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How to bias HK990
How to bias HK990
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Old 28th October 2013, 06:20 PM   #1
Febox is offline Febox  Italy
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Question How to bias HK990

Hi Guys I am not new on this forum but this is the first time I write here. I'd need and help checking the bias of my new (used) Harman Kardon HK 990. I am sure nobody checked it for years and now I want to do this to be sure about it is correct.
The manual says:

AMP Adjustment Iding Adjustment
Precaution for handling measuring instrument.
The ground side of the measuring instrument to be connected to the speaker terminal of this unit must be kept in floating condition because this unit is equiped with the floating balanced power amplifier.
Condition
* Start adjustment 5 minutes or more after the power is turned on.
* Non loaded condition.
Idling Adjustment
Adjust R53 so that the DC voltage
of TP1 becomes +38mV
I have some questions. What do they mean when they say " The ground side of the measuring instrument must be kept in floating condition" ? Do they mean I need to find a ground somewhere in the amplifier and tip it with the black terminal? TP1 from the service manual pic is composed by 2 tips TP4 and TP8 should I point the instrument on them contemporary? I biased some tubes amplifier but it was a bit different to do and i know it is normal but I never had difficulties doing that. Have you some advice for me? Any helps will be appreciated.
The best regards
Febox
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Old 28th October 2013, 07:03 PM   #2
Febox is offline Febox  Italy
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This is the page from the service manual
Attached Files
File Type: doc HK990 BIAS.doc (123.0 KB, 72 views)
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Old 28th October 2013, 08:48 PM   #3
Mooly is offline Mooly  United Kingdom
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How to bias HK990
It will mean that the test equipment must be isolated (not a mains powered and grounded DVM)

For the actual measurement it sounds as if the speaker negative terminal is one point and the test point the other. It wouldn't matter if you reversed the meter leads and saw -38mv because you can't have a negative bias here.

They make it difficult... if you look at the circuit you are probably just measuring across an emitter resistor in the output stage.
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Old 29th October 2013, 08:34 AM   #4
Febox is offline Febox  Italy
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Than you very much Mooly, I will try to bias my amplifier with your helps and hope it will works!
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Old 29th October 2013, 08:45 AM   #5
Mooly is offline Mooly  United Kingdom
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How to bias HK990
OK

Just be careful you don't overbias it. With most SS amps the bias can vary quite a bit with temperature and its easy to think its too low.
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