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KLH Research 10 Series Model SCX-3a repair and recap


Carlspeak
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I received two slightly different crossovers recently from a customer. They were from the Research Ten series and had slightly different model numbers. Both had very bad Aetna-Polak 5 ohm rheostats in need of replacement and those pesky black caps with the red ends (see my cap test thread). 

I managed to find some 5 ohm 25W Rheostats of similar design and construction to Ohmites but at considerable savings from what the Ohmites cost today. Also replaced 10 ohm 5W resistors with 10W versions. There's also a 50uF NPE (replaced with a 0.1uF bypassed ERSE 50 uF) and a 25 uF NPE with a Dayton film cap.

The only difference between the two xo's was the tweeter cap. The Model SCX-3A had a 2.5 uF cap and the model SCX-A had a 5 uF cap.

Below is a pic of the face of the terminal board and the opposite side.

 

klh recap a.jpg

klh recap b.jpg

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10 hours ago, owlsplace said:

Nice ... looks like NOS Phillips wirewounds.

I suppose the bypass cap is to improve transient response?

Roger

For me, bp caps are st'd practice when the uF value of the NPE is too large and costly to use a film cap. 

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Carl, since you have substantial experience using bypass caps, can you speak further to these issues? And, do you use bypass caps for woofers circuits as well?  

1. how to determine bypass cap value (uF) in relation to main cap (uF)

2. same question, re: voltage rating

3. type(s) of cap (polypropylene, polyester, film/foil, PIO) used for bypass

 

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I generally use 0.1 uF. Can't remember using one in a woofer circuit. I just don't see the value.

Voltage 250V or higher

Definitely Film Foil.

Dayton has a small selection of FF caps that are a very good value. The ones in the KLH xo's are 0.1 and 400V

If you want to spend more money, get AudioCap Theta's. They were a favorite of the late Zilch. Some audiophiles swear by FF. The AudioCaps come in even uF values above 0.47uF, unlike many others.

JBL was one of the first to start using them. I have some JBL xo's to upgrade that have what look like orange drop types. They are probably mylar. Jon Bau of Spica fame used similar bypass caps to JBL Snell used pF value orange drop type caps in the Model A crossover to help glue the NPE bundle to the board.

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Thanks, Carl, I was just curious about your own approach to the notion of bypass caps. I am somewhat familiar with JBL's frequent use of this technique, as well Zilch's staunch advocacy of Theta's, but I believe he normally paired them with regular polypro Dayton's and not NPE's. And yes, the Dayton film/foil caps do appear to be very good values.

Interesting to note that you normally use the same value (0.1uF) regardless of the main cap value - - - others seem to recommend the bypass cap be within a range of 1 to 10% of main cap value. Have you ever used any vintage caps for this purpose? I have some small value NOS caps - - - Sprague Vitamin-Q's, specifically - - - that I've been curious to try out for this purpose.  

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No, never used those other caps. With regard to bypassing, I would discourage those who may blindly follow North Creek's methodology of cascading bypass caps. They sure must have sold a lot of caps with that sales pitch. :-)

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  • 2 weeks later...

I, too, agree that Dayton caps offer excellent value, and I would certainly expect that any modern-day caps purchased as "new" should measure reasonably close to stated values. While I don't know what North Creek or cascading is all about, I guess my question was trying to get at why the 0.1uF cap was deemed to be the "one-size-fits-all" value for bypass cap regardless of the value of the main cap. And even though I don't fully understand exactly what perceived or measured benefits are purported to be gained with this process, I may still conduct my own limited experiments since I have some very good small cap values on hand.

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  • 11 months later...

I'm resuscitating a pair of Research Ten SCX2 speakers, and would like to ask where you sourced the "5 ohm 25W Rheostats" you mentioned at the start of this thread?

Add: same fellow that sold me the EMITs i replaced on these has 4 pristine Pollaks coming my way soon - yay!

Edited by Resyn Thesis
changed info
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  • 2 months later...

Carl, I just came across this discussion and  it looks like you're the crossover guy.  This is all out of my wheelhouse so any help would be appreciated.  I was given a pair of Research Ten series SCX3's and seem to be having problems with the crossovers. The drivers have been re-foamed, tweeters and mids sound good (as best I can tell) but are cracking in and out as you rotate the adjustment knobs . I guess this because of the bad resistors. Is there a simple way to bypass these or is it more involved?  

IMG952893.jpg

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