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Yamaha CA-1000 runaway DC offset

Discussion in 'Solid State' started by Mico, Nov 7, 2018.

  1. Mico

    Mico Active Member

    Messages:
    363
    Location:
    Montreal
    I have just recapped amp modules on Yamaha CA-1000 replacing three electrolytic caps.
    Everything seemed fine, easily adjusted 15V on both sides, A/B and A class idle currents are stable as well and DC offset on the right side. But left side DC is acting up, on cold startup it takes off from -150mV and than starts the countdown: in about 5min DC drops to - 65mV and continues to move in that direction.
    After 15min. value is already zero , only to stabilize around 15-20mV. when full operating temperature was reached. Relay clicks in immediately even at -150mV so all this does not trigger protection. DC is the same regardless of the operating mode (A/B or class A) and amp/preamp bridge has been disconnected.
    Never seen this before and not 100% sure if it was the same before recap, probably not as I would have noticed the extreme.
     
    Last edited: Nov 7, 2018

     

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  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,684
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    Was it doing this before you replaced the parts?
     
  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,684
    Location:
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    Did you use a bi-polar capacitor for C603 -- 47µf/50v ?
     
  4. mbz

    mbz Super Member

    Messages:
    2,175
    Location:
    Melbourne, Victoria
    Did you check the fusible resistors are in spec on the power amp boards (replaced in some versions with ordinary resistors).
    As a side question, did you replace the 2SC458LG on the heat sink.
     
  5. avionic

    avionic " Black Knights " Subscriber

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    That would cause bias issues if it was bad.
     
  6. avionic

    avionic " Black Knights " Subscriber

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    Lets find the problem before willy nilly throwing more parts at it.
     

     

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  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    For drifting, slow to stabilize offset . We need to focus on TR602 and 603 long tailed pair.
     
  8. mbz

    mbz Super Member

    Messages:
    2,175
    Location:
    Melbourne, Victoria
    The drop from -150mV to 0mV as the amp warmed up pegged my interest. Yeah, unlikely to be cause however
    the transistor does control how hard each half of the amp works, maybe missing some +mV
     
  9. avionic

    avionic " Black Knights " Subscriber

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    Location:
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  10. Mico

    Mico Active Member

    Messages:
    363
    Location:
    Montreal
    Yes C 603 is bipolar 47µF/50V and the rest is properly installed too. I am pretty sure DC was not that wild before.
    Fusible resistors tested fine, will look into TR602 /603.
     
  11. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Look your board over real good for stray solder bridges and iffy solder connections around those transistors.
    The transistors need to be pretty closely matched for Hfe.
     

     

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  12. Mico

    Mico Active Member

    Messages:
    363
    Location:
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    It seems I measured fusible resistors twice on the same side... After repeating everything on the left channel, 680Ω fusible measured 310Ω .
    Will order new one and see what happens, looks like problem was there before recap.
     
    Last edited: Nov 10, 2018 at 1:57 AM
  13. mbz

    mbz Super Member

    Messages:
    2,175
    Location:
    Melbourne, Victoria
    Fusibles "always" drift high. Hope there is no confusion with R619 68ohms. The 680 ohm is R622.
     
  14. avionic

    avionic " Black Knights " Subscriber

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    Take it out of circuit and remeasure.
     
  15. Mico

    Mico Active Member

    Messages:
    363
    Location:
    Montreal
    R622 measured 310Ω outside the circuit, new 680Ω resistor is already in. No change in DC stability, still takes 30min. to stabilize completely.
    Will replace TR 602/603 with matched pair, can not see anything else being wrong.
     
    Last edited: Nov 11, 2018 at 1:32 AM
  16. mbz

    mbz Super Member

    Messages:
    2,175
    Location:
    Melbourne, Victoria
    Try to match both hfe and Vbe.

    If you are bored, could you measure the Vdc at TP3 and GND/chassis. Repeat for TP4 and GND. Which if any is bouncing
    Repeat for Pin C, note if it's stable/bouncing...
     

     

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  17. Mico

    Mico Active Member

    Messages:
    363
    Location:
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    4am here...not the best time to fry the outputs, will do it latter. What really bugs me is that temperature shift. When I measure dead-cold amp DC starts at 150mV, if I warm up heat sink with hear dryer for 15- 20 sec.and than measure,
    DC starts way lower, around 50-60-70mV.
     
  18. mbz

    mbz Super Member

    Messages:
    2,175
    Location:
    Melbourne, Victoria
    Need to sleep on that. Not sure that's real significant.
    The outputs and bias transistor are on the heatsink. The heatsink is quite large, did you heat it up evenly, maybe one output hotter etc
    or the bias transistor thermally out of whack with the outputs. Transistor Vbe drops as temp increases. A good observation anyway.
     
  19. Mico

    Mico Active Member

    Messages:
    363
    Location:
    Montreal
    While there I will also replace 2SC458 bias transistor with KSC1845, can not hurt to try as those Hitachi devices have pretty bad reputation.
     
  20. Mico

    Mico Active Member

    Messages:
    363
    Location:
    Montreal
    Did not change anything yet but did some thermal testing. While lightly cooling TR602 with compressed air, DC offset immediately goes down relative to initial value. When doing the same on TR603, DC starts moving back to its cold start position. Will see what happens when two new KSA 992 go there.
     

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