Blew AMP, can't seem to locate problem - diyAudio
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Old 5th August 2009, 03:17 AM   #1
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Default Blew AMP, can't seem to locate problem

Link to schematic for reference.

I have an out of warranty Alpine PDX 1.1000. About 3 months ago, it quit working. Found the fuses blown, replaced, but they blew as soon as the system was turned on. Ordered the schematic and found two FET's had shorted. Replaced all four FET's in the array Q401, Q402, Q405, Q406. Attempted to bench test AMP, had no output. Started checking voltages at the FET's and found to be out of spec. Decided to swap PWM driver IC307 with IC308 and found a short in IC307, replaced IC307. Now I am back to immediately blowing the fuse when I turn on the AMP, and have found Q306 to be getting extremely hot.

I can't seem to locate a data sheet on Q306 (KTD718-O), but there is no obvious short. While Q306 may be bad, I am thinking that something else is shorting, causing it to cook. I have vary limited test equipment, and am debating on just replacing the amp as I have already spent over $100 with out success. Dose anyone have any suggestions on what else to check on the amp before I hit up ebay for a new one?
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Old 5th August 2009, 06:35 AM   #2
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http://www.keccorp.com/data/databook...Eng/KTD718.pdf

Do you have a large (25w+) 2 ohm current limiting resistor? An H6054 headlamp will also work in most cases.

Have you tried removing the rectifiers from the board to confirm that the problem is in the audio section of the amp?

Can you break the connection between the VDD terminal of IC307/308 and Q306?
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Old 6th August 2009, 08:44 AM   #3
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Thank you Perry Babin for your guidance.

I am working on getting a resistor for testing.
The Q306 rectifier tests good.
There isn't a trace that I can get to to break the VDD connection, but I could run Q306 externally without connecting the VDD. From what I can see, that would remove positive supply to the PWM drivers and isolate that portion of the circuit. I will post back with my findings.
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Old 6th August 2009, 04:06 PM   #4
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I didn't get much time last night, but I did get the amp to power up (lights came on) without Q306.
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Old 6th August 2009, 06:11 PM   #5
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If you only replaced IC307, IC308 may be causing the problem. When 307 shorted, it may have been due to high voltage feeding back into it from the outputs. When it shorted, it could have sent that same voltage to IC308 via the VCC line.
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Old 9th August 2009, 04:22 AM   #6
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Slowly narrowing it down. Re-installed Q306 and pulled the T401 choke which has a faint burnt smell. Fuse did not blow. I know it couldn't be the choke, now I just have to determine if it's before or after it.
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Old 9th August 2009, 04:32 AM   #7
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What happens if you install T402 in place of T401 (leaving the inductor previously at T401 out of the circuit)?
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Old 9th August 2009, 09:00 PM   #8
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Made a few new discoveries on that line. R404, R405 and R406 are open and C320 was cracked. Also discovered that the grounds are not connected on the PCB, but rely on the screws to complete the circuit to the chassis. I hadn't been installing them for my tests. Now ordering some caps and resistors, at least they aren't as expensive as the FET's and IC's.

I am going to hold off swapping the T401/2 chokes as they are sucking up all the heat making it difficult to desolder with my equipment.
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Old 10th August 2009, 01:53 AM   #9
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check for any bit of black dust on parts.
most on capacitors theyre maybe are blown (they can be blow)
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Old 10th August 2009, 03:52 AM   #10
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Thank's ozracing. I have been over the entire board with a magnifying glass and there is nothing obvious that I can find. The only reason that I found the broken cap is because I got no reading when testing it and when I went to desolder it, part stayed on the board. BTW, this is a SMT ceramic cap.
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