Help with G-22000 Bias and Offset/missing test voltage?

Discussion in 'Exclusively Sansui' started by tder1, Apr 17, 2017.

  1. tder1

    tder1 New Member

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    Hey all ! I finally had my 22 running good after 3 months and many hours. Kind of a tough piece to learn on. Pantera for about an hour on 3-4 is a good enough test and about all your ears can take on a 22 at that volume. So I figure on to refinements. Installed some Bourns Trimmers. Left side no problem. 15 Bias 0 Offset nice and stable. Right side,hook up meter and just no voltage at test points. Both read 0v and moving trimmers does not move meter? I adjusted the offset for min. distortion with no speaker movement and bias for cool outputs but have no idea where I am ? Looked everything over but I'm out of ideas. Anybody have any idea what or where I should be looking for. Any ideas/questions appreciated . Thanks
     
  2. ghamilton

    ghamilton Super Member

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    I just went through something kind of like that with my 919. You can read my thread. Someone should chime in soon that can help.
     
  3. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    A few things to check.
    Trimmer value correct ? (Bias trimmer not swapped for DC Offset trimmer?)
    Trimmer orientation correct ? (trimmer 'slider' in correct PCB hole?)
    Solder joints ?

    Try removing the trimmers and testing with meter on 'ohms' that they do actually adjust.

    Note that you might find that you will need to turn the trimmers for one channel in the opposite direction to those fitted in the working channel. Also if the replacements are Bourns type, for information - when you are at the end of the trimmer track you will hear a faint clicking noise coming from the trimmer.

    Are you saying this is what you did with the channel that you can't set up after the trimmer changes???

    Not sure what you mean, you should always adjust DC offset for minimum voltage at the output with no signal, which might very likely be at minumum distortion if you subsequently pass a signal through the amp and measure it. And similarly with Bias - you adjust for a specific current through the output transistors - heatsink temperature is an extremely inaccurate way to set Bias, unless you have an accurate means to measure heat, and want to spend days doing it.
     
    Last edited: Apr 18, 2017 at 4:00 AM
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  4. tder1

    tder1 New Member

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    Hi Hyperion, nothing like having one of the best,
    Your strategy makes sense as they are acting like they are not there at all.
    Trimmer value correct ? (Bias trimmer not swapped for DC Offset trimmer?) 200ohm DC 100ohm bias confirmed
    Trimmer orientation correct ? (trimmer 'slider' in correct PCB hole?) Sure
    Solder joints ? Always check to next joint in line, ruling it out, but i will look again. Iv'e checked all my work under 5X

    Try removing the trimmers and testing with meter on 'ohms' that they do actually adjust. They do adjust, I set them close before install but ill pull them and recheck.

    Note that you might find that you will need to turn the trimmers for one channel in the opposite direction to those fitted in the working channel . Hadn't noticed as i was just turning as looking at meter.
    WILL ADVISE
    The Offset and Bias were set under correct conditions, on Aux no connections, min volume, tone defeated, speakers off except for offset. I only used sound when I had no choice to adjust the right side 1) till it stayed out of protection 2) minimized distortion. I know my Outputs run 2 degrees over room temp so thats what i was shooting for with a Temp Gun.

    Exactly what does the Bias Adjust.Just Emitters on Outputs? Is there another point to test it beside the test posts on the Driver ?
    Thanks Guys
     
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  5. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    What is 'bias' current?
    Bias adjustment sets the required voltage at the bases of the driver transistors - and therefore onward to the OP transistors to overcome the turn-on voltage required to get these transistors conducting. The reason for this is to reduce or eliminate crossover distortion. This is the distortion caused when the AC output voltage approaches 0V from either the +ve or -ve direction, this distortion is dramatically reduced by setting the bias current correctly. You can measure this as a current in mA (by breaking the circuit and inserting an ammeter) or a voltage dropped across a resistor, usually an emitter resistor, as this gives a direct reading (after using a little Ohms Law) of the current flowing in that resistor.

    How is bias current set?
    The means of setting bias current is usually via a trimmer which (in most cases) to increase bias turns off a transistor connected between the bases of the driver or pre-driver transistors. As the bias transistor turns off, (thus increasing the bias current through the OP transistors), it pushes the voltage at the bases of the driver transistors closer to the positive rail (for the positive driver), and towards the negative rail (for the negative driver). The opposite effect is seen when the trimmer is adjusted to turn on the bias transistor, this has the effect of shorting the base of the positive driver to the base of the negative driver thus 'pinching off' the supply of current to the drivers and also the OP transistors in turn.

    The above is a rough description, there are variations on this theme, depending on amplifier topology (and class), but the overall operation is the same.
     
    Last edited: Apr 18, 2017 at 6:01 AM
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  6. JOMARK911

    JOMARK911 Super Member

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    Make sure your problematic test points have continuity on the circuit . I had it some time ago , not on my G22K but on my B2301. a COLD SOLDER on the test point.
     
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  7. dr*audio

    dr*audio Fish fingers and custard!

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    The bias test points are usually across one or both emitter resistors in the output stage. If they are properly connected you should measure 1 Ohm or less across them with the power off.
     
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  8. tder1

    tder1 New Member

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    Plenty to think about for me. Are you gentlemen familiar with the 22000 Service Manual notorious for mistakes? OK, By the Manual the Bias is set by voltage across the test posts on Driver Board F2901 and adjusting VR2 which is smack in the middle of 2901. I followed the Test Posts up thru the molex connector and have .2Mv at the connector. Now the stumper unless it's wrong in the Manual, I can move this voltage by turning VR1 which is for Offset? These two test posts go right to main board 2903 and end up at the connector with the grey and blue wires from the OP's. Turning VR2 does nothing here. This is one of those throw your hands in the air moments for some of you and i wouldn't blame you. Is the Manual wrong? I am still moving VR2 to drop my OP's temp now at 92 degrees. Sorry for the sideways pics. Always in a hurry
     

    Attached Files:

    Last edited: Apr 18, 2017 at 8:15 AM
  9. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Not familiar, but looks straightforward. :idea:

    What about this?
     
  10. tder1

    tder1 New Member

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    Hey Ya, Ghamilton do you have a link to your problem ? Cant find it and I'm very interested ?
     
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

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  12. JOMARK911

    JOMARK911 Super Member

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    I do own the service manual in a hard copy , original. If you read it carefully you will be able to adjust bias correctly , unless your unit has a problem.
    Vr 01 on f 2900 2901 adjusts dc offset.
    Vr 02 on f2900 2901 adjusts bias .
    What exactly you can't do?
     
  13. LBPete

    LBPete Rolling Along Subscriber

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    Be sure you have a good connection between your meter probes and the test terminals. I've had issues with oxidized test points.

    - Pete
     
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  14. LBPete

    LBPete Rolling Along Subscriber

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    Almost forgot, please post the serial numbers for the database.

    - Pete
     
  15. dr*audio

    dr*audio Fish fingers and custard!

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    Please use your ohm meter to verify connection from the test points to the emitter resistors, as I said. This is important before we proceed. If you measure around 1 Ohm or less across the test points then you have a good connection and you are missing the bias current.
     
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  16. kevzep

    kevzep Its all about the Music Subscriber

    Also check on the Driver boards F-2900, that R33,34 are in spec, I believe they are fusible resistors and their value can drift as they get old and complete so many heat cycles.
    I have had problems with those before. Their failure in this way will exhibit the symptoms you are having.
     
  17. tder1

    tder1 New Member

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    Location:
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    I'll certainly be back. Slip of the Probe Damn it. Don't you just hate that! Got some stuff to go over including 701, rough slip on the Driver Board. This Amp just does not want to cooperate with me.
    BTW Hi There Kev.
     
  18. dr*audio

    dr*audio Fish fingers and custard!

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    Always put tape on your probes so only the very tip is exposed.
     
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  19. tder1

    tder1 New Member

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    Yes, and I have covers for them too. Stupid, amateurish mistake.
     
    Last edited: Apr 20, 2017 at 1:38 PM
  20. kevzep

    kevzep Its all about the Music Subscriber

    Whats happened with the slip of the probe? What were you measuring?
    Are any of the rail fuses blown?

    There's a real art to not slipping, BUT don't feel too bad, we all slip......
     

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