3. Output transistor bias issue?
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Author Topic: Output transistor bias issue?

May 10, 2022, 03:59:13 PM
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MDR

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Output transistor bias issue?
Hi,

Just built a 573, all is working and the unit seems to sound good.

However, when I adjusted the bias of the output transistor, I couldnt get TP4 to go below 3.8v.  It started off around 5v, but wont adjust lower than 3.8v.
Is this a problem or within tolerance? 
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May 11, 2022, 11:50:09 AM
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JPK

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Re: Output transistor bias issue?
This is not normal and the transistor Q6 will get too hot.
Please report the DC voltages shown here: https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5
in particular in points G to L.
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JPK
May 14, 2022, 05:05:17 PM
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MDR

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Re: Output transistor bias issue?
Ok I dont know what happened but when I went through measuring all of those points, I found the bias voltage was down around 1v and was able to adjust it up to 3v.  So it seems to be fixed.  Perhaps I was measuring the wrong point the first time around.  
Thanks for your help.
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