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This is my first and only post on a very old and kinda sad topic, but as I looked for my solution I realized that there might still be some interest in solutions as well as my perspective (I'm a retired electrical engineer). To say that the KSPs are not a part of Klipsch's "greatest moments" might be an understatement. This series has been fraught with issues revolving around the amp for the 15" subs since their introduction. Suspect many owners have either given-up, swapped in another amp into the KSP's carcass, or parted ways with them! Some, like me, may have persevered and actually solved their problems. I have gone through several iterations of a re-build on mine over the years and had to do so without the benefit of a schematic; which I now have thanks to one being posted online by a kind member of this forum as well as writing to Klipsch directly and obtaining a copy. It would have been invaluable on my earlier rebuilds which involved replacing OP amps as well as the power transistors. Hum was only a nagging, intermittent issue partly solved by the deletion of the ground lug on the power cords. This is an extremely POOR solution to a ground loop problem that Klipsch should have seen coming. Granted, when new, my KSPs did not hum with the ground intact, but that issue really reared its head as they got older. OK, so what did I find that might help someone else? First, you do not need to be an electrical engineer or electronics tech to bring these speakers back to life w/o a hum! But, you better be damned good with a 40 watt solder pencil! The great news is that all the parts you need can be had on Ebay... probably cost $40 total for both speakers and I have extras of many parts. You can "cherry pick" which parts you try, but I suggest all of these or you may not solve the issue or may have to go in again just to save a few bucks. The amp design is marginal at best. These components are really under-sized as spec'd by Klipsch and I will get into that. Also, be VERY, VERY mindful that the tracings on the circuit board are meant for initial assembly by Klipsch and are easily torn from the circuit board. You may remove a capacitor (for example) and install a new one and flow solder that appears good only to power-up and nada! Visually verify that the trace copper on the board is still there and verify each connection with a multimeter. I used the extra lead length of the new components to "bridge" to the nearest circuit path. This is especially true of power transistors and regulators. Ok...the meat: Replace C4 & C15 6800 uF electrolytic capacitors with bigger (if possible) and 100V rated. I found 10,000uF @100v in 35 x 70mm package- audio quality 4pcs for $29 on Ebay. 80V rated caps have barely 12% "headroom". No wonder they puke their guts in short order. Ditto for C3 & C5 rated at only 25V and C9 & C10 rated at just 16V. I replaced with 35 V for the few pennies they cost over their predecessors. While I was at it, I replaced 7812 & 7912 with L7812CV and L7912CV rated @ 1.5 amps ea. I believe original plain 7812 & 7912s are rated at just 1 amp. Replaced D1 with a 50V bridge rated at 1.5 amp vs. 1 amp incumbent. Just a side note.....My 55" Samsung TV as well as tens of thousands of other Samsung models had a chronic failure where they would not turn on every time after often less than a years use! They used measly 16V electrolytics which failed almost their entire model sequence of big screen TVs when the solution would have been to install even 25V caps. Space was NOT an issue. They saved probably a dime per TV set sold. The denied most repair claims and just google Samsung TV won't turn on to get a scope on the size of the issue. Sure, a 16V cap in a 12V environment *should* work......YMMV. Emphasis on should. One power surge and these marginal components give-up the ghost in everything electronic. All to save $.05 times a 100,000 units sold and failing.... Kinda puts the price on brand reputation IMO. I digress. The parts are so easy to find and cheap the point is not "why" should I replace them, but why not? I did both speakers for around $40 total in parts and I will not have to worry anymore unless output transistors go again...But, at 65 years of age, I rarely risk my ear drums any more 😃 Also cured my "auto" light being on with no drive from my receiver. I cannot reiterate enough that after un-soldering and re-soldering components on these boards that you absolutely verify that the original circuit board foil remains intact. If there is any doubt, use new component legs as "hardwire" to make sure connections are maintained. Just because your solder bead may look shiny and bright, does not mean there is any foil left under the solder bead to make connection "go anywhere". Really hope this helps someone. I tried to attach the schematic pdf here, but keep getting upload error. Maybe I lack privileges. File is only 780kb?? May try in a sep. post. KSP-400 Service Manual PDF.pdf