Hi There,
I'm working on a GK 400RB (the schematic is attached). Originally the amp had blown output transistors and Q7 had blown up (and Q10 looked suspect) as well as some of the surrounding resistors (22ohm and 1k, I also replaced the 33ohm 5 watters as the values were way off). I've replaced everything that was damaged and the amp makes noise, but it's not working properly. There is a distorted output when you start to get into any usable volume range. I suspect that there is a problem with the bias network, however I can't track down what the cause is. I basically was reduced to a shot gun approach, where I was just replacing parts that I suspected could be the next culprit, but the problem persists.
Here are my voltages:
Q7
B 1.6V
C 57V
E 1V
Q10
B -.4
C -56.7
E .171 (yup positive)
bias network
R15 .025V (with the trim pot cranked I can move it to -.2V, but then R18 is still wonky)
R18 .29V
The junction between R17 and R18 is -28.62
I'm getting 6.7mV across R26-27
I'm also getting my + and - 15V at points A and B
Let me know if there is anything else that I can report. Thanks!
I'm working on a GK 400RB (the schematic is attached). Originally the amp had blown output transistors and Q7 had blown up (and Q10 looked suspect) as well as some of the surrounding resistors (22ohm and 1k, I also replaced the 33ohm 5 watters as the values were way off). I've replaced everything that was damaged and the amp makes noise, but it's not working properly. There is a distorted output when you start to get into any usable volume range. I suspect that there is a problem with the bias network, however I can't track down what the cause is. I basically was reduced to a shot gun approach, where I was just replacing parts that I suspected could be the next culprit, but the problem persists.
Here are my voltages:
Q7
B 1.6V
C 57V
E 1V
Q10
B -.4
C -56.7
E .171 (yup positive)
bias network
R15 .025V (with the trim pot cranked I can move it to -.2V, but then R18 is still wonky)
R18 .29V
The junction between R17 and R18 is -28.62
I'm getting 6.7mV across R26-27
I'm also getting my + and - 15V at points A and B
Let me know if there is anything else that I can report. Thanks!
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