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Old 12-24-2013, 09:48 AM   #34
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Originally Posted by Khron666 View Post
Let me put it this way: silicon devices are FAR more prone to damage / "death" than transformers. And if the T1 was indeed at fault, there's a pretty good chance the damage would've been visible.

Given that, is the UC384x working? What's the Vcc reading? And also, which revision of the power supply is in your pair? If it's the first version, the C8 startup cap is between some power resistors and the FET heatsink, so it's quite likely "cooked" - replacing that (and relocating that, as per one of my earlier posts) is a mandatory first step.

You say the MOSFETs were damaged and replaced. Are the gate-driver transistors (and associated passives) still ok?
Hi Khron666,

Thanks for the reply. Yes, I saw your original post. My C8 is right next to the power resistors and the Q1 FET/heatsink. I did replace this component already per your earlier post. But, there were other parts that were visibly damaged when I began to investigate.

So far, I've replaced:
-C8, R3, R4
-Q1 FET, Q3, Q4, C15, R15, R11 (all showed visible signs of damage)

NOTE: I mispoke in my previous post. I did not replace both FETs. Only Q1, as it was noticeably damaged, along with Q3 and Q4.

I don't see any physical damage to T1, like burn marks, for example.

And yes, I'm learning my lessons here, in that I'm replacing parts without fully knowing how they were damaged in the first place.

I'm away on holiday, so I can't measure Vcc on UC3844 at the moment. I'll be able to revisit this at the end of the week.
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