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Kennwood KA-3020 distorted
Kennwood KA-3020 distorted
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Old 2nd January 2018, 01:37 PM   #21
SuperSKD is offline SuperSKD
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Voltage between TP 7 and 8 is 0.02 V DV or 0.005 V AC
between 9 and 10 it's 0.03 V DC or 0.004 V AC.
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Old 2nd January 2018, 01:52 PM   #22
Mooly is offline Mooly  United Kingdom
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Kennwood KA-3020 distorted
Those readings sound good. That at least shows the output stage is biased correctly.

As you have a scope we can do more detailed tests. No speakers attached initially.

You should apply the test signal (1kHz sine for example) to one of the inputs, and then look at the output voltage on R101 and R102. Adjust the volume to give around 4 volts peak level corresponding to 1 watt into 8 ohms.

Do this for both channels and confirm the signal is distorted or not.

Also you can check that both channels are receiving the same input signal and that this signal is pure and undistorted. Measure on R71 and R72 for this.

If the output is good for channels then you now need to test with a load attached, preferably an 8 ohm resistor and see if the distortion only appears under load.

Before you do any of this, have you proved the speaker relay is OK ??? you can easily short it out when the amp is playing and see if the distortion stops.
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Old 3rd January 2018, 07:05 PM   #23
SuperSKD is offline SuperSKD
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Okay yesterday I realized that mechanical tapping on the amp had an effect on the crackling, sometimes there was crackling on the left channel and sometimes there wasn't. I tried to find the source of this but it seemed to happen allmost regardless of where I tapped. So I resoldered a lot of the connections and cleaned all of the pots.
After putting the amp back together there were changes in the behaviour:

- I measured the voltage between TP7 and TP8 again and this was now 0.000 VDC so I tried to readjust it with trimp pot VR1 but this didn't have any effect. I desoldered the VR1 and it's working as it should.

- When I have an input signal on the amp and play sound either over terminal A or B and turn up the volume I hear the relay click and the sound is gone for a few seconds, then the relay clicks and the sound is back and this behaviour repeats. It happens only when theres a speaker connected to the left channel. I measured the supply voltage of the relay and at low volume it's 24VDC. When I turn the volume up above about 50% the voltage drops to zero leading the relay to switch back. Now I'm not quite sure what to look for? Is this some over current protection (maybe related to IC13 (µPC1237HA) or something different?

By the way after I put the amp back together I couldn't see any effect of tapping anymore.
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Old 3rd January 2018, 07:13 PM   #24
Mooly is offline Mooly  United Kingdom
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Kennwood KA-3020 distorted
It sounds like you have a few things going on with this amp. Losing the bias voltage between the test points suggests that something has happened during your resoldering.

IC13 will disconnect the speakers if either a DC offset appears or an overcurrent situation (or both).

You need to check all the work you have done and make sure no solder blobs or anything like that have occurred.
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Old 3rd January 2018, 08:19 PM   #25
turk 182 is offline turk 182  Canada
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it doesn't hurt to look at all solder joints under a magnifying glass either (i check my own work this way) things like cracked solder joints will cause all sorts of problems and don't always correspond to physical stimulus and from the behavior your describing, the cycling of the output relay with increased volume means something is going open under current like filter cap ground's or somesuch.
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Old 3rd January 2018, 10:47 PM   #26
MAAC0 is offline MAAC0  Portugal
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You've replaced UPC1298V ? Those aren't manufactured for a long time. Are You sure they are genuine parts ? One day I had a Sony Amp for repairing and couldn't find those ICs, well I could find, but they they had an obnoxious price. So I replaced the board with two LM3886 boards and the costumer is happy.
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Old 3rd January 2018, 10:49 PM   #27
Max Headroom is offline Max Headroom  Australia
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Location: West Australia, near to the beach, natural ambient sounds mostly.
Quote:
Originally Posted by SuperSKD View Post
......- When I have an input signal on the amp and play sound either over terminal A or B and turn up the volume I hear the relay click and the sound is gone for a few seconds, then the relay clicks and the sound is back and this behaviour repeats. It happens only when theres a speaker connected to the left channel. I measured the supply voltage of the relay and at low volume it's 24VDC. When I turn the volume up above about 50% the voltage drops to zero leading the relay to switch back. Now I'm not quite sure what to look for? Is this some over current protection (maybe related to IC13 (µPC1237HA) or something different?

By the way after I put the amp back together I couldn't see any effect of tapping anymore.
http://www.tecnotre.it/audio/tecnica/upc1237.pdf
The cap on pin 4 is small physical diameter cap and prone to drying out and this will render the overvoltage protection overly sensitive.
I have seen this fault numerous times over 35 years.
Not to say this is your fault without reading the rest of the thread.

Dan.
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Old 13th February 2018, 06:40 AM   #28
SuperSKD is offline SuperSKD
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Okay, I'm back again
I soldered the power transistors out (Q21..26) and checked them with the multimeter --> several didn't test correct relating to the routine I found online to check transitors. I think I might have destroyed them myself by accidently creating a short circuit while measuring. So I ordered all 6. Yesterday I soldered them in again and it seems like it fixed the whole problem. No distorted sound anymore, actually it's sounding very nice!

There's one thing that's still bothering me: I tried to set the bias voltage again and I'm still measuring 0.00 V independent of the trim pot setting (at both channels). But since the amp sounds good to me and it's not even buzzing at standby I guess I will just stick to it.


Quote:
Originally Posted by Max Headroom View Post
http://www.tecnotre.it/audio/tecnica/upc1237.pdf
The cap on pin 4 is small physical diameter cap and prone to drying out and this will render the overvoltage protection overly sensitive.
I have seen this fault numerous times over 35 years.
Not to say this is your fault without reading the rest of the thread.
Dan.
Thanks for this answer, too. I will keep this mind. Are you talking about C51/C52 or about C53/C54?
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Old 13th February 2018, 07:54 AM   #29
Mooly is offline Mooly  United Kingdom
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Kennwood KA-3020 distorted
That's great news

(It is up to you if you want to investigate the lack of bias adjustment, just say if you do )
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Old 15th March 2018, 08:56 PM   #30
SuperSKD is offline SuperSKD
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Quote:
Originally Posted by Mooly View Post
That's great news

(It is up to you if you want to investigate the lack of bias adjustment, just say if you do )
Okay, after using the amp now for some time I realized that there's a little distortion on higher frequencies on the right channel. While listening to music it's hard to hear but when I play Zelda on the Switch it gets quite disturbing in the ambience sounds. Also the amp has a little hum, nothing major but I can hear it when the amp is idling at a higher volume setting.
So I would indeed like to investigate the problem. What would you suggest to start with?
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