Kenwood KA-400

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Hi gecko,
Those voltages look awfully low. It might be that there isn't enough time for them to come up. I'd be more interested in teh bias current measurements. I think that's where the problem is. Meantime, I'll try to look up the values of those zener diodes.

-Chris
 
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Hi gecko,
Okay, I would expect a little higher than 2.2V but there readings are in the ball park. I was thinking of another thread when I referred to them as zener diodes (they are varistors).

Carefully watch the voltage across one emitter resistor (white thing with three legs) and carefully turn it's bias control one way a bit, then the other. No more than 1/4 turn. If you get a reaction then adjust the reading to 5mV to start. Watch it to make sure it's stable, then do teh other side. Let it sit for 10 min and check them both again. You are okay if they remain stable for 1/2 hour. Later try 10 mV, then 20 mV and listen to it.

-Chris
 
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Hi gecko,
Odd. Something should have happened.

So let's check the voltages from the center of the white resistor to each emitter of the drivers. R43, R45 are 100 ohm resistors, we are checking on each side. You should see 0.55 VDC approximately. Also, adjusting the bias control should change whatever voltage you do read.

-Chris
 
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Hi gecko,
Okay, let's take a step back and look at this again.

What I want you to do is check all the junctions of your transistors. Use the diode test feature on your meter. Make sure you have the unit unplugged and discharge all the filter caps.

-Chris
 
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Hi gecko,
What I am concerned about is an open junction or shorted E-B connection.

You should measure a diode drop on your meter when testing base to emitter and base to collector. Example: an NPN type. Positive on the base, when the negative lead is touched to the collector and emitter you should read a diode drop. Try in circuit first.

-Chris
 
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Hi gecko,
Sorry, not good enough.

You need to confirm two diode drops as I described. If you don't get that answer, remove the part and test it out again. If it checks okay, write down the location number so you know you have checked it. If not, it is more than likely bad. You can have it tested again by a tech, or just replace it.

Sorry, I wish I was there. We could get through this rather quickly.

-Chris
 
Yeah, I'm clueless about this problem :xeye: I changed two output transistors from one channel for two that I know that are working fine. but again the same distortion comes out, I believe it should be from something that is common for both channels because I hear the same distortion in both channels. must be about the bias current because the bias pot don't change anything. but more than that I can't tell what is wrong with this :smash:
 
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Hi gecko,
Right, but that is not systematic. You may have other damage that hasn't been found yet.

Sometimes it's easier not to think, but just go through and recheck. No matter how positive you are you checked something. That's how I get unstuck. I just look at the problem like I've never seen it before.

Take a break and clear your head a bit (works for me). Then just look at it as if it just came in the door.

-Chris
 
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Hi gecko,
It's very important to ensure all the transistors are okay so far. You must check the transistors again. Most of the checking can be done while the parts are on the board.

If you need to take a part off, use a solder sucker to remove most of the solder (and heat) followed by some solder wick. You shouldn't damage the board this way. Remember to not force anything, let the heat work for you. Use a razor to lift bent over leads.

-Chris
 
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