Kenwood KR-4200 - High DC Offset, Quirky Problems [X-POST]

pascal21

New Member
Hello everyone! Thanks for this great resource, it has been a great help so far trying to figure out some problems I've been having.

I am fairly novice when it comes to electronics, and almost completely new to diagnosing electronics issues. I have been doing some research and am not really sure what my next step should be so I am hoping someone may be able to point me in the right direction.

I got a Kenwood KR-4200 from my parents. They bought the unit new in Germany and save for an American plug the unit hasn't been modified, recapped or upgraded in any way (as far as they can remember).

My initial problem has recently taken an interesting twist, but I'll start here from the top. Whenever I would power up the receiver it would work for about two minutes at which point the speaker relay would click off. All the lights would stay on and the headphone output still worked.

After cutting the power for about 30 seconds, the unit would work again for about 10 seconds. The longer it was left off the longer it would work, but never for more than that original 2 minutes, and that was only if it was left off for a significant amount of time.

I tested with all sorts of different speaker setups, and each speaker has the same error on each channel and each A or B output. The unit even clicks off with no speakers connected. At some point during all of this, the receiver started popping pretty heavily on power on and off, something new or that I at least hadn't noticed.

So, I opened everything up and started looking for bad connections and what not. I noticed that when the output stopped working, if I quickly powered the unit off and then back on, the relay would move slightly but never fully engage.

I looked at this diagram, and with a little help found that in sector 6-I the voltage should be what looks to be 15.5v. I measured the voltage across the relay, basically across the D1 diode, and it came in at 25v, and dropped to 0 when the audio stopped working.

http://i.imgur.com/6sbSBpn.jpg

After a little bit of research I measured the DC offsets for the speaker outputs and they both came in around -100 mV, which I guess is a lot.

Here's where things get interesting. I haven't really DONE anything save for these tests. It's possible I jostled something, but no hardware has been changed or resoldered. I was going to, on a whim, replace the thermal paste on some of the transistors that are attached to a large heat sink. I asked my roommate to fire up the receiver and see if they were getting hot (I was out of town) and, lo-and-behold, the receiver stayed on for over an hour!

I've been listening to it over an hour myself now, and some measurements have changed. The relay is still at 25v, but the DC offsets are 830 mV for the left channel and 1130 mV for the right channel, crazy! Additionally the right channel no longer outputs audio on either speaker A or B setting.

As an additional note, the sound seems to come through the working channel pretty 'full' for brief moments, with more depth, but then flattens out a little bit. The pots are a bit crackly so maybe I need to clean those? First order of business though is the other problem.

tl;dr, My receiver was seemingly overheating, but now it is staying on but the DC offsets are even higher than before, with the right channel no longer working.

I don't really fully understand how the transistor and safety circuit stuff works to adjust voltage and whatnot, so if anyone has any good resources I would love to see them.

Thanks in advance for any help!

- Kevin
 
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