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Old 21st July 2009, 10:18 PM   #1
nchakos is offline nchakos  United States
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Default 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?
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Old 22nd July 2009, 01:48 AM   #2
nchakos is offline nchakos  United States
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anyone ... ??
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Old 22nd July 2009, 03:39 AM   #3
djk is offline djk
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Three hours and you're complaining?

Check the fuse, if it's OK then replace the cap in the feedback loop.
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Old 22nd July 2009, 11:27 AM   #4
nchakos is offline nchakos  United States
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thank you .. ill wait longer next time...
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Old 28th July 2009, 10:08 PM   #5
nchakos is offline nchakos  United States
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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..
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Old 31st July 2009, 12:11 AM   #6
nchakos is offline nchakos  United States
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I just now replaced all the 47uf caps and that fixed the problem!


thank u djk
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Old 31st July 2009, 05:56 AM   #7
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****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
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Old 5th May 2012, 03:17 PM   #8
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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?
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Old 6th May 2012, 11:22 PM   #9
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The input cap is best if a 2.2F~10F film type, whatever fits.

The feedback cap is best to be a 47F non-polar electrolytic with a 0.1F film bypass.
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Old 7th May 2012, 03:23 PM   #10
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awersome - thanks!
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