Fisher 400 Rebuild Project

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

  1. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    860
    Location:
    Melbourne, FL
    Kinda like the black caps. How did you clean the chassis?
     

     

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  2. tekuhn

    tekuhn New Member

    Messages:
    21
    Location:
    East Texas
    I used Brasso there too. It works pretty well and didn’t touch the silk screening. The chassis had a few spots where something was spilled on it and corroded it. I still need to work on that a bit.
     
  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,196
    Location:
    Glen Burnie Md.
    Kinda like darker streaks? That's the cadmium from when the chassis was dunked originally. Some chassis streaked and some didn't. Go figure. When you get done shining it up to USMC standards, you'll need to put some straight carnauba wax on it. Butcher's Bowling Alley Wax or Johnsons Paste Wax (yellow can) will do it. Give it a couple coats as the brasso has stripped off alot of the cadmium plating.
     
  4. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    860
    Location:
    Melbourne, FL
    The chassis on mine was so bad Brasso wouldn't touch it. It was pitted through the cadmium layer down to the iron underneath all over. I ended up using a rust remover, but I did put wax on it when I was done to try to keep the rust from coming back. A lot of my stenciling is unreadable now. I think I'm going to find stenciling fonts and try to replace some of the missing text.
     
  5. tekuhn

    tekuhn New Member

    Messages:
    21
    Location:
    East Texas
    Thanks for the tip on the carnauba wax. Mine mostly had lots of oxidation - especially where finger prints were left, but there are about 3 sections where something got on the surface and attacked it and then dried. A razor blade worked pretty well on those, but can't get to all of it. Not a big deal - I care a lot more about how it sounds than looks, but still like to polish stuff up where I can.
     
    Last edited: Apr 13, 2018 at 10:18 AM
  6. tekuhn

    tekuhn New Member

    Messages:
    21
    Location:
    East Texas
    I plan on converting the Spacexpander jacks to Pre-Out and Amp-In Jacks. There is a resistor network prior to each jack. My question is where to insert the shielded cable? I'm wondering if one way or the other would make the long cable runs less susceptible to picking up stray signals and audio degradation?

    jacks.jpg


    My guess is B for the Pre-out side and A for the Amp-in side to keep the signal level in the cables as high as possible
     
    Last edited: Apr 13, 2018 at 1:02 PM
  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,300
    Location:
    Ball Ground, GA
    Tek -- In my unit, the shielded cable was installed at points A and C. The components of the buffer -- including IC1, the .22 uF cap, 100Ω, and 220K resistors -- are all installed on a small perf-board type circuit board, with shielded cable running from the junction of the 100Ω and 220K resistors on the board, and the preamp output jack. For the power amp input jack, the 1M resistor is mounted at the power amp input jack, with the cable running between the jack and R100, which is located at the tube socket.

    As for which ends of the cables are grounded, as a rule, it is always best to ground only one end -- unless continuity is required in the ground lead to carry the ground to the other end of the cable. In my unit (as I recall), the ground for the new preamp output jacks was already established at the jack end from the original build, which I maintained in their conversion to Pre-out jacks. The shield for the cable to this jack was therefore grounded at the jack end, and left floating at the circuit board end. For the power amp input jack the cable is clearly grounded at the driver tube end, and (I believe) carries through to provide the ground for the Power Amp In jack as well.

    I hope this helps!

    Dave
     
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  8. tekuhn

    tekuhn New Member

    Messages:
    21
    Location:
    East Texas
    Exactly what I needed - thank you!
     
  9. tekuhn

    tekuhn New Member

    Messages:
    21
    Location:
    East Texas
    Everything is really tight in the corner where the input jacks are, so I decided the drill the rivets and remove the Spacexpander panel and do a good job of cleaning things up to prepare for new cable. I'm glad I did and I already have in my head the path I'm going to use for the cable runs. The inventor of desoldering braid deserves to be rich.
    IMG_3729.JPG
     

     

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  10. tekuhn

    tekuhn New Member

    Messages:
    21
    Location:
    East Texas
    I received most of my parts this week. Here's one channel of the power amp finished. I find with a high wattage gun, the chassis will take solder just fine. I am using that technique to add terminal strips as necessary to support new components. I used a piece of desoldering braid to attach the upper end of the metal shield to the chassis after desoldering it from the High Filter switch.

    IMG_3738.JPG
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    IMG_3731.JPG
     
    Last edited: Apr 18, 2018 at 3:05 PM

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