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Old 17th March 2013, 10:38 PM   #1
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Default Harman/Kardon HK870 blowing fuses

I have a Harman/Kardon HK870 that I have been sitting on for some time now. What happens is one of the channels (left or right I don't quite remember) keeps blowing a fuse. I was seeing if anyone can help me figure out what is wrong. I do have a service manual and a volt meter is there any other tools that may be needed.

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Old 17th March 2013, 10:49 PM   #2
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Check output transistors of the channel that is blowing! Most the time they blow and can take other parts with them too.
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Old 17th March 2013, 11:16 PM   #3
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Thanks for the input, a local tech around town recommended the same thing. I will take a look at them some time this week.
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Old 19th March 2013, 02:24 AM   #4
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I tested the transistors on the channel that was blowing the fuses, I found that between c and e there was continuity on both npn's so I pulled them out and turns out only one of them did, I assume it was showing both with continuity because they are on the same circuit. I don't know if both have to be replaced or if they have to be matched. Since these are getting hard to come by I will replace all 4 of the npns, if the amp works maybe I will seek couple pairs of pnps. What do you think.
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Old 19th March 2013, 02:35 AM   #5
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Best choice is to replace all the outputs of the channel that is blown, make sure they are "matched" and "original". After all is done check for bias and dc offset voltages on both channels. Good luck!
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Old 19th March 2013, 07:31 AM   #6
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Default Should Be Easy...

No need to replace all the outputs, just the one that is blown.
Also check the driver transistors and all resistors throughout the driver/output stage....fusible resistors can look ok even when open circuit.
Resolder the driver/output stage being sure to include the bias monitor transistor....HK's suffer dry joints ime.
Connect a 40W lamp across the mains fuseholder and run it up...if all is ok the lamp will barely glow.
If ok, refit mains fuse and adjust dc offsets and bias currents.

Good luck, Dan.
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Old 14th October 2013, 11:41 PM   #7
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Its been awhile but i finally replaced one of the transistors, it would blow the fuse but would take a while. Turns out i had a 3amp fuse instead of the 5amp it calls for. Would the 2 amps make a differnce. I will go ahead and replace the other. If for some reason they are not matched what will happen.
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Old 15th October 2013, 12:27 AM   #8
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Well that didn't go, seem to be warming up fast, then when I went to go check the idle current, pop! The fuses blew. So I will order the rest of the out put transistors, hopefully they are matched and are real. But without any good known source its all I have to go off.
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