Kenwood KR-7600 - I Need a Plan!

Hi all,

I've been lurking on these boards for a while, and I'm just amazed at all of the great knowledge on this board. I was hoping a few of you might chime in and offer some advice and insight.

About a year and a half ago I picked up a KR-7600. It seemed to be in pretty good condition, and it played great for a few months before I began to have problems with the right channel.

Once the receiver gets warmed up, the right channel will cut out. It doesn't cut out completely but the volume drops to a very low level, while the left channel continues to play normally. The drop out (drop down?) happens after it has been playing for 20-40 minutes, though I've had it play normally for as long as four hours before the problem recurs. It happens on all inputs, including the radio.

The interior is pretty clean. I sprayed deoxit in all of the pots, and I replaced some obviously damaged caps in the power supply and amp board. I'm planning to replace the power filter caps as well, and frankly the whole thing could probably stand to be recapped. I saw some minor carbon build up, and a lot of the solder joints should probably be retouched as well.

I'm admittedly a noob, but I have slightly more time than money on my hands. I'd like to be able to fix what I can on my own and use this as a learning opportunity.

I talked to a tech about it a while back, and he thought that it might be a damaged transistor somewhere in the power supply or amp board that had failed or was close to failing and that it was going into protection mode once things heated up. Does that sound plausible? If it is a bad transistor, what's the best way to go about finding and replacing it?

For now, I'm planning to recap the power supply and amp board (because it probably needs doing anyway). There are also a pair of damaged (leaking brown goo) caps on the control board that I'll replace. I'll also retouch any solder joints that need it. Then I'll... start pulling transistors on the amp board and testing them?

I'd really appreciate any advice you can offer, especially if you're familiar with this model. Any other theories or possible solutions are welcome. I can post some pics later. Thanks for your help!

Before you mess around with transistors, replace the power protection relay. I think most of the Kenwoods from that era used the same type, which are still easily sourceable today.

The relay has little pins inside that build up oxidation, and dropping channels is a very common symptom. It's best to replace the whole relay rather than attempt to clean one. Especially since a replacement would be easy to find and cost you only a few bucks.

This is just a guess, but your relay is probably compatible with digikey/mouser part number MY4-02-DC24. The KR-6400 and KA-8006 both used this relay.

And that brown goo you see is probably just glue. It's also very common. If your caps were damaged enough to actually leak, there is no way in hell it would even produce any recognizable audio.

This is a photo I took of the old relay that was used in both of those units mentioned above next to the new part. They're basically identical:



PS - don't be afraid about asking lots of dumb questions. I am still admittedly a newb in many areas, but I've received a lot of help and learned a lot of tips from these forums.

Thank you! I'll definitely be checking that out. Great advice.

Also there is a speaker selector switch to the back of the unit that gets dirty too ... I believe there's a shaft that goes back there from the front panel .. clean that too..
Sometimes more than one cleaning is necessary on these old babies .. LOL

John M

There are three switches on the back panel. One is operated by the tape knob via a shaft and the other is operated by the selector knob via a shaft, in addition to the speaker selector. I don't know what they do, but they're there! Here's thread on how to access them if you haven't already done so. This on a KR-6600, which is identical to a KR-7600 in that regard. http://www.audiokarma.org/forums/showthread.php?t=515817

I believe EW Glenn has called this one a "techs nighmare" LOL I had one when they first came out, bought new. Good receiver.

JM

SicMan said:

I believe EW Glenn has called this one a "techs nighmare" LOL

JM

Click to expand...

Yeah, it's not exactly easy access.

Trambone said:

Hi all,

Once the receiver gets warmed up, the right channel will cut out

Click to expand...

Maybe it's the input selector, as suggested: mine had an erratic behavior, after a while one of the channels got very distorted, moving the selector to another input and back fixed the issue temporarily, so I was able to determine it was needing a good cleaning.

Andrea

Thanks for that too. I have some parts on the way. Hopefully I'll be able to find some time this weekend to work on it. I'll look at the switches in the back of the receiver and give them all a good cleaning.

I hope you all don't mind too much if I try to resurrect my old thread, but life (in the form of my now 2-year-old daughter) got in the way. I've finally got the time and space to work on this project again.

So just over a year since my first post, I finally was able to replace the relay as iamthejeff suggested. This seems to have corrected the problem, or at least it's allowed me to hope that it has. I've been able to listen to the receiver for several extended sessions without losing the right channel.

However, I did lose the right channel briefly yesterday. When the sound dropped out, I turned the balance all the way to the right and turned up the volume. After a few seconds, the sound came back to the right channel, which seemed to "fix" the problem.

I've read in a few places that this could be symptomatic of a dirty pot or switch or maybe a cold solder. Does that sound right? It only happened the one time after 6-7 trouble-free listening sessions, but I'm sure it will come up again. I'll try it again after work tonight and see if I can recreate the issue.

Thanks again for all of the good advice.

When I clean pots, sliders, switches, and jacks, I sometimes find it takes about three times to finally get the problem point clean. Sliders can be especially problematic.

My KR-5150 had an nearly absent channel, that came back with a three-stage cleaning of all the pots, sliders, switches, jacks. It didn't have the change with warm-up aspect you originally had, but you seem to be beyond that now.

It makes perfect sense to post to your existing thread. No need to apologize for that IMO. I know that in some forums people jump all over someone who posts to an old thread. Posting to your own thread about the same project is the most logical way to proceed.

I agree with what illinoisteve said. Contact cleaning without disassmbly, while relatively easy, is not always efficient. More likely that some contact areas still have some type of corrosion than a bad transistor causing your issues IMHO. And a bad solder joint is the next most likely problem. Do you know how to visually identify bad solder joints?

If you are able to determine which knob(s) and/or switch(es) have some affect on the cut-out, that can guide you as to where to apply your cleaning efforts. A schematic can be useful too, as sometimes trying alternate signal routing can help to verify where a problem is.

I am not saying that a transistor could not be to blame. I am saying that unsuccessful 1st attempts at Deoxit'ing are very common. If you do ever get to a point where you change any transistors, the one that drives the relay is a prime candidate for renewing as it is a common point of failure. But failure of that transistor would result in longer and longer delays for the relay to click, which is not what you are describing here.

Trambone, I'm working on a 7600 right now and had this exact same symptom with mine. As a matter of fact I'm ordering the caps right now and came over to the fourm!

Try this...

Look at the 4 electrolytics clustered around the balance knob on the control amp. Use an insulated tool/probe of some sort and move them around slightly and see if one of them causes that channel to go in and out. I was losing the left channel on mine inexplicably, and found one of these caused that problem. One is either a bad cap or has a bad solder joint. You could be experiencing the same issue. Hope this helps!

Thanks again for all your advice. Excellent suggestions. When I get a little free time later this week, I'll tackle the cleaning. If there's still a problem, that's probably where it is. There's a little scratchiness when I manipulate the balance knob. Same story with the volume. I've only lost the right channel once since replacing the relay, so maybe it's just a little dust or something in there.

I'll have to look at those caps, bossman. Hopefully all it needs is a good cleaning.

re: bad solder joints. Some of the joints seem to have shrunk over time and could stand to be retouched. I think I can spot a bad joint, but I'm still a novice.

If moving certain pots or switches causes noise then there is a good chance that is where your problem lies. It is the most common problem in vintage gear and it is not mere dust, it is corrosion on metal surfaces that can be stubborn to remove. As an example, take a look at the pic on post #25 of this thread That is the inside of a switch from a '70s Kenwood piece. Next pic shows what it looks like after vigorous cleaning by hand. Without dis-assembly (I am not advising you to dis-assemble pots/switches at this point) and by simply squirting Deoxit inside a pot/switch, the cleaning process is less efficient and less likely to succeed, especially on one attempt.

Bad solder joint on one of the pots you mention could still be an issue, as could the caps mentioned above as different models seem to have their own particular quirks. But my money is on corrosion inside pots and/or switches.

Appearance of bad solder joints can vary. Often it will be a hairline crack in the joint that looks like a halo around the middle of the joint. Good pic in post #9 this thread. BUT, cracks or other solder issues will not usually be as obvious as those shown in that pic. You should google to find other examples if a thorough deoxit job doesn't resolve the problem.

Proper deoxit application may require some dis-assembly of the receiver and/or creativity to get the deoxit inside the pot/switches and as close as possible to where the contacts are. Some pots/switches are open and the contact areas are visible, others can be a challenge...

Here is the link to AK's Deoxit sticky thread

good luck :thmbsp: