Kenwood KC-208 and KM-208 please help

T

t0nito

New Member
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 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.
 
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General advice for testing an amp:

  • Are there any fuses, in-line with the speaker outputs, that could be blown
  • Are all the outputs of the power supply producing voltage stated in service manual / schematic
  • Are all the outputs of the power supply reaching the boards they are supposed to go to
  • Are any electrolytic caps looking bloated or leaking any goo
  • Do any transistors or resistors show signs of over heating or flame
  • Do any of the areas near all the transistors and resistors show any signs of over heating or flame

You can always feed a low level input, like from an ipod or phone with volume turned way down, directly into the amp to see if the problem lies in the preamp

Just some general ideas to help get you started. Good luck!
 
Thanks for the reply. It's a preamp so there are no speaker outputs or fuses, visually there is nothing wrong with it, there are no bloated or leaked capacitors, smoked resistors, charred or cracked transistors, I tested the diodes on the the preamp output section and found no shorts. Both -15V and +15V rails are present.

I tried feeding low level signals on the amp but it's dead silent, my theory is that the preamp's faulty -7V output burnt the amp's input, someone tried swapping channels and burnt the other input too.
 
I've only fixed one km-208 and one kc-208 so i'm not an expert.

if you have -7V on one channel output of the pre, i'd look fixing that first. I can't really tell were the -7v is and why it is a problem.

these preamps are pretty simple. my guess an opamp failed. there is a kc-209 thread on the Kenwood sub forum i created that discusses opamps. I couldn't find any difference between the 208 and 209 and used the 209 service manual to fix the 208. I might even have the 208 on the shelf. I'll take a look at the schematic link above.

since you have a scope, you can measure the output of the pre to verify. you can back up to see which one is failing.
 
i looked at the schematic.

upload_2022-8-15_11-23-29.png

if you have -7V on the cathodes (band) of D55 and D54, check the output of IC11 (pin 7). It might be around -7V.

upload_2022-8-15_11-25-16.png
 
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once you get the pre working, we can look at the power amp.

As doug listed above, you can always use a 3.5mm stereo to RCA cable and test your power amp using your cell phone.

play something on your phone, turn down the volume to zero, plug in to amp and slowly raise volume until you see a signal on your scope. I start at the input and trace the signal until it stops. Does the protection relay engage? you should hear a click. if not, maybe the outputs are fried and need to be replaced.
 
rickl said:
i looked at the schematic.

View attachment 2639769

if you have -7V on the anodes of D55 and D54, check the output of IC11 (pin 7). It might be around -7V.

View attachment 2639770
Click to expand...
Thank you so much, for helping out, I do measure -7V on the cathodes of the diodes but I measure 0V on the IC pin 7, but on that capacitor C62 right after the pin 7 output I measure 0V on the positive side and -7V on the negative side. Comparing with the right channel I get 0V on the negative side and 40.4mV on the positive side of C63.

As for the amp I tried what you suggested with the phone and cable, it's absolutely silent, only the red lights are displayed at the front, there's no audio or VU activity so I guess the audio isn't even getting in, but I agree, I want to address the preamp before the amp.
 
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not a lot of components left between IC11 and Lout. Q5 might be generating the -7V. what is the voltage at D38? note this is common base to both Q5 and Q6. you could measure the bases of both. you know the emitter of Q5 is -7. i guess you could swap Q5 and Q6 to see if the problem moves to the other channel. I don't like doing this since it might fail and now both transistors are bad. your call.

EQ off? On/Defeat button? i've only used CD Direct which bypasses the EQ. does this help?
 
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rickl said:
not a lot of components left between IC11 and Lout. Q5 might be generating the -7V. what is the voltage at D38? note this is common base to both Q5 and Q6. you could measure the bases of both. you know the emitter of Q5 is -7. i guess you could swap Q5 and Q6 to see if the problem moves to the other channel. I don't like doing this since it might fail and now both transistors are bad. your call.

EQ off? On/Defeat button? i've only used CD Direct which bypasses the EQ. does this help?
Click to expand...
I'm getting the -7V on D38 and on the bases of Q5 and Q6 and 0V on the emitter of Q6 and -7V on the emitter of Q5, does this mean the Q5 is bad?
 
I believe I found the culprit, I swapped the transistors and now the problem is on the other channel, interestingly my tester says both transistors are npn transistors, but one had a value o Hfe=31 Ube=704 mV and Ic=6.0mA the other said Hfe=647 Ube=633mV and Ic=3.1mA, I don't know what those numbers mean but they're quite different.
Without the transistors I had -14V on the pads of both bases and 0V on the emitter pads. These are C2878s I tried replacing them with stc1740 and got -13V on the base and -1V on the emitters, so looks like I have to order a couple of C2878s.
 
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just an FYI but pulled the cover and measured a few voltages on my kc-208.

I'm getting -15v on D38 zener (band side) not -7v. you might have some other muting problems (or Q5 is affecting the voltage) :beatnik:.
 
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rickl said:
you probably know this but both are NPN on the schematic:
View attachment 2639870
Click to expand...
Yes what I meant was that the tester was testing both as transistors, and not some random thing like a diode or resistors when something is shorted. Without the transistors I was getting around -14 V on the diode too, it was with out a doubt the transistor pulling the voltage up to -7V for some reason. Hopefully the new transistors will fix it when they arrive. I tried other npn transistors but got -13V on the bases and -1V on the emitters. But at least both transistors were showing similar values to each other this time.
 
rickl said:
just an FYI but pulled the cover and measured a few voltages on my kc-208.

I'm getting -15v on D38 zener (band side) not -7v. you might have some other muting problems (or Q5 is affecting the voltage) :beatnik:.
Click to expand...
Also, thank you so much for taking the time for opening your unit to check for voltages in order to help me. When the transistors arrive and are installed I'll keep you posted if it's fixed or not. Thanks once again!
 
I would look at the output transistors in the power amp. If the preamp doesn't have a mute function or the mute function didn't keep the -7V off the output, likely the power amp got railed and smoked an output. Pretty unlikely a failed output would also get the preamp, but I suppose anything is possible.

Power amp should have a protection relay, If you don't hear it engage a few seconds after powering on, it needs some work. It will contain the differential pairs and bias control circuit, drivers, and output transistors. My experience with Kenwoods in protection is that usually there is a serious offset if the relay won't engage but it does power up without blowing a fuse. Blown fuse is almost always a sho
rted output or two.
 
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