20K Self Inflicted Pre Amp Issues

Discussion in 'Exclusively Sansui' started by Fhamre, Jun 18, 2017.

  1. Fhamre

    Fhamre AK Subscriber Subscriber

    I have a nice AU-20000. It came to me partially recapped and with two variants of driver boards. Thanks to Kale I now have two of the correct driver boards. Added new KG Super Tune power supply caps. All good.
    Noticed how hot the power supply board got and that the two 5 watt resistors were literally cooking.So thx to Echowars suggestion I mounted two 25 resistors to the chassis to better deal with the heat. Tested and all great.
    Noticed that the unit was full of A726 transistors in the pre amp so even though a little voice said leave well enough alone I did what I felt needed to be done.I opted to go in one more time and replace them with A992's and C1845's for the compliment.Did the Tone Amp board and Flat Amp boards first as they were easy to access. Tested fine after.
    Then went in and did the filter circuit board ,the EQ board and the Tape Copy board, pulled the meter board as well just to look to see if any diodes needed replacing plus the previous recap per had damaged some traces and I wanted to get a better look.Had to do all three boards at once as it is not easy getting in. Lots of connections that need to be paid attention to. I have lots of pics and I marked the connectors. Took my time replacing the transistors as I didn't want to go back in. The originals are BCE the replacements both ECB.
    Well now when I power up (on my DBT)I get no output. Plus one of the 25 watt resistors on the power supply is immediately cooking. If I disconnect the pre section the amp section works fine.
    So I went back in and methodically checked my work . Could see no issues. Also paid attention to ensuring I have all the connectors in the right place.I studied all my photos.
    Now I am looking for help and advice on what I should be looking for.
    I have been looking for broken wires etc but nothing found as yet.
    May start to pout soon!


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  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Hertfordshire, UK
    You've got something in 'back to front' or with wrong pin orientation, check your work, assume nothing.

    Unplug the power to the boards one by one until you lose those odd symptoms.
  3. Fhamre

    Fhamre AK Subscriber Subscriber

    Yes, unplugging the boards is my next plan. To help narrow things down.
  4. Fhamre

    Fhamre AK Subscriber Subscriber

    The symptoms went away when I unplugged the filter board. The filter board has two smaller boards that attach to it F-2498 one for each side.I pulled those and found I got my audio back.
    Now these little boards still look correct to me. I have a before pic to help compare and they both are exactly the same and appear to be correct . I will keep the focus on these two boards and try and see what I'm missing.
  5. wlhd1610

    wlhd1610 Penny and her new friend Subscriber

    upstate new york
    If one of them is installed reversed(very easy to do) it will cause the exact problem you're experiencing.

    Hyperion likes this.
  6. Fhamre

    Fhamre AK Subscriber Subscriber

    Got it sorted out . My work on the boards was fine I just installed them incorrectly. The picture shows the correct way. I installed them the way the two side by side connectors matched the board. Nope.


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  7. Fhamre

    Fhamre AK Subscriber Subscriber

    I was typing this as you sent your post. You are absolutely correct.
    Thx, I will now have a good Father's day!

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