KLH subwoofer repair

[AzGuy]

Hi all. I need some help to repair a KLH subwoofer mod: ASW80-80. It has a constant hum with nothing connected. The only connections that could be made are high-level inputs from the amplifier and out to speakers or R & L low-level inputs, via RCA jacks.
I would suspect a bad filter capacitor but without a schematic, I have no idea what I'm dealing with. Looking at the circuit boards/components, I see no indication of burnt components, hot spots on the board, or stressed electrolytic capacitors. I'll continue to look for cracked solder joints and measure some components to see if there may be an open (unless I catch a parallel device), but otherwise, I'm shooting from the hip.
Does anyone have experience with this type of problem or even a schematic? I have electronic measuring equipment, but again, without a schematic, I don't know what signals or voltage levels I should be reading.
Thank you.

[AVGresponding]

Caps don't always show physical signs of failure. If there's one near a heatsink, it's an immediate suspect.

Dry joints are not uncommon, especially if heavy components like filter caps and heatsinks aren't properly secured. Reflowing solder joints is the cure for that, with the addition of proper mounting/goop of choice to prevent re-occurance.

You might want to have a look as the "12voltvids" channel on youtube; he does a lot of audio repairs, and may even have one of the model you're dealing with!

[andy3055]

Ground the inputs and see if it is still there. If it goes away, then you have a very sensitive input. You may have a bad ground  point or bad power supply caps. Check those large caps with a meter.

[james_s]

In my experience most of the time bad capacitors show now visible signs, in fact most bad components in general visually look fine. A capacitor has to get REALLY bad before it bulges or leaks and it takes a catastrophic failure for a semiconductor to crater. Usually parts just stop working due to internal faults.

[Stray Electron]

 Constant humming in audio is often caused by bad electrolytic caps in the DC power circuits. Often the larger value caps simply dry out and quit capacitoring (a technical term), aka filtering out the AC.  An easy way to test to is to use a similar value cap or equal or higher voltage and jumper it across any cap in question. If the hum stops then you've found the problem.  We used to keep several caps around the shop with a long pair of jumper leads with test probes attached to them and we could quickly put the probes on the cap under test and within seconds we'd know if it was the problem.

  If you really want to test the circuits, then test the various DC power test points with a DVM set on DC volts and then test again with it set on AC volts.  There should be very little AC on the DC test points.