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Fisher 800-T

Discussion in 'Fisher' started by mpr2000, Aug 11, 2018.

  1. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    I'm working on a Fisher 800-T. This one has the multi-tap power transformer and the owner bought it in Germany new and brought it back here (US) in the 70's.

    I'm going through it for him, and had a question on a cut wire at the power transformer. All the schematics I've seen are for the US market 500/800 units.

    See the pic below - there is a wire that is Green/Light green stripe cut short and hanging. I don't imaging this would have left Fisher like this and the green series wires are usually secondary side and lamp windings/6v.

    Any thoughts? Anyone have an 800-T specific schematic with the multi-voltage transformer? More of a curiosity than anything. I'me wondering if he used it while in Germany at 240v and had it re-wired for 120 when home, but it's not clear. One more indicate it may have been worked on is that some of the wires from the transformer are soldered to the power board, where it appears all others are push connectors.


    IMG_0318.jpg

    IMG_0319.jpg
     

     

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  2. fred soop

    fred soop Super Member

    Messages:
    2,029
    The schematic shows a white/green wire which is at the end of the primary winding and used only for the highest voltages, 240 V or 130 V. The secondary 6.3 V heater wires are green and yellow. Looks like you could stick a meter probe or a short piece of wire to connect a meter probe into the end of that wire, then measure ac voltage to another primary wire.
     
  3. AudioWizard

    AudioWizard AK Subscriber Subscriber

    Messages:
    196
    Location:
    Reno, NV
    Is that a large piece of wire, soldered in where the fuse are supposed to be?
     
  4. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Thanks - that helps. I will probe the one wire and see if I see AC there. I'm betting that's the 240v winding.

    The fuses are fuses, and are soldered in place for some reason. Good call though; I will look that over more carefully. There is the one (externally mounted) main AC line fuse that is the correct value correct for 120v.
     
  5. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    More on the fuses mentioned above. They are in fact "Fuse Jumpers" (only on the 500-TX and 800-T). There is a service bulletin (146) that makes a change. Each emitter of the output transistors has a 4 amp pico fuse in series at each transistor. The bulletin specifies jumpering these fuses, and replacing the two fuse jumpers with 5 amp slo-blo fuses. This is B+ and B- supply.
     
  6. AudioWizard

    AudioWizard AK Subscriber Subscriber

    Messages:
    196
    Location:
    Reno, NV
    So the factory replaced the fuses with soldered-in buss bar (" Fuse Jumpers") and then went back to slo-blow fuses ?
     

     

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  7. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    It appears that way. See attached bulletin.
     

    Attached Files:

  8. fred soop

    fred soop Super Member

    Messages:
    2,029
    Good design per what we know today is there should be fast fuses of 2 or 2.5 A but they should be in the power leads to the amplifier boards, not on the emitters of the output transistors as originally connected. The slow blow fuses will only protect the power supply after a disaster in the output stage. When the original emitter fuses blow, the drivers are still connected to the load and this can cause additional problems, especially if a shorted load was the original cause. With fuses at the power supply connections to the boards, a blown fuse will shut down the entire amplifier.
     
  9. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Good info - I will be doing the SB work to remove the emitter fuses and adding fuses at the PS.
     
  10. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Continuing work on the 800T. I'm working on the power amp board and have a few questions.

    (1) one of the output transistors is very leaky and looking to replace all four. Reading here indicates the MJ15003 and MJ15004 would work well. Looking to confirm this.

    (2) CR801, 802, 803, and 804 are series diodes - 3 diodes in each. Assume I can replace with series 1N4148?
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,894
    Location:
    Glen Burnie Md.
    I put MJ21193G and 94G's in my 500-TX. Massive overkill but that's me. The 03 and 04 should be fine.

    Replace ONLY if you get a low voltage reading on the diode string. 1.2v per diode IIRC.
     

     

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  12. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    So as I am going through this receiver, I have a pile of questions...

    Power Supply Board

    1 - All electrolytics will be replaced
    1a - C975 and C976 - I've found some low ESR 3300uf 75 volt caps. Bother with small by-pass film caps?
    2 - Replace the Zener's?
    3 - I'm replacing the bridge diodes with fast recovery (MUR460)
    3a - CR975 replaced with 1N5402
    4 - Good idea to install an inrush limiter?
    4a - CL60 or CL90?
    5 - Power cord. Install a polarized or 3-prong grounded? Original cord is shot.
    5a - If polarized or grounded, which side neutral? I'm used to seeing the hot side switched.
    5b - Is the 820k to ground something I need to consider with polarized/grounded options?
    5c - Leave alone and replace with non-polarized plug?
    6 - Last one here for now... Snubber (QuenchArc) across the power switch? The one I have on hand is .25uf/100ohms.


    [​IMG]



    Power Amp Module

    1 - Mentioned above in the thread, don't replace CR801, 803 or 809 unless needed
    2 - Any advantage or issue replacing C801 with a film cap?
    3 - Good idea to replace CR805, the zener? 1N5935
    4 - I plan to replace the bias pots with 1k multi-turn
    5 - Should the NTC150 varistors be replaced?
    5a - On this unit and pics I see of others, they just kind of hang out there and not in a specific position. Assume this is ok or is there a component they should be hovering above/near?

    [​IMG]
     
  13. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Anyone? Thoughts, guidance?
     
  14. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,816
    Location:
    Calgary, Alberta
    Not sure on most, but a CL90 would be too large? From my reading, I plan on using a cl60 for my hafler dh-200. I used a cl-90 in my Fisher ,800c , but it's tube .
     
  15. mpr2000

    mpr2000 Active Member

    Messages:
    295
    Location:
    Rochester, NY
    Thanks - I did read elsewhere a CL-60 is a good choice.
     
  16. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,816
    Location:
    Calgary, Alberta
    Possibly cl-40 may be more suitable?
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,894
    Location:
    Glen Burnie Md.
    WHOA!! The CL-80 is plenty enough for the 800-T and most all of the FISHER U.S. Built Solid State Receivers and Amps. Having a 500-TX (Same unit without the Multi-voltage trannie) 1- cl-80 is plenty enough to handle the 500-TX/800-T. You match up the amps the varistor handles with the amps the unit draws on startup. The 500-TX/800-T draw about 1.8 to 2.0A on startup (charging caps, etc.) 50ohms resistance will bring it up in a second or 2. And that is all you need for the switch.
     

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