TA-500 with No Phone Stage

Discussion in 'Exclusively Sansui' started by mklein357, Jan 15, 2019.

  1. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    I agree with Slimecity. Also, Sansui used their own part numbers for a lot of their chips. If you can read a different number directly from the IC or transistor you may be able to search for a cross reference that way. Make sure it is not functioning before replacing it though.
     

     

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  2. mklein357

    mklein357 AK Subscriber Subscriber

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    OK. Tested and replaced all capacitors on the power supply. I found 2 suspect caps: Both were 330uf caps that were measuring at around 260 uf. Also replaced the 3 4.7 ohm resistors and checked functionality of the transistors. All transistors are working properly. I hooked back up last night and ran through DBT and have good output on aux and tuner. Will set DC offset and bias voltages tonight and test out the phono stage. Fingers crossed.
     
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  3. mklein357

    mklein357 AK Subscriber Subscriber

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    Well, getting closer. I dialed in DC offset and bias voltages and gave her a test. Auxiliary output: Check. Tuner output: Check. But as for the phono section, it is working but only at low volumes Anything into moderate volume levels causes the sound to sputter and panel lights to flicker. I notice the issue is exacerbated by increased bass, loudness function on. Problem is less noticeable with subsonic filter on but is still present. Any ideas out there?
     

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    Last edited: Feb 18, 2019 at 10:02 AM
  4. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    Since you recapped I believe, most likely one of the transistors is weak and breaking down. I replaced all with no problems after that. Check for cold solder joints as well. Caps all in correctly?
     
  5. skippy124

    skippy124 AK Subscriber Subscriber

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    Hi,

    Do you have the unit running through the dim bulb when this happens - you will have the symptoms you describe if the dim bulb is in circuit and you are trying to run the unit at normal listening levels, the dim bulb is limiting the amount of power that can be drawn.

    Cheers

    John
     
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  6. mklein357

    mklein357 AK Subscriber Subscriber

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    The dim bulb was not connected at this time. I have re-capped the power supply but have not re-capped the phono board. The only thing I did to the phono board is replace the 4 fusible resistors. Looks like I'm on to re-capping and replacing transistors on the phono board. I will say that what I have heard so far through the Aux. port is very good so this encourages me to keep moving forward with the restore. Very happy with the quality of sound.
     

     

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  7. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    I really liked the sound of mine when finished as well. Pay clause attention to cap polarity and ECB pin outs of BJT’s. Your almost there. Don’t want to fry those nearly unobtanium AN7070 driver amp IC’s on this last leg of your journey!
     
  8. mklein357

    mklein357 AK Subscriber Subscriber

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    Getting ready to do some more work on this project. I checked voltages on the phono board last night. All match the schematic except at the transistor as hi-lited below. I am getting about -.4 volts here. I'm assuming the +40volt label shown below is incorrect?


    upload_2019-2-18_11-5-40.png
     

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  9. slimecity

    slimecity Super Member

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    I wouldnt assume its incorrect - rather it suggests that F3015 isnt getting the power it shoudl be getting at pins 5 and 6.

    To double-check - measure the voltage that you are getting at the collector junction with R33 of the equivalent transistor (shown above the transistor you reference in the schematic) and tell us what it is.
     
  10. mklein357

    mklein357 AK Subscriber Subscriber

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    At the junction of R33 and its related transistor, I get -3.87volts.
     
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    There is no chance of it being 40 Volts at that point - I think it is a misprint for 4 Volts, or actually -4 Volts, judging by the reading you are getting. ;)

    How about the other voltages - all correct?

    Have any of the electrolytic capacitors in the EQ amp been changed yet?
     
    Last edited: Feb 20, 2019 at 11:51 AM

     

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  12. mklein357

    mklein357 AK Subscriber Subscriber

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    Here are the voltages on the board that I have gathered. I've replaced all the electrolytics and transistors. I have not changed out the ceramic and polystyrene cap's which are C15 through C20.

    upload_2019-2-21_8-28-35.png
     
  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    OK, those voltage readings look ok - As I understand it all inputs sound good except Phono, which is still as per your post #23 ?

    And what are you using as an input to test the Phono input?
     
  14. mklein357

    mklein357 AK Subscriber Subscriber

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    Correct. All inputs except phono are good. I'm using my turntable which plays fine at all volume levels through my standard receiver. On this TA-500, phono output is OK in the low volume range, but at medium to medium-high listening levels, that is when the problem starts. Sound cutting out and the display lights simultaneously flickering. The problem gets worse as volume is increased. Problem also gets worse as bass is increased.
     
  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    That really sounds like something power supply related - but I'll be danged if I know what!

    My first though was that the phono input is being severely overdriven, :idea: but you say it works fine with another receiver - Soooo, what turntable, cartridge, and receiver do you have that work OK together - and what input on the receiver are you using. :idea:
     
    Last edited: Feb 21, 2019 at 11:35 AM
  16. maxhifi

    maxhifi AK Subscriber Subscriber

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    I wonder if you have a lot of rumble from your turntable, or possibly acoustic feedback from the speakers, which is causing the amplifier to be overloaded when you crank it up. How does it sound, when you listen to a record using headphones, speakers off, and turn it up loud?
     
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  17. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Good thinking!
     

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