1. Rest in Peace Paul (Kegger) If you would like to help the family in this time of great sorrow and need, you may donate on their GoFundme page: https://www.gofundme.com/mckechnie-medical-and-funeral-fund?
    Dismiss Notice

AU-317 Large Voltages Swings From Control Amps

Discussion in 'Exclusively Sansui' started by rd48sec, Mar 16, 2017.

  1. rd48sec

    rd48sec Active Member

    Messages:
    134
    Location:
    Eau Claire, WI
    When I power the AU-317 on I get large voltage swings from the pre-amps. Both amps exhibit this behavior. Measuring with a voltmeter, the voltages swing from +/-2V and decrease to 0V. I have not looked with an oscilloscope, but it appears to be sinusoidal.

    After the voltages calm down, I install the pre to main jumpers and I can hear some crackling coming from both speakers. Playing music with the volume up a little ways drowns the crackling out.

    I checked the +/- 28V supplies. They appear to be rock solid.
    I have attached the left channel pre-amp/control amp schematic. The chart shows turned on after being off for 2 hours, then turned off, then turned on, and then turned off.

    Any places to start diagnosing the pre-amps?

    LeftChPre.jpg

    Chart.jpg
     
    Last edited: Mar 17, 2017
  2. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,316
    Location:
    Long Beach, CA
    Did you make the voltage graph or is that in the service manual? What made you check the voltage originally?

    - Pete
     
  3. rd48sec

    rd48sec Active Member

    Messages:
    134
    Location:
    Eau Claire, WI
    I made the graph. I checked the voltages because I heard loud static/noise from both channels when I powered it up after replacing the fuse resistors. I replaced the fuse resistors because one of them was at 166K (150ohm is normal) and that channel was at the negative rail preventing the output relay to engage.

    Maybe these voltage swings are normal? There is a possibility that the main amp has a bad transistor or two. I am open to suggestions.
     
  4. moefuzz

    moefuzz Active Member

    Messages:
    267
    Location:
    Peeyong Bangyou
    Hmmm, Used to be when I powered my cold (unplugged/disconnected) G-9700 up, the lights in the old house would simultaneously dim, as in power surge.

    thinking outside the box - and as many here know, my knowledge off electrickcity is based on hitting things with a hammer (and I'm not afraid to use it) :rolleyes:....


    moe in Santartica
     
    Last edited: Mar 20, 2017
  5. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,316
    Location:
    Long Beach, CA
    There is a big difference between a 317 and a 9700, the 317 is rated at 50wpc, the 9700 is 200. A good working 317 will not have a giant power surge on startup and no special wiring or other start up rituals are necessary.

    Replacing fuse resistors with a correct replacement value won't cause noise so I don't think there is any correlation between your voltage surge and the resistor replacement.

    The spike in your graph will cause an audible pop in your speakers. Since it appears at the 22 second mark, I'll assume that the protection relay has already engaged so I don't think it's normal.

    So, we need to find where it's coming from. Do you have a media source connected? Does it still pop when no sources are connected? I have a an AU-555a connected to my PC. It causes a whole range of strange noises depending on what's on the screen.

    - Pete
     
  6. rd48sec

    rd48sec Active Member

    Messages:
    134
    Location:
    Eau Claire, WI
    At 22 seconds is where I switched it back on. 3 seconds later is when the relay clicks on. There is still a "swinging" voltage when the relay clicks. There is no media connected during this test. I have grounded the inputs with the same results. That is happens on both channels and the same amplitude is puzzling as well.

    I am replacing C09 & C05 (and the same caps on other channel). I will see if that improves anything.
     
  7. rd48sec

    rd48sec Active Member

    Messages:
    134
    Location:
    Eau Claire, WI
    Replaced C09, C05, C10, C06. That helped decrease the voltage swings.
     
  8. inductor

    inductor Super Member

    Messages:
    1,387
    Location:
    uk
    the transistors in the preamp\tone\phono cct usually need replacing, a real pain
     
    Karl vd Berg likes this.
  9. rd48sec

    rd48sec Active Member

    Messages:
    134
    Location:
    Eau Claire, WI
    Started another thread here which says just that. I found only one that was noisy however. This is a repair for someone else so not going to replace all of them now.
     

Share This Page