Kenwood KA 6000

[Felix123]

Hello everyone!
I recently purchased a Kenwood KA 6000 amplifier and a Pioneer TX 6200 tuner.
I am amazed that an almost fifty year old amplifier sounds this good! It actually puts my 2006 Kenwood receiver to shame. The depth and detail of sound is incredible.
There's one problem though... The output transistors on the right channel get extremely warm even at low volume. The left side runs nice and cool even at high volume. Right now I'm running four 8 ohm KLH bookshelf speakers with the a+b selector.
My question is... Could this be just a bad transistor or a more serious problem? I plan to re-cap this in the near future. I was just wondering if you guys have had this same issue with this amplifier.
Thanks !

 

[chazix]

Welcome to AK!

The output transistors on the right channel get extremely warm even at low volume.

I can't remember if I've worked on any examples of this particular model, but in general, this could be caused by a severe mis-adjustment of the bias/idling current. The adjustment procedure is in the service manual, which is available on the splendid hifiengine site:

https://www.hifiengine.com/hfe_downloads/index.php?kenwood/kenwood_ka-6000_service_en.pdf

You need to register with hifiengine to download manuals, but it is free of hassle or cost (unless you choose to make a donation).

The adjustment procedure calls for the use of a variac and an ammeter, so it's rather fussy. You can probably do without the variac, but using a "dim bulb tester" rig would be a really good idea when making an initial adjustment. (If not familiar, you can learn about DBTs by searching AK.) It would also be a good idea to clean the adjustment pots (and maybe replace them, if you're going to be working on the amp boards anyway). If the initial adjustment with the DBT goes OK, set the current at somewhat less than the service manual says, and then make the final adjustment with the amp powered up normally.

Hope this helps,

chazix

 

[Felix123]

Welcome to AK!



I can't remember if I've worked on any examples of this particular model, but in general, this could be caused by a severe mis-adjustment of the bias/idling current. The adjustment procedure is in the service manual, which is available on the splendid hifiengine site:

https://www.hifiengine.com/hfe_downloads/index.php?kenwood/kenwood_ka-6000_service_en.pdf

You need to register with hifiengine to download manuals, but it is free of hassle or cost (unless you choose to make a donation).

The adjustment procedure calls for the use of a variac and an ammeter, so it's rather fussy. You can probably do without the variac, but using a "dim bulb tester" rig would be a really good idea when making an initial adjustment. (If not familiar, you can learn about DBTs by searching AK.) It would also be a good idea to clean the adjustment pots (and maybe replace them, if you're going to be working on the amp boards anyway). If the initial adjustment with the DBT goes OK, set the current at somewhat less than the service manual says, and then make the final adjustment with the amp powered up normally.

Hope this helps,

chazix

Thanks for the welcome and the reply!
I'm not too familiar with this procedure, only read about it on this site. I don't know alot about electronics but I'm sure I could get er dun!
I want to play with my new toy but I don't want to damage it . This was apparently made the same year I was born ...1969... So that's really cool . I'm going to download the manual on it , and replace most of the caps and transistors. Where's a good source to buy high quality parts? I really appreciate your help with this.
Have a great day !

 

[chazix]

Where's a good source to buy high quality parts?

Mouser or DigiKey are what I think most U.S. AKers would consider the Gold Standards.

I'm going to volunteer that not all AKers would recommend changing transistors (unless there's some basis for suspecting that they need changing). I wouldn't, myself.

 

[Felix123]

I know what you mean, "If it ain't broke, don't fix it"! I noticed someone in here saying that this amplifier is full of 2SC458 transistors and that they were troublesome. Maybe I can get by without too much work. Do you think I should even change the capacitors? I'm no audiophile but this thing sounds great as is.
There is a little bit of hiss at high volume with no input but it's not noticeable with music playing. Maybe because of the low gain pre-amp high gain amp setup it has?
I'm going to try what you said and see if it's just a bias issue . I'm good at changing components and soldering but don't really know enough to diagnose problems.
The tuner and amplifier are both in almost new condition . Almost no dust inside and not one of the pots are scratchy like most old equipment. The best part is... I only paid $25 each ! I'm still trying to figure out how to post pictures on here. Thanks again for your help!
Felix

 

[spark1]

If you are confident in your capabilities, there are about a dozen 2SC458s in that unit (tone amp/preamp/head amp/power amp). They are quite notorious for becoming noisy with age, and are a likely source of the hiss you are hearing. The preamp and tone amp boards are difficult to get at....all the point to point wiring can be quite challenging. I recently completed a major rebuild of a KA-7002, to which your amp is a predecessor.

Good luck with it. One other thought...if you are going to be doing the service adjustments in the service manual (eq, center voltage, idle current), you might want to think about replacing the trimmer pots first...especially the bias pots.

I would suspect a bias problem as the best guess as to why it's running hot on one channel. I think this amp puts the speakers in series when running two pairs, so, I would not think it is a load issue.

 

[Felix123]

Thank you spark1 ! Your absolutely right, this looks like it's going to be hard to get to anything in here ! This thing is crammed full of stuff ! It looks about as complicated as the space shuttle...ha...ha. I can't imagine how hard this was to manufacture. Almost everything was done by hand, not like todays stuff that's all made by robots. I'm gonna go through it and order the parts I'll need and upload the photos so other people with similar issues can repair them.
I appreciate you taking the time to respond!
Felix