KR-2400 Rebuild

Starquest

Super Member
I'd like to rebuild a KR-2400 I picked up this weekend. The seller told me that the right channel had bad sound, but I cleaned all the switches, and it sounded great with headphones, so I thought it was maybe okay.

But then I connected it to some speakers tonight, and sure enough, the right channel is distorted. Again, headphones are fine.

Is this bad output transistors? This model uses a 2SC789 and 2SA489 in pairs. Evidently these commonly go bad:

https://www.iamthejeff.com/post/22/kenwood-kr-2400-receiver
 
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Well that's cool...you bought it this past weekend and already you have blog comments from 2014! ;)

On another note, the headphones are tapped directly off the outputs...did you clean the speaker selector switch and check the wiring/soldering? ...what's the wiring/soldering integrity between the speaker selector and the binding posts? And of course, have you swapped the speakers around? I know, but if I don't ask, I won't know how far you'd troubleshot it.

I thoroughly cleaned all the switches and buttons. Tried A & B speakers -- same distortion. The "binding posts" on this model really suck -- they're screws, and they're junky screws. In fact, the phono ground screw won't tighten, so I grounded my turntable to one of the FM antenna screws :) (Which worked of course.)

Threads on the same issue with different amps seem to indicate that various output transistors could be at fault.
 
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I'm sorry, I should've been clearer...did you swap the speakers from R to L channel? Just tryin' to rule out the speakers. The first thing I'd do before looking at the components would be to pull all the connectors and clean the pins. And I took a look at photos of the rear of one of these and they all show binding posts...skinny ones with red and black plastic covers, but nonetheless. Do the speaker screws/posts tighten okay?

Thanks for the reply.

The screws are somewhat difficult to turn, but they tighten fine. Speakers are fine too; they sound good on another receiver.

I could try blasting the speaker selector with another round of cleaner. I did get it good the first time, though, because initially it didn't turn very nicely. You know that mushy way that selectors and pots feel when they're really dirty. That improved after I cleaned it.

If there is a component problem, could it possibly damage the speakers if I keep trying them? I'd rather not risk that.
 
Headphones don't draw much current so they are easily driven by the drivers of the output stage. Speakers draw much more current and are not happy with only the drivers so this means you probably have a problem in the output stage on that channel. Could be bad output transistors, open emitter resistors or both. Needs to be troubleshot.

Craig
 
Headphones don't draw much current so they are easily driven by the drivers of the output stage. Speakers draw much more current and are not happy with only the drivers so this means you probably have a problem in the output stage on that channel. Could be bad output transistors, open emitter resistors or both. Needs to be troubleshot.

Alright, here is the schematic for the main amp

Screen Shot 2018-01-24 at 18.00.48.png

The output transistors are Qe19/21 and Qe20/22. It appears I need to look for +20v on the collector of 19/20, and -20v on C of 21/22. Beyond that I can't tell. Can you please help?
 
OK, I'm getting +23.1v on the collectors of Qe19/21, and -23.1v on the collectors of 21/22. Perhaps a little high, but at least it's consistent.

I'm getting +1.2v on the base of both Qe15/16, which is right in line with +1.3v on the schematic.

Ideas?
 
"The first thing I'd do before looking at the components would be to pull all the connectors and clean the pins." You might also take a chop stick or drum stick and tap components in the bad channel to rule out a mechanical cause.

Can you define the "distortion"? Cracks, pops, static, muddy sound, high-pitched noises, ...? 100% of the time? If not...does it matter if it's hot or cold? Does it change with volume? In the absence of other information, your process of comparing one channel's voltage against the other is a good strategy.

I did a little tapping around with a chopstick -- nothing. The distortion is throughout the frequency range, especially in the mids. It sounds like it's being overdriven; that's the best way I can describe it. No cracks/pops. Just overdriven. No change when switching to mono. Also it appears the A/B speakers are wired in series or something, because there's no sound on A when I switch to A+B.

Now, check this out. The weirdest thing happened. I was messing around with the tuner. I was somewhere around 92 FM, and the Rockford Files theme comes on. That's already weird. So I let it play thru (I love that show) and the little jingle tells me I'm listening to 103.something! I figured the tuner was all messed up, but I moved it up to 94.5, and that station was right on. Don't know if I hit a pirate station or if the receiver is haunted and channeling James Garner. I guess the vintage of the stereo matches that show.
 
Pins 7 & 8 look good and Pins 9 & 10 are zero Vdc??? Congrats, you have the first vintage amp without trimmers that I've seen with no DC offset.

This amp does have VRs, but only for the bias adjustment. I haven't touched those. I did attempt to measure the bias, but I couldn't tell where to place the probes, so I got zero voltage all around. Is it possible the bias is out of whack?

And there is plenty DC offset, as I noted in post #13 :) -43 on the left and -5 on the right (i.e., the bad channel)

Have you cleaned the pins on all the plastic connectors and visually inspected the solder (bottom) side of the board for these connectors?

Haven't done this, but I can.

Does the distortion change with volume?

I don't think so, but I can check it again.

If the easy stuff looks good, than as llwhtt alluded to, it'll be time to start from the back and work forward. A transistor check starting at the output transistors (19-22) and a resistor check starting at the .47 ohm emitter resistors (R38, 40, 42, 44) would be in order.

I don't have a transistor tester; is there anything I can do with a DMM? If I get those transistors and resistors out, wouldn't it be prudent to just replace them? They've gotta be dirt cheap. Or is that shotgun approach still bad in this case?
 
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Pins 7 & 8 are the signal outputs from the amp board and are indicative of DC offset. If you had -43V offset on the good channel and -5V on the bad, your speakers would be fried. Where did you measure -43V? Maybe -.43 and -.5? You may have had the DMM set to mV instead of V? Even so, you would have seen it on pins 7 & 8...red lead to pin, black lead to chassis ground.

Yes, that's a typo. Those numbers are in millivolts :)

Yes, you can check the transistors with a DMM set to diode check. For a PNP, the black lead goes to the base and the red lead goes to both emitter and collector. For an NPN, the red lead to base and the black lead to E & C. You'll need to know the pin assignments of the transistors...some are ECB, some CBE, some EBC, pins 1,2 & 3.

Thank you. General question...the numbers I'm seeing on those transistors in-circuit appear within spec. However, am I correct in saying that doesn't imply that the transistors are good?

Also, can the diode check be done in-circuit?
 
@rjsalvi - could you (or someone) help me with some measurements?

First, I tried the diode test on all four outputs. Three of them read around 600-700 (roughly) but the fourth one, Q20, is over 1000. Is that a problem?

Second, I tried getting the bias voltage. Connecting my DMM to the white circled legs of the emitter resistors gives me around 7mv on the left channel. I get about half that when connecting the positive probe to the red circled leg. (Guessing the higher reading implies a correct connection.)

I get nothing on the right channel, on any combination of resistor legs. Am I connecting something wrong or is there a problem there? Which legs should I be connecting to? Thanks.

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It's possible one of the emitter resistors is toast on the R CH. Test their resistance individually and check the solder joints on the bottom side...then we'll go from there.

Alright. So the odd reading on that one transistor doesn't indicate much?
 
Hi All,

Same issue here. I just bought my wife a great record for Christmas and when we went to play it, right channel sounds distorted and thought I friend the speaker.

I did some troubleshooting. Both speakers sound great in the left channel but both sound distorted in right channel. The distortion just sounds like a bad signal on a radio and does not change quality with a change in volume.

Speaker selection seems to work fine.

Here are the voltages on amp board outputs:
Pin 7 is -.15v
Pin 8 is +.25v
Pin 9 is .40 volts
Pin 10 is .35 volts

I've looked through the schematic and have noticed that Q19 & Q20 are NPN and Q21 and Q22 are PNP. Can you please explain in more detail how to test them? It seems that you are running a diode check and a voltage check on all of them.

Voltage check on Q21, Q22 read 7 (on 2000m setting) from B (black lead) to C (red lead) on DMM. Is this the breakover voltage?

On diode check, testing same as before, they read close to 500.

Can you explain how to test these in most simple terms possible? Are two of these related to the right channel and two to the left?
 
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