Bias Current Adj on a G-6700

Discussion in 'Exclusively Sansui' started by 91XR7, Sep 27, 2010.

  1. 91XR7

    91XR7 Active Member

    Messages:
    123
    Well i thought before i closed it up for hopefully the last time for a long time, i would Do some Adjustments.. I was following the instructions in the SM got the DC-Offset? done went to do the Bias Current.. the instructions said to use `Top view on page 10' for the G-5700, G-6700 But you go to that page, it's titled for the G-5700 only?

    NOW on Page 11 has the top view labeled for the G-6700 and 7700 but in the instructions it says that one is only for the G-7700??

    Thanks..
    Kevin
     
  2. thermocouple

    thermocouple New Member

    Messages:
    45
    Location:
    Albany Oregon
    Maybe page 13

    The copy of the service manual I downloaded from the site has a page 13, labeled 6-2 Audio Section <G-6700>, which is for the G-6700. Maybe you should download the manual again? There is also a correction page on the next to last page. Note you are looking for ~3.3 mV for the bias voltage (Not 5 V).
     
  3. 91XR7

    91XR7 Active Member

    Messages:
    123
    Well this is confusing..

    If you go by the instructions For the L-Ch Bias Current adjustment:

    the G-6700 it says: Between Terminal 85 and Point A (in the picture it shows you an A with Blue under it) i can't figure out what would be point A, and nothing `blue' is in that area?

    For the G-7700; it says to go between Terminal 89 and 87; the 6700 doesn't have anything there for an terminal 87
     
  4. 91XR7

    91XR7 Active Member

    Messages:
    123
    Well one i downloaded myself and another that a member on here (kevzep) sent me.. both are pretty much exactly the same..

    Also thank you for reminding me about the correction page.. DOH! Printing that out as i type

    Thank you very much for pointing this out for me.. NOW maybe before i go to bed, i can close it up :yes:

    Okay um.. there are FOUR of those 3W 0.33Ohm Resisters, Two per side.. does it matter which one you probe?
     
    Last edited: Sep 28, 2010
  5. kevzep

    kevzep Its all about the Music Subscriber

    No it doesnt matter which one you meter off................

    Welcome to the confusion that is 'Sansui's weird and wonderful manuals"......

    You are looking for 5mv across the emitter resistor, not 3.3, I don't know where that came from, but 5mv is the correct voltage.

    Page 6 called driver board adjustments, its fairly straight forward, I know the test points for the 7700 are one on each side of the driver board, but it doesnt matter because you can just do it across the emitter resistor.........

    Oh and make sure you do the adjustment on the corresponding side to the emitter resistor you are measuring, you dont want to be cranking up the bias and have your meter on the other channels resistor, and do it in small increments.........nice and easy...........
     
    Last edited: Sep 28, 2010
  6. 91XR7

    91XR7 Active Member

    Messages:
    123
    Confusion is correct :sigh:

    Well i have it set up for @3.3, the thing is it kept creeping up in Voltage thou.. I left it on for a few minutes before i started my adjustments.. Did the left side first, then went to do the right side, then came back to the left side to double check and it went from like 3.2mV to 5.? mV

    So i'm thinking i may end up opening it up again these coming days after running it for a fair amount of time and adjusting it again..

    OH and i was not thrilled by the location of those Resisters either:no:

    thank you to the both of you for the help :)
     
  7. kevzep

    kevzep Its all about the Music Subscriber

    It will creep up a little as the unit warms up, but it shouldnt increase that much unless you did it when it was stone cold.............
    Thats why they have the test points, because they are difficult to get to......I cant imagine they too different from the 7700, there will be one on the bottom side of the board will be where the grey wire and the blue wire connect, and on the top side there should be a spade terminal near the driver transistor towards the outside edge of the driver board.
    The driver transistors are the ones with the small heatsinks on them........
     
  8. 91XR7

    91XR7 Active Member

    Messages:
    123
    Ahh okay.. Well i set them to around 3.1-3.2mV before i closed them, but this was after the unit was on (no load thou) for least @ 4-5 minutes. This friday i have off so when the wife is away, i may listen to it for 30 minutes and then try it again?

    And as thermocouple Mention about the Correction page, it said it on it that it's suppose to have been 3.3mV so. But with so much incorrectness in the Shop Manual, even the `correction page' could even be wrong :sigh:

    On the plus side, the wife likes the Reciever:banana:
     
  9. kevzep

    kevzep Its all about the Music Subscriber

    Hmm not sure why they would say 3.3mv, 5mv works fine, the op's dont run too hot, if it is sounding fine I wouldnt worry about............
    Could be a number of reasons why they revised the bias current...........
     
  10. 91XR7

    91XR7 Active Member

    Messages:
    123
    What truly would be the difference between 3.3 and 5.0 mV??

    greater output?

    I know after the adjustments, that both side, least by the meter, seem to be equal now in their outputs :)
     
  11. jd2005

    jd2005 New Member

    Messages:
    22
    Was just fixing a sansui G5700 and when coming to adjust the bias came accross the same puzzling manauls...

    The bias current does not change anything in the receivers balance or power. it is meant to position the working point for class AB to overcome the distortion created when working below the current cutoff point.

    Well. I also think that it should be 5mv.. nevertheless let's look at this from transistor working point perspective. 5mv over 0.33Ohm means that the bias current through the transistor should be around 13-15mA. even 3.3mV should be kind of OK, with 10mA through the transisor.

    Looking at the datasheet http://www.svntc.com/TPDF/2111.pdf or better this one https://www.semicon.sanken-ele.co.jp/sk_content/2sc4468_ds_en.pdf which shows the ICE-VCE graph and even without this since this is a common current for many class AB power amps means that this current should put it well above the current cutoff and inside in the working point for class AB operation. which is OK...
     
    Last edited: Aug 12, 2017
  12. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,692
    Location:
    Sykesville, MD
    Welcome to AK, and thanks for sharing your experiences.

    Anyone who is attempting to set up a receiver/amp and is not completely certain about what they are doing with the DC Offset and Bias adjustments should read enough of EchoWars sticky: http://www.audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/ . It is kinda large (nearly 4500 posts), but start at the beginning, hit the high points, then go back and read the parts that are relevant to what you are trying to do.

    Trimmer pots in many of the "vintage" receivers are OLD and may not have been designed to last this long. My suggestion is to replace them with multi-turn Bourne style trimmers before you make any attempts to fire it up or make these adjustments. Just make sure that you know which way to install them so that CW or CCW turns will increase the bias in accordance with the instructions in your Service Manual (SM).

    Here is my understanding of bias (and I'm sure I'll be corrected as necessary):
    Transistors exhibit distortion whenever they transition between the OFF & ON states. The distortion begins as the transistor approaches the transition and starts to diminish as the transition is completed. This is not a 1/0 thing!
    Bias is applied to the base of the transistor to ensure that it is always enough above the transition point that it does not get close enough to get into the region of distortion.
    You adjust the bias with volume set at zero (max attenuation) so that there is minimum "signal" flowing through the output TRs, AND after the unit has 'warmed up'.
    The bias trimmer is adjusted to the point where there is a minimal acceptable voltage applied to the base of an OP TR to turn it on enough to let the described bias current flow through the emitter resistor. This usually indicates that the OP TR has been biased above the transition point to satisfy the manufacturer's specs (when built!).

    This is a delicate dance.
    The bias depends on the connected circuitry, line & P/S voltages, temperature, and other factors . . . Changes in any of these contributors can affect the bias. Like changing some components or component degradation, letting a unit sit for 6 months (winter/summer), and many others.

    I wonder if there is anyone out there who keeps small digital ammeters permanently attached to their receiver/amp so they can easily check these levels with a glance.

    Clear as mudd?
     
    Hipocrates likes this.

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