My New PL400
- Thread starter Zach C.
- Start date
In Zach's initial post he mentioned that C6 shorted. If it shorted before Q5 failed that could lead to very high offset, but would not likely take out Q5. So down the road suppose C6 shorts again just because it could. Even if the clamp diodes are in place, they will have no effect because a shorted C6 would trump the diodes. I've seen C6 open several times but never short. So that begs the question, why did C6 short?
In Zach's case I think the failure sequence started with a failure of Q5 for whatever reason. As Joe has pointed out that sends the output to the negative rail. This negative voltage would present itself across C6, in this case a reverse voltage. A reverse voltage across C6 of more than a few volts would cause it to quickly fail.
R25 also failed open. It is in the V/I limit circuit. Although I rarely see them open, I've seen many that have been hot and sometimes charred. I would guess that it may have failed earlier and went unnoticed.
It's also entirely possible I missed something. If so everything above is null and void.
In Zach's case I think the failure sequence started with a failure of Q5 for whatever reason. As Joe has pointed out that sends the output to the negative rail. This negative voltage would present itself across C6, in this case a reverse voltage. A reverse voltage across C6 of more than a few volts would cause it to quickly fail.
R25 also failed open. It is in the V/I limit circuit. Although I rarely see them open, I've seen many that have been hot and sometimes charred. I would guess that it may have failed earlier and went unnoticed.
It's also entirely possible I missed something. If so everything above is null and void.
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Maybe I should back up here. C6 measured 16uF on my BK810B handheld cap meter.
I think that's short, right?
Don, thanks so much for the further analysis. Man, I gotta re-read all that amp stuff. Been so long I hardly remember it, but one doesn't really need to understand everything that's going on in there to troubleshoot. Now that I'm sleeping again, that might actually happen.
Also, who knows how long any given section was acting up, since the gentleman wasn't using that channel, but had the input shorted and just happened to notice the meter pegged one day.
Zach
I think that's short, right?
Don, thanks so much for the further analysis. Man, I gotta re-read all that amp stuff. Been so long I hardly remember it, but one doesn't really need to understand everything that's going on in there to troubleshoot. Now that I'm sleeping again, that might actually happen.
Also, who knows how long any given section was acting up, since the gentleman wasn't using that channel, but had the input shorted and just happened to notice the meter pegged one day.
Zach
I was going to clarify what a "short" was and then decided to keep the post as brief as possible. In the troubleshooting classes I teach, many electricians (often with years and years of experience) struggle with the concept of short circuits.
Usually a shorted device such as capacitors, transistors, transformers etc has failed such that it's resistance is zero ohms. In power amps the most common failure is shorted output transistors. So If C6 changed in value from 100uf to 16uf it would not be considered a short. More accurately it would be closer to an "open". The affect of this value change would be a substantial reduction of low frequency response. The common recommendation is to increase the value of C6 to 470uf to further extend the bass response.
As for R25 it is part of the negative half cycle of the protection circuit. I think that it often burns due to the amp being over driven or possibly oscillating or maybe when the amp fails. Lee, with all of the mods you've done I'm sure you've seen heat stress in the area of Q8/Q9 on the original boards.
Usually a shorted device such as capacitors, transistors, transformers etc has failed such that it's resistance is zero ohms. In power amps the most common failure is shorted output transistors. So If C6 changed in value from 100uf to 16uf it would not be considered a short. More accurately it would be closer to an "open". The affect of this value change would be a substantial reduction of low frequency response. The common recommendation is to increase the value of C6 to 470uf to further extend the bass response.
As for R25 it is part of the negative half cycle of the protection circuit. I think that it often burns due to the amp being over driven or possibly oscillating or maybe when the amp fails. Lee, with all of the mods you've done I'm sure you've seen heat stress in the area of Q8/Q9 on the original boards.
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