On Monday, March 31, 2014 3:19:13 PM UTC-5, chuck wrote:
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
...
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:
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.
I replaced the bad components but it failed again. I think I missed a few 1K fusible resistors that are open. They connect to the gates of the power fets so I think the gate was floating on 2 fets when I fired it up.
I'll replace all the bad parts again but this time I'll bring it up slowly at my work since I have better equipment there.
I do have 2 questions:
1. On page 32/35 of the schematic there is a 2K pot in the near the middle of the page. I assuming this is used to adjust the bias of the power stage to some level. I have a function generator to help with this, where should I place my scope probes when adjusting this pot and what am I looking for?
http://elektrotanya.com/infinity_ove.../download.html
2. On the same page there is a 1K 2W (R68) resistor on one of the relay contacts, what is this for?
thanks,
-jason