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Kenwood KR-6400 help

Eastham

Eastham

More Class-A than ever!
Hi guys, new guy to the forum here!

So I recently purchased a Kenwood KR-6400 with the intent to repair and use, the ebay listing stated that the chassis was live and that they hadn't tested it for about 2 years, when I received the receiver plugged it in and powered it the chassis wasn't live but, there was a lot of DC offset in the speaker outputs not enough to kill my test speakers but enough to give the woofers a workout, and as the receiver powers up the transformer would make a low buzz and progressively get louder until the speaker relay clicks on. I replaced all the capacitors on the power supply board and the filter capacitors but now the receiver is blowing fuses, I replaced the caps with all the correct values mad sure they're in the right orientation assuming the stripe on the cap means negative. I can see a trace that burns before the fuse blows it does not have any components of which I replaced on it I'm assuming that it is related to the fuse blowing.

The burned trace.
UfzdrO7.jpg


The only thing i can see on that trace is these two diodes. (I think they're doides)

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It should be okay to hook up the filter caps like this for the time being right? I plan to "stuff" the old filter caps with these new ones to keep the original look.

V4PKyiN.jpg


A picture of the power supply board.

Pvr5kvo.jpg


My knowledge is still quite limited, so please don't get to technical. :)

Thanks.
 
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I would check diodes 1,2,3,4, the four in the center of the board and resistors R2 and the one next to it looks cooked, couldn't make out the number. Also the filter cap you have connected to the negative side of the supply seems to be backwards, the white wire is negative and ground is positive in respect to the negative voltage.
 
I clipped the two offending diodes out checked them and sure enough they're shorted, I proceeded to power it on and the wires I used to hook up the capacitors started to melt and billow smoke, I guess this is because either a. I'm stupid and powered it without the diodes or b. Because like you say I have that cap wired backwards, or both. Any recommendations on what diodes I should use to replace these ones? I note in the schematic they're part number U05B, but I would imagine after 40 years or more nobody stocks these anymore.

Thanks.

Edit/ Update.

I got some UF5405 diodes to replace the U05B's.

Here's hoping that I haven't blown anything else...
 
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I think those diodes will work,I would replace all 4, hope it gets it going, good luck. Don't forget to check those resistors, they look toasted.
 
Thanks, I have checked all the resistors and they all seem to be within tolerance within 2 ohms give or take, I replaced all the blown bulbs with warm white 9 - 12v leds, I powered the unit on for a second to check them, they light up the dial face perfectly not to bright not to dim, I can't wait to get this receiver working, I ordered all the capacitors I will need to re-cap the amplifier board, it sounded great with the DC offset I can only imagine what it will sound like working 100%. :music:
 
Thought I'd take some pictures.

My attempt at wiring the led's.

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Holes drilled into the base of the old cap and the new cap stuffed inside, my intention is to run the wires trough the holes onto the old capacitor leads.

OUZEGEt.jpg


Is the brown goo around the bottom of these capacitors old glue or the electrolyte leaking out from them? I am replacing these ones when the diodes arrive.

WWi3iny.jpg
 
No parts came today, should be here tomorrow hopefully. Decided to kill time by putting the new caps inside the shells of the old and wiring them in.

L1X2R4q.jpg


mamEVXX.jpg


End result is pretty good, what I was aiming for and they're functional too, proving a good mounting place for the new caps whilst making the insides of the unit look nice and tidy, hot glue them in place and away we go! I always try my best not skimp on the quality of my work, it is a reflection of me. :)
 
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Well the didoes didn't come today. :/ But all the capacitors for the amp board did! So I got all of them replaced with the exception of one 100uf 10v bi-polar cap I'm having a hard time finding a replacement for.

pueK7ed.jpg


Crusty old caps.

neSRfOD.jpg
 
Pretty much, always safe to go up one or two ratings higher. Use caution when dealing with Tuner and Phono sections though. A lot of the respected techs here use ratings 1 higher as a rule of thumb. (16v in place of 10v, 63v in place of 50v...).
 
So the diodes came in today, great! Now the unit powers up and produces sound! But now the bad part. I was listening to the radio with it when I noticed a bit of DC offset, well after a while a bit of DC offset turned into a lot of DC offset almost blowing my test speaker, luckily I yanked the cables out in time, so after that I turned it off and started probing around and smelled something noticeably hot, so instead of using my multi meters temperature probe I start touching things with my finger and when I hit the left most transistor on the amplifier board lets just say it was hot, burning a curious pointer finger hot. So after burning my finger I probed the transistor with my multi meter, 92c :eek: so my question now is, is this transistor bad?

The offending transistor.

JtoUWOj.jpg


Shortly after it cooled down I tried to power it on again and it indeed blew the fuse, after replacing it, it powered on again.
 
Right, so for a while it would power on but wouldn't kick the speaker relay, after removing that transistor it will power on and produce sound again so I'm guessing that transistor is kaput, there is still a noticeable hum coming from the test speaker I have hooked up, any idea's what could have caused that transistor to blow and what could be causing this hum?
 
Check the values of resistors, you need to lift one end to get accurate readings. But in your case the amp board is mirrored so you could just compare the values on left and right channels without having to lift one end for a quick and dirty comparison. A bad diode or transistor could cause runaway voltages also or you may have already found the problem. The hum is generally PS caps in my limited experience. That cap next to the transistor on opposite channel of the one you circled looks original, maybe the culprit. Start compairing voltages at diff points between the channels is a good starting point. Not sure if you seen this nice thread by Echowars on testing transistors http://www.audiokarma.org/forums/showthread.php?t=43186. Before putting in new transistor be sure to check that channel out as you could fry the new one.


Note on Hum. How are them main filter caps you replaced currently hooked up? If they are still using them jumpered leads as shown in early photo then that could very well be a problem.
 
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I removed the transistor and tested it out of circuit and I did get a reading on both emitter and collector, as for the filter caps, yes I am still using wires but they're much thicker gauge and shorter about 3cm. the filter caps have been shorted once or twice by accident and one was wired in backwards by me on accident. I decided to check the voltage across one cap and notice it was extremely erratic jumping from about 6v to 3ish to 0v and back to 6v, I wonder if it's an issue with the transistor on the power board, guess I'll warm up my soldering iron and pull it out of circuit to test it.

*Edit*

I almost forgot, that cap that is original is the 100uf 10v bipolar capacitor.
 
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I started probing some of the resistors around the transistor, theres a 4.7kohm resistor re22 on the data sheet that reads 1.5kohms and a 2.2kohm re20 that reads 1.4kohms, Not sure if these would be causing my issues but I will be replacing them for sure.
 
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