Harman Kardon HK6950R left channel problem - diyAudio
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Old 21st September 2006, 08:54 PM   #1
difflvl is offline difflvl  United States
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Default Harman Kardon HK6950R left channel problem

I just got my hands on a Harman Kardon HK6950R. Everything works except the left channel preamp. If I bypass the preamp, both channels work equally the same. When I don't bypass the preamp, the right channel gets a lot louder, but the left stays the same.

How can I fix this? What should I look at?

Thanks
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Old 22nd September 2006, 01:39 AM   #2
difflvl is offline difflvl  United States
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I have tried cleaning the boards, resoldering joints. Don't know what to do next.

I know the driver and output stage work fine, because when I put the right output into the left channel driver and output stage, it works fine.

The problem seems to have something to do with the preamp.

This particular model has a switch which lets you bypass the amp. I checked the switch as well, and it works fine.

Can anyone give me some pointers?

Thanks
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Old 22nd September 2006, 01:51 AM   #3
Leolabs is offline Leolabs  Malaysia
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First check for semiconductors.Then check for passive components,esp caps and resistors.
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Old 22nd September 2006, 02:40 AM   #4
anatech is offline anatech  Canada
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Hi difflvl,
A schematic would help. But from here it looks like a simple case of signal chasing.

Do you have a 'scope and a DVM??

-Chris
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Old 22nd September 2006, 05:57 PM   #5
difflvl is offline difflvl  United States
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Nevermind found a faulty transistor on the tone board. Removed it and it works fine. Wierd.

Now I need to know where the test points for the bias is at. I know where to adjust it from, but I dont know where to measure from.
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Old 22nd September 2006, 06:02 PM   #6
ostie01 is offline ostie01  Canada
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You can take measurement between the emiter resistor on one of each side but don't think you need to do that since the only change you made was to replace a resistor on the preamp board. But if you want to adjust bias, look for about 10 to 12mv on the emitor resistor,
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Old 22nd September 2006, 06:04 PM   #7
anatech is offline anatech  Canada
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Hi difflvl,
Glad you found it. One down then.

Bias is normally measured across the emitter resistors with no load connected to the amplifier. Measure cold and then allow it to settle down after it's warmed up and measure again. Aloow time for the amp to settle in after making an adjustment.

You really should have the manual for this type of adjustment.

-Chris
Edit: Ostie01, HK normally runs hot compared to the average amp.
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Old 22nd September 2006, 08:31 PM   #8
difflvl is offline difflvl  United States
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Here is the service manual. http://manuals.harman.com/hk/Service...6950R%20sm.pdf

Which ones are the emitter resistors?

The part I took out was actually a N channel Mosfet. Q517 on PCB2 Tone Control. Part 2SK364. There is actually another one right by it, which I left in.

Wierd thing is that I took it out and it works. What is it for?

Thanks for all the help!
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Old 22nd September 2006, 08:31 PM   #9
difflvl is offline difflvl  United States
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If someone can give me the part designators of the dc bias, and idling current adjustment measurement points, I would really appreciate. I have tried searching, and browsing the internet, but I still cant find it.
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Old 22nd September 2006, 08:48 PM   #10
anatech is offline anatech  Canada
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Hi difflvl,
R487/489 would be the emitter resistors, and the others like that. I also notice some R15 resistors. They may be measuring across there. If you look at page 4 (of 56) in the manual, it directs you to pages 8, 10, 11 and 25. You need to look at the picture, not the PDF page number.

You may be further ahead to print the darn thing and tape it together so it makes more sense. That's what I would do if I were you.

-Chris
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