SA-7500 2sk34

Discussion in 'Pioneer Audio' started by kingmace, Sep 13, 2013.

  1. kingmace

    kingmace New Member

    Messages:
    10
    I got myself a beautifull SA-7500 amplifier, still in good working condition.

    I've cleaned all the pots and switches ultrasonicly. Also I've recapped all the boards, replaced the 2SC1451 transitors with the KSC3503, the 2SA725/2SA726 with KSA992 and the 2SC1312/2SC1313 with KSC1845 because I've read they can become trouble.

    But now I found two 2SK34 n-fets (Q1 and Q2) on the Control Amplifier Board.
    According to the Jaeger Elektronik table the 2SK34 is listed as (LF,Uni, 30V, Idss=0.3...12mA, Up=0.3...6V) and a possible replacement is BF245 (NF/HF...VHF, 30V, Idss>2mA, Up<8V) or 2SK381 (Uni,50V, Idss>0.3mA, Up<6V). But when I search the forums here I find that the 2SK34 is 50V. Is it possible to use the BF245 replacement in this situation (they are easy to get here in Europe)? I can't read schematics so I don't know whats the purpose is of the fet in the Control Amplifier.

    [​IMG]
     
    Last edited: Sep 13, 2013
  2. princever

    princever AK Subscriber Subscriber

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    Location:
    Bel Air, MD
    As far as I am aware, there isn't really a good replacement for the 2SK34. If yours is working, I recommend leaving it. I have done an amp where I replaced the rest of the transistors but kept the 2SK34.
     
  3. kingmace

    kingmace New Member

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    Thanks for the advice. I already intended to keep them in place but they have black oxidized legs and I want to have a replacment just in case they go bad. If there is no replacement for them what should I do then when they go bad?
     
    Last edited: Sep 13, 2013
  4. princever

    princever AK Subscriber Subscriber

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    260
    Location:
    Bel Air, MD
    In your case it is used in the amp. The JFET you quoted is a 30V JFET it looks like. I think the original 2SK34 was a 50V one. In cases where it is used in the power supply as a current source, there are other alternatives, such as a current regulator diode or custom built current source.

    I am not sure if a 30V JFET would work in this position for you, but my best advice would be to leave it and only begin to investigate if it fails at some point. I don't believe the black tarnish on the 2SK34 leads is known for causing failures. I know on some other transistors has problems with cases that weren't properly sealed around the leads and moisture and corrosion could get up in there, but I think these were ok.

    I know you want to have modern replacements for everything in there, but unfortunately there are a lot of custom ICs, transistors, bias diodes, etc, in vintage equipment that there is no modern equivalent.
     
  5. kingmace

    kingmace New Member

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    I know the manual also mentions the 2SK30 (in the same place) wich is for sure is 50V (Jaeger rates this one also as NF/Uni, ra, 50V, Idss>0.3mA, Up<5V), but I can't find any conformation about the 2SK34 being 50V.

    Someone else has any thoughts about it?

    I know, I've killed a very nice Goldstar GSA-8500 by doing this.
     
    Last edited: Sep 13, 2013
  6. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    Leave the jfet alone. I do. DON'T sweat it...

    They VERY rarely fail in those positions, they are mistreated when used in power supplies.

    Sometimes if a 2sa725 that they are paired with fails, it kills them - but that would give you silence...
     
  7. kingmace

    kingmace New Member

    Messages:
    10
    Thanks guys for all the advise. Is there anything else I could (should) do, to keep it playing as long as posible?

    Beside the full recap and the transistor swap I've put in a new speaker releais and changed the bias/off-set pots for Bourne multi-turn pots.
     
    Last edited: Sep 16, 2013
  8. princever

    princever AK Subscriber Subscriber

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    260
    Location:
    Bel Air, MD
    That sounds like good maintenance to me. I never do more than that unless there are some metal films that go hi-Z with age/heat. Make sure you deoxit all the switches and you should be good to go.
     
  9. Watthour

    Watthour Electron Rancher - JS3600 Subscriber

    I've had good success replacing the 2SK34 in muting circuits with the BF256B.
     
  10. kingmace

    kingmace New Member

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    10
    Good to know, is this how they are used, in the schematic above?
     
  11. rcs16

    rcs16 Super Member

    Messages:
    2,858
    Location:
    Baldwin, Ontario, Canada
    No, the ckt, as shown, is a jfet/bjt composite amplifier with a AC gain of around 6.
    Sub/replacing this jfet,2SK34, would require some ckt design/simulation/testing to come up with a working ckt. 2SK117 is a good candidate, which if you look was used on the SX-1280 to replace the 2SK30AW, as was used in the SX-1250.
    Newer parts such as LSK170 probably could be made to work, but not to bother until someone has a failed 2SK34 & can not find an original part.
     

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