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AU-4900 trouble

Discussion in 'Exclusively Sansui' started by jerseymc, Aug 10, 2018.

  1. jerseymc

    jerseymc New Member

    Messages:
    29
    I received some good advice here on the forum once before. I was directed to replace my 2sc726 and 2sc1313 transistors and all was good until it wasn't.

    I got the magic smoke.

    So I replaced the burnt resistors that I saw. I replaced the bias transistors with paired ksa992's. I also went ahead and did a 99 percent recap. I didn't replace the large filter caps.

    Right now I only turn it on with the dim bulb tester. When I power up the bulb wont turn on at all. The red power light on the front of the amp lights up but the relay wont click. I have not even attempted to hook up speakers yet.

    Any tips on where to go next?
     

     

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  2. jerseymc

    jerseymc New Member

    Messages:
    29
    I got the relay to click off protection. It was a diode that I pulled to check that was re-installed backwards. I’m still on dim bulb. It won’t light at all. I set the bias. Music plays but with static. If I flip the -20db lever off- or I turn up the volume. The relay clicks off.
     
  3. slimecity

    slimecity Super Member

    Messages:
    3,009
    Location:
    New Zealand
    The dim bulb is a good sign! It means no short.

    Ensure your bulb value is something like 60W though - if so, you are good.

    So move on and try to ID the issue - when you say "the relay clicks off" if you turn up the volume - does that mean it goes into protect? When the unit is not in protection - measure the DC offset at both spkr terminals.

    What do you mean by "static" - is this crackling and popping? In both channels?

    You really need to ID an issue and sort it before doing a global recap.
     
    Oldsansui441 likes this.
  4. jerseymc

    jerseymc New Member

    Messages:
    29
    Thank you for your response. My dim bulb is 72w. It does not light at all.

    Yes, My relay clicking “off” means it goes into protect when I turn the volume up

    The static is not cracking and popping. It is More like a weak radio station. In and out. Both channels.

    I’ll measure dc at the terminals in the morning.
     
  5. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,742
    Location:
    Victoria, Australia
    I'm assuming you mean the main amp input differential pairs? Good call by slimecity on the DC offset.

    Would also be a good idea to check all power supply voltages to make sure they are in spec.
     
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  6. jerseymc

    jerseymc New Member

    Messages:
    29
    I measured my dc outputs.

    3.6 mV on the left channel
    5.7 mV on the right

    Oldsansui441: Yes I replaced the main amp input differential pairs (2sa798) with paired ksa992's. Although, the 2sa798's still test good. I pulled them based on a recommendation from my last problem.

    I will need help on checking the power supply voltages. I want to do it correctly and safely. Pics coming.
     

     

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  7. jerseymc

    jerseymc New Member

    Messages:
    29
    So I need to test my power supply voltages. Where is the best place to do this? Do I use the square pins in this pic?
     

    Attached Files:

  8. slimecity

    slimecity Super Member

    Messages:
    3,009
    Location:
    New Zealand
    You need to go to Hifiengine.com - register, download the service manual - it will tell you the correct pins to test WRT power supply voltage.

    If the character of the noise is exactly the same in both channels I would be looking at replacing any large transistors in the power supply.
     
  9. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,915
    Location:
    uk.. the middle bit
    i never heard of an incandescent lamp with that rating before :idea:
     
  10. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,742
    Location:
    Victoria, Australia
    As slimecity suggested the service manual has all of the power supply voltages listed both on the block diagram and the schematic. They are the DC rectified voltages referenced to earth, they are B1-B6. You will have to do this without the dim bulb tester.

    Best to have unit turned off before making any connections, you may even feel safer unplugging power cord before making connections as you will still have AC at one side of power switch.

    Did you experience any of the above problems before you replaced any caps/transistors?
     
    Last edited: Aug 12, 2018
    slimecity likes this.
  11. jerseymc

    jerseymc New Member

    Messages:
    29
    I'm using a 72 watt halogen bulb for my DBT, I also have a 43 watt. Real incandescent bulbs are hard to come by these days in the US.
     
    slimecity likes this.

     

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  12. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,915
    Location:
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    as i suspected a halogen .. from what i have read it will let the current pass more quickly so you might not see the initial flash as the unit powers up . i have never tried one so cant comment further . i get my lamps off local freecycle groups . as banned here too .apart from heavy duty ones that are hard to find .
     
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  13. jerseymc

    jerseymc New Member

    Messages:
    29
    Good Morning. Off the dim bulb, all 6 points for the power supply B1-B6 measured correctly.

    I checked the bias again. The left channel can be dialed in to 13mV. The right channel only hits 15mV with the dial turned all the way turned counter clockwise.
     
  14. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Location:
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    Last edited: Aug 13, 2018
    slimecity likes this.
  15. jerseymc

    jerseymc New Member

    Messages:
    29
    Correct, 15mv is the lowest I can get the voltage on the right channel. Turning it back clock wise increases the voltage quickly, just a hair and it jumps to 17mv.
     
  16. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,742
    Location:
    Victoria, Australia
    That is good news. Could we possibly have a bit more info regarding if this same problem existed before you started any work on unit?

    Does this unit have fusible resistors on the main amp board towards the heatsink area? I have worked on three AU-4900's and one did use the fusible resistors and the other two didn't, so there are two different versions.

    If it does use these resistors they will be the ones that are standing up off the board (four from memory) to allow for air flow. They almost always are out of spec (they drift very high in value) and cause problems in the circuit.
     
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  17. jerseymc

    jerseymc New Member

    Messages:
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    http://www.audiokarma.org/forums/index.php?threads/new-member-sansui-au-4900-problems.825014/
    This was the thread my original problem.

    Within a day after this issue had resolved. I got smoke from my amp and it went silent.

    Full disclosure: I failed to check the bias after I replaced the transistors on the tone board.

    Later today I plan on pulling/testing questionable trim pot.

    In this picture I'm assuming the 2 larger gray resistors next to the heat sink are the fusible ones?
     

    Attached Files:

  18. jerseymc

    jerseymc New Member

    Messages:
    29
    Never mind. Google and a sticky here told me that they are the fusible resistors. I’ll check them later too.
     
  19. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,742
    Location:
    Victoria, Australia
    Thanks for the photo, your amp is the version that doesn't have the fusible resistors, they are not the grey 1/2 watt resistors in your photo, l know l did say standing up off the board.

    Your bias wouldn't have changed after replacing the transistors on the tone/pre amp. The faulty 2SA726's causing the popping noises could have caused spikes in the DC offset when noisy though.

    I can see all of the 1/4 watt resistors you have replaced after they smoked, there must have been something that caused excessive current flow in the RH channel. Have you only replaced those resistors after the failure?
     
    slimecity likes this.
  20. jerseymc

    jerseymc New Member

    Messages:
    29
    I was aggressive in replacing the resistors. Only 2 were visibly burned. Answering your question: I only replaced those resistors after I got the smoke.
     

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