Kenwood KR-7600 problems

[ande13]

Recently I bought a Kenwood KR-7600 amplifier and quite soon I realised that the left channel was quieter than the right one. Sometimes it was better and sometimes almost silent.

I have never before fix anything from any amplifier but I decided I'm gonna try to fix it.
I did clean everything and I did lube the pots etc. Then I changed power supply a capacitors and some diodes and transistors, a kit I bought from ebay. The Qk1 was looking like it was time to change it.

Then I checked the bias and after I adjusted both channel to 40mv I noticed the other channel started slowly grow from the 40 mv to 41 and so on. Other one was steadier.

I think I succeeded with everything but when I started to play music the problem was still there but it was in the different channel so now the right channel is quieter than the left one. If I push reverse button it changes the weaker channel.

I wanted to explain everything I have done and if anyone have any idea for what to do next all help would be welcome.

Also in the service manual there is that if there's a problem in one of the channel you should check from the control unit Qi1,2 transistors and IC1, They looked alright but I don't know to check them, transistors are cheap but if I change them and it doesn't fix the problem what next? And same question is there benefits recapping the whole ampilifier?

Thanks a lot!!

Antti

 

[Blue Shadow]

you should check from the control unit Qi1,2 transistors and IC1, They looked alright but I don't know to check them, transistors are cheap but if I change them and it doesn't fix the problem what next? And same question is there benefits recapping the whole ampilifier?

Pretty easy to check a transistor. There is a sticky in DIY about BJT testing. The IC could be easy to check, too if you have the datasheet to know what to look for.

there are a number of steps to go through before a shotgun recap is the answer. If you mess up just a little bit, you now have the original problem whatever other problems you build in with an error in recapping. Best to resolve the issue, first. Can't be trying to find two hidden issues.

 

[turboyam]

I dunno if the reverse switch is before or after the volume control on a 7600? but my KR-6600 has a louder left channel, maybe 3db's louder. I verified this on my scope, with all inputs and i found that the volume pot, is un-equal, from say position ''1'' to about 4... I blame it on volume pot wear ( yes, it's been Faderlubes a few times).

 

[ringading]

I would start by cleaning all of the controls with contact cleaner and/or Deoxit. A dirty volume or balance control can cause lower volumes in one channel. If that is unsuccesful I would reverse the main in/pre-out jacks in the back.

If the problem stays in the left channel then the problem is on the main amplifier board. If it changes to the right then the problem is in the pre-amp/tone control. Likely suspect is a failing transistor in the main amp. If you can confirm that the problem is in the left main amp I would start with the drivers. Blue Shadow alluded to checking the transistors (you need to remove them from the board first). This can be done with a simple DMM or with a transistor tester if you have one. Lots of threads here on how to do that. Keep in mind that unless the transistor is blown, it "may" test as good with a DMM as the amount of current used to test it is not as high as what it receives when in the unit. If you need to replace a transistor, you can find replacement suggestions here on AK.

Good luck - that is a nice unit!

 

[ande13]

Thanks a lot for all the advices!

Right now I've been using my amplifier without any problem, after cleaning and the power supply fixing I made I took the amp from the cold garage back to inside and I'm not sure if the problem was just that it needed some time to warm up?

We'll see if the problem stays away. I need to buy that Deoxit D5 cleaner, I did lube the pots with PRF7-78 which should clean and lube, I'm not sure if it's enough?

It's a great amp and it's been fun to learn how to fix it. I'm still thinking if I should change the caps at somepoint, they are quite old

 

[turboyam]

I agree for the ''re-cap'' but be aware that those KR- X600 are NOT the easyest to work on.

 

[ringading]

Warming up the unit may have helped but I wouldn't think that the problem would go away permanently on its own. I did work on an old Magnavox Astrosonic console that seemed to only work correctly when it was warmed up.

Recapping is usually a good idea at some point. Some would argue that it isn't needed (take an afternoon and read some of the debates here on AK about it - entertaining stuff). I look at it as insurance against a future failure that may or may not damage other parts of the unit. I think about it like a car battery - best to replace it when it starts to near its end of life. Easier than being stuck somewhere with no juice.

This line of Kenwoods is not the easiest to work on like some units with removeable plug and play boards but also not the hardest either. The larger Marantz and Sansui quad units are some of the worst.