Trouble with the bias B&K ST140

mdelrossi · 20th September 2009, 10:00 PM

Hey all,
I'm having trouble biasing my ST-140.
Prelude, I've rebuilt a Sansui au-999 and others so I kinda know my way around.
I have the schematics
As per the instructions I got from B&K remove the + rail fuse attach dvm set to milliamps (fluke77) adjust p2 to 200 milliamps.
On one channel I can't adjust it past 35ma, the other just pegs the meter to OL
then drops after power off.

This happened on the 999 and I had to replace the driver tranistor, is this the same deal?

BTW the amp plays fine and sounds good, I just want to make sure it's in spec.
and not about to blow the outputs, as they are Unobtainium (I replaced them about 12 years ago).

thanks.
mdr

bountyh · 21st September 2009, 12:58 AM

Post a schematic of the output and bias chain?

mdelrossi · 21st September 2009, 01:12 PM

Here you go:

Thanks.
mdr

B Cullingford · 21st September 2009, 05:21 PM

the bias is controlled by the voltage dropped across P2 by the current set by the constant current source consisting of Q10/R9/the LED

so check those parts and voltages on them.

I'd say compare channels but it sounds like both channels are wrong, still it can't hurt to compare

oh and other things could cause your problem, output MOSFETS for one

mdelrossi · 22nd September 2009, 06:29 PM

Thanks I'll check those.
mdr

mdelrossi · 23rd September 2009, 03:05 PM

OOOPS.

I had the probes in the wrong holes on the meter

Everything works great now

I'm going to be re-caping this guy any way.
I'll post with results.
thanks again.
mdr

jackies · 02:59 PM

Cool!
I have a st-140 too, but never messed with it. I get an occasional rustling noise in one channel, so I was thinking to recap it too. It seems to use some sort of Arcolytic nonpolar caps. What are those, some obsolete brand?

mdelrossi · 24th September 2009, 03:12 PM

What you see is what I got.

metzb1 · 3rd July 2010, 12:54 PM

This is an old thread, but I was hoping to see if you ever recapped your ST-140. I have one also and am considering some refresh work.

Brian

Quote:

Originally Posted by mdelrossi

OOOPS.

I had the probes in the wrong holes on the meter

Everything works great now

I'm going to be re-caping this guy any way.
I'll post with results.
thanks again.
mdr

defec · 8th September 2010, 12:35 AM

Hi, Does anyone happen to know where the test points are for setting the bias with this amp are? Thanks.

20th September 2009, 10:00 PM	#1
mdelrossi diyAudio Member Join Date: Feb 2005 Location: ny	Trouble with the bias B&K ST140 Hey all, I'm having trouble biasing my ST-140. Prelude, I've rebuilt a Sansui au-999 and others so I kinda know my way around. I have the schematics As per the instructions I got from B&K remove the + rail fuse attach dvm set to milliamps (fluke77) adjust p2 to 200 milliamps. On one channel I can't adjust it past 35ma, the other just pegs the meter to OL then drops after power off. This happened on the 999 and I had to replace the driver tranistor, is this the same deal? BTW the amp plays fine and sounds good, I just want to make sure it's in spec. and not about to blow the outputs, as they are Unobtainium (I replaced them about 12 years ago). thanks. mdr

24th September 2009, 02:56 PM	#7
jackies diyAudio Member Join Date: Nov 2007	Cool! I have a st-140 too, but never messed with it. I get an occasional rustling noise in one channel, so I was thinking to recap it too. It seems to use some sort of Arcolytic nonpolar caps. What are those, some obsolete brand? Last edited by jackies; 24th September 2009 at 02:59 PM.

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21st September 2009, 12:58 AM	#2
bountyh diyAudio Member Join Date: Sep 2009	Post a schematic of the output and bias chain?

21st September 2009, 01:12 PM	#3
mdelrossi diyAudio Member Join Date: Feb 2005 Location: ny	Here you go: Thanks. mdr

21st September 2009, 05:21 PM	#4
B Cullingford diyAudio Member Join Date: Mar 2005 Location: California	the bias is controlled by the voltage dropped across P2 by the current set by the constant current source consisting of Q10/R9/the LED so check those parts and voltages on them. I'd say compare channels but it sounds like both channels are wrong, still it can't hurt to compare oh and other things could cause your problem, output MOSFETS for one

22nd September 2009, 06:29 PM	#5
mdelrossi diyAudio Member Join Date: Feb 2005 Location: ny	Thanks I'll check those. mdr

23rd September 2009, 03:05 PM	#6
mdelrossi diyAudio Member Join Date: Feb 2005 Location: ny	OOOPS. I had the probes in the wrong holes on the meter Everything works great now I'm going to be re-caping this guy any way. I'll post with results. thanks again. mdr

24th September 2009, 03:12 PM	#8
mdelrossi diyAudio Member Join Date: Feb 2005 Location: ny	What you see is what I got.

8th September 2010, 12:35 AM	#10
defec diyAudio Member Join Date: Jul 2004 Location: !	Hi, Does anyone happen to know where the test points are for setting the bias with this amp are? Thanks.

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