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Sansui AU-20000 balance potmeter

Discussion in 'Exclusively Sansui' started by jeromach, Aug 9, 2018.

  1. jeromach

    jeromach With sufficient thrust, ducks fly just fine

    Messages:
    1,178
    Location:
    O<erveen, the Netherlands
    I did a full rebuild on a Sansui AU-20000 and encountered a lot of issues.

    Fortunately, after ages of time, I managed to solve everything, but there's one thing that really gives me a headache.

    Unfortunately this amp apparantly had been "treated" in a butchery, since a lot of things were a big mess. I saw it was worked on before I accepted the work on it, but boy, it was much much worse than I had seen at first glance.

    Well, let's not waste too much time complaining, it's nearly ready and in the end it is the result that counts.

    This one last big issue is the balance pot. The butcher replaced it for a non original part and it did not work as it should, so I have been testing some replacements, but none seem to be working okay.

    A normal (100k) balance pot I took from a parts amp (probably an alps pot) made the sound come to the left when turning the knob to the left, to the right when turning the knob to the right, but no sound at all in the middle.

    I then tried a regular (100k) pot, but now the sound is present when the knob is in the middle, but the left channel is a lot louder than the right channel.

    My question therefore, does it need a special pot? And if so, would that still be obtainable somewhere?

    Another issue, although a little less urgent; one of the heat sinks on which the power transistors are mounted gets a lot hotter than the other (almost too hot to touch). I adjusted the bias current according to the manual, so that is the same for both power stages.

    I suspect that it might be caused by power transistors from different batches. The slaughter replaced the original transistors with MJ15022 and MJ15023. For the MJ15022 the batches are 9223 anhd 9607, for the 23 they are (BM)0213 and (BM)0331. Especially the first ones differ a long time in their producton period.

    Any thoughts on that?

    Here a pic of the culprit...

    [​IMG]
     

     

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  2. Sansuiman

    Sansuiman Active Member

    Messages:
    311
    Location:
    St. Louis, Missouri
    Balance pots are a specific type of pot, and have the gangs (sections) arranged such that the match point between the two gangs is only at dead center, and each gang will have a diverging equal but opposite difference in resistance at a given point in the rotation of the shaft, save for the 12 o'clock position. A well designed balance pot will have a detent at the center position as well.

    A volume or tone control pot has its gangs paralleled so that the resistance on each gang is identical (in theory, provided they were made to a precise specification) at any given point in the rotation of the shaft.

    Therefore, a standard (volume or tone type) two gang pot of a given resistance value - 100K in the present case - won't work as a balance pot, the pot selected and used has to be a balance pot.

    I don't know what the spec is on the balance pot for the AU-20000, but if 100K is the factory value then a 100K balance pot can likely be sourced from any number of donor components since it is a common value, if you aren't overly concerned about having a factory correct part and just want a properly functional balance control. Obviously there are differences in mounting (solder lugs vs. PCB mount, shaft length and broach type, etc). Those details need to be taken into account if using a part scavenged from another component.

    Finding a factory part won't be easy, as the AU-20000 isn't often seen being parted. A CA-3000 balance pot may be compatible, but not many of those get chopped either, so could be looking at a long wait for either.

    I just ran a quick search on "100K balance pot" over at that auction site which will remain nameless here. That renders a number of results that may be usable. Full disclosure: I have no listings over there, and cannot vouch for any you may find. Just offered as a starting point.
     
  3. jeromach

    jeromach With sufficient thrust, ducks fly just fine

    Messages:
    1,178
    Location:
    O<erveen, the Netherlands
    I had this project off the work bench for a couple of days, but today started to work on it again. The problem with the balance pot has been solved.

    The whole issue was a difference between the print positions and the pot positions. Apparantly there are differences in the several used balance pots throughout the years, brands and models, although it seems like nearly all of them in Japanese vintage stuff are produced by Alps.

    Anyway, after reversing the connections by removing the pot connections from the print and wire them seperately the amp works fine.

    However, one thing, it gets rather hot. The DC bias voltage is 35mV, it's adjusted to that value - sounding a lot better than with the initial 10mV - what are the experience of Sansui AU-20000 owners, is it normal it gets rather hot?

    This amp has been "overhauled" somewhere around 2006 and the power transistors used are although from the same brand and model rather different in production dates. I did not replace these transistors since they measured fine and the amp sounds perfect, but now I wonder if these far away production dates could cause additional heat.

    PS: I thought it needed a 100k pot, but the SM indicates 50k, just mentioning it before someone makes a mistake.
     
    Last edited: Aug 15, 2018

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