SA-8100 protection circuit issue

Discussion in 'Pioneer Audio' started by KeithD, Jul 2, 2017.

  1. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    Thanks for that explanation, that was very helpful. The function of that part of the protection (that we've been discussing so much) now makes sense to me. I figured Q5 was essential, but I couldn't see why there were two paths (q3 & q4) to it. It also now makes sense why I should try lifting Q4 C, then measure the voltage at the solder pool for Q4 C. If Q4 is faulty, the voltage should come up to equal Q3 C, right?
     
  2. petehall347

    petehall347 the brandy coffee man Subscriber

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    uk.. the middle bit
    yes that voltage should rise . maybe to more like 35v .
     
  3. LesE

    LesE 110284 Subscriber

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    577
    Location:
    Ontario, Canada
    Yes, the voltage on the Q4 solder pad should be equal to the supply voltage since there will no longer be any current flowing through R14. But, I would expect that the Q3 collector voltage will drop since the collector current will increase when the Q4 collector current is interrupted.
     
  4. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    OK, will try that in a little bit.

    A friend who fixes electronics had NTE123AP transistors on hand that he said is equal or better specifications to the original 2SC945 in Q3 and Q4. Would you concur? If so, I could try those once I confirm the voltages comes up on Q4 C with the collector lifted.
     
  5. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    All connections in except Q4 C out.

    E, C, B
    Q3: +0.3 (variable), +34, 0
    Q4: +0.1 (variable), +9 (variable), 0
    amp stayed in protection; however after 2 or 3 minutes while making measurements, it moved in and out of protection. Got some readings during this time:

    Q3: -0.6, 17, 0
    Q4: 0.5, 34, 0

    Should I try putting the NTE123AP.
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

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    q4 emitter being positive in second reading is now telling me q3 might well be the culprit .
     
  7. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    OK, put Q4 C back in and pulled Q3 C

    E, C, B
    Q3: -0.8, 34.2, -0.2
    Q4: -0.6, 6, -0.1

    (Collector for Q4 is measured at the solder pad.)
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

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    interesting .. a new pair should hopefully fix the issue . if your meter has hfe test i would be interested what hfe is for q3 q4 . after pulling them of course .
     
  9. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    Put in the NTE123AP transistors for Q3 & Q4:

    E, C, B
    Q3: -0.6, +18.6, 0
    Q4: -0.6, +18.6, 0
    Q5: 16.4, -1.2, 19.1

    Haleluiah!! Amp comes out of protection as it should. Q5 C should be -0.7, but I assume that reading is ok?
     
  10. petehall347

    petehall347 the brandy coffee man Subscriber

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    hallelujah ... dont sweat q5 c .. for now . i need to look at this some more .
     
  11. KeithD

    KeithD Active Member

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    178
    Location:
    La Crosse, WI
    My DMM does not have functions for testing transistors. I can use the diode test, but that's it. I think one lesson here is that 1845 can't be used to replace a 945. I did look at the spec sheets before I put the 123s in, and those are a much better match for a 945 than the 1845.

    Q5 C might settle down? Those were just initial readings. I was too excited to wait longer! :banana:
     
  12. petehall347

    petehall347 the brandy coffee man Subscriber

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    r22 might be high ohms .
     
  13. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    Actually in post 174, LesE's simulation came out with -1.5 V for Q5 C. His values for Q3 & 4 base also line up. Mine were negative but in the single digit milivolts, so I called that zero. He has -8.7 and -8.3 mV.
     
  14. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    Actually, it was testing low: 22 kohms in place. Should be 150k.
     
  15. QSilver

    QSilver Super Member

    Messages:
    1,033
    Great that the transisitor changed sorted it, but I would steer clear of NTE parts. Most people would avoid them on here... best to get some modern 945's from somewhere like mouser. KSC945CGTA.

    945's can sometimes be changed to 1845's but it does depend as they have different "sensitivities" as maybe we've seen here in the protection circuit. And protection circuits are very sensitive anyway.

    The 945 issue plagued me quite a bit until I compared the datasheets...
     
  16. KeithD

    KeithD Active Member

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    178
    Location:
    La Crosse, WI
    Can you tell me why?
     
  17. gslikker

    gslikker Super Member

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    Location:
    Close to Alkmaar, Netherlands
    I guess the NTE story is something from the past/happening in the old past?

    A reputable company like Newark supplies them also.
     
  18. QSilver

    QSilver Super Member

    Messages:
    1,033
    I couldn't find anything that MTF or Echowars have said in the past quickly, but installing NTE is never a good idea. Best to get transistors from a good and reputable company like Fairchild/OnSemi for one. No affiliation.
     
  19. KeithD

    KeithD Active Member

    Messages:
    178
    Location:
    La Crosse, WI
    OK, thanks for that. The protection circuit/amp is working at this point, but I will put some KSC945CGTA's on my next mouser order and replace the NTEs. Interestingly, the NTEs are 5 -10 times more expensive than Fairchild.
     
  20. QSilver

    QSilver Super Member

    Messages:
    1,033
    Better quality AND cheaper! :rflmao:

    Just check the data sheet for the 945CGTA's when you put them in. I think the "C" signifies that the center leg is the Collector. so they are ECB rather than the usual 945's which are EBC. IIRC...
     

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