Fisher 400 Rebuild Project

Discussion in 'Fisher' started by tekuhn, Apr 10, 2018.

  1. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    I got the EFB(tm) board installed tonight - everything connected and mounted, except the MOSFET which turned out I did not have any screws smaller than 6-32 which is slightly too large. Will have to make a trip tomorrow for some 4-40. What's left is to do the tone bypass to the High Filter switch and then connect the buffer board. I'm starting to see the finish line!

    IMG_3760.JPG IMG_3761.JPG IMG_3762.JPG
     
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  2. thornev

    thornev Active Member

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    289
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    I'm in awe of you people who design your boards so efficiently and aesthetically. What's the trick to doing designs so well? I guess it's an art that evolves like any creative process. Thorne
     
    Last edited: Apr 27, 2018
  3. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    It's not something I do very often, but I basically started with the size board that fit the space where I wanted to mount it, and seemed like had a reasonably good chance of holding all the components. Then I started at the corner with the diodes and started fitting components as efficiently as I could while looking ahead and trying to avoid any traces that would have to jump each other. Dave's nice hand-drawn schematic helped tremendously because he drew it so that no connections had to jump. Once I figured out that I could do a ground line around the perimeter of the board, that helped too in deciding orientation for some things. Honestly, a wire jumper to get across another trace isn't a big deal either, I just wanted to see if I could figure out a way to do it without any jumpers. It probably took me 3-4 hours to get it laid out and I was sweating it by the time I got to the last component - that big 1.8K resistor!
     
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  4. thornev

    thornev Active Member

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    289
    Location:
    Mid Hudson Valley, NY
    te - Have you ever considered using a breadboard? I have one with an integrated power supply, but the architecture of it (rows are limited to 5 pin holes which means having to pack components tightly or having to add jumpers to lengthen the rows) makes it extra difficult.
     
  5. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    I have no idea what the price range is, but surely there are software packages to design PCB layout. Even if you weren't planning to manufacture a board, just letting it design the layout would be quite useful for hand wired boards too.
     
  6. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    MOSFET attached to rear chassis for heatsink...

    IMG_3763.JPG
     

     

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  7. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
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    I have some start-up questions. With the output section complete, I'm really tempted to fire this up and use another pre-amp to put a signal through it...

    - I know I've read this, but can't find the post. On the AC balance adjustment - in the absence of the proper distortion measurements, wasn't there a suggested setting for the pot using either a resistance or voltage measurement that should be close?

    - Is it fine to power the unit up with no tubes installed? My main concern is not knowing if the EFB circuit can be damaged if operated without a load.

    - Assuming it is OK to power up - what is a good target initial setting for the bias voltage? Will it drop (become less negative) with the tubes installed? Of course, I will use the cathode measurement once the tubes are installed to arrive at a final setting. I just want to be sure and have a good conservative initial setting.

    Thanks in advance for your help!

    Tom
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,351
    Location:
    Ball Ground, GA
    The AC Balance control for the modified design was designed so that under normal conditions, the center setting is (virtually) the lowest distortion set point, or about 45.2K. This setting accounts for a slight imbalance in the OPT between each half of the primary winding.

    It is perfectly fine to power the unit up with no output tubes installed. Under this condition, the main B+ will be higher, which means that the screen grid will be higher, as will the output of the bias supply as well. The two EFB™ regulators could really care less if the tubes are installed or not, because they are not responding to the tubes themselves but rather, to the main B+ supply and how IT is affected by the tubes.

    With the output tubes removed, the screen grid voltage should be about 73% of what the main B+ voltage is. For example, if with no output tubes installed, the main B+ rises to 455 vdc, then the screen grid voltage should be about 332 vdc if the EFB Screen Grid Regulator is operating correctly. By the same token, the -15 volt output should increase to about -16 vdc if the EFB Control Grid Regulator is operating properly. This strikes at the heart of what EFB does: As the main B+ supply increases or decreases due to external electrical conditions or internal ones due to current draw demand by the amplifiers, the regulators work to keep the plate, screen, and control grid potentials all relatively the same between the three elements at all times. This in turn maximizes power output, and minimizes distortion.

    Understand therefore that the EFB regulators are not absolute voltage regulators as most regulator designs typically are, but relative voltage regulators that act to maintain an established relationship once set.

    I hope this helps!

    Dave
     
  9. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    It definitely helps - thank you!

    I was noticing that the 22uf electrolytics that we add to bypass the cathode resistors are all installed with negative to ground even though some are showing a negative voltage at the cathode on the schematic. Is the orientation correct?
     
  10. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    I finished all the mods LATE last night. Dave is right - that is a lot of shielded cable! Hopefully tonight I will be able to spend some time with the variac and measure voltages to make sure all is well. Then I can start plugging in tubes. More to come.....

    IMG_3777.JPG
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  11. thornev

    thornev Active Member

    Messages:
    289
    Location:
    Mid Hudson Valley, NY
    TE... Is that a unity gain mod in the lower picture? Nice looking! By any chance did you take a picture of the underside of the board? I'd like to see how you did the traces. I just finished my board and it looks totally NON-symmetrical. Thanks. Thorne
     

     

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  12. tekuhn

    tekuhn Member

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    Actually, I did take a picture, but never posted it. I just drew on the components for clarity. Hope it helps!
    IMG_3744.JPG
     
  13. thornev

    thornev Active Member

    Messages:
    289
    Location:
    Mid Hudson Valley, NY
    Super..thanks ! I thought there are 2 electrolytic caps, but I see only 1 on your board. Can you explain?
     
  14. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    IMG_3778.JPG
     
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  15. thornev

    thornev Active Member

    Messages:
    289
    Location:
    Mid Hudson Valley, NY
    Thanks, TE. This is VERY helpful. The left and right outputs... you attached them to the power amp section, right? Where is that connection on your 400? I looked at the bottom picture of your chassis that you posted but can't make out to what the leads are connected.

    Also, the trace at the bottom to which the smaller electrolytic and other parts are connected, what is that trace connected to off the board? I assume it's at least connected to the larger electrolytic. Is it also attached to the third .1 uF cap and diode IN4007?
     
    Last edited: May 3, 2018
  16. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    This should help....
    Untitled.png IMG_3777.JPG
     
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  17. thornev

    thornev Active Member

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    289
    Location:
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    TE - I'm not planning to use the preamp/power amp connections so I assume those circuits will end with both 220K's going to ground. I do have a question...

    The 1M resistor attached to pin 5 of the op-amp, where does the other end of that resistor connect? My thought is that it connects to the non-grounded side of the 47K and 10uF/35V parts which is the -7.5V side of the 47K that connects to pin 4 of the left channel (where it says "to other channel). Correct? I'm also thinking I could connect it to the negative side of the large electrolytic cap, but I'm not sure the voltage is correct there.

    I'll post an image of where I mean.
     

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    Last edited: May 3, 2018
  18. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    Your first statement is right. The second 1MΩ connects to pin 5 and the other end connects to the first 47KΩ which is dropping the supply voltage from pin 4 (making it less negative). My first picture above your post shows it pretty clearly with both channels of the op-amp wired. Also at that same connection point are the 10uf and 2nd 47K - both to ground, and the first 1M which goes to pin 3.
     
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  19. thornev

    thornev Active Member

    Messages:
    289
    Location:
    Mid Hudson Valley, NY
    One last post TE... thank you so much for your help. I've been struggling with the last few details as this is the most complicated schematic I've implemented and thanks to you I can finally add the traces and install. I was a fellow Texan (Austin) between 1995 and 2005. Had to move back to NY though. Family issues. Absolutely loved Austin. Big sky ! Big state !
     
  20. tekuhn

    tekuhn Member

    Messages:
    55
    Location:
    East Texas
    You're certainly welcome. That seems to be what this site is about - sharing knowledge! I'm sure glad to be in Texas. Grew up outside D.C., then New Orleans, and now Texas since 1981.
     

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