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Old 14th January 2012, 02:15 PM   #1
Nite is offline Nite  Europe
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Default Subwoofer amplifier MIK

Hi there,
I have an amp that is built into a sobwoofer case that I haven't been able to fix. Maybe you guys can help me on you on this one? I have fixed a few amps before, but this particular amp is giving me a hard time. When I received it, both power supply mosfets (IRFZ44N) and four output transistors with one preamplifier transistor were shorted. So, I replaced them, but were unable to find the exact match for the preamplifier transistor B631K, for wich I used BD140 as a temporary replacement.

The amp powers up and everything seems to be fine: I get +-35V on the rectifiers and the led is green. However, as soon as I connect it to the speaker, the output transistors and preamps begin to get warm gradually to the point where they are so hot that I have to disconnect the power. I'm using a power supply with a weak current output of about 2A, so I have time to measure voltages before things get critical. I have checked other transistors in the audio section, but couldn't find any more shorts. What do you think? Thanks in advance for you time. Please see the attached pictures for reference.
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Old 14th January 2012, 03:51 PM   #2
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is it attached to a heatsink during testing?
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Old 14th January 2012, 04:03 PM   #3
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Quote:
Originally Posted by djQUAN View Post
is it attached to a heatsink during testing?
No, it's sitting on my table without heatsink. The limited current output from the power supply doesn't let it get hot quickly. But maybe I should afterall attach the heatsink in order to protect the transistors I installed.
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Old 14th January 2012, 06:12 PM   #4
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What is the DC voltage across the wires that go to the speaker with the amp on but with no input signal?

Have you checked the other driver for leakage (would have to be pulled out of the board)?
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Old 14th January 2012, 06:25 PM   #5
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Hi Perry,
DC voltage across the wires is 0.39 V. I checked the other driver for shorts in circuit. Do you think I should remove it?
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Old 14th January 2012, 06:40 PM   #6
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A leaky driver could cause problems and you can't check it for leakage in the circuit.

Do you have a scope?

Do the output transistors heat up if the amp is on with no speakers connected?
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Old 14th January 2012, 06:45 PM   #7
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The amp doesn't heat up when the speaker is not connected. The trouble start as soon as I connect it. Unfortunatelly I don't have a scope, so I can't diagnose the amp as thouroughly as I would like to.
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Old 14th January 2012, 07:11 PM   #8
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Do all of the outputs heat up or do only 2 (of the same part number) heat up?

Check the driver.
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Old 14th January 2012, 07:15 PM   #9
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I removed the driver that I didn't replace before, but it isn't shorted when checked out of circuit. I turned the amp on briefly with the speaker connected, and the outputs are still heating up. Two A1694 transistors dissipate most heat.
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Old 14th January 2012, 07:32 PM   #10
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Is it leaking (not shorted but conducting in a way that it should not)?

The heating is because those transistors are passing the DC to the speaker.
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