Current Source replacement of 2SK34 in SX-780 PSU

Discussion in 'Pioneer Audio' started by RFmaster, Oct 26, 2015.

  1. RFmaster

    RFmaster Member

    Messages:
    80
    Location:
    SoCal
    Working on another SX-780 that came in with blown 1 AMP fuses. After checking the +/- 39.5 supply rail is dead. Replaced both Q19/20 (KSC2073/KSA940) but it clear that positive side will not start. After probing around - Q23 (2SK34) appears to have shorted internally. Well, what to do??
    2SK34 is a JFET that has been been long discontinued. There are no exact replacements for this device. Originally this part was designed as VHF LNA. Pioneer used this part all over the place as well as using as a current source in the pass through regulator of SX-780 (and others as well) PSU. Search of ebay result in couple of hits, but I am not in a mood to pay $14 for pair of 2SK34's. A modern alternative is a bit more desirable!
    A while back Mark posted a recommendation - CRD E202 - here the thread.


    http://www.audiokarma.org/forums/index.php?threads/sx-d7000-poppity-pop-up-in-smoke.453275/page-4

    But OP did not go forward with CRD E202 installation. l willing to go through and document this retrofit. From Semitec data sheet it would appear that Anode (the marked end) of E202 should be coupled the base of Q19 (+39.5V rail), same for the negative rail (-39.5V) the Anode should be coupled to the base Current Source modification SX780.jpg //RF
     
    dlucy likes this.
  2. SYK3S

    SYK3S Active Member

    Messages:
    234
    I know a direct replacement for 2sk34 jfets i have used in my pioneer sa 9800 simply use NTE459 did the trick for me
     
    dlucy likes this.
  3. markthefixer

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

    Messages:
    20,564
    Location:
    Bensenville,Illinois
    CRD are fine, or EchoWars current sources. Originally this was a resistor and capacitor, used to supply low ripple current to make the regulator operate.

    look at the awr-101 power supply in the sx-850 and sx-950, the resistor is 1.5 k ohms and the cap is equal to the energy storage cap to run the supply, 470uf!!

    Either the ripple is RC'd out or it is servo'd out by the current source.

    The pictures I posted are gone.... although I did save them long ago, and I need to figure out how to get them back in. WHEN I get time.
     
    dlucy likes this.
  4. RFmaster

    RFmaster Member

    Messages:
    80
    Location:
    SoCal
    Mark

    Thank you for chiming in! CRD appear to be a reasonable solution other than aforementioned 1.5k 470/63 cap used in by 850/950 voltage regulator circuit. I don't believe there is sufficient room on the PCBA to drop another 470/63 cap in the SX780 board. Interestingly enough in SX850 negative side rail uses 100/63 cap for charge storage. My only other concern is CRD connection in the negative supply rail circuit - base of Q20 is at higher potential (-40V) vs. unregulated -53.4 Volt input. CRD data sheet does not indicate that E202 current source is bidirectional device.
    There is another possibility - exercise a nuclear option and replace discreet voltage regular circuits with modern linear voltage regulator - currents are <1amp. But that would be bad-bad thing to do...
    Priced CRD 954-E-202 $2.31(Mouser), while NTE459 comes in at hefty $6.60 to 13.0 from various surplus places. Updates to follow once I get parts in in about 3 to 4 days.
     
  5. markthefixer

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

    Messages:
    20,564
    Location:
    Bensenville,Illinois
    I should have looked closer at your diagram, your negative regulator CRD connected to the base of Q20 is in backwards.

    The Pioneer diagram shows the 2sk34 common gate/source short connected to q19 base in the positive regulator
    and the 2sk34 common gate/source short connected to q20 COLLECTOR in the negative regulator.
     
  6. RFmaster

    RFmaster Member

    Messages:
    80
    Location:
    SoCal
    Schematic - corrected CRD interconnect.
    B3 supply SX780_correct.jpg
     
    dlucy likes this.
  7. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    23,043
    Location:
    uk.. the middle bit
    i have never tried one so don't blame me if it doesn't work .. 2SK381
     
  8. RFmaster

    RFmaster Member

    Messages:
    80
    Location:
    SoCal
    I can try (do not mind to try) 2SK381 - but it has a similar fate as 2SK34 - no longer in production and has been discontinued. Ebay has couple available for $9.50. Data sheet is bleary photo scanned one pager.
     
  9. RFmaster

    RFmaster Member

    Messages:
    80
    Location:
    SoCal
    Update 20151101.
    It works! getting solid +/- 41 volts on both rails. So if you have blown 2SK34 in the power supply circuit this is the way to go. Also, when these FET go out it is a good idea to replace MZ140. In my case I had a dead MZ140 compounding the problem!
    SX780 2SK34 Replacment.jpg
     
    dlucy likes this.
  10. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    23,043
    Location:
    uk.. the middle bit
    did you use the 2sk381 then ?
     
  11. RFmaster

    RFmaster Member

    Messages:
    80
    Location:
    SoCal
    Nope. The grad idea was to use low cost CRD current source diode. I would be happy to try 2SK381 - its an easy swap!

    //RF
     
  12. jblmar

    jblmar JBL & marantz

    Messages:
    5,569
    Location:
    Brooklyn, New York
    RF,
    Glad you repaired your SX 780.
    Where did you source the MZ 140?
     
  13. RFmaster

    RFmaster Member

    Messages:
    80
    Location:
    SoCal
    Thank you Jblmar.
    Mouser (is) was my source for 14 volt 0.5 zener mz-140 512-1N5244B

    2SK381 looks like another obsolete JA transistor - Evil bay has some 'NOS' stuff.
     
  14. jblmar

    jblmar JBL & marantz

    Messages:
    5,569
    Location:
    Brooklyn, New York
    Thanks, RF.:thumbsup:
     
  15. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
    329
    Location:
    philippines
    is that crd a direct replacement for 2sk34 in all scenarios?
     
  16. princever

    princever AK Subscriber Subscriber

    Messages:
    260
    Location:
    Bel Air, MD
    No, it is not. In this case it works because it is being used as a current source. The CRD is a diode.
     
  17. princever

    princever AK Subscriber Subscriber

    Messages:
    260
    Location:
    Bel Air, MD
    I would like to add my solution for a similar problem in case anyone needs it. I have a SX-780 which, like yours, had a blown 2SK34. I stole the 2SK34s out of the FM muting circuit to put in the power supply. They lead a very gentle life in the muting area so they were working perfectly. Then I put BF256B in the muting circuit (lead arrangement!) and those work perfectly there.
     
  18. EchoWars

    EchoWars Hiding in Honduras

    Messages:
    28,783
    Location:
    Kansas City
    Nearly any FET would do the job here, but you might have to pad the Idss value down with a resistor.

    [​IMG]

    Just decide on the desired current value (manual calls for a K34C or D, so I'd shoot for about 3mA) and if you have a FET that can do at least 4 or 5mA Idss, set it up with a resistor and adjust the value to your heart's content.

    Bottom line is that, although TO-92 FET's are a severely endangered species, a discreet FET (if you can find one) will nearly always be superior to a CRD in just about every measurable category, especially noise, which is important in this application.
     
    Last edited: Nov 4, 2015
    dlucy likes this.
  19. SargentRicko

    SargentRicko New Member

    Messages:
    30
    The Pioneer SX780 JFET 2sk34 at location Q23 & Q24 on the AMP PCB are identified on the Parts List as being 2SK34-C or D. Pioneer Electronics web site indicates the following substitutes for the 2SK34-C: 2SK34 and 2SK381. Today (June 2016) Pioneer Electronics apparently still sells the 2SK381 for $12.25 and MCM electronics apparently sells the 2SK381 for $2.36 ea. Fairchild Semiconductors, P/N J113 is an equivalent replacement for the 2SK381. The J113 is available at Mouser Electronics $0.36 each.

    As a reply to "SYK3S" regarding NTE replacement for 2SK34.
    Looking at location Q23 & Q24 on the SX-780, the Transistors have a TO-92 package
    .
    NTE has a cross reference parts search engine, but be careful to look at all the specifications.
    NTE cross for 2SK34C --> NTE312 which sells for $2.07 ea
    NTE cross for 2SK34 --> NTE459 <----> WARNING this is a TO-72 package - not recommended....
    NTE cross for 2SK381 --> NTE458 which sells for $8.21 ea
    NTE312 vs. NTE458 - Which one is the best cross for the 2SK34-C? I don't know, I lack the knowledge.
    I know that many don't recommend using NTE products. Sometimes when dealing with obsolete parts, with no known equivalent replacement other than what NTE is offering, it can be very frustrating.

    Several manufactures have cross reference P/N search engines, like Fairchild, Toshiba, Panasonic, ON Semiconductor, Vishay, Rohm, Taiwan Semiconductor, Central Semiconductor, etc. When an alternative p/n cannot be found, try and contact the manufacturer, and ask them if they have a substitute p/n for your semiconductor.

    I had a P/N in which I was having a hard time finding a substitute for, I contacted Fairchild Semiconductors about an equivalent P/N, they told me to post my question in their "community" web page, stating that their engineers constantly read and reply to the questions that are posted there. Oh, and just as a side note; ON Semiconductor is currently trying to purchase Fairchild.

    The specifications I found on the 2SK34-C in English that I have found are as follows:
    Construction: NJD
    Package and Pinout: TO-92 A
    VOS Max: 30V
    Lo Max: 20mA
    Temp: 125C
    Ptot: 150mWF
    Vgs: 0.3/6V
    Loss: 1/3mA
    GPS: 1/3mS
    CISS Max: 8PO
    USE: ALG
    MNF: MIT
     
    Last edited: Jun 16, 2016
  20. ZOOM

    ZOOM Well-Known Member

    Messages:
    782
    Never use NTE parts for anything. They won't do what you want them to.
     

Share This Page