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Kenwood KR-9400 Humming Sound - HELP

Semanon said:
With the power off, Pull fuses Fk4 and Fk5. These are 500ma fuses that supply power to the low voltage preamp, tuner, tone amp, etc. With these fuses pulled, plug it in and test it again...
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OK. Forget I said this. Just traced the power. If you pull the fuses the protection circuit won't have power and the protection relay won't work
 
ExtraCharity said:
Was able to test DC offset. Holding steady at 05.2mV. Poking around with dowel. Haven't found anything yet. Hum seems to be coming from the transformer area?
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Channel A Left: 05.2mV
Channel A Right:14.8mV

Channel B Left: 05.3mV
Channel B Right: 15.3mV

Channel C Left: 05.2mV
Channel C Right: 14.9mV
 
Semanon said:
Offset looks very good. The imbalance between channels isn’t worth doing anything about.
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Awesome! So the receiver is “healthy”. What would be my next step? As I mentioned before the buzzing/hum seems to be coming from the transformer area.

Overall though I only hear it when I put my ear next to it. When music is playing it still sounds great no audible buzz. Wondering it I should not go down the rabbit hole and just enjoy my 50lb beauty
 
coonmanx said:
A little confused now. Is the buzzing coming from the area of the transformer or from the speakers?
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Light buzzing is emanating from transformer area of the receiver but i think is being picked up by the speakers and presenting as an audible hum when the volume is increased and no music is playing.
 
Fairlane said:
Did you check to tsee if the screws for the transformer are tight. Worth a shot.
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But then it most likely would not also be coming from the speakers. That is what has me confused. I usually think that a low level hum from speakers could be something like bad filter caps.
 
I'm a little confused, too. The hum from the transformer will have the same general sort of sound as a 60 or 120 Hz hum from the speakers, but the cause is almost certainly unrelated.

The hum from the speakers is caused by unfiltered/unrectified line frequency being where it's not supposed to be, while transformer noise is caused by magnetostriction.
 
Semanon said:
I'm a little confused, too. The hum from the transformer will have the same general sort of sound as a 60 or 120 Hz hum from the speakers, but the cause is almost certainly unrelated.

The hum from the speakers is caused by unfiltered/unrectified line frequency being where it's not supposed to be, while transformer noise is caused by magnetostriction.
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Here is a video. Again I am a novice here. Not sure if this is within the optimal performance range or if something is occuring.
 
Thanks for sharing the video. The speaker hum sounds like 120 Hz which would make is likely to be a filter capacitor or other post-diode power supply problem. This is an electronic problem

The transformer noise is annoying and could be quieter, but this happens in some amps. It can be reduced. However, that's a mechanical problem and was probably there before you got the amp.

My advice is to fix the speaker hum, then timker around with the transformer hum later.
 
A few questions.

- what kind of meter do you have? Can it test capacitance?
- do you have a soldering pencil or iron?
- how good/bad are your soldering skills?
 
ExtraCharity said:
Here is a video. Again I am a novice here. Not sure if this is within the optimal performance range or if something is occuring.
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What input was selected in the video...?... not terribly unusual for a low level hum to appear on an open phono input at high gain, sometimes they come with shorting plugs on high gain inputs.... the unit has a microphone input if I remember correctly with a control labeled 'sound injection', is that control completely turned down? If you put your ear close to the speaker you can hear it change with the volume control, right? It does sound like 120hz, not 60hz, which is big clue heavily suggesting power supply issues. Just wondering, are there any sources of electrical interference nearby... a light, timer, other electrical devices. You mentioned it was not there before using compressed air to blow dust out of the unit, is that correct? Just because, try reversing the plug in the outlet and see if it makes a difference.... measure the ground side, cold, of the mains at the plug to a known good ground for excessive voltage, it's ac so some will be present measured with a high impedance meter like a DVM...or try another outlet.. kind of a reach but it's worth investigating. Every now and then a curious problem with a unit appears on AK that I'd like to have on the bench, yours is one of them.
 
Everything @spicer said. Something got dislodged or moved or changed. Trying to do non-invasive steps first before digging into the circuitry. Some got changed and we need to figure out what.

After doing Spicer's steps, open up the unit, and post some inside pics. Then, with the amp on, start moving wire bundles with wooden dowel or chopstick to see if you can change the loudness of the hum.

After that, start gently pushing on components in the power supply. A solder joint may have cracked or become intermittent. Gently is the operative word. Don't want to create new problems.

Finally, look over the PCB with a magnifying glass and very good light. Look for bad solder joints. Annular cracks are hard to spot.
2013-04-05_195034_solder_cracks.jpg fracturedsolderjoint1front.jpeg
 
Semanon said:
Everything @spicer said. Something got dislodged or moved or changed. Trying to do non-invasive steps first before digging into the circuitry. Some got changed and we need to figure out what.

After doing Spicer's steps, open up the unit, and post some inside pics. Then, with the amp on, start moving wire bundles with wooden dowel or chopstick to see if you can change the loudness of the hum.

After that, start gently pushing on components in the power supply. A solder joint may have cracked or become intermittent. Gently is the operative word. Don't want to create new problems.

Finally, look over the PCB with a magnifying glass and very good light. Look for bad solder joints. Annular cracks are hard to spot.
View attachment 2437571 View attachment 2437572
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Yes, and remember while gently poking around that it is a high powered unit that could easily damage your speakers if you find the problem and it produces a large spike in the output as you disturb it ... you might even fuse one side of each speakers wires with a one amp in line fuse should it suddenly spike the amp. On the bench it would be feeding a dummy load monitored with a scope.... or, in a tech shop, the price of a speaker driver is just the cost of doing business, something one wants to avoid at home. Actually, our benches had in line fuses for the speakers with a bypass switch built into the test panel. Also, not sure about that unit but sometimes you'll get a hum with the bottom off. Very good example and pictures of bad solder joints, sometimes they are very hard to discern.
 
Semanon said:
Everything @spicer said. Something got dislodged or moved or changed. Trying to do non-invasive steps first before digging into the circuitry. Some got changed and we need to figure out what.

After doing Spicer's steps, open up the unit, and post some inside pics. Then, with the amp on, start moving wire bundles with wooden dowel or chopstick to see if you can change the loudness of the hum.

After that, start gently pushing on components in the power supply. A solder joint may have cracked or become intermittent. Gently is the operative word. Don't want to create new problems.

Finally, look over the PCB with a magnifying glass and very good light. Look for bad solder joints. Annular cracks are hard to spot.
View attachment 2437571 View attachment 2437572
Click to expand...

excuse my lateness I have been traveling for work and will be back EOW. Unfortunately, I do not think my meter can read capacitance. I have solder many an electrical component in the past and am quite good if I say so myself.
 
spicer said:
What input was selected in the video...?... not terribly unusual for a low level hum to appear on an open phono input at high gain, sometimes they come with shorting plugs on high gain inputs.... the unit has a microphone input if I remember correctly with a control labeled 'sound injection', is that control completely turned down? If you put your ear close to the speaker you can hear it change with the volume control, right? It does sound like 120hz, not 60hz, which is big clue heavily suggesting power supply issues. Just wondering, are there any sources of electrical interference nearby... a light, timer, other electrical devices. You mentioned it was not there before using compressed air to blow dust out of the unit, is that correct? Just because, try reversing the plug in the outlet and see if it makes a difference.... measure the ground side, cold, of the mains at the plug to a known good ground for excessive voltage, it's ac so some will be present measured with a high impedance meter like a DVM...or try another outlet.. kind of a reach but it's worth investigating. Every now and then a curious problem with a unit appears on AK that I'd like to have on the bench, yours is one of them.
Click to expand...

Input selected was AUX. I do not recall if the sound injection was turned up but I normally keep it off when not using. The unit is located right below a dimmer light switch and right next to my desk with a monitor and laptop, it was not before.
 
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