kustom kps-pm100 hums

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I own a computer repair service and a friend of mine brought in a PA for me to check out. It's a Kustom kps-pm100 setup. It hums... All the time when turned on and either speaker is plugged in. It doesn't matter what you do with any of the settings, even master volume. The hum stays the same volume and frequency. The hum comes out of both speakers or either one if hooked up individually. My intuition says it's somewhere in the power circuit, but I'm not seeing any cracked or broken connections anywhere and all the grounds are tight. Has anyone else dealt with these PA systems and had this issue?

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Not familiar with the name (as in never heard of it :)) but the symptoms are classic of any failed power amplifier stage.

Yes, it could be a supply problem, but more likely its a failed output stage which could be 'discrete' meaning its made from individual transistors, or its a monolithic output IC (an integrated circuit). The latter is usually easily replaced.

You need to look and see what it uses (IC or discrete) and take it from there. If discrete then just do a quick check on the output transistors which will probably read dead short.
Looks like I get to do some disassembly, then. They didn't design this to be easy to work on. I will check the output transistors, though I think it's a single ganged transistor. STK404-100 ring a bell? That one I can reach the solder points on the underside of the board and nothing is shorted on it. Will pull the assembly and check other components as well as get some photos. Thanks for the reply.

Troubleshoot the problem to isolate it. First the type of hum. DC on the outputs will make a hum from the resulting severe ripple. It also is bad for your speakers, so leave them disconnected for now. No signal input, and all controls to zero, measure between the speaker wires for DC voltage. There should be very little, like under 250mv. Now check for AC on the outputs. Some amount would be there unless you already found DC. it would be there causing the speakers to reproduce it. If it is a lot of hum, a lot of AC, then you may have a lost filter cap.

Kustom is alive, you ought to contact them and ask for the schematic.

I usually do not see STK chips in pro audio power amps, but I will believe what you saw.

FInd the data sheet for the IC, and look at the power pins, or for that matter find the main power supply filter caps and see if both polarity supplies have similar voltages and relatively free of ripple.

Not only verify the main voltage rails, which I will assume are in the 30-40 volt range maybe, but also the low voltage rails for the op amps, which will usually be 15v, once in a while 12v.

There is an "insert" jack on the front, is there not? That is a send and return in one jack (a TRS or "stereo" jack). I can't tell you if the tip or ring contact is send or return. But it breaks the signal path from the preamp (the mixer portion) and the power amp. Plug something in and out that jack and see if it interrupts the output hum. A plug either all the way in or stopped at the halfway in point - the "first click" - will put a signal into the power amp, bypassing the preamp. That way you may test the power amp by itself.

OK I looked it up myself, the insert jack is lower left corner, and the tip contact is the power amp input, so push a plug all the way in to apply signal to the powr amp directly

Here is the owner manual, which includes a block diagram of the system.
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