From: jjkooner@gmail.com   
      
   On Sunday, March 30, 2014 1:37:06 PM UTC-5, Mark Zacharias wrote:   
   > "chuck"  wrote in message    
   >    
   > news:ag1bj9ld72u9vvplr531cq967mhh7hor9c@4ax.com...   
   >    
   > > On Thu, 27 Mar 2014 21:23:14 -0700 (PDT), Jason Kooner   
   >    
   > >  wrote:   
   >    
   > >   
   >    
   > >>Hello,   
   >    
   > >>   
   >    
   > >>I have a pair of infinity overture 2 speakers. They have an internal amp    
   >    
   > >>to power the woofer section. However, I don't here anything from the    
   >    
   > >>woofer. There is a midrange and tweeter that are not powered and they work    
   >    
   > >>fine.   
   >    
   > >>   
   >    
   > >>I unplugged one of the speakers and opened it up. There is some black dust    
   >    
   > >>on the pcb near the power fets. There are 4 fets and they are all shorted.    
   >    
   > >>The fets are Fairchild IRFS250B. I couldn't find these (too old) so I    
   >    
   > >>replaced them with ST Micro STW40NF20 fets. Also the power fets are    
   >    
   > >>grouped in 2 pairs with one pair having an NPN transistor between them,    
   >    
   > >>why doesn't the other power mosfet pair have this NPN transistor? The NPN    
   >    
   > >>is a Fairchild 2012, it measures ok with a meter but I went ahead and    
   >    
   > >>replaced it anyway.   
   >    
   > >>   
   >    
   > >>I plugged in the speaker and tested it. The woofer is alive now but sounds    
   >    
   > >>terrible, it's all distorted. The midrange and tweeter are fine.   
   >    
   > >>   
   >    
   > >>I plan to bring a scope and thermal camera home from work for the weekend    
   >    
   > >>to take some more measurements.   
   >    
   > >>   
   >    
   > >>Questions:   
   >    
   > >>   
   >    
   > >>1. Was that wrong to sub in the ST Micro fets? I did a datasheet    
   >    
   > >>comparsion and didn't see anything special about the Fairchild fet.   
   >    
   > >>   
   >    
   > >>2. Does anyone have a schematic for this pcb?   
   >    
   > >>   
   >    
   > >>3. What other parts should I look at?   
   >    
   > >>   
   >    
   > >>I think I was too quick to clean the black dust and should off spent some    
   >    
   > >>more time trying to figure out the source!   
   >    
   > >>   
   >    
   > >>Cheers!   
   >    
   > >>   
   >    
   > >>-jason   
   >    
   > >   
   >    
   > >   
   >    
   > > Check for open resistors and open drive transistors.   
   >    
   >    
   >    
   >    
   >    
   > UGH. Had to work on one of these once. Reminded me of a car amp.   
   >    
   > Don't forget to use an isolation transformer.   
   >    
   >    
   >    
   > The manual can be downloaded from Elektrotanya or you can email me at:   
   >    
   >    
   >    
   > mzacharias@labolgcbs.net   
   >    
   >    
   >    
   > and reverse "labolgcbs" to read "sbcglobal".   
   >    
   >    
   >    
   > They do get a number of open resistors. I my case, a predriver looked good    
   >    
   > but wasn't. Caused additional heartache. Got it working, but it wasn't worth    
   >    
   > the trouble for me. If business hadn't been slow I would have just turned it    
   >    
   > away.   
   >    
   >    
   >    
   > Mark Z.   
      
      
      
      
   I have the manual now ...thanks. Also I did find some 0.22ohm half watt   
   resistors that were open. There is also one BF469 NPN transistor that is bad   
   but I can't find it online. I tried searching for recommended alternate but no   
   one carries those either...   
   too old.    
      
   Can I just lookup the Hfe, voltage and ft on the datasheet and search on   
   mouser od digikey for the same specs? Or does anyone know of a BF496 sub that   
   I can get in 2014?   
      
   thanks,   
      
   -jason   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)