Ka-4002 fubar

danj

modern primitive
In the middle of a great song -the Kink's "20th Century Man", the left channel on the 4002 gets louder than it should be and then, after about fifteen seconds, the channel goes dead. I rebuilt it when I first got it a couple years ago. It worked fine until today.:thumbsdn:

Strange thing is there was no warning and no smoke or foul odor came from the unit. The speaker wasn't harmed but my blood pressure rose a few points.

Thankfully I had a replacement for it - a nice vintage receiver - but I'm gonna miss that sweet little integrated. I know I'll fix it someday 'cuz I'm a glutton for punishment.

I think it happened because just yesterday I was boasting to a friend about how reliable these little amps are so Karma came and kicked me in the behind, leaving me with another SNAFU.:yes:
 
Any of you Kenwood experts have any ideas why this might have happened? I've yet to pop the cover to investigate. I hope it's a fuse but don't figure I'll be so lucky. I have a schematic somewhere........now, where is it?

Actually, what I need is to go through my files and get all that straightened out b4 I do any equipment troubleshooting. I have a bad habit of putting things away where it's convenient and not where they belong. It's a stupid thing to do and sure makes it hard to find things. My file system would frustrate Rube Goldberg.

My uncle George always told me that a clean desk is a sign of a cluttered mind. I wonder if a cluttered desk is a sign of an empty mind.....:scratch2:
 
I had a problem with a receiver lately that had too much volume on one channel.
Turned out to be an open cap.
Could be a cap failed and took out a driver transistor.

I have a KA-4002 as well. I use it with Minimus 7"s for computer audio.

Good luck with the repair.

Rob
 
I'm gonna start some troubleshooting later this week and hopefully that's all that going on.

I had to repair it when I first bought it a couple years ago. The main problem was that someone else had attempted to repair it but they didn't know what they were doing (if I can judge by the horrible soldering they did) and they had messed it up:sigh:. The original problem seemed to be a simple bypass circuit and a resistor and diode that were shot. Three simple components that took about 20 minutes to replace plus a couple hours to undo the ham-fisted mess up the previous owner had inflicted upon it. He/she/it apparently couldn't read a schematic and/or didn't know how to read & decipher component color or letter codes. It's a wonder they didn't electrocute themselves.

Who knows? Maybe they did and that's why the Kenny was in the thrift shop.:yes:
 
Hi danj,

I've got 2 off KA-4002A myself in my stockpile for future rebuild (both work, but sound like crap, lots of caps to replace). Anyway, do you have the 'A' version, i.e. KA-4002A? They are basicly identical to the KA-2002A models (got 2 of them as well, one of them over 10 years my PC output amp, listening right now), except for a knob more in the tone control part, and some different output transistors,

The first thought I would have when the volume goes up is that the bias is increased, until the output protection circuit pops in and shuts the output stage down (yes, this little amp has a decent design). There is a good possibility that your output trannies are not fried yet, but I would recommend to disconnect the ones from the faulty channel before doing any analysis. Not the ones from the working channel, because you are gonna need the other channel for reference.

A potential cause for your trouble is the MV-13 diode in the bias spreader, which might have failed in a shorting way ('suicide diode' as mentioned many times in the Sony TA-4650/5650 VFET amps, killing the precious VFETs). It's very difficult to find any data/datasheet on the MV-13 diode, but I think it's in fact a 'double diode' which should provide around 2x0.65V=1.3V voltage drop (hence the '13').

This is why you have to keep the working channel alive. Measure the DC voltage (no signal, volume at 0) across the MV-13 (De1 and De2) of each channel and see if they are the same.

Funny enough, in the KA-2002A amps, the PCB shows a symbol and space (+ holes) for two diodes, as where a single MV-13 diode overlaps the space for two diodes. If your MV-13 diode has failed, then you can replace it with 2 ordinairy 1N4148 diodes in series (if the working channel shows that it is indeed a 2in1 diode). And of course, replace them in the working channel as well afterwards.
 
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Thanks for the info. I'll locate that diode and check it out.

My amp is a KA-4002. I do have another KA, a Trio KA-2000A, which is a different design than the 4002.
 
Thanks for the info. I'll locate that diode and check it out.

My amp is a KA-4002. I do have another KA, a Trio KA-2000A, which is a different design than the 4002.

oh yeah, the KA-2000A is different from the KA-2002A (I mentioned the 2002). I don't know how different the KA-4002 from the KA-4002A is; often the 'A' was an externally upgraded version of the non-A model, i.e. the champagne colored bottom part of the front and aluminium knobs and push buttons instead of black plastic.

Well, let's see what you find.

I have to add that the MV-13 diode can also fail in a non-shorting way, i.e. open loop. But in both cases (shorting or non-shorting) you can check what it should be on the working channel.
 
The amp in question....

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I may have fixed it. After some troubleshooting and repairs there's now sound on both channels. Will to be able to thoroughly test it soon. Right now I have too many projects in the fire....

Cheers!
 
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No Longer FUBAR

Found a bad diode and some minor wiring deficiencies. I repaired them and also flowed some fresh solder to a few questionable spots and wire wraps.

It's been undergoing testing for the last two days and it's survived with no smoke or blue-white flames. Sound is full and balanced. :banana:
 
Found a bad diode and some minor wiring deficiencies. I repaired them and also flowed some fresh solder to a few questionable spots and wire wraps.

It's been undergoing testing for the last two days and it's survived with no smoke or blue-white flames. Sound is full and balanced. :banana:

Good to hear. Was it the MV-13 (or equivalent) diode?
 
I did my troubleshooting without a schematic, which I could not find. I have since procured another schematic. You know what? It was MV-13; marked D2e on the circuit board.

That, my friend, was a great long-distance diagnosis.

Oh, and it appears the 4002 and 4002a differ only in cosmetics. I'd say the 4002a is the better looking of the pair.
 
I did my troubleshooting without a schematic, which I could not find. I have since procured another schematic. You know what? It was MV-13; marked D2e on the circuit board.

That, my friend, was a great long-distance diagnosis.

Oh, and it appears the 4002 and 4002a differ only in cosmetics. I'd say the 4002a is the better looking of the pair.

Well, that was an easy one for once; some jobs go on for ages and are hard to fix. The little kennies are highly underrated, they have a decent circuit design in it, and many friends keep asking me to find one when they hear them as my PC audio output. Due to their little size they go well on a desk. Enjoy :music:
 
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Kenwood kr 3200

I did my troubleshooting without a schematic, which I could not find. I have since procured another schematic. You know what? It was MV-13; marked D2e on the circuit board.

That, my friend, was a great long-distance diagnosis.

Oh, and it appears the 4002 and 4002a differ only in cosmetics. I'd say the 4002a is the better looking of the pair.

I have a kr3200 with the same MV-13 which seems to have smoked it.
I have a 500 pack of 1n4148 of which I used 1. Can I put 2 of em the right way in place of the mv-13 ?

Thanks guys.
Srinath.
 
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