1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Sansui 3000a protect light

Discussion in 'Exclusively Sansui' started by The Fuxtor, Aug 26, 2018.

  1. kevzep

    kevzep Its all about the Music Subscriber

    The amplifier is not operating correctly, lets do one thing at a time......lets get the amplifier operational and then worry about the protection circuit.
     

     

    Please register to disable this ad.

  2. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Ok, so replacing those old trimmers is probably a very good idea! Would old coupling caps be cause for concern as well?
     
  3. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,084
    Location:
    uk.. the middle bit
    think i would start with voltages at fuse 3 and 4 etc .
     
  4. KentTeffeteller

    KentTeffeteller Gimpus Stereophilus!

    Messages:
    25,928
    Location:
    Athens, TN
    Yes, and do check the coupling caps. They're at the age where they need replacement. Biased up properly once serviced, you should be good for a long time. Do begin with voltages at fuse 3 and 4.
     
  5. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,084
    Location:
    uk.. the middle bit
    can you point out the coupling caps ? maybe my schematic is wrong .
     
  6. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Ok, they are both on the same channel tho? I can check the voltages on all 4.
     

     

    Please register to disable this ad.

  7. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    They are part of the service bulletin...
    Look back to post from earlier, a schematic was uploaded of it by Bluestime
     
    KentTeffeteller likes this.
  8. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,084
    Location:
    uk.. the middle bit
    is the bias ok?
     
  9. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    I was able to set the bias to 25mv measuring at emitter resistors ...
    My meters fuse blew previously, so no ma measuring at fuses....
     
  10. Bruno Primas

    Bruno Primas Active Member

    Messages:
    172
    Correct me if I'm wrong, but the 3000A is capacitor-coupled. You can't measure DC at the speaker terminals. You need to measure off of the collector of the output transistors. There are 2 for each channel, and one should be set 1/2 voltage of the the other one. For example, start with the left side. One should be somewhere around 70v or so, then set the other left one at 35v. Then do the right side transistors.

    Also, the DC must be set before setting bias because adjusting DC balance WILL affect bias. If you turn up the DC balance voltage, the bias will lower, and vice-versa.
     
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    Well you can try to measure DC offset on a capacitor coupled output stage, but you might get erroneous results without a suitable load connected. ;)
     
    Last edited: Aug 30, 2018

     

    Please register to disable this ad.

  12. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    The unit has no DC offset adjustment,
    I believe by adjusting both rails of each channel equally, the DC should be minimized.....I think?? You need 2 meters to adjust bias, as both adjustments affect each other.
    Original design wouldn't be considered cap coupled ,would it? The service bulletin safety mod added in the caps afterwards to prevent speakers from catching fire.
     
  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,556
    Location:
    Hertfordshire, UK
    Yes, Kev touched on this in post #16 - seems like 2 meters would be a very good idea.
    That is my understanding. (personally I wouldn't even consider putting one of these into regular use without performing the Service Bulletin modifications).
     
    Last edited: Aug 30, 2018
  14. Bruno Primas

    Bruno Primas Active Member

    Messages:
    172
    On my 4000, adjusting the bias didn't affect the DC balance all that much.

    IIRC, the DC affects the bias directly. (Although conversely. )

    I only had one MM, so it was a pain going from the fuse holder to the transistor collectors, but it can be done. Work carefully.
     
  15. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,774
    Location:
    Cleveland, Ohio
    If you don't have a load on the speaker terminals the caps can have a DC voltage there because there is nothing to discharge the cap except the input resistance of your meter which could be over 10M Ohms. You have to check the DC offset at the input side of the output coupling caps. There are 2 adjustments, which are really both bias adjustments, and they affect each other. You get the bias close on both transistors, then tweak one of the adjustments for the DC offset to be 0V.
     
    Hyperion likes this.
  16. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,774
    Location:
    Cleveland, Ohio
    The 4000 is totally different. Bias has no effect on offset on the 4000.
     
    Bruno Primas likes this.

     

    Please register to disable this ad.

  17. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,774
    Location:
    Cleveland, Ohio
    Good luck finding a replacement trimpot for the ones used on the 3000A. They are high wattage because of the higher current running through them. Just clean them.
    The best solution for the 3000A is to replace the power amp completely with a new board or module and get rid of the interstage transformers. That's what I did on mine. The tuner on this model is superb so it's worth upgrading the amp to make it reliable.
     
  18. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Ahhh. Ok ! Now I have something . Makes good sense.... I initially had a load on speakers, but I was also checking the dc at speaker outputs....
    I had adjusted bias to 25mv at emitter resistors? Fuse method is a pain! Does 25mv sound reasonable??
     
  19. kevzep

    kevzep Its all about the Music Subscriber

    I thought he had a load on it which is why I was wondering whether the mod was actually implemented, how else would there be that much DC on the output?
     
  20. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    I did have dummy loads attached for the first measurements, but from what Dr.Audio states, I need to be measuring st the inputs of the coupling caps?
     

Share This Page