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Hafler xl-280 pop on startup and higher DC offset o
Hafler xl-280 pop on startup and higher DC offset o
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Old 13th November 2017, 07:12 PM   #1
rory957 is offline rory957
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Join Date: Nov 2010
Default Hafler xl-280 pop on startup and higher DC offset o

Hi,I need help.
I bought Hafler xl-280 with damaged both channels.
Left channel repaired with replacing output FET-s with BUZ900-BUZ905 and it works fine with bias 300 mA and DC offset 0,01 V. On left channel I replaced output FET-s with BUZ900, BUZ905 and Q9, Q10, Q14 and Q15.Problem is pop during startup. Can not regulate bias lower then 320 mA and DC offset is about 0,15 V. Thanks.
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Old 14th November 2017, 04:54 PM   #2
trebolo is offline trebolo  United States
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Location: Oregon Coast
I would check the input fets. And you may have a oscilation causing the dc offset and making the bias adj difficult. What did you replace transistors with

Last edited by trebolo; 14th November 2017 at 05:00 PM.
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Old 14th November 2017, 08:32 PM   #3
rory957 is offline rory957
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I did not check input's FET-s. I bought transistors from: HAFLER XL-280 XL-600 PC-40 REPAIR KIT | eBay and replaced Q9 with 2N5550, Q10with 2N5401, Q14 with 2N3440 and Q15 with 2N5416. Testing amp with 10 % lower mains voltage passed with same pop on speakers, lower bias and almost same DC offset. I"ll check FET-s on input. Thanks for help.
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Old 14th November 2017, 09:49 PM   #4
MAAC0 is offline MAAC0  Portugal
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Transistors from ebay... Are they genuine parts ?
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Old 15th November 2017, 03:45 AM   #5
trebolo is offline trebolo  United States
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Join Date: Aug 2014
Location: Oregon Coast
Do you have a Scope? TO look for oscillation. C11 is a variable 120pf cap. If you could adjust a little it may help to adjust bias current, maybe get a second opinion on it first

Last edited by trebolo; 15th November 2017 at 03:49 AM.
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Old 15th November 2017, 04:23 AM   #6
msb64 is offline msb64  Canada
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That eBay seller is qua-co ( Ed Fantasia ), a former tech for Dynaco and project engineer for Hafler back in the day, he really knows his stuff and sells quality components.
I have purchased items from him before, from components to upgrade boards, and he was always helpful in response to my questions.
It wouldn't hurt to ask him about your issue.
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Old 15th November 2017, 06:15 AM   #7
rory957 is offline rory957
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Join Date: Nov 2010
Yes, I have scope and I'll look for oscillation and tried to adjust C11.

Thanks to all for helping me..
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