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Kenwood KR-a3080

E

Eivind

New Member
Hi, and thank you for a great forum with lots of awesome discussions!

I picked up a Kenwood receiver from around 1998 for 25 dollars, including Kenwood speakers. Now, the problem is that there is distortion in it. But it seems to be just the first minute. At first there is no sound, and it gradually becomes more and more sound, but it is distorted. More distorted in the left channel, but some distortion in the right channel as well. When I connect a second pair of speakers, the distortion is a lot worse. Sounds like clipping, but it happens regardless of input or output volume.

It was really dusty inside, I have vacuumed (I know, maybe not the preferred method), and cleaned up the whole main board, both sides, with something called electronic cleaner. Didn't help. There was some nasty salt-like layer almost over the whole thing, I think it could be dust that has dissolved in the cleanup and then settled as a thin layer. I got rid of most of it with q-tips and a paintbrush. Or could it be a cap leak maybe? Although I haven't found any apparent caps that is bad, from the looks of it.

I am thinking it is the power supply caps that could be in need of replacement, especially since it goes away after a while.
Or could it be something else?
 
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I'd start with a basic health check of the power supply. Check/measure at main filter caps and the regulated supply
 
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I couldn't find anything that seemed wrong to me, with a multimeter and my basic electronic skills. Except maybe that one of the two main filter caps drops much quicker in voltage after I turn off the power.
Could that mean that it the cap is leaky/faulty?
And also, I found out that it the distortion is much worse with low impedance speakers (6ohm) than with 8ohm speakers, and when trying to connect two pairs of 8 ohms, the distortion is absolutely horrible. With only one pair of 8ohms, it sounds pretty decent after a couple of minutes of being powered.
 
Carefully measure dc voltage at wire jumper/link W94 expect +11.6Vdc.
Repeat for W92, expect -12.5Vdc. Black probe connected to chassis/Ground.
upload_2020-3-3_19-49-3.png
 
B-/Negative voltage rail will collapse quicker than the positive rail
since it also feeds the tuner and pre-amp, ie, more load. You would have
seen this measuring the negative terminal of the main filter cap.

upload_2020-3-3_20-12-53.png

Dirty switches are a common cause for channel drop outs/noise/distortion. However, based on your comments
that the amp sounds ok/good after a warm up would point to something else, maybe silicon... Suggest continue with
basic health check... carefully measure the bias/idle current as per ADJUSTMENTS, page 13 of manual

upload_2020-3-3_20-17-28.png

Wise to use mini grabbers or similiar to connect to the test points.
upload_2020-3-3_20-20-13.png
 
if the distortion seems in time with the music check the soldering on the main filter capacitors . also clean and check soldering on any fuse holders .
 
I did check voltage at the test points, it was very close to correct voltage. I did turn the knobs slightly to perfect the voltage, but no difference in audio.
When you say silicon, do you mean as in a transistor or a diode? Or as in silicon paste :D ?
Will check the other suggestions when I get the chance.
 
Eivind said:
When you say silicon, do you mean as in a transistor or a diode?
Click to expand...
yes, could be either

Measure the dc voltages at jumpers W92 and W94, this is the +/- supply to the opamps in the preamp.

Do you have a tester that can measure large capacitance, good idea to check that main cap.
 
W92 measures 10.60V and W94 measures -12.71V.

No I don't have anything else than a cheap multimeter, and an oscilloscope someone is borrowing, will have it back in a few weeks.

So, W92 is a little bit off... What could that mean?
 
Now, after about 10 minutes of power, W92 has risen to 10.69V. Don't know if it that means anything.
 
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Seems that you've got multiple issues, the low opamp supply does not
appear related to the distortion issue however you should investigate
to ensure all ok. Suggest measure DC voltage at D56 cathode (+11Vdc)
and then at Q52base/C211/R205 (+12.8Vdc) as per below,
upload_2020-3-5_8-3-55.png


Your oscilloscope will probably be needed to track this down. While
waiting, there is a mute and attenuate circuit which could be at the
threshold between ON/OFF so may cause distortion (long shot).

Immediately after power on, measure DC voltage at R122/R123 junction
(-2.2Vdc). Monitor voltage for 2 minutes. Repeat for R233/R236/R237
junction (+0.7Vdc?)
upload_2020-3-5_8-21-36.png
 
Hmm... D56 cathode is 10.68V, and Q52 base is 12.57V.

Maybe you are on to something with the mute and attenuate circuit. At R122/123 the voltage starts at 2.17V and rises steadily to 2.20V after 2 minutes, and stabilizes there. However, it is positive 2.20V, not negative.

I lost track of the components and had to run to work when looking for the that last junction, have to check when I get home.
 
I don't see the low power supply voltage or mute/attenuation circuit as the cause for the distortion.

Eivind said:
Hmm... D56 cathode is 10.68V, and Q52 base is 12.57V.
Click to expand...
D56 11V zener, 10.68 is within tolerances. Q52B 12.57 v service manual 12.8, again within tolerances. Expect 0.65V drop
at Q52be and another 0.65C drop at Q51be so expect about 11.2Vdc rail rather than the 10.69V you are seeing. It should
be investigated further but again don't see it as cause of distortion (after distortion clears, rails voltage is about the same
lower value)

I will have another look at the mute circuit later in the day (bit busy...).

Probably time to build an audio probe? Use an old meter probe however bare/soldered wire will work ok. series connected
0.1-1uf/400V+ film capacitor then connected to another (cheap) amp via AUX-RCA (ie, use old RCA cable). Cheap amp
has cheap/disposable speakers connected. Connect GND of RCA cable to test amp chassis. If you don't have a second amp
then you can connect directly to a cheap speaker. Then probe the audio path of test amp to work out where distortion is introduced.

Both my tektronix scopes need "recapping", I've used this toy a few times, better than an audio probe
https://www.ebay.com.au/itm/DSO138-...crylic-Case-DIY-Kit-SMD-Soldered/192004290393
 
The R233/236 junction is 2.20V, on the other side of R236 there is 0.7V.
Audio probing with a speaker sounds smart (and fun!), never thought of that.
Though I am not sure how much more time it is worth putting into this amp, I don't have too much spare time, and it's not the most expensive or important amp in the world. Would be satisfactory to find the error, but I'm getting quite tired and frustrated with it! I want to listen to music now... ;)

Thank you so much for your help, I appreciate it. I never thought anybody would put this much effort into it! I'm impressed with your knowledge.
If I feel like it, I will probe it with my oscilloscope when I get it, or make the audio probe. Ordering stuff from ebay takes weeks from where I live (Norway), so I won't bother for now :)
 
Hi Eivind i come from North Macedonia i have the seame reciver with seame fault and one more fault louseing memory if i power off mechanikal bouton.What are you do with you reciver
 
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