can verify for sure if that is the concern or not.

    Dave
     

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