An Attempt At Fisher 400C Transformation...

Discussion in 'Fisher' started by Tysen, Apr 16, 2018 at 2:36 AM.

  1. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
    Los Angeles
    Hey folks!

    I have an early version of the 400c (no dual tone knobs). I recently had a tech (who will go nameless) restore it (caps, out of spec resistors, and selenium rectifier)...I got it back and it worked fine for literally 2 weeks. This was back in November. While working, it produced a lot of noise which I read prior to the restoration was normal for this unit.

    Well, now I have a pre that produces barely audible distorted noise, even at full volume.

    I've completed a couple of projects since then and I feel confident I can fix this. I also feel confident I can perform dcgilliespie's mods, but not without help from this great community!

    So, I'm starting this thread to document the mods in case I run into problems.

    Here's my starting point:

    IMG_5401.jpeg


    I'll be starting the restoration on Wednesday, but I hope to gather base voltage readings beforehand to see if I can pinpoint my current problem of almost inaudible distorted output. I know it's not the rest of system because everything else works with my passive pre in place. I also should point out that the distorted sound happened somewhat gradually—at first it was a volume imbalance so I swapped tubes between V3 & V6 thinking that my 12AT7s were fried. The imbalance remained. I'm nervous to put in another new set of my 12AT7s and risk killing them too...

    Dave suggested that I inspect the integrity of the tape monitor jacks. Which I still need to do. I also will try cleaning the volume pots to see if that helps.


    Cheers!

    —Tysen
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Those yellow Wax/paper caps need to GO, Unless they are new construction.Gradual degradation of sound can be tubes (not too likely) and leaky caps. The noise you are hearing is most likely from the yellow caps . Get some good quality Polyprop caps (AES has a good selection of Illinois axial caps) and change out the Filter caps if they haven't been changed out. I'd also replace those electrolytics with Nichicon UKL (signal path) or UPW/UHE in the power supply. Panasonic FC series are also popular among some guys.

    Clean ALL POTS, SWITCHES on the Front with DeOxit D5 and follow up with Faderlube F5 or Shield S5. Faderlube or Shield will replace the DIRTY lubricant that the D-5 washed out. See this thread " The Idiots Guide to De-Oxit...Revisited ".
     
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  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Humm. Looks to me like the original can cap for the DC heater supply is still being used. It has been shown to have a very long life, but has also shown that when it goes, it goes, and fails completely.

    Dave
     
  4. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
    Los Angeles

    Thanks for the link and tip! In regards to the yellow cap, they're actually poly film caps made by C.D.E. (in the 90s I think ?) I have purchased NOS Russian PIO caps to replace most of the C.D.E's to flatten the frequency response, per Dave's suggestion.




    I noticed that too, @dcgillespie. I purchased new discrete caps to replace whatever is going on there. Gonna keep the can on top though for looks!
     
  5. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
    Los Angeles
    Okay, cleared up the DC cap situation—the original terminals were used as tie points, and then cut off from the actual can. It's appears safe, but I'm going to install some new 1000uf caps with the ground lifted. @dcgillespie you mentioned in @notdigital 's thread that "When you install my modifications, the voltage that the filter caps in the DC heater supply actually "see" remains unchanged from that of the original design." So, 35v is okay, correct?

    My last question for this evening is related to C49. All 3 caps are still connected to chassis ground, right? Using 47uf, is okay too?


    Edit: I'm using a 8a 400v bridge, that's okay too, right?
     
    Last edited: Apr 20, 2018 at 1:29 AM
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    A 35V rating is perfect for the caps use at C50. 47uF for the three sections of C49 is fine as well, and yes, these sections are still grounded. Only the caps at C50 no longer connect to ground. An 8A bridge is a bit of overkill for a circuit that draws about 1A maximum. As long as it is not so overly large physically so that it won't radiate PS noise into the adjacent audio circuits, it will be fine

    Dave
     
  7. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
    Los Angeles
    Thanks Dave! the 8A appears similar in size to the one you used. Couldn't find a 4A that you could screw into the chassis. I'm going to post photos of my progress tonight.

    Tysen
     
  8. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
    Los Angeles
    Well, I think I'm done with the DC Bias, along with the new bridge rectifier.

    I had to get creative adding the third 47uf cap as the JJ only had two sections...I think I've hooked it up correctly. And don't worry, the leg of the 1.2k resistor is safe from the negative lead of the cap (has tubing on it).

    If anybody sees any egregious errors in my work so far, let me know!


    Now off to the DC supply...really wish I would've just purchased a can cap for C50....
     

    Attached Files:

  9. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
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    Update. I believe I've completed the DC power supply section. It's not necessarily pretty, but considering I didn't have a can cap, I'm pretty happy. (Note, it's hard to tell in the photos but I do have clear shrink tubing in tight areas as a precaution).


    To the positive leads of C50: A) 10ohm 2W between the terminals; B) The three white wires and the + wire from the light panel to the same 1000uf; & C) The positive leg of the shielded wire coming from bridge is connected to the other 1000uf cap.

    To the lifted terminal: A) negative side of the 10uf cap from pin #2 of the rectifier socket; B) negative side of the shielded wire from the bridge; C) Heater wire from the phono section; & D) ground wire from the light panel.

    Connected to the chassis ground in that area is 1) the shielding for the bridge connecting wires (only connected one side); & 2) the grounding wire from pin #5 of V2.

    Am I missing anything? it's hard to tell from @dcgillespie 's original "transformation" thread if anything else is connected to the chassis ground in that area.



    Assuming nobody spots any errors; I'll be re-capping & changing the grounding scheme in channel A tomorrow.
     

    Attached Files:

     

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  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Ty -- The negative side of the 10 uF 100 volt cap should be grounded. The negative side of the DC heater system should be returned to the POSITIVE side of the 10 uF 100 volt cap. That point represents about 60-65 vdc which then lifts the DC heater supply by that amount above ground to bring the heater/cathode voltage in line with the ratings of the tubes for that parameter. If any point of the negative side of the DC heater supply is grounded, it will short out the bias voltage so that the supply will not be lifted above ground.

    Dave
     
  11. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
    Los Angeles
    Dave,

    I see the error of my ways.

    Thank you!
     
  12. Tysen

    Tysen Active Member

    Messages:
    201
    Location:
    Los Angeles
    Update:

    Drilled out all the rivets for the terminals on Channel A and installed the nylon hardware. I also changed some caps, per Dave's instructions. Just waiting on the the four 1ufs. Should arrive on Monday.

    All that's left is to ground the terminals to their counterparts on channel B and install the remaining caps.

    Attached are some photos. Let me know if anyone spots anything that's off.
     

    Attached Files:

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