Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: MDR on May 10, 2022, 03:59:13 PM

Title: Output transistor bias issue?
Post by: MDR on May 10, 2022, 03:59:13 PM

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? 

Title: Re: Output transistor bias issue?
Post by: JPK on May 11, 2022, 11:50:09 AM

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.

Title: Re: Output transistor bias issue?
Post by: MDR on May 14, 2022, 05:05:17 PM

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.