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I've been working on an Alesis powered monitor which had the following components blown on the SMPS board;
D1, D2, D3, D4, Q1, Q3, Q4, R15, U1.
I thought this was going to be a 'standard' fix - I replaced the damaged components and checked for any other shorted/open components on both sides. I also changed R10 & R11 to be sure. All seemed well, so I reinstalled the unit and powered it via a 100w bulb. It ran for something like 5 seconds and blew the same components again, which I wasn't expecting with the limiter. During this brief period the front LED came on OK and there was a normal level of hiss (level turned to max), so I'm thinking that it was putting out operational voltages while it was working. I would have thought that the feedback from R15 would have self-limited the current through Q1 via pin 3 of U1. Clearly it doesn't. I found another post elsewhere where someone had experienced the same problem (no resolution though). I wonder if anyone has worked on these?
I've only been able to locate a partial schematic;
D1, D2, D3, D4, Q1, Q3, Q4, R15, U1.
I thought this was going to be a 'standard' fix - I replaced the damaged components and checked for any other shorted/open components on both sides. I also changed R10 & R11 to be sure. All seemed well, so I reinstalled the unit and powered it via a 100w bulb. It ran for something like 5 seconds and blew the same components again, which I wasn't expecting with the limiter. During this brief period the front LED came on OK and there was a normal level of hiss (level turned to max), so I'm thinking that it was putting out operational voltages while it was working. I would have thought that the feedback from R15 would have self-limited the current through Q1 via pin 3 of U1. Clearly it doesn't. I found another post elsewhere where someone had experienced the same problem (no resolution though). I wonder if anyone has worked on these?
I've only been able to locate a partial schematic;
Attached Files