B&k St-140 trouble shooting question

[nchakos]

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?

 

[nchakos]

anyone ... ??

 

[djk]

Three hours and you're complaining?

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

 

[nchakos]

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

 

[nchakos]

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]

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


thank u djk

 

[east electronics]

****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]

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]

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]

awersome - thanks!

 

[claudeb960]

djk -

Dumb question, why is the input cap best if a 2.2µF~10µF film type? My question goes to a higher value - why not match the original 47uf or exceed it?

Thanks for any feedback/thoughts.

 

[claudeb960]

One more question, on my model C8 is a 266 pf cap - It looks like a mylar cap - any thoughts on a replacement for this one?

Thanks!

 

[djk]

Input pole R2=24.3K + 47µF = 0.14hz
Feedback pole R6=1K + 47µF = 3.4hz

1.2µF + 24.3K = 5.46hz

Size and dominant pole.

I would actually choose 1.5µF so the input pole is at leas 1.414 time the feedback pole.

How big are the filter caps?

The pole formed by the filter caps and the load should be lower than the feedback pole.

 

[claudeb960]

Filter caps are 300uf nichikon muse.

 

[claudeb960]

Thank you for your input!

 

[djk]

The main filter cap will be at least 10,000µF (or bigger) I would imagine, it won't run properly if they are smaller than about 6,800µF.

 

[claudeb960]

Djk,

Thanks for all your help. I was confused on the filter caps. The main caps are bp computer grade 19000uf 100v and I added some 3uf polystyrene bypass caps to them.

 

[claudeb960]

I had to hold off on finishing the amp mods due to some other projects. I'll keep you posted. Thanks again!

 

[djk]

19,000µF + 4Ω = 2hz.

That should work well with:

Input pole R2=24.3K + 1.2µ = 5.46hz

Feedback pole R6=1K + 47µF = 3.4hz

 

[claudeb960]

Hi there djk,

The feedback pole R6 is also a 24.3K resistor.

I am finally getting around to finishing the amp! LOL I had to take it "down" and research the source of a DC pulse on power up. I believe I found the cause and am waiting for some parts from PCX.

On another note - does anyone have a schematic for the ST-140? My understanding is that there is about 3 different REVs on the ST-140 schematics.