Kicking It with a Kenwood KA-2000! - diyAudio
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Old 18th May 2017, 01:21 AM   #1
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Default Kicking It with a Kenwood KA-2000!

Hi everyone, James is back with his next project.

I picked up a Kenwood KA-2000 at a garage sale for $3 last summer. I cleaned it out and it worked fine. Yesterday I noticed it had developed a "hiss" in the speakers even at rest. I did some research and found a schematic on HIFIengine.

I studied the schematic and compared it to the unit and there are differences. The schematic is for the Trio version of the amp (British market) and has circuits for one phono and two aux channels. My unit has two phono - phono 1 and phono 2.

And, there are differences in what is in my amp on the pre-amp board and amp board component-wise.

First question is does anyone have a service manual for the American version of the KA-2000? I have searched the internet and find only the Trio one.

My best guess right now is a transistor is failing and I originally thought it must be in the phono circuit because the hiss is louder when set to either phono circuit with phono 2 being quite a bit louder than phono 1. Whereas, the aux and tuner settings are quiet.

Thoughts and suggestions are invited. I pulled the four transistors in the pre-amp section (2sc458) (schematic calls for 2sc350r and y) and they tested fine though one side had a slightly lesser hfe than the other. Each of the legs on the transistors was quite black. (I got a Peak DCA55 tester for my birthday).

The transistors on the amp board are four 2sc984 (little metal cans) where the schematic I have calls for 2sc734 transistors.

The one common factor is the schematic has a 2sc458 at Q9 and so does the unit.

?

James
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Old 18th May 2017, 02:04 AM   #2
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The old 2sc458's are known to become leaky and noisy.
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Old 18th May 2017, 02:08 AM   #3
spind is offline spind  Canada
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Over on AudioKarma the 2sc458's have a reputation as being quite the 'troublemaker.' They don't appear to age well


Edit-ooops, jerluwoo beat me to it while I was verifying what I remembered.
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Old 18th May 2017, 03:35 AM   #4
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Thanks for the replies. I pulled and tested each of the transistors with my nifty Peak transistor tester. Each one tested fine and none had any leakage. That said, given their age it may be time anyway.

Any suggestions as to a replacement for them?

Thanks James
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Old 18th May 2017, 03:49 AM   #5
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The schematic calls for 4 2sc350 on the main board. On my unit there are 4 2sc458s all of which tested fine. They are not terribly similar though if I compare the data sheets and no online database names them as equivalents.

It looks like someone was in here before as I see indications as minor scorch marks on some wires where a soldering iron touched and the like. Does it make sense that since this unit has two phono options whereas the schematic only has one that is why I have different components? Confusing I must say.
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Old 18th May 2017, 04:27 AM   #6
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Quote:
You can save your 2SC458s. The problem is silver oxide needles that form between the 3 legs. Note the black legs? With a fine stainless steel tool scrape the area between the 3 legs, and most transistors will be magically restored!
www.tapeheads.net/showpost.php
Conflicting info in the post, I have always just changed out 'black leg' transistors and no more troubles.
That said the noise caused is flicker noise and not 'hiss' IIRC.

Dan.
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Old 18th May 2017, 04:29 AM   #7
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I use freeze spray to find the noisy component. Just spray one part at a time and in a minute you'll find it.
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Old 18th May 2017, 04:32 AM   #8
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Hi, Thanks for the reply. When you say "flicker" noise is that the same thing as oscillation? Oscillation seems to point to my power capacitors which look to be original.

Thanks, James

Quote:
Originally Posted by Max Headroom View Post
www.tapeheads.net/showpost.php
Conflicting info in the post, I have always just changed out 'black leg' transistors and no more troubles.
That said the noise caused is flicker noise and not 'hiss' IIRC.

Dan.
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Old 18th May 2017, 04:34 AM   #9
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Default freeze spray

I guess I will need to get some freeze spray and give that a try. That helps identify the culprit but not what I should replace with I think?

Does anyone have thoughts on the difference in components I have mentioned?

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Originally Posted by prairiemystic View Post
I use freeze spray to find the noisy component. Just spray one part at a time and in a minute you'll find it.
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Old 18th May 2017, 05:02 AM   #10
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Hi, I just found another schematic over on AudioKarma. Problem solved. Now, I just need to get replacements for the 458s and new caps. I smell better sound!

James
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