Kenwood amp in protect, stumped - diyAudio
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Old 3rd October 2011, 03:54 AM   #1
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Default Kenwood amp in protect, stumped

A friend of mine decided to run his kenwood amp at 1 ohm when it's only 2 ohm stable. The amp didn't go up in smoke, just went in protect. I took it apart and tested the power fets and the output transistors and some driver transistors and they all tested good. The only thing I'm questioning is the rectifiers. They are reading about .221 voltage drop which seems a little low.

Powering up the amp doesn't blow the fuse, nor will it blow the 5 amp fuse I have inline with the B+. Any suggestions? The amp is the kenwood KAC-8104d.
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Old 3rd October 2011, 05:57 AM   #2
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Figured out the protect problem but the amp won't produce any audio.
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Old 3rd October 2011, 08:17 AM   #3
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Does it have positive and negative rail voltage on the output transistors?
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Old 3rd October 2011, 08:31 AM   #4
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Yea it has 12 volts positive and negative on the outputs. I also tested the power fets and I got 61 volts on 2 of the 4 fets. The other 2 read nothing.
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Old 3rd October 2011, 08:44 AM   #5
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The 'power supply' FETs are the ones with 12v.

The ones with 61v are the outputs. Did you check all 3 legs of the other outputs to see if any had -61v?
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Old 3rd October 2011, 08:55 AM   #6
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I always get them mixed up. I do get -61 volts on the 2 outputs that don't have positive voltage on the center leg. I get -61 volts on legs 1 and 2 of both those transistors.

I also tested these output transistors part numbers C4883A and A1859A. I get 11 volts on the first 2 legs of A1859A and -9.8 volts on the first 2 legs of C4883A. The set of 4 outputs are FB31N20D's.
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Old 3rd October 2011, 11:50 AM   #7
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The 4883 and the 1859 are not output transistors. They are being used as the pass transistors for the 15v voltage regulators.

On Q411 and Q413, you should have had -rail voltage on legs 1 and 3. Recheck this.

In the future, give the circuit board designations (silkscreened on the board next to the transistor) for all transistors that you mention.
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Old 3rd October 2011, 04:29 PM   #8
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Q411 and Q413 did have - rail voltage on legs 1 and 3.
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Old 3rd October 2011, 04:35 PM   #9
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Do you have a scope?
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Old 3rd October 2011, 04:36 PM   #10
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That's one thing I don't have yet.
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