Kenwood KR-2400 Project

S

Starquest

Super Member
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Hi guys, I'm starting a Kenwood KR-2400 project and looking for assistance with parts. There's distortion on one channel and I can't measure the bias on that channel. Apparently it's common for the output transistors on this model to die, so I'd like to replace all four.

Question 1: Outputs - it seems the TIP41 and TIP42 are acceptable. (DigiKey part numbers)

2SC789 > TIP41B - TIP41BGOS-ND
2SA489 > TIP42B - TIP42BGOS-ND

There are A, B, C models. I'll probably get the B model as that seemed to work for this guy - https://www.audiokarma.org/forums/index.php?threads/kenwood-kr-2400.442280/

Question 2: Emitter resistors. I don't know if I can adequately test mine, so I thought I'd just replace those too. They are wirewound 0.47 ohm at 2W. Closest I could find is this Yageo 5W: https://www.digikey.com/product-detail/en/yageo/SQP500JB-0R47/0-47W-5-ND/18623

I'm guessing that would work, but not sure if it's right.

Question 3: Bias adjust VRs. These are the kind you adjust from the top (see photo), but there are so many options of Bourns I'm not sure what to get. What's the best for this application?

Ugx1omVh.jpg


Finally, new mica insulator pads. Looks like this will work for TO-220: https://www.digikey.com/product-det...yd-corporation/56-77-11G/56-77-11G-ND/1625385

Assuming this repair goes well I'll recap the entire amp (minus the tuner). yes, it's only 16 wpc, but I think it's a good first project. I just got a Hakko FR-301.

Any help is greatly appreciated. Thank you! :)
 
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Suggest first track down the cause of the distortion/bias issue. You are "guessing" that it's the output transistors. You
might be right, or not. The kenwood guru's, knowing the amp, might agree with you...
you don't get much for 2c...
 
Agree w/ mbz. Diagnose first, before changing out any components (and possibly complicating already existing issues.) If it is indeed all factory, everything is in the right place right now. You just need to find which ones aren't working right. Changing things out willy-nilly only increases the number of variables.

Have you checked the outputs?
 
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avionic said:
Not according to the service manual .. R45 and R43 are swapped around.
Is this the channel that won't bias ?
Click to expand...

Not sure what you mean. R43 is mounted sideways.

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I think I can measure the bias on this channel, but I was uncertain exactly how to do it. The other one channel I get nothing no matter where I put the probes on the resistors. I also think I shorted one of the transistors while doing the measurements, yet somehow the readings didn't change. Hah.
 
I am using pin 3 on the power supply for a ground, as indicated in another post on AK.

I get around +/- 23V on the collectors of all outputs. No voltages on any other pins.
 
Starquest said:
I am using pin 3 on the power supply for a ground, as indicated in another post on AK.

I get around +/- 23V on the collectors of all outputs. No voltages on any other pins.
Click to expand...
Thats a good sign..
 
avionic said:
If you have mini grabbers connect them to the emitter of Q19 and the emitter of Q21 . And adjust VR1
Click to expand...

Shouldn't I be connecting to the pins of the emitter resistors? Seems that would be easier. I don't have mini-grabbers unfortunately. I have some alligators.

What can I use to adjust the VRs? Can I put electrical tape on a small screwdriver?
 
Starquest said:
Got it. I moved the clip to the other leg of R43 and now I'm getting 3.0 mV
Click to expand...
I need to see the solder side of R43..To see which end of R43 is connected to the emitter of Q21.That would be the correct lead to use.
 
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