KR6160 adjustments

ducati2

ducati2

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I am still a rookie, so bear with me . . I recently acquired 10 Kenwood KR6160 receivers in various state of condition. I have built something of a list of caps and have been rebuilding them, but have not yet truly adjusted the main power, the dc offset or the bias.

I have rebuilt 6-7 marantz 2270s and have done these adjustments with great success, but can not seem to find out where to adjust for this kr6160 unit. I have searched for several weeks online and am now hoping someone here will have an answer. A photo of which pot to turn for what would be great.

I found one set of kenwood instructions that refers to 'common',,, do they mean the main power or dc offset or what? again I am a rookie that is hooked on playing with these old units and need all the guideance I can get. thanks
 
In the KR-6160 there are two trim pots on the power amp board. They are symmetrical clipping adjustment pots, VRe1 for the left channel, and VRe2 for the right channel. If you don't have a scope you can adjust for 50% of the positive supply voltage (around 86V) at the + side of the output capacitor (around 43V).

There is no bias adjustment. The bias current can be anything between 5 and 100mA according to the schematic.
 
This seems way different than the marantz procedure. No bias adjustment I understand. But power is set w one pot on the marantz, so on the kenwood I set both sides at half? Ok w that I guess.

Now, what about DC offset? Reason I am tackling this is I am trying to clear up one channel w distortion and clipping after a rebuild. I measured mv at the speaker terminals w speaker switch set to off and get about .01mv. Then I tried it w switch set to spkr set A and the meter was all over the place.
 
Ok I adjusted vr1 and vr2 to 43v per side. It was off a bit for sure. But still have major distortion on one channel. It sounds just like it does when u mistakenly have some speaker wires shorting out. not as loud, fades in and out and some crackling.

I still have not adjusted the DC offset as I don't know how. Any thoughts on whether this could be my problem?
 
This receiver is cap coupled at the output. No offset adjustment. It is normal that you have a large DC voltage at the output when no speaker is connected but the speaker switch is set to ON.

For the distorted sound my first impression is that you need to replace some capacitors. There are pre-out and main-in jacks at the back. Try to feed the R ch amplifier with the L. ch preamp, and feed the L. ch amplifier with the R. ch preamp. If the distortion comes from the same side as before, the problem is in the power amp section. If the distortion switches to the other side, the problem comes from the preamp section.

Do you have the schematic at least?
 
ecluser said:
Do you have the schematic at least?
Click to expand...

Oh yes, I have the schematic and have replaced pretty much all caps except the am fm boards. I must have a bad solder or connection somehow. I am more of a machinist,, I am used to very different schematics, but learning.

Thanks for advice, I will check those boards today.
 
I don't know if you replaced the output capacitors but the noise might come from these capacitors. When output capacitors are within the feedback loop, like they are in this amp, they can produce a lot of noise if they are bad.
 
If you mean the two largest matching caps, yes I replaced them. I have been checking my work to see if I did it correctly. All seems well. Wish I had checked the new parts before install w my cap checker. I probably put too much faith in the manufacturer.
 
Ok found a cap I installed backwards on the main 384j board so I reversed it. Same symptoms remain, but now wondering what reversing that 33uf 16v cap at the Ce5 position might screw up? I am thinking maybe a transistor, but where the heck would I start? Wish I had a transistor checker....
 
ducati2 said:
Ok found a cap I installed backwards on the main 384j board so I reversed it. Same symptoms remain, but now wondering what reversing that 33uf 16v cap at the Ce5 position might screw up? I am thinking maybe a transistor, but where the heck would I start? Wish I had a transistor checker....
Click to expand...

The schematic is wrong for Ce5. The + side of Ce5 should be on the emitter of Qe3.

Did you try to switch the channels at the pre-out/main-in jacks? That's important the you do it because we don't know for sure if the problem is in the power amp stage or in the preamp stage.
 
I have not tried to switch the main and pre amp jacks since I am unclear how to do so. I assume u mean do something w RCA cables on the back? Attached is a pic of the back and it also has a switch from normal to separate.... Do I mess w that?

Also, need to tell you I was probing around checking some voltages and accidentally touched PIN number 2 on the main board and POP! Lost all quality on the 'good' channel. So now I have a real mess on my hands.
 

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ducati2 said:
I have not tried to switch the main and pre amp jacks since I am unclear how to do so. I assume u mean do something w RCA cables on the back? Attached is a pic of the back and it also has a switch from normal to separate.... Do I mess w that?
Click to expand...

Yes, with RCA cables. Put this switch to "separate" and connect the left output from the preamp to the right input of he amplifier.

Sorry for the accident on the reference channel.
 
Update- found a transistor not connected well, re soldered and now have reference channel back. I did the preamp to the main swap and the distorted channel stayed on same side. So now to the preamp section to see what I can find... IF I can first figure out how the heck to get this board out. I have always wondered what I was going to do when I needed to remove a board that is wire wound at the pins.. do I just unwind each, label and maybe solder them back when I am done?

also, what is the trim pot for on this board?
 

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ducati2 said:
I did the preamp to the main swap and the distorted channel stayed on same side. So now to the preamp section to see what I can find
Click to expand...

No, no... If the problem stayed on the same side, the prblem is within the power amplifier section.

Can you try with headphones alone (no speaker connected)?
 
yes, the power section. I meant to say power. Anyway, I have checked all connections, even removed and tested many components , then even swapped the power boards from another unit and still get same results.

however, listening to headphones with speakers disconnected and speaker switch to off, it sounds great. what gives?
 
ducati2 said:
however, listening to headphones with speakers disconnected and speaker switch to off, it sounds great. what gives?
Click to expand...

OK, the problem is in the output stage. With headphones alone you have good sound because the driver transistor are able to drive the headphones.

Check the power transistor (measure their hFE) and the large 0.47 Ohm resistors. At least one of them is bad. This stage is needed to drive the speaker.
 
are you talking about the large top hat shaped output transistors ? I have a transistor checker that measures the HFE, but what is normal, healthy reading?
 
Yes, these ones. Should be marked as 2SC1115

Typical hFEs will be of the order of 50, but it may vary from one specimen to another and it depends also about your transistor checker.

If the 0.47 Ohm resistors are good, you can measure the DC voltage across the resistors. That will tell you how much bias current flows in these resistors. If one transistor is bad (open) you will have no bias current in the resistor connected to the emitter of that transistor. It is another way to tell you if the transistors are good, without removing the transistors.
 
Ok tested all output transistors and the all hover around 56-58 Hfe . They all all newly installed Mexican units and they measure fine.

All 4 (2 on Q2 and 2 on Q3) .47 ohm 2w resistors measured at .8 ohms..... Not sure what to think about that. So, I ordered 4 new resistors, but that does not explain why one side is solid and clear, while other channel is lame and crackling.
 
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