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Kennwood KA-3020 distorted
Kennwood KA-3020 distorted
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Old 16th November 2017, 04:37 PM   #1
SuperSKD is offline SuperSKD
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Default Kennwood KA-3020 distorted

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/repertoi...ice-Manual.pdf

Last edited by SuperSKD; 16th November 2017 at 04:44 PM.
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Old 16th November 2017, 05:04 PM   #2
SuperSKD is offline SuperSKD
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some pictures from the inside
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File Type: jpg 20171116_185810.jpg (608.4 KB, 272 views)
File Type: jpg 20171116_185823.jpg (547.8 KB, 268 views)
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Old 17th November 2017, 09:13 AM   #3
SuperSKD is offline SuperSKD
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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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Old 17th November 2017, 10:16 AM   #4
Ian Finch is offline Ian Finch  Australia
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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
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Old 17th November 2017, 11:02 AM   #5
SuperSKD is offline SuperSKD
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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...
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Old 18th November 2017, 10:59 AM   #6
SuperSKD is offline SuperSKD
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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?

Last edited by SuperSKD; 18th November 2017 at 11:12 AM.
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Old 18th November 2017, 11:48 AM   #7
Mooly is offline Mooly  United Kingdom
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Kennwood KA-3020 distorted
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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Old 18th November 2017, 06:24 PM   #8
synonymous is offline synonymous  United States
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vacuum that thing!
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Old 18th November 2017, 07:32 PM   #9
Ian Finch is offline Ian Finch  Australia
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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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Last edited by Ian Finch; 18th November 2017 at 07:54 PM.
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Old 20th November 2017, 08:38 AM   #10
SuperSKD is offline SuperSKD
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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.
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