Klipsch Promedia 2.1 HF amp failure

[cadamwil]

A while back I noticed my Klipsch ProMedia 2.1 speakers had a failure.  One channel didn't work.  The sub and other channel seemed unaffected.  I swapped speakers to see if that resolved anything and checked connections.  None of this resolved it.  So I opened up the enclosure and the damage was obvious.  The familiar smell of expensive and elusive smoke.  It seems one of the HF amp boards had blown components.  A resistor of unknown resistance and the left MOSFET were blown in half.  After doing some research, I determined the value of the resistor and that the MOSFET was a TIP41C.  I ordered electrolytic capacitors for all on the board, the TIP41C, TIP42C and a capacitor on the underside of the board that looked a little crusty.  My suspicion for the cause of the failure was solastic (sp?) going conductive.  This HF amp board had a good coating that had run off from another component, but the other unaffected HF board had none.  I scraped all of the run off solastic off, replaced the resistor, electrolytic capacitors and pair of TIP41/42C chips.  I reassembled it all, powered it on and in a flash and poof, the smoke came out of this set of components, frying the TIP42C and the resistor.  This failure is similar, but of the opposite chip, as to what I found initially.  So, as I have run out of my depth in electronic repair, I thought I would beg help here on what could be the failure point.

The schematic linked is the one I used to guide my repair, and looks correct so far.  I am assuming the resistor frys just a bit too late to save the chips, but any ideas on what's gone wrong in the circuit?  My initial thought was something got grounded that shouldn't have and cooked a component, but that doesn't appear to be the case, or at least not in the method I thought.  I have replacement parts, but, I don't was to blindly try again.  R17 has blown both times, Q9 was initially blown in half, and Q10 did it this time.


http://christopherbradshaw.net/The_Project_Bin/Klipsch%20ProMedia%202.1%20HF%20Amplifier%20Schematic.gif

Adam

[SpecialK]

I don't know.  I would guess that if you keep blowing the 220ohm R17 between the bases of the two output transistors that might suggest something wrong in the preceding stage.  I'd check out Q7 and Q8.  I suspect a collector-emitter short in one or both.

[cadamwil]

I thought they might be bad as well, but since I couldn't find direct replacments for them, I didn't swap them.  I bought the following transistors, does anyone have opinions if they will work, and if not, suggestions on different (available) parts. 

Q407 = 2SC2235 purchased a KSC2383
Q408 = 2SA965 purchased a KSA916

Thanks for your help.

Adam

[SpecialK]

The problem I see is that KSC2383 and KSA916 are not listed as complementary to each other.  The 2SC2235 is the complement to the 2SA965. 

Can you check the existing transistors with a multimeter?

[cadamwil]

I'll do that when I get home.  I couldn't find any complimentary pair of transistors with similar specs, but I don't know what I'm doing either, so...

Adam

[CaptDon]

The TIP41 series are not MOSFETS. With Complementary Symmetry amplifiers
you need to verify all transistors after a blowup. They are all direct coupled
and if you just replace the 'smoked' parts it will smoke again as you have
proven. Go through it carefully. I haven't looked at your schematic, but sometimes
the bias diodes can be two or even three diodes in series in one diode package.
They will read strangely on an ohm meter. They are very common in the
older Carver PM-2.0T and similar amps and I have two decades of service experience
on those units from Clair/ShowCo.