From: chuck@deja.net   
      
   On Mon, 31 Mar 2014 10:19:55 -0700 (PDT), Jason Kooner   
    wrote:   
      
   >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   
      
      
   NTE108.  Mouser Electronics or MCM may have it in stock.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)