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Ground Loop Kenwood KA7002

oldvinyldude

oldvinyldude

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Hi:

I just finished resolving another issue (no amp output), by cleaning the speaker selector switch and reapplying Deoxit. I also did the same for the tape monitor\dubbing switch, as it was still noisy.

Next up-I have a ground loop somewhere-120 hz tone heard anytime a speaker is switched on. It does not vary with volume knob. I don't know if this was present prior to my above work.

I did find a contact arm that I had lifted on the monitor\dubbing switch and corrected. I have dubbing and monitor functions on A and B. I have speaker output on all three speaker connections on rear.

The unit has a non polarized (two prong) plug, and swapping it's polarity does not change condition.

Thoughts?
 
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Yes-did these now, and still have the hum.

I will feed the preamp output to another amp, and see if I can isolate it better

EDIT-I have isolated it to the power amp. No hum when using it as a preamp.
 
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Ground loop would be 60Hz, if you are sure it's 120Hz then that's rectified frequency, bad main filter capacitors would be my first guess. The preamp section has it's own regulated power supply so that's why it's working correctly.

Craig
 
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I have the issue on both channels, and across all 3 speaker outputs.

I thought a ground loop was evidenced by a 120hz tone, not a 60hz one.

I am not at all sure how to check for bad grounds-trying to learn though.

I can check the main filter cap values-but I was going thinking if these were bad, I would be hearing a 60hz tone. No?
 
Full wave rectified ripple on the main filer capacitors will be 120Hz, twice the line frequency. Sounds more like a buzz than a hum. Got a scope?

Craig
 
Although im a complete noob, I just wanted to say I had exactly the same type of buzz/humm when I connected teh main filter caps wrong on my KA7300, so my shitty guess would be those caps too, pull em out and test?
 
No scope to use-on my someday soon list.

I will desolder and test filter caps-it's tight in there though. Sure wish they had used screw terminal types.....
 
Well-it seems my available soldering irons can't generate enough heat for these large lugs for the filter caps. Looks like I will need to put the unit aside until my tools and skills have improved.

Thanks for the suggestions.

Paul
 
I think you need some flux. Put some new solder on the tip then put the iron on the lugs. Or use a flux pen
 
Hi Guys,
I am also new to the forum. I have the same amplifier and am trying to upgrade it too.
Regarding the el caps of the main stage PS: I had to cut the loose. Since I have replaced them that wasn't a worry. Doesn't look so neat but these Japanese guys back in the 70's really knew how to solder something in "for life". And there is miles of wires sitting in the way of where you need to stick the iron.

I have a similar hum/buzz issue but will start a new thread, because I don't want to hijack this one.
Mine is singled down to the pre-amp stage but also 100 Hz (Europe ;-)). So I suspect it comes from the PS somewhere.
As to the tools: cheap scopes can be found on e-bay. Little calculator sized ones that are either ready made or available as a kit. Cheap 'n cheerful....You also find component testers there that 'll do transistors diodes caps etc etc. Not top notch but good enough for everyday troubleshooting :-)
 
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