Kenwood KA-6000 oscillating

loplott

New Member
Hello folks,
I have a problem with the Kenwood KA-6000. Instead of describing it, I think it's better to just show you what I mean: https://www.youtube.com/watch?v=eb-FRdBor_Q
The bang at the beginning is ok, but the bumm, bumm, bumm not.
Now the story is:
1. When I remove the Pre Out/Main In Jumper both main amp and pre amp work well independantly.
2. When I hook up my Equalizer between Pre Out and Main In the amp works fine (the EQ is set to pass through so the signal isn't really effected).
3. When I push one of the Low Filter, the speaker movement fades away.
4. I replaced C304 and C307 (2x 470/25), because they were damaged.
First the problem was solved but after a week the bumm, bumm, came back.
I tested C302, C303 and C306 and they were weak so I replaced them, but it didn't make any difference.

Nevertheless the Equalizer thing works, so I use the amp for about 2 years now.

One week ago I replaced the 2SC458 with 2SC2240. I read they cause noise and after the replacement the amp was quiet (at least that was successful:D)
BUT the old problem was back (and even increased): now neither the Low Filter nor the EQ set on signal pass can prevent the speakers from oscillating.
I have to set the EQ on (all faders in mid position), then it works.

So the main point is: Main amp and pre amp work, but not together.


Of course I sent the device to a local hifi workshop, before I've done all this, but they said the amp is ok... well, funny thing is that the shop doesn't exist anymore...
Now I hope someone has an idea what might be the problem.
 
That is a most interesting video. And those are most interesting symptoms.

1. Does the problem occur on both channels?

If the answer is yes, that would suggest a power supply related problem. The fact that you have replaced several power supply capacitors is good but, you may need to dig a little deeper. I would also preplace C711 on the H. P. 40HZ board and C615 on the NF TONE AMP board.

I would also check to see if there is any DC voltage on any of the PRE-OUT or MAIN IN jacks.
 
Last edited:
Thanks for the quick reply!

@bktheking: I replaced all 2SC458 except from the two on the head amp board (18 in total).

Yes, the problem is same on both channels.

I checked the voltage on the jacks:
Pre Out:
on startup nearly 400mV, after a few seconds it settles down to 1.5mV
Main In:
260mV ... 2.5mV

ok, I will check C711 and 615

I tested a little bit and it also doesn't matter if I just connect one speaker or both.
When I remove ONE of the jumpers it disappears. So the symptoms just occur when BOTH jumpers are connected and at least one speaker is connected (when I turn the selector to Phones, the headphones work fine).

very strange...
 
Do you know what caps were in there?

What caps did you replace them with?

I recently had a similar problem with ultra low impedance caps. I think it is effecting the circuit damping. I'm going to re-replace them with regular general purpose caps, the original type, and see if that helps.
 
I used Panasonic FC caps as substitutes. I used the same values like the original ones.


To return to the C711 and C615 caps:
The schematic diagram says they're both 47/25V.
But on my KA-6000 the C711 is a 100/25V.
 
I used Panasonic FC caps as substitutes. I used the same values like the original ones.


To return to the C711 and C615 caps:
The schematic diagram says they're both 47/25V.
But on my KA-6000 the C711 is a 100/25V.
The applied voltage on C615 should be 24V. The applied voltage on C711 should be 22V. A 25V rated capacitor leaves little safety margin. Depending on the line voltage, the applied voltage on either of those two capacitors may be greater than 25V. I would go the next higher voltage rating, if they would fit.

Do you have an oscilloscope? It would be handy to see if there an oscillation at either the PRE-OUT or the final output of the amplifier. Check resistors R173, 10 Ω, 2W and R273 10 Ω, 2W to see if either one is open.
 
I checked the resistors R173 and R273, but in the schematics I have, they are rated at 4.7 Ω. In the amp they have the value of 4.7 Ω and they are ok.

And I checked the voltages:
There are 33V on C711 and 35V on C615!
...well, I think tere is a fault.

Unfortunately I don't have an oscilloscope.
 
Is this amp set to run at 120V? There's no zener to set the voltage...only dropping resistors. Fairly lame setup.
 
I checked the resistors R173 and R273, but in the schematics I have, they are rated at 4.7 Ω. In the amp they have the value of 4.7 Ω and they are ok.

And I checked the voltages:
There are 33V on C711 and 35V on C615!
...well, I think tere is a fault.

Unfortunately I don't have an oscilloscope.
The higher voltage is consistent with my experience. All the DC voltages tend to run a bit high.

The 4.7 Ω resistors are correct. Kenwood made a few changes to the schematic over time. I was going by the schematic that I received with my KA-6000.
 
Is this amp set to run at 120V?

I'm from Germany, so I have a European model with the "AC 110-120V / AC 220-240V" switch set to 220-240V. The switch is protected, so you can't move it.

tcdriver said:
The higher voltage is consistent with my experience. All the DC voltages tend to run a bit high.

But I checked the resistors around the power supply and R302 820 Ω /4W is open. I think, the voltage used to be lower with the resistor.
The voltage at the test point is 84V which is even a bit lower than the voltage in the shematic.

tcdriver said:
The 4.7 Ω resistors are correct. Kenwood made a few changes to the schematic over time. I was going by the schematic that I received with my KA-6000.


Ok, that explains also why in my KA-6000 the C711 is a 100uF instead of 47uF.


Well, I will order substitutes for C711, C615 (with higher voltage rating) and R302 and replace them in the coming days.

Btw. thank you all for your help and patience with me and my problem:D
 
Hi, from my limited experience it looks like a "motorboating" issue. Kind of explains why the problem only occurs with the speaker and with both the preamp and the amp connected together.

As tcdriver suggested, it's probably caused by the changes to the power supply filtering and/or any ground connection changes that you may have done.

Cheers,
Sant
 
Hi,
yes I heard about this "motorboating" thing.

sant_ka said:
As tcdriver suggested, it's probably caused by the changes to the power supply filtering and/or any ground connection changes that you may have done.

But the problem was there before I've done any changes.

Anyway, I have to tell you that this is not the main problem anymore.
Yesterday one of the 2SC898 on the right channel got a short circuit with the heatsink. The mica insulator was broken and half of it was nearly falling off the transistor.
I checked the other insulators and they look realy frayed too. Next time this should be one thing to check first when getting an old amp, but I've never heard about scuffed mica insulators before.

Well, unfortunately Q6, 2SC898 and Q2, 2SC680 are damaged and to get substitution won't be easy.
 
I know this post is quite old, but I just want to give a quick update in case anyone comes across this thread.
I finally managed to get NOS 2SC898 and 2SC680. I replaced them and R302 as well.
As I mentioned above I already replaced a few caps, however I never removed the clamped ones.

A few years later now I checked all of the big clamped caps again (power supply and coupling caps).
Capacitance and leakage current are actually quite good.
So instead of replacing them I added parallel ones to increase capacitance within the power supply and slightly increase the capacitance of the coupling caps.

Final result: The amp works better than ever! No oscillation, no noise, the amp works stable on full load and the sound is just amazing!

In the end I don't know the exact cause of that weird oscillation. Perhaps the broken R302, causing higher voltages down the line? (All voltages now match the ones in the manual).
Anyways, it works now and I'm happy with that.
I cleaned it and together with the KT-7000 it just looks awesome!
 

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Having paralleled the PS caps are you concerned about inrush current on the power switch? If so, you might consider using the triac switch configuration suggested in another thread somewhere in this sub-forum(?).
Caveat - I posted a question about noise and one of the respondents asked if I had a triac switch. I am guessing they can inject some noise but with all the capacitance you now have that may be a moot point.

Have fun and enjoy.

Dave
 
Thanks for the replies!
I thought and read about inrush current, but I increased the main 4000uF cap only to 4940uF, so it probably won't make a huge difference.
And the other PS caps have resistors in series.
 
Final result: The amp works better than ever! No oscillation, no noise, the amp works stable on full load and the sound is just amazing!
Thanks for the follow up. I am glad to hear that you got it all sorted out. The KA-6000 looks mighty fine sitting on top of the KT-7000. :thumbsup:
 
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