Is worth replacing the 2SK30A's in my SA-7500?

Discussion in 'Pioneer Audio' started by Eastham, Sep 11, 2018.

  1. Eastham

    Eastham More Class-A than ever!

    Messages:
    2,116
    Location:
    Liverpool, UK
    Decided to complete a full rebuild on this amp since I think I'm gonna keep it a while, I've had it for a few months and unlike any other amps, I don't feel like I'm missing something.
    I've already gone thru the amp and replaced all the transistors but I've hit a crossroads with these two fet's in the pre-amp. I can't seem to find a replacemetnt for them but I'd like to replace them for the sake of reliability. Anyone got any suggestions on whether it's worth replacing them and if so, what would be a good substitute?
    Also, just morbid curiosity... Any reason Pioneer chose to use mosfets in this section of the amp instead of a regular bipolar?
     

     

    Please register to disable this ad.

  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
    42,091
    Location:
    Fort Dodge, Iowa
  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
    42,091
    Location:
    Fort Dodge, Iowa
    Not MOSFET..... JFET High gain very low noise.
     
    slimecity likes this.
  4. Eastham

    Eastham More Class-A than ever!

    Messages:
    2,116
    Location:
    Liverpool, UK
    Oops, my bad. Thanks Avionic!
     
  5. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,643
    Location:
    Cleveland, Ohio
    They don't often go bad unless you have a powerline hit or ESD strike. Leave them alone.
    Buy spares if you want. If you insist on replacing them, wear a grounded anti static wrist strap, use an ESD safe soldering station, wrap a wire around the leads until the transistor is in the board. If this is for a differential pair, the new ones should be matched for IDSS
     
    merlynski likes this.
  6. Eastham

    Eastham More Class-A than ever!

    Messages:
    2,116
    Location:
    Liverpool, UK
    Nope, it's not a differential pair, just the input of the pre amp. Are FET's really that static sensitive? Might just leave them be if they're reliable. I have a ESD safe soldering station but I haven't had a wrist strap since my early days as a computer tech and after a little while I just stopped wearing it and grounded myself to the chassis before touching components.
     

     

    Please register to disable this ad.

  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
    42,091
    Location:
    Fort Dodge, Iowa
    Yes.
     
    merlynski likes this.
  8. Eastham

    Eastham More Class-A than ever!

    Messages:
    2,116
    Location:
    Liverpool, UK
    Much more sensitive than transistors then I guess. This is the first amp I've had / worked on with FET's I'll admit.
     
  9. gslikker

    gslikker Super Member

    Messages:
    2,072
    Location:
    Close to Alkmaar, Netherlands
    The fet can be measured in-circuit, for this just apply power and look if the DC drain voltage is at least "more or less" as the manual states (10 Volts) .
    Also compare to the other channel.
    If there is a real big difference, you can assume one FET is bad, since they will have come from the same batch.

    I have a SA900 where one of the FETs was compromised.

    I replaced using BF245A which is probably still around in UK.
    But, as @dr*audio stated, leave alone if no reason to replace.

    Note, a Peak DCA75 tester computer graph is really great for easy comparing low voltage JFET graphs!
     
    dlucy likes this.
  10. rcs16

    rcs16 Super Member

    Messages:
    3,794
    Location:
    Baldwin, Ontario, Canada
    I agree leave the 2sk30 jfet alone unless it going noisy or faulty.
    Pioneer used this jfet since it has very low noise/high Z thus it has little loading on the previous stage which is usually the volume control. this allows one to use a small coupling cap allowing for a film cap to be used. jfet's can be damaged by static, but are no where near as static sensitive as a isolated gate MOSFET, still good to use good anti-static procedures.
     
    Eastham likes this.
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,763
    Location:
    Glen Burnie Md.
    Plus the one's that pioneer used were the 50 (I can't remember what the value meant) and the replacements are usually 30's. MTF also says to leave them alone if working ok.

    Larry
     

     

    Please register to disable this ad.

  12. Eastham

    Eastham More Class-A than ever!

    Messages:
    2,116
    Location:
    Liverpool, UK
    I've come to an conclusion! The 2SK30's stay. They seem to be working perfectly fine and there's no excessive hiss aside from what I'd put down to noise floor with headphones.
     
  13. rcs16

    rcs16 Super Member

    Messages:
    3,794
    Location:
    Baldwin, Ontario, Canada
    Good idea to keep the original 2sk30 installed.
    Toshiba made this part over a number of years, changed to a TO-92 package later with the 2sk30ATM, we have a spec for that.
    jfets are graded by the zero gate-source (shorted) voltage current spec called Idss, Toshiba used values like "R", "O", "GR", "Y", "BL".
    Pioneer used the "Y" grade of the part for the pre-amps, a 2sk117Y will also work as that is what they used in the sx-1280 iirc.
     
    Eastham likes this.

Share This Page