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Kenwood KC-208 and KM-208 please help

t0nito

May 8, 2016
7
Joined
May 8, 2016
Messages
7
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.
 

t0nito

May 8, 2016
7
Joined
May 8, 2016
Messages
7
I ended up getting help from another forum and from Reddit, turns out that Q5 that is leaking -7V from and dropping another -7V from the -14V that comes from D38, in case someone needs to know in the future.
 
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