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Behringer BX3000T repair : heavy distortion for a clean bass amp
Behringer BX3000T repair : heavy distortion for a clean bass amp
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Old 2nd June 2019, 05:44 PM   #1
Astuck is offline Astuck
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Join Date: Nov 2010
Default Behringer BX3000T repair : heavy distortion for a clean bass amp

Hi Folks, I'm repairing a bass amp Behringer BX3000T for my friend.
During a rehearsal, volume dropped and the bass sound was totally distorted in a "83 - Kill'em All" way.
I opened the beast and put a DMM on the power supply section :

+17v/-17v rails are OK and are powering the preamp zone.

+15v/-15v rails had a problem with T10 (mesured +6v/-15v). Changed T10 (original BC546B was dead short)
and now, I read +20v/-15v, is this due to hFE mismatch of this new transistor ?


R33 and R48 out of the board are ok. The 15v zener diodes around seem OK (reacted like a standard diode),
D10 was mesured in place, 0,65v one way, blocking the other way.
Before D10 I read +15,5v. Between D10 and R48 I read +0,63v. After R48 I read +20,5v.


Second question, VR1, VR2 and VR3 seem to be bias setting ? How to use them to calibrate the amp ?
Service manual cannot be found the schem is attached here.

Thanks for any help.
Attached Images
File Type: jpg Schem repair.jpg (491.6 KB, 65 views)
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Old 2nd June 2019, 10:08 PM   #2
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
VR1 is bias, adjust like any other solid state amp. VR2,3 are adjustments for current limiting. Just leave them alone and they will be fine.
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Old 2nd June 2019, 11:38 PM   #3
Astuck is offline Astuck
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Join Date: Nov 2010
Thanks Enzo !
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Old 29th June 2019, 12:20 AM   #4
Astuck is offline Astuck
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Join Date: Nov 2010
Hi folks,
Finally got it ! Zener D18 15v was dead too !
Now reading are +14,5v/-14,5v, no hum, no shut down every 10 min due to heat.


New zeners are 5W, damn too big for the holes. I had to use old resistor legs to adapt to the correct size.
It was a long process due to lack of precious time but it works well now,
my friend will be happy to play bass again.
Attached Images
File Type: jpg Schem repair part3.jpg (461.5 KB, 23 views)
File Type: jpg IMG_7865.jpg (648.1 KB, 21 views)
File Type: jpg IMG_7867.jpg (771.1 KB, 19 views)
File Type: jpg IMG_7872.jpg (693.2 KB, 20 views)
File Type: jpg IMG_7877.jpg (983.6 KB, 8 views)
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