[seavan]

Hi, there is a low level 'bright' sounding hum coming out of both speakers with my RM-10 MK1. I tried replacing all tubes and nothing changed. I can replace parts myself. I would appreciate any advice.

[Speedskater]

For hum & buzz problems, see the troubleshooting section of this Bill Whitlock paper.

"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

[seavan]

Speedskater, thank you for the link! The hum is there even if the amp is not connected to any source. I also tried in different houses. I am looking for a practical advice specific to this amp, like a most likely capacitor to cause it.

[powerfast]

Most likely from the transformer lamination's.  I tightened up the screw/nuts holding the lamination -easily done.   I also removed the transformer and secured it to the chassis with isolator's made of doubled up electrical tape. This reduced the noise significantly to a low acceptable level.  Hope this helps.

[G Georgopoulos]

Check power supply and power supply wiring,also ps wiring in the amplifier...

[tubegroove]

I had a case of low level hum (although only from one channel) and it turned out to be a break in the solder to one of the pins on the input tube of that channel.  A little solder to that pin sorted the problem
First however short the inputs to eliminate any noise coming from the inputs either via interconnects or airborne
Hum could also be a result of bad input filter caps (I have a Mk2 but assume the MkI also has these).  See this link and scroll down for a photo of the insides. 
http://www.6moons.com/audioreviews/musicreference/rm10_2.html
The input filter caps are two grey ones at the bottom right of the photo.  Check if these look leaky or puffed.  This could be letting in the noise from the AC power input (50/60Hz)
If that doesn't help you may want to prod the solder joints of each of the input tubes (use an insulated probe like a wooden chopstick) to see if you can stop the hum
Hope this helps

[HAITIMAN]

For hum & buzz problems, see the troubleshooting section of this Bill Whitlock paper.

"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

Excellent read. Much appreciated.

[seavan]

Thank you for all advices!  I will try them next week and report back.

[roscoeiii]

I would also recommend calling Music Reference. You will likely get Roger on the phone.

[seavan]

Thank you everyone for your advices! I have to say that the hum was really low, I had to be one-two feet close to a speaker to hear it. Visual observation did not reveal any issue, caps looked just fine. Following powerfast advice I tried tightening the transformer and I discovered that one bolt was loose. After tightening it, the hum level went down and I can only hear it a couple inches away from a speaker. I am happy now, but I am still wondering if this is normal to have any hum on this amp.

Powerfast, I did not try to put the electrical tape between the transformer and chassis (did not have time). I will definitely try it later.

[pehare]

you shouldn't be able to hear anything at idle thru the speakers - should be dead quiet.  A tiny bit of transformer hum at the amp when no music is playing is normal - let's you know it's humming along fine.

[GMuffley]

Low level hum can be a sign of a filter capacitor going bad.  If so, it will gradually worsen.

[Ericus Rex]

Is it a pitched 'hum' or is it a 'shhhh' sound?

[seavan]

It is a pitched hum.

[G Georgopoulos]

Low level hum can be a sign of a filter capacitor going bad.  If so, it will gradually worsen.

Could be that. Also check joints of such caps...

[Ericus Rex]

If pitched hum is coming from the speakers the first thing I'd suspect is a ground loop.  Do an AC "ground loop hum" search and you'll get lots of tips to fix this.  If nothing there works than you'll have to go internal and inspect solder joints and look for leaking caps, burnt resistors and such.  Good luck and keep us informed!

[Roger A. Modjeski]

Hi, there is a low level 'bright' sounding hum coming out of both speakers with my RM-10 MK1. I tried replacing all tubes and nothing changed. I can replace parts myself. I would appreciate any advice.

Thanks for all the responses from readers. Firstly lets be clear about a few things. Hum out of the speakers has nothing to do with mechanical hum from a power transformer, unless there is a horribly microphonic tube.  When an RM-10 is in good shape the hum is about 0.2 mV. which is 20 dB lower than some amps out there.  Tightening up the transformer bolts has nothing to do with what comes out the speakers but it can reduce mechanical hum. Besides the bolts that hold the transformer to the chassis I have found tightening the bolts that hold the transformer together can help. It is very very difficult to have a transformer be totally silent. I feel if you cant hear it a few feet from the amp then why worry.

As I have said many many times the only way to assess hum in an amplifier it to disconnect the input and PUT IN SHORTING PLUGS. An open input is not a proper test.  If you didn't save the shorting plugs from your Pioneer receiver (not those silly RCA cover caps that are being sold now, they do nothing) you can make some. Take an old cable, cut it about 2 inches from the end, strip the hot and ground and twist them together.

Once you have confirmed the amp is quiet you can start adding components to the system one by one till you find the one that causes the hum.  In general, only the power amp should be grounded to avoid ground loops.

As to capacitors, the two nearest the power inlet are for the left and right channel screen supplies and can cause hum if they are open or low in value. If just one fails it will cause hum in just that channel as they are independent. They may look good but not be. They are easy to replace. The main caps can also cause hum but are less likely. Some of these amps are now approaching 20 years so we often replace those on a regular service/tune-up.

You can also send the amp to us. We turn things around in less than a week for regular repairs, mods take longer. Call us at 805 687 2236 or use the email on the website.

[seavan]

Roger, thank you very much for your response!

I apologize for my ignorance. I did not have shorting plugs and I tried the amp with open inputs. Putting in the shorting plugs removed the hum completely. The amp is dead silent now. I will try chasing down the hum when the amp is connected to a source when I have time.

[Roger A. Modjeski]

Roger, thank you very much for your response!

I apologize for my ignorance. I did not have shorting plugs and I tried the amp with open inputs. Putting in the shorting plugs removed the hum completely. The amp is dead silent now. I will try chasing down the hum when the amp is connected to a source when I have time.

Don't feel bad, you are #1001 who I have told about shorting plugs. Glad to hear the amp is OK. Now just follow the path back to the source.