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Old 31st January 2013, 07:24 AM   #1
tony Ma is offline tony Ma  United States
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Default newbee need help

Dear all,

Can any one help me to understand better how to adjust dc offset and bias on B&K ST-140?

I want to learn here....

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Old 31st January 2013, 08:12 AM   #2
Mooly is offline Mooly  United Kingdom
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DC offset normally involves adjusting for 0.000 volts DC as measured across the speaker output.

Bias current will be specific to the model and circuit so unless you have a manual I would advise not to touch.
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Old 31st January 2013, 09:29 AM   #3
amptech is offline amptech  Scotland
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hello and welcome to the forum...adjustments on amplifiers need to be done with caution as you could fry the o/p by applying to much bias..

Some amplifier's don't have off set trimmer's as there dc of set is low say 30mv dc at the speaker o/p.

Adjustment off set voltage is straight forwards to do if you have the schematic/service book to hand, so before opening the unit place your test meter with its voltage selected to 20 or 2v volts dc and connect the test leads to one speaker channel only. now power on the amplifier with no signal and make a note of it's reading it should be 30mv but can be as high as 100mv's.. Using the offset pre-set trim you can get close to o volts +/- even 10mv is ok...

It takes a little more setting up of the bias voltage's, again this is done via placing your test meter leads between one of the 0.47 5 watt resistors and the amplifiers o/p, these will be at the power transistors emitters leg's.(readings of 15mv to 39mv can be seen, or check current draw of the amplifier under test via removing one of the power rail fuse's and placing your meter in current mode and connect the leads across the fuse holder and look for a reading of 100ma or more and adjust the bias trim pot to this current reading.

if you have scope a test load and signal generator to hand to check that all is well, you want to see a good clean sine wave at 1khz.. in put voltage to the amplifier should be set to 775mv ac...

if you check on the forum there will be more comments regarding this topic..

regards A.
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Old 31st January 2013, 09:34 AM   #4
amptech is offline amptech  Scotland
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If all is well just leave the amplifier in it's working state as out lined in last wrong adjustments can be a costly...more advice would be get books etc etc...
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Old 31st January 2013, 10:29 AM   #5
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Google is your friend - the schematic comes up easy and this thread shows P1 is to adjust offset.
P2 is to adjust bias current. The factory kit should have adjustment instruction for this.

Last edited by Ian Finch; 31st January 2013 at 10:31 AM.
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