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Old 7th November 2005, 04:57 PM   #1
gecko is offline gecko  Argentina
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Exclamation Kenwood KA-400

Hello! I got a Kenwood KA-400 solid state amplifier and I found some problems on it, I wonder if someone here knows about this problem and how to fix it I have the schematics for this amp which helped me a lot, I have one channel working now but with a strange sound comming out, it's like when you record something in the sound recorder of windows using a low frecuency, or when you unplug an amp and keep earing the sound but with lower quality until it dissapear.. :s I believe it could for any dried capacitor, but I'm not sure
And for the other channel, I can't use it because it causes the protective circuit (HA12002) to open the output, I measured the output and there's around 12 vdc coming out from that channel =/

any help with this?
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Old 7th November 2005, 07:13 PM   #2
anatech is offline anatech  Canada
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Hi gecko,
It sounds like your amp is motorboating. Open supply caps can cause this. Can you send the schematic so I can give you a hand?

-Chris
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Old 8th November 2005, 03:25 AM   #3
gecko is offline gecko  Argentina
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oh, I'll check that! thank you
and you can get the schematic from here:


KENWOOD_KA-100_Amplifier.zip
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Old 9th November 2005, 07:33 AM   #4
gecko is offline gecko  Argentina
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It is still not working, I tried with the main filter caps, but it still not works, actually it's not like a motorboating sound, it's more like a little distortion in the sound
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Old 9th November 2005, 05:03 PM   #5
gecko is offline gecko  Argentina
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Well, I found a faulty driver transistor that was causing the 12v on the output, so now I have both channels working but.. both with the same distortion I wonder if there's a easy way to bypass the bass and treble controls..
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Old 9th November 2005, 06:19 PM   #6
anatech is offline anatech  Canada
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Hi gecko,
The tone controls are part of the feedback network. They use the gain of the amplifier stage to function, so you are not going to find your problem there.

What makes you think the problem is with the tone controls? What transistor did you change? There is a protection circuit, so you should not have had any sound with a 12 VDC offset.

Have a look at C101, C102 and D18, D19. D18 and D19 are zener diodes to stabilize the supply for the tail currents for the diff pairs. C101 and C102 help decouple this supply for noise.

-Chris
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Old 9th November 2005, 08:14 PM   #7
gecko is offline gecko  Argentina
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Thanks Anatech, I'm checking those right now
I got sound in one channel because I opened the circuit just after the Pin of the HA12002 so it couldn't detect those 12v coming out from the faulty transistor , the transistor I changed was the driver 2sc1735 from the channel that was not working.
I was not really sure why it could be from the tone control, I just wanted to take that circuit aside just to make sure, but if you say that it couldn't be from there so I'll believe you
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Old 9th November 2005, 09:49 PM   #8
gecko is offline gecko  Argentina
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Well, I changed C101 and C102, D18 and D19 seems to be okay.
but the distortion is still there, I noticed this: I only can hear the distortion when I'm listening a song or something, and this distortion seems to be in the high frequency range.
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Old 9th November 2005, 11:24 PM   #9
anatech is offline anatech  Canada
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Hi gecko,
Have you measured the bias current? Also the voltage across those two diodes?

-Chris
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Old 10th November 2005, 01:28 PM   #10
gecko is offline gecko  Argentina
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I'm.. not sure how to measure the bias current
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