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This amp came right after I was done with the one in this previous thread.
http://music-electronics-forum.com/t21635/
Just lettin' Enzo and all other gracious helpers know what was wrong with this one. It took me a couple hours vs a month to figure this one out thanks to all who helped in the previous post.
It came in muted. I checked to see if Q9, Q10, and Q11 were on with the amp muted and they were not. So that was correct. I then checked power supplies for the pa and also junction tested Q14-21 and all seemed fine. So I plugged into J1 and unmuted it to take some more measurements. It seemed like the amp was still muted so that lead me to the thermistor and muting circuitry. R82 had 0v across it. Q8Vbe was 0v, yet Q8's Vb was -41v. That led me to R72 which had 21v across it. That told me current was flowing through Q7. Q7 Vbe was -645mv... wo why was it on? To cut to the chase everything checked out except for the -41v on Q8's base. It seems that Q8's base should be anywhere between -41.4v to -31.4v. I lifted one side of D20 and the amp came back to life so I alligator clipped a new 10v zener in there and it's all good. So just wanted to share with yall.
Note I'm still not COMPLETELY privy to the workings of the muting circuitry, however my ss troubleshooting skills led me to the issue. Now writing all this I'm not quite sure how I got to where I did.. :0 HA!!!
Oh, I guess if Q7 was on that I thought Q8's base SHOULD be -32vdc.
http://music-electronics-forum.com/t21635/
Just lettin' Enzo and all other gracious helpers know what was wrong with this one. It took me a couple hours vs a month to figure this one out thanks to all who helped in the previous post.
It came in muted. I checked to see if Q9, Q10, and Q11 were on with the amp muted and they were not. So that was correct. I then checked power supplies for the pa and also junction tested Q14-21 and all seemed fine. So I plugged into J1 and unmuted it to take some more measurements. It seemed like the amp was still muted so that lead me to the thermistor and muting circuitry. R82 had 0v across it. Q8Vbe was 0v, yet Q8's Vb was -41v. That led me to R72 which had 21v across it. That told me current was flowing through Q7. Q7 Vbe was -645mv... wo why was it on? To cut to the chase everything checked out except for the -41v on Q8's base. It seems that Q8's base should be anywhere between -41.4v to -31.4v. I lifted one side of D20 and the amp came back to life so I alligator clipped a new 10v zener in there and it's all good. So just wanted to share with yall.
Note I'm still not COMPLETELY privy to the workings of the muting circuitry, however my ss troubleshooting skills led me to the issue. Now writing all this I'm not quite sure how I got to where I did.. :0 HA!!!
Oh, I guess if Q7 was on that I thought Q8's base SHOULD be -32vdc.
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