From: mark_zacharias@labolgcbs.net   
      
   "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.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)