Hi, I do some electronic repairs as a hobby, but I'm no expert, a friend of mine asked me if I could take a look at his Kenwood Amp, as it's totally dead silent, I said sure and took it home with the pre amp.
I started to test things out and in fact the amp (KM-208) lights up but absolutely silent, the VU bars don't even show up, so I suspect it's the input that's bad and not the output. But before I came to that conclusion I took an oscilloscope to see if there was any audio output on the preamp (KC-208). One channel is indeed working but the other has a DC voltage of -7VDC.
I downloaded the schematic and started probing things. I have both the -15V and +15V supplies present. I've measured the diodes for shorts, all measured OK, there are 2 diodes on each channel D54, D55 and D56, D57, between D56 and D57 is one output which I measure 0V, and -15V and +15V on each side of each diode, between both D54 and D55 I measure -7V and also -15V and +15V on each side, I don't know where the -7V is coming from. The schematic is easily accessible if anyone would care to help.
I haven't been in the actual amp yet but I suspect this -7V probably damaged the amp's inputs.
I started to test things out and in fact the amp (KM-208) lights up but absolutely silent, the VU bars don't even show up, so I suspect it's the input that's bad and not the output. But before I came to that conclusion I took an oscilloscope to see if there was any audio output on the preamp (KC-208). One channel is indeed working but the other has a DC voltage of -7VDC.
I downloaded the schematic and started probing things. I have both the -15V and +15V supplies present. I've measured the diodes for shorts, all measured OK, there are 2 diodes on each channel D54, D55 and D56, D57, between D56 and D57 is one output which I measure 0V, and -15V and +15V on each side of each diode, between both D54 and D55 I measure -7V and also -15V and +15V on each side, I don't know where the -7V is coming from. The schematic is easily accessible if anyone would care to help.
I haven't been in the actual amp yet but I suspect this -7V probably damaged the amp's inputs.