Sansui AU-X901 Protection Mode

Discussion in 'Exclusively Sansui' started by sajjad amer, May 18, 2018.

  1. sajjad amer

    sajjad amer Active Member

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    I have replaced all trimmers, capacitors. I will shortly post a close up picture so that you can verify my work.
     

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  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    OK, please lift one end of kD51 and kD53 diodes and verify that they are not shorted, resolder them if ok.

    If the above checks out ok please remove and test kQ57 & kQ59.
     
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  3. sajjad amer

    sajjad amer Active Member

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    KD 51 and 53 check fine on diode test and replaced back. forward the diodes read 0.68 reverse 0L
    KQ 57 and 59 Read fine and replaced back
     
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  4. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Check that kR64 (560Ω) is OK - lift one end to measure.

    The measurement I requested for kQ5 - there are 2 kQ5's shown on the schematic! - you may have measured C-E voltage on the wrong kQ5. I should have said kQ51 see board layout diagram on pp15 SM for its location - near the OP transistors, can you perform this measurement again and try adjusting the respective trimmer - kVR51 ?
     
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  5. sajjad amer

    sajjad amer Active Member

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    KR64 is ok, I installed it brand new as it was burnt. I have lifted one leg again and it measures fine.
    There are 2 KQ5 , you are right. One in each channel. EC voltages across both measure -44.32
    I have checked on KQ 51, EC voltage is 3.8v and it does respond to KVR51
    Thank you for holding my hand, I appreciate it
     
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  6. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    That is great news!
    I find this kind of direction difficult, I just hope I can 'pull this off' as the saying goes. ;)
     
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  7. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Next is to perform the same bias adjustment checks on the Left Channel - as before reading the C-E voltage and adjusting the appropriate trimmer to check that the bias control trimmer can vary its C-E voltage. - all of this still on DBT - and as before - Be Careful with your probes.

    So far, Well Done Sajjad!
     
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  8. sajjad amer

    sajjad amer Active Member

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    Thank you, so much
    Voltage across EC on KQ51 and KQ52 is 2.8 v each and it does respond to KVR 51 and 52
     
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  9. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    That's interesting, now can you try setting the bias again? Try the Right and Left channels, measuring at the TP, multimeter set to mV - and adjusting for 16mV. Note that you might have to turn the VR's quite a long way before you get any reading - then it's quite sensitive - be ready for that. (Note this is 2 settings for each channel).

    Let me know how you get on.
     
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  10. sajjad amer

    sajjad amer Active Member

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    Now this is where I am confused. There are 4 Test Points, 2 on each channel

    I have used TP1 and TP2 Pin 1 and 4 (out out) to adjust DC Offset with KVR1

    Bias adjustment procedure is on the legs of out put resistors, manual says on both outer legs adjust 17mv, this forum says middle and outer leg adjust 8mv

    kindly advise me where should the meter probes be for me to adjust this?
    Many Thanks
     
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  11. sajjad amer

    sajjad amer Active Member

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    I have used both outer legs of output resistors and adjusted the BIAS on 3 of out put resistors to 16mv.

    However on Right Channel KVR51 has no effect on the voltage across the concerned resistor's outer legs.

    When I check on EC of Q51, I can adjust the voltage with same trimmer, BUT lowest I can get is 3.79, I cannot go lower than this, as trimmer starts to click .

    The voltage across the "normal" EC is 3.6v.

    After BIAS adjustment I have reset the DC Offset to 00.17 and 00.21 mV.

    The amplifier is still stuck in protection mode.
     
    Last edited: Jun 2, 2018
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  12. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Good information - we are getting closer
    Ok - and don't worry too much about protection mode at present
     
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  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    To explain, the resistors have the same current flowing through them, the resistors drop around ~8mV each. Measuring across one gives 8mV (of course) and measuring across two gives 8+8mV or ~17mV. ;)
     
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  14. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Try changing kQ51 - should be a 2SC1845 (or KSC1845) - make sure you have turned the trimmer back to minimum bias - then try again - you are looking for the same degree of C-E voltage adjustment as with the other three ;)
     
  15. sajjad amer

    sajjad amer Active Member

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    That explains it all, thank you
     
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  16. sajjad amer

    sajjad amer Active Member

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    KQ51 C1845 was indeed shorted.

    I have replaced all 4 C1845 , however symptoms are still the same, I cannot adjust Bias with KVR 51. Rest of all three Bias VR do their job as intended.
     
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  17. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Please can you say which was shorted, B-E, C-E ?
     
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  18. sajjad amer

    sajjad amer Active Member

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    I use a transistor tester to check the transistor, I do not use the diode function on multi meter.

    However I could have checked it but unfortunately I have binned it and cannot find it despite going through the bin. Apologies.
    Is there any other way to diagnose this issue further please?
     
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  19. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Ok, not a problem, nice to know but not essential. :)

    You say you can't set bias - but what about the voltage adjustment between C-E of kQ51 - How is that looking now?
     
  20. sajjad amer

    sajjad amer Active Member

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    3.3 v and it does respond to KVR 51
     
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