I had one channel fail while doing LFE duty. The strange thing is, all of the .33R/5 watt output transistor emitter resistors failed on the PNP side with only 1 transistor failure out of 5. The driver on that side also failed as did its 2.7R/2watt resistor.
On the NPN side, 2 output transistors failed as did their corresponding emitter resistors.
Refer to the photo where failed transistors are facing up.
The other strange thing, R51 (which roasted) on the NPN side is listed as a 1K8R half watt as is its complimentary R52 (good) on the PNP. What is installed in those two positions are 180R / half watts.
R51 and R52 on the good amp module both have 180R as well however, every picture that I come across clearly has 1K8 installed. I`m thinking the amp must have left the factory that way as I can confirm that the amp was never opened by anyone witnessed by the tamper proof seal that was still intact on the heat sink modules.
Would that type of subsitution be responsible for that kind of failure? It`s worth noting the emitter resistors on the bad module were manufactured by SRECO and the good module has COLBER`s.
I do believe that BGW had issues with emitter resistors at some point but I dont recall the particulars.
Your thoughts would be appreciated.
On the NPN side, 2 output transistors failed as did their corresponding emitter resistors.
Refer to the photo where failed transistors are facing up.
The other strange thing, R51 (which roasted) on the NPN side is listed as a 1K8R half watt as is its complimentary R52 (good) on the PNP. What is installed in those two positions are 180R / half watts.
R51 and R52 on the good amp module both have 180R as well however, every picture that I come across clearly has 1K8 installed. I`m thinking the amp must have left the factory that way as I can confirm that the amp was never opened by anyone witnessed by the tamper proof seal that was still intact on the heat sink modules.
Would that type of subsitution be responsible for that kind of failure? It`s worth noting the emitter resistors on the bad module were manufactured by SRECO and the good module has COLBER`s.
I do believe that BGW had issues with emitter resistors at some point but I dont recall the particulars.
Your thoughts would be appreciated.
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R51/Q6/Q4 and R52/Q7/Q5 appear to be output protection circuits. My guess is that Q6 will turn on when there is more than about 2A in R38, and thereby shutdown the driver stage. Why it only protects Q12+Q13 I am not sure; presumably it's a transient-overcurrent protection and not expected to prevent issues in the case of output device failure.
I'm not sure what a different value of R51 would do; probably make Q6 turn on harder and more suddenly? If R51 is cooked, that implies that there was high base current through Q6. Something very bad was happening in that amplifier and while the value of R51/R52 may be "wrong" (or may have been adjusted to provide more sensitivity in the short-protect circuit), I don't think they are the root cause of its demise.
Q12 shorting or R38 going open for example would cause Q6 to turn on pretty solidly and may have cooked R51?
I'm not sure what a different value of R51 would do; probably make Q6 turn on harder and more suddenly? If R51 is cooked, that implies that there was high base current through Q6. Something very bad was happening in that amplifier and while the value of R51/R52 may be "wrong" (or may have been adjusted to provide more sensitivity in the short-protect circuit), I don't think they are the root cause of its demise.
Q12 shorting or R38 going open for example would cause Q6 to turn on pretty solidly and may have cooked R51?