B&k St-140 trouble shooting question

nchakos · 21st July 2009, 10:18 PM

hi all

I bought this amp on ebay ..the left channel has less mid and low bass than the right.. the highs are at the same volume level..

any ideas?

nchakos · 22nd July 2009, 01:48 AM

anyone ... ??

djk · 22nd July 2009, 03:39 AM

Three hours and you're complaining?

Check the fuse, if it's OK then replace the cap in the feedback loop.

nchakos · 22nd July 2009, 11:27 AM

thank you .. ill wait longer next time...

nchakos · 28th July 2009, 10:08 PM

I checked all fuses and all are ok.. I ordered new caps 4 x 100uf 100volt an 4x 47uf 16volt.these are all the electrolytics there are ...the 100uf caps arrived yesterday i put them in ..
the left side still sounds week , hopefully the when the other caps are in it will correct the problem..

nchakos · 31st July 2009, 12:11 AM

I just now replaced all the 47uf caps and that fixed the problem!

thank u djk

sakis · 31st July 2009, 05:56 AM

****replace all the small electrolytics .....
****replace also electrolytics in amplifier boards ( not only tone control)
**** given the oportunity you may upgrade some of them to something better

finally take a look at this

vintage amplifier repair upgrade manual

claudeb960 · 5th May 2012, 03:17 PM

nchakos,

Quick question for you on the 47uf caps - I am modding my ST-140's now and I noticed last night that the 47uf (position C1 on the board, there is also another I believe in C4 position) is a bi-polar non polarized cap. Did you use a polarized replacement cap? Just curious.

It was suggested by a friend of mine that I replace with a non polarized cap (usually poor audio quality) or go with a polypropylene cap (or caps paralleled) since polarity is a non-issue for polypropylene caps (at least as it was read for the C1 position - supposedly this is an input cap).

Any thoughts on this approach?

djk · 6th May 2012, 11:22 PM

The input cap is best if a 2.2µF~10µF film type, whatever fits.

The feedback cap is best to be a 47µF non-polar electrolytic with a 0.1µF film bypass.

claudeb960 · 7th May 2012, 03:23 PM

awersome - thanks!

21st July 2009, 10:18 PM	#1
nchakos diyAudio Member Join Date: Jun 2006	B&k St-140 trouble shooting question hi all I bought this amp on ebay ..the left channel has less mid and low bass than the right.. the highs are at the same volume level.. any ideas?

22nd July 2009, 03:39 AM	#3
djk diyAudio Member Join Date: Feb 2001 Location: USA	Three hours and you're complaining? Check the fuse, if it's OK then replace the cap in the feedback loop. __________________ Candidates for the Darwin Award should not read this author.

31st July 2009, 05:56 AM	#7
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	**replace all the small electrolytics ..... replace also electrolytics in amplifier boards ( not only tone control) ** given the oportunity you may upgrade some of them to something better finally take a look at this vintage amplifier repair upgrade manual __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

6th May 2012, 11:22 PM	#9
djk diyAudio Member Join Date: Feb 2001 Location: USA	The input cap is best if a 2.2µF~10µF film type, whatever fits. The feedback cap is best to be a 47µF non-polar electrolytic with a 0.1µF film bypass. __________________ Candidates for the Darwin Award should not read this author.

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22nd July 2009, 01:48 AM	#2
nchakos diyAudio Member Join Date: Jun 2006	anyone ... ??

22nd July 2009, 11:27 AM	#4
nchakos diyAudio Member Join Date: Jun 2006	thank you .. ill wait longer next time...

28th July 2009, 10:08 PM	#5
nchakos diyAudio Member Join Date: Jun 2006	I checked all fuses and all are ok.. I ordered new caps 4 x 100uf 100volt an 4x 47uf 16volt.these are all the electrolytics there are ...the 100uf caps arrived yesterday i put them in .. the left side still sounds week , hopefully the when the other caps are in it will correct the problem..

31st July 2009, 12:11 AM	#6
nchakos diyAudio Member Join Date: Jun 2006	I just now replaced all the 47uf caps and that fixed the problem! thank u djk

5th May 2012, 03:17 PM	#8
claudeb960 diyAudio Member Join Date: Oct 2011	nchakos, Quick question for you on the 47uf caps - I am modding my ST-140's now and I noticed last night that the 47uf (position C1 on the board, there is also another I believe in C4 position) is a bi-polar non polarized cap. Did you use a polarized replacement cap? Just curious. It was suggested by a friend of mine that I replace with a non polarized cap (usually poor audio quality) or go with a polypropylene cap (or caps paralleled) since polarity is a non-issue for polypropylene caps (at least as it was read for the C1 position - supposedly this is an input cap). Any thoughts on this approach?

7th May 2012, 03:23 PM	#10
claudeb960 diyAudio Member Join Date: Oct 2011	awersome - thanks!

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