Kennwood KA-3020 distorted

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Hi,
I'm currently having a problem with my Kennwood KA-3020 Amplifier. The sound is very silent even if I turn it up and it's also distortet unless I don't turn it very silent. The problem occurs on left and right channel but one channel is even more silent than the other.
Since the amplifier is working I don't really have a clue where to start. I guess I would need an oscilloscope, right?
But maybe somebody already has some advice for me?
Thanks!

This is the schematic, allthough I have the non-SE version:
http://www.vintageshifi.com/reperto...php?pdf=Kenwood-KA-3020-SE-Service-Manual.pdf
 
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some pictures from the inside
 

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Could it be possible that the problem is in the supply voltage circuit? My idea is that the voltage is lower than it should be and because of that the signal is already clipping while still being on a low level.
Would this make sense? And what could cause this? Maybe a faulty capacitor?
 
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In any event, you will need some form of test instruments. Start with the basics and a reasonable quality digital multimeter. Set it to a low DC range or let it DC autorange if it's that type and test for any DC voltage between the output terminals of each channel (those you usually have speakers connected to).

First though, turn the volume down and remove any input signal for this test. As you otherwise have some sound and the protection system still enables speaker connection to the amp, probably nothing disastrous has occurred. You can verify this by the relay click, a second or so after power on and the fact you hear at least something.

If the DC measurement is very low, like much less than 100mV, the next thing to consider is the output relay itself. These are prone to pitting and grime accumulation on their contacts and this breaks the connection over time but I suspect you won't be able to see inside or remove the cover to check.

With some program playing again, tap the top of the relay case with an insulated object like a hard plastic screwdriver handle and see if the audio is affected. It may even be that simple but that would probably still require a replacement relay to effect a good repair. See how you go and keep safety in mind when poking around with live circuits and mains voltage :eek:
 
Hello Ian,
Thank you very much for your Post. Of course I have a digital multimeter, the thing is just, that I am not quite sure what I am looking for. But I will check the relays when I get home today. I actually read of similar problems caused by relays. Otherwise I would be able to get an osciloscope in the next days.
I will report after I checked...
 
Okay, I just measured the idle voltage as on page 3 of the manual. I measured 20 mV and then adjusted it to 16mV (value should be 10 mV). The trimmer pots didn't let me choose a lower value. I guess this is pretty much what you told me to do as well, right?
I also tried tapping on the relays while music was playing but I couldn't hear any effect. But what's very strange is that the output through the headphone sounds as it should. When listening through headphones only the sound is normal, but when I press the button for speakers A (or B) the sound is distorted again also on the headphones. And I'm also not sure if it's supposed to play sound via headphones AND speakers at once. Other devices often mute speakers when headphones are connected. In the schematic I found relay K3 that obviously switches the headphones but I don't know when or how this is supposed to switch. Could my problem be related to this relay?
 
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The headphones appear to be controlled via their own relay.

If the sound via headphones distorts only when speakers are enabled then I suspect you are looking for a fault elsewhere.

You can easily eliminate any doubt on the relay by simply dabbing the + speaker lead of 'speaker A left channel' onto the junction of C69, R101 and L3. If the sound is still distorted then its not relay related.

Odd that both channels are the same although in a way that could be good as it doesn't directly implicate the two monolithic driver chips at this stage.
 
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I think there are more than a few differences between your amp and the SE version which the schematic refers to. The output protection relay with its speaker/headphone switching relay arrangement could well be quite different too. I can see 2 relays on the SE PCB layout but only 1 in the pic. of the inside of your amp.

I suggest that you don't take the bias current setting or the arrangement of the output protection circuitry, switches and relays as being the same or equivalent in the two models. In the details, they may be quite different. Probably then, the 20 mV bias setting was correct - not that it's of much concern until the real problem has been dealt with.
 
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Yes, you're probably right. It seems as if I only have one relay for both speakers + a headphone relay probably (didn't see it yet because its on another pcb and probably on the backside). Unfortunately this was the only Schematic I could find. I will try what Mooly suggested today or tomorrow.
 
Hello again, I still didn't find the fault but I took some measurements. This is what the Signal looks like:


I observed that IC2 (right channel, upc1298V) was way hotter than IC1 (left channel). After that I ordered a new one and just replaced it. Now the Gain is higher but the right channel is still distorted. The IC is also still getting quite hot while the left channel stays cool.




After that I measured all the Pins on both ICs (Speakers connected and on, volume at middle, but no sound connected):

Left Right
1 44,3V 44,3V
2 44,3V 44,3V
3 38,1V 38V
4 0,09V 0,03V
5 0,09V 0,03V
6 -42V -41,5V
7 -2,1V -1,3V
8 -0,03V -0,7
9 -44,2V -43,7V
10 -44,2V -43,9V
11 -0,4V -0,6V
12 0,02V 0,6V
13 0V 0V
14 0V 0V

Pins 7, 8 and 12 differ, the other ones are pretty much the same.
I also noticed something else: Last time I soldered a bridge over the pins of the speaker relay to make sure it's not the relay. This bridge is still there. Now when I switch the Amp off while music is playing I can still hear the music for about 2 seconds on the left channel while the right channel is instantly gone. Could this be a faulty cap?
 
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Lets start again from the beginning because tbh I couldn't really understand your first post description of the fault :)

What does this mean ?

The sound is very silent even if I turn it up and it's also distortet unless I don't turn it very silent.

Have both channels got a problem, or is it just one channel ?

Can you measure (with no speakers attached) the voltage between TP 7 and 8 and TP 9 and 10 which will give a clue as to what current is flowing in the output stage.
 
Okay, I try to explain again (sorry for my bad english!). When I started the volume level of the amp was too low (compared to the volume level that was adjustet) and one channel had even lower volume than the other. If I remember this right both channels hat this problem although the effect on one channel was stronger than on the other. When I turned the volume above ~40% the sound started to become very distorted.

Now after I replaced the upc1298V (right channel), the volume on the right channel is higher again but Im still having the distortion problem and I can still feel that the IC (the new one) is getting very hot while the left one stays cool.

What do you mean with TP? The Pins of the UPC1298V?
 
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Those readings are fine (low DC offset) but what we really want is the voltage between the test points so that we can calculate and derive the bias current. That might just give a clue as to whether there is a problem around that area of the circuit.

The TP's are across the pair of 0.22 ohm emitter resistors of the output transistors and so a reading in the region of 20mv to 45mv would be in order.
 
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