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MC225 slight buzzing right channel

Alfmonster

Alfmonster

Active Member
I recently started playing the MC225 through some horn speakers and started picking up slight buzzing in the right channel.

I switched RCA L/R plugs as well as the speaker wires to make sure and have found that it is definitely coming from the MC225 Right channel.

another trouble shooting I did was to clean the pots with a little DeOxit but again nothing changed.

The buzzing is not loud but it is definitely there.

Any help from others who may have run into this issue would be wonderful. Thank you!
 
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I think there's a 12AX7 shared in the first stage of both channels, it's possible there may be heater to cathode leakage in one of the sections. Otherwise, one of the other small tubes in the affected channel.
 
Changing out the 12AX7 with a new one can check it. Swapping the other small tubes from one channel one at a time can tell which, if defective, by moving the buzz to the other channel.
 
Pio1980 said:
I think there's a 12AX7 shared in the first stage of both channels, it's possible there may be heater to cathode leakage in one of the sections. Otherwise, one of the other small tubes in the affected channel.
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Oh interesting, okay I’ll try moving the tubes around. I tried the 12ax7 and the buzzing continued. However I’ll move around the other 2 sets of 12au7 and 12bh7’s.

would the 7591 output tubes be worth checking out as well?
 
Alfmonster said:
Oh interesting, okay I’ll try moving the tubes around. I tried the 12ax7 and the buzzing continued. However I’ll move around the other 2 sets of 12au7 and 12bh7’s.

would the 7591 output tubes be worth checking out as well?
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Heater to cathode leakage/short might caused it there.
 
Alfmonster said:
I moved around/changed out all the tubes and have come to find the buzzing is still there. I’m now worried it might be something internal?
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If it didn't move channels with the tubes or disappear entirely, then that is possible. The 12AX7 would be the most likely suspect.
 
Pio1980 said:
If it didn't move channels with the tubes or disappear entirely, then that is possible. The 12AX7 would be the most likely suspect.
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Thank you so much @Pio1980 - yeah let me try some more 12ax7s.
It is strange when the 225 starts up it’s silent and then at one point while one of the tubes are heating up, I’ll hear the buzzing kind of flicker on.

I did try my the 3 spare 12ax7s I had and they all seemed to buzz - although one kind of flickered/buzzed on startup. They are all vintage so it is a good chance there is a leak…which would be heartbreaking bc of the tubes.
 
Are the input RCA jacks clean and still tightly swaged to the chassis. I have seen one instance where the RCA jack was loose and caused a bit of noise. Have you ever had any restoration work done to it? They aren't young any more. Possibly a deteriorated solder connection or a failing resistor or capacitor. Buzzing vs. humming is usually a power supply noise so it's helpful to know if it is a 60hz hum or 120hz which has a moe buzzy sound to it. Try putting a shorting plug into the offending channel's input jack and see if it changes.
Happy hunting,
John
 
nerdorama said:
Are the input RCA jacks clean and still tightly swaged to the chassis. I have seen one instance where the RCA jack was loose and caused a bit of noise. Have you ever had any restoration work done to it? They aren't young any more. Possibly a deteriorated solder connection or a failing resistor or capacitor. Buzzing vs. humming is usually a power supply noise so it's helpful to know if it is a 60hz hum or 120hz which has a moe buzzy sound to it. Try putting a shorting plug into the offending channel's input jack and see if it changes.
Happy hunting,
John
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Hi, I’m not sure exactly if it’s a 60hz hum or a 120hz. When nothing is playing it’s slightly audible - like a buzzing noise floor. Honestly, when the music is playing it’s not very evident. The buzzing become more evident once the 225 got hooked up to the horn speakers.

The unit was gone over twice by AudioClassics who completely restored it at one point and then went over the unit again to take care of some additional issues.

But as you said it’s an old unit and I have listened to it quite a bit since. So I imagine something may be in need of replacement.

What is a shorting plug? And how do I clean the RCA jacks out?

I wanted to exhaust all the non-invasive remedy’s before having to resort to the invasive ones.
 
nerdorama said:
Shorting plug is just an RCA plug with the center pin shorted to the shell or a smaller value resistor connected between the two. It's a way to isolate the unit from upstream equipment to be sure it isn't a ground loop. Let's you see if it is really in the unit you are testing.
https://www.takefiveaudio.com/contents/19-rcainputshortingplugs
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Thanks for sending this, I may try to make one of these as I just cleaned out the RCA jacks and plugs with a little DeOxit and the noise is still there in the Right channel.
 
SaturationPt said:
Can you attach a short sound file here?
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Sure, I've uploaded a Vimeo link of the audio. I did this with my phone so please excuse the quality of this.


693216809
 
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It's pretty quiet, but to me sounds more raspy and not like a clean power-supply sine-wave. Are there any flourescent lights nearby? That could make a similar sound. Still, being one channel points to a problem, even if the light/ballast is getting through it shouldn't be.

Are you confident that all tubes are seated well in clean sockets?
 
SaturationPt said:
It's pretty quiet, but to me sounds more raspy and not like a clean power-supply sine-wave. Are there any flourescent lights nearby? That could make a similar sound. Still, being one channel points to a problem, even if the light/ballast is getting through it shouldn't be.

Are you confident that all tubes are seated well in clean sockets?
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there are no fluorescent bulbs or anything (I don’t think that is creating a sine wave. I did clean the sockets a little while back but will clean it again and reseat the tubes.

do you have a suggestion on how to properly clean the tube socket/tubes? Audio classics mentioned they just spray some DeOxit into the sockets and put the tubes in.

it sounded a little messy, so I’ve been carefully putting the DeOxit on the tube prong and then inserting them into the 225. - but this may not be enough to fully clean it.
 
I have taken an old tube, spray the pins on the tube, insert it a couple of times and let it sit, remove/wipe and do the same to the next socket. If any seem loose you can slightly adjust (bend) them to tighten.

Still, I think it needs a trip to a tech at this point.
 
SaturationPt said:
I have taken an old tube, spray the pins on the tube, insert it a couple of times and let it sit, remove/wipe and do the same to the next socket. If any seem loose you can slightly adjust (bend) them to tighten.

Still, I think it needs a trip to a tech at this point.
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Thank you - that is really helpful.

If this DeOxit trick doesn’t work, I’m afraid you might be right about taking it in. I guess I’m just trying not to have to send it in bc the wait time of like another month or two- regardless probably for the best.
 
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