Kenwood KR-9400 Humming Sound - HELP

[ExtraCharity]

Hey Folks, I believe I am the cause of this now annoying Humming sound. Received a KR-9400 for Christmas and have a pair of infinity rs525s hooked up and have been having a blast these couple of days. Got it from the original owner. Took it upon myself to remove the top cover and use some compressed air to remove decades-old dust. Turned it back on and now I have a loud humming coming from my speakers.

I am deadset on fixing this as the listening experience pre-hum was euphoric and I know my 50lb monster is capable of better.

Hum is coming from both speakers, gets louder as volume is increased. FYI

Please lend me the advice and experience to remedy this nuisance.

 

[ExtraCharity]

Would really appreciate help on this. While I am an engineer by trade and very mechanically inclined, I am very hesitant to perform any work on this as I do not know much of anything about servicing it, nor do I have the proper equipment. Guidance would be appreciated. I assume the hum is associated with age components past their lifecycle.

 

[spicer]

Would really appreciate help on this. While I am an engineer by trade and very mechanically inclined, I am very hesitant to perform any work on this as I do not know much of anything about servicing it, nor do I have the proper equipment. Guidance would be appreciated. I assume the hum is associated with age components past their lifecycle.

Does it hum with nothing connected to any of it's inputs? Does it go completely away with volume turned all the way down? I used to own one of those, bought it brand new... good set and has a great tuner. Nice avatar.

 

[ExtraCharity]

Does it hum with nothing connected to any of it's inputs? Does it go completely away with volume turned all the way down? I used to own one of those, bought it brand new... good set and has a great tuner. Nice avatar.

Hi Spicer, thanks for the query. Currently have a iphone->lightening adapter->3.5mm plugged into the AUX. Upon removing said input the hum is still present. Hum has a direct relationship to volume control. So yes volume down cant hear it. Volume increases hum gets louder.

This was a gift from my father to me for Christmas as he knows I have been wanting my own HI-FI system. I am very appreciative to have been blessed with such a receiver. It was his childhood friends. He had it new since 75. They used it all the time when they were young growing up in Queens. He bought it off him a week ago to give to me. Thanks the avatar is me.

 

[ExtraCharity]

I have a sneaking suspicion this might be a cap issue

 

[spicer]

Hi Spicer, thanks for the query. Currently have a iphone->lightening adapter->3.5mm plugged into the AUX. Upon removing said input the hum is still present. Hum has a direct relationship to volume control. So yes volume down cant hear it. Volume increases hum gets louder.

This was a gift from my father to me for Christmas as he knows I have been wanting my own HI-FI system. I am very appreciative to have been blessed with such a receiver. It was his childhood friends. He had it new since 75. They used it all the time when they were young growing up in Queens. He bought it off him a week ago to give to me. Thanks the avatar is me.

The avatar... really, very classy picture, I thought it a still from a movie.

Well... the next thing you might do is remove the pre/main jumpers from the back, power off, and see if the hum is still there. If so makes me think main power supply capacitor or related. If not either some portion of the regulated part of the power supply to signal level boards, or same loaded down by one of the boards. You might get a copy of the schematic for this and, being an engineer, don't forget to apply what you know to the problem... include in your thinking it is both channels. That is not a simple set but that kind of problem should not be difficult to track down by one such as yourself but you should have the schematic and, for this, a scope. Also, an open or loose signal level ground might be the problem... wire wraps are not an uncommon source of bad grounds... check with a volt meter or scope for small voltage where it shouldn't exist. The schematic... I think available on hi fi engine, a free site that is a wealth of information often containing the entire service manual, but of course the schematic is of primary interest.

 

[Semanon]

Welcome to AK.

If you don’t get traction here, try reposting in the Kenwood/Trio/Accuphase forum. Plenty of talented folks there. Some of these take a few days to get rolling, especially during the holidays.

A couple questions:

• In both channels?
• Is it on every input? Do you hear the hum with the tuner and phono?
• Does the audio play as it did before, but with added hum?

The pre/main jumpers are for Dolby and don’t act like jumpers on other amps. It does not separate preamp from the amp. It is for processing and is placed before the volume control. It will be helpful to understand, but not as useful as a true pre/main jumper.

 

[ExtraCharity]

The avatar... really, very classy picture, I thought it a still from a movie.

Well... the next thing you might do is remove the pre/main jumpers from the back, power off, and see if the hum is still there. If so makes me think main power supply capacitor or related. If not either some portion of the regulated part of the power supply to signal level boards, or same loaded down by one of the boards. You might get a copy of the schematic for this and, being an engineer, don't forget to apply what you know to the problem... include in your thinking it is both channels. That is not a simple set but that kind of problem should not be difficult to track down by one such as yourself but you should have the schematic and, for this, a scope. Also, an open or loose signal level ground might be the problem... wire wraps are not an uncommon source of bad grounds... check with a volt meter or scope for small voltage where it shouldn't exist. The schematic... I think available on hi fi engine, a free site that is a wealth of information often containing the entire service manual, but of course the schematic is of primary interest.

Thank you. Pre/main jumpers have been removed. They have never been removed since 75, they were an absolute mother to get out. Thank god for the concept of leverage. Used that moment to also clean them up with some emery cloth. Powered unit back on. Hum is still present with the volume turned up. . Unfortunately, this is the extent of my vintage receiver knowledge.

 

[Semanon]

I am very hesitant to perform any work on this as I do not know much of anything about servicing it, nor do I have the proper equipment.

Folks here will guide you through the process and take you as far as you want to go. You will need a camera and a multimeter to get started. Also download the service manual. There is a free one at HiFi Engine, but you will need a free account. There may also be one in the AK Database. You can also find the user manual and other documentation at HFE.

Good luck.

 

[ExtraCharity]

Welcome to AK.

If you come get traction here, try reposting in the Kenwood/Trio/Accuphase forum. Plenty of talented folks there. Some of these take a few days to get rolling, especially during the holidays.

A couple questions:

• In both channels?
• Is it on every input? Do you hear the hum with the tuner and phono?
• Does the audio play as it did before, but with added hum?

The pre/main jumpers are for Dolby and don’t act like jumpers on other amps. It does not separate preamp from the amp. It is for processing and is placed before the volume control. It will be helpful to understand, but not as useful as a true pre/main jumper.

Thanks, I will repost. Yes, both channels. Yes, all inputs. Audio plays as it did before but when not playing audible hum is present. Not discernable when playing music.

 

[ExtraCharity]

Folks here will guide you through the process and take you as far as you want to go. You will need a camera and a multimeter to get started. Also download the service manual. There is a free one at HiFi Engine, but you will need a free account. There may also be one in the AK Database. You can also find the user manual and other documentation at HFE.

Good luck.

Much appreciated. I have obtained a pdf of the service manual. I have a multimeter and most common tools just not more advanced equip such as a scope. I am confident that with the right guidance I can her back in optimal working order.

 

[Semanon]

I have a sneaking suspicion this might be a cap issue

Thanks, I will repost. Yes, both channels. Yes, all inputs. Audio plays as it did before but when not playing audible hum is present. Not discernable when playing music.

It is likely in the power supply. Probably a wire that got knocked loose or a cold solder joint.

I had a Kenwood that developed a hum. One of the wire wrap wires broke when I was fixing something else. Chased the problem for three hours until I started being methodical and checking everything. Then I noticed the wire.

There is a saying ‘round here: ‘If it ain’t broke, fix it ‘til it is.’

 

[ExtraCharity]

It is likely in the power supply. Probably a wire that got knocked loose or a cold solder joint.

I had a Kenwood that developed a hum. One of the wire wrap wires broke when I was fixing something else. Chased the problem for three hours until I started being methodical and checking everything. Then I noticed the wire.

There is a saying ‘round here: ‘If it ain’t broke, fix it ‘til it is.’

Understood. This is good! I am willing to check. Could you possibly pinpoint me to the area in which I should be looking? Any pictures or images based on your experience?

 

[Semanon]

My skills will only take you so far, but here goes.

Test DC offset. Let the unit warm up for ten minutes. With a meter set for DC millivolts, measure for DC at the speaker terminals. Volume at zero, no input, aux selected. Be sure the speaker output is selected. Voltage should be in the low millivolt range. This is general test to help understand the basic health of the receiver.

Turn the amp off. With a chop stick or wooden dowel, gently (very gently) probe around looking for wires that have become disconnected.

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, listening for hum. If there is no hum, the power amp is probably not the problem, and we’ve eliminated something,

Here is the power supply section of the receiver. Fuses are circled in green.

 

[Semanon]

One absolute necessity is to have minigrabber test leads. Alligator clips or bare test leads are a recipe for disaster. One tiny slip and you create a whole new disaster. Ask me how I know…

Alternately, you can put electrical tape on the probe tips so only 1/8” shows. This will also do in a pinch, but requires more care.

 

[spicer]

Welcome to AK.

If you don’t get traction here, try reposting in the Kenwood/Trio/Accuphase forum. Plenty of talented folks there. Some of these take a few days to get rolling, especially during the holidays.

A couple questions:

• In both channels?
• Is it on every input? Do you hear the hum with the tuner and phono?
• Does the audio play as it did before, but with added hum?

The pre/main jumpers are for Dolby and don’t act like jumpers on other amps. It does not separate preamp from the amp. It is for processing and is placed before the volume control. It will be helpful to understand, but not as useful as a true pre/main jumper.

Thanks for clearing that up, I was mistaken about the jumpers... and I owned one bought new.

 

[REMINGTON700]

What was your source of compressed air - spray can or compressor with hose? First step as stated earlier is a careful and close visual inspection for loose/broken wiring/connectors. Wouldn't hurt to rotate unit upside down in case the compressed air lodged an old screw, washer, copper wire clipping etc., into the electronics.

 

[spicer]

Thank you. Pre/main jumpers have been removed. They have never been removed since 75, they were an absolute mother to get out. Thank god for the concept of leverage. Used that moment to also clean them up with some emery cloth. Powered unit back on. Hum is still present with the volume turned up. . Unfortunately, this is the extent of my vintage receiver knowledge.

Those jumpers do not separate the main amp from the pre as I said, it is an insert much like the tape monitor jacks. I was hoping to get the conversation started so others would jump in. Since the hum is still there with jumpers removed it does serve to indicate the problem is after the insertion point of the jumpers and before the volume control. Spend a few minutes studying the schematic for signal flow and you ought to able to isolate it to a board though if a ground problem sometimes it will manifest in some strange ways... remembering to note that it is in both channels... since it plays music fine it sounds like a signal ground problem 'somewhere'.

 

[Catmanboo]

Never use compressed air, you can introduce dust & crud where there was previously none, and in the hardest-to-reach places. Vacuum & brushes way better. Chance that you did such, and/or knocked something astray. May help to pull Amp cards & Vac out the newly accessible real estate, then reinsert cards & try again.

 

[Semanon]

Thanks for clearing that up, I was mistaken about the jumpers... and I owned one bought new.

I thought the same about my KR-6600 until I tried to use the jacks only to find they were pre-volume control. A case of engineers screwing things up by over thinking and trying to do something different.