Sansui 1000A Repair

Discussion in 'Exclusively Sansui' started by Agorax, Nov 23, 2018.

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  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If it's running ok, then you most likely did it right. But verify your voltages at the plates, screens, and Bias points on the OUTPUT TUBES. (Pin3 (Plate), 4-8(Screen), Pin 6 (Bias Voltage), and pin 5 (Cathode amperage).
     

     

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  2. Agorax

    Agorax New Member

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    I'm thinking about throwing in a set of output tubes, leaning toward the Tung Sol reissues. Seems to be slightly better overall than the JJ's and Electro Harmonix from the research I've done.

    Before I spend that money though I'd like to at least get some bias readings on the original tubes, I'll start with a new multimeter battery and go from there.

    Are the bias pots something I should give a good cleaning with contact cleaner and Deoxit? I haven't touched them at all since I've gotten the 1000a
     
  3. bhamham

    bhamham AK Subscriber Subscriber

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    I'd go with the Tung Sols or the EHs. I put a quad of new JJs in my 1000A and two red-plated. I then put a quad of EHs and they worked great.

    Nice work on yours!
     
  4. Agorax

    Agorax New Member

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    Thanks bhamham, I'm trying my best here.. I've sorted out my bias problem and have set a baseline at 260mV.

    However I am only able to get a reading on 3 of the 4 output tubes, one is giving me nothing.

    Comparing the temperature of the side of this tube to the other three, its running cold, maybe I'll swap two and see if the zero bias reading follows.

    In case anyone is interested in knowing, my original readings before adjustment were 138, 165, and 253 mV.

    Edit : The zero bias reading follow the tube, thinking I should order that set of Tung Sols I mentioned earlier.
     
    Last edited: Jan 5, 2019
  5. Agorax

    Agorax New Member

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    I have the Tung Sol 7591a's installed and biased around 285mV, they biased with ease. 472v on pin 3 all around.

    Sound quality has increased dramatically, especially at higher volumes. Unfortunately my right channel is still weak output wise.

    With the balance at 3 o'clock (equal balance) the receiver sounds wonderful, plenty of power and lots of tight bass.

    As much as I'd not like to talk about it, the right most transformer still doesn't get warm, perhaps next I should remove it's leads and figure out the best way to test it.

    Thinking about throwing some tubes in V14, V18, V15 and V19 as well.
     
    Last edited: Jan 11, 2019
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Bring them up to 330mv (33ma.) That will bring them up into the middle of the 70% to 80% range. Right now the total effective wattage is 12.03W based on your 285mv(28.5ma) then subtract 3ma for screen dissipation for a total of 255mv (25.5ma). This is too cold and will cause increased distortion.
    Using 330mv (33ma) your Plate dissipation with 472v on the plates is 15.57w or about 81%. Subtract 3ma(screens) from the 33ma and you get 30ma effective total dissipation of 14.16W per which is very close to 75% dissipation.

    I run mine with 478 on the plates @ 34ma. Subtract 3ma from plate dissipation for Screens. So 31ma effective total dissipation

    478 x .031 = 14.818W per tube.

    Swap the 6AN8's and check balance again. If balance stays the same the problem is in the circuit further back. If it moves with the tubes, then it's the tubes. If it's the circuit, swap the 2 12ax7's. (also 1st check to see if the 2 sections per tube are fairly equal. If grossly out of spec(non equal) replace with a balanced tube. Then listen to it again with the balanced tube.) If still unbalanced, swap the 12ax7's and listen for the imbalance following a tube.

    If it's still further back prior to the 12ax7's DEOXIT EVERY SWITCH, POT, or CONTROL on the FRONT PANEL! If it goes Side to Side, in-out, up-down, or Round and Round, DE-Oxit it. On gear this old, multiple applications with vigorous exercising of the controls IS WARRANTED!
     
    Last edited: Jan 12, 2019

     

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  7. bhamham

    bhamham AK Subscriber Subscriber

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    Great advice ^^^^
     
  8. Agorax

    Agorax New Member

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    Thanks again for the info, I"ll post what I come across next.
     

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