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MA2275 Phono Stage Interference Help - Stumped!

Matt The Cat

Matt The Cat

Prowlin' At 78 RPMs
Hi,

I'm stumped and I really could use some advice on how to fix possible RF, EMF or grounding issues on the phono stage of my MA2275.

It was just in the shop for 2 blown transistors controlling the bias of one of the KT88s. That issue was fixed, but as soon as I hooked it up, I noticed a much increased noise floor on the phono stage, V4 and V5 on this amp. I have tried a variety of remedies to try and bring this interference down. I've recorded the noise with my cell phone to show how loud it is and what it sounds like. Here is a link to the 5 secs of the phono stage interference.
http://www.mattthecat.com/MacPhonoDistortion.wav

The phono stage sounds fine, except for the interference. All line level inputs are perfectly quiet. The turntable is grounded and the interference doesn't change AT ALL when the turntable is disconnected. I've used shorting cables and they bring the interference down slightly. Thinking it might be a grounding issue, I've taken out the grounding pin on the amp, I've moved both the amp and the turntable to the same circuit...then moved them to different circuits. NO CHANGE AT ALL. I've upgraded my power strip to a Furman PS8 to try and clean up my AC. NO CHANGE AT ALL.

THE TUBES: I've changed out the tubes in V4 and V5 many times and found NO CHANGE. I bought some tube shields and installed them. They seemed to make things worse. When I touch the tube in V4, the interference becomes MUCH worse with more static. Then when I separately touch the tube in V5, a loud hum is introduced. This happens the same on every 12ax7 that I have tried. Touching the ax7s in the line level section has no affect on anything.

I've talked to the McIntosh certified tech who worked on the amp about this and he assures me that he didn't do anything while he was inside the amp. He said that by working on the transistors under a power tube, he didn't mess with anything in the phono section. He said the boards are completely separate and there is no grounding in there to mess with. Can anyone else confirm that there isn't a simple grounding fix within the amp?

The 2275 is not located near a TV, antenna or wifi router. I've unplugged the only light dimmer in the room. I've shut down all computer equipment and baby monitors. I can't for the life of me figure out how to solve this problem.

Please let me know if you have any ideas or can comment on possible grounding issues in the phono stage itself. Is there anything I haven't tried? This is starting to drive me crazy.

Thanks so much!

Matt
Salem, MA
 
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I assume you hear the interference the same when the cage is installed and screwed into place, correct?

Since this appeared after the power amp was repaired, I think that your tech may have damaged something or knocked something loose or some such. Did you ask your tech about it?
 
"Did you ask your tech about it?"

Matt The Cat said:
I've talked to the McIntosh certified tech who worked on the amp about this and he assures me that he didn't do anything 1while he was inside the amp. He said that by working on the transistors under a power tube, he didn't mess with anything in the phono section. He said the boards are completely separate and there is no grounding in there to mess with.
Click to expand...
 
That's a great question regarding the tube cage. When I place the tube cage back onto the amp, the distortion is slightly reduced.

Not only did I ask my tech about it, but I took the hour and a half drive back down to drop it off again. He was convinced that he didn't do anything while he was inside the amp and that the noise was brought on by a microphonic tube. I have changed the tubes out several times and I know I don't have a microphone tube in there.

Would I harm the circuit at all if I installed a, say 12au7 tube in the phono stage to bring the gain down slightly? Maybe as a way to determine if the tubes are involved in this distortion or if it is solely coming from grounding issues?

Thanks,
Matt

TSmith8605 said:
I assume you hear the interference the same when the cage is installed and screwed into place, correct?

Since this appeared after the power amp was repaired, I think that your tech may have damaged something or knocked something loose or some such. Did you ask your tech about it?
Click to expand...
 
Matt The Cat said:
. . .Would I harm the circuit at all if I installed a, say 12au7 tube in the phono stage to bring the gain down slightly? Maybe as a way to determine if the tubes are involved in this distortion or if it is solely coming from grounding issues?

Thanks,
Matt
Click to expand...

No don't mess with wrong tubes.

damacman said:
Time to take it back to the tech ...
Click to expand...

Maybe a different tech . . .
 
Yeah, if I have to take it in again, it will be with a different tech. I was only using this shop far away from home, because they are the closest authorized McIntosh repair place. If I take it in again, I'll just take it to Boston/Cambridge. I guess I really didn't need to use an authorized tech since my 2275 is not under warranty or anything.

When I went to pick it up concerning the phono stage noise, I discovered that the tech was only using headphones (some 1960s/70s cans) to monitor the noise issue. He had never hooked it up to speakers. That's a problem as the 2275 has a pretty weak headphone amp and under regular listening conditions, you have to turn the volume up to the 1 or 2 o'clock when using headphones. So I don't think he even properly heard the issue.

At 77lbs, I really don't want to have to move this beast again. I've already done so 4 times and my listening room is in the attic of a very old house, up a very narrow attic stairway.

Is anyone familiar with the inside of a 2275 and how the phono stage is grounded to the chassis?

Thanks,
Matt

TSmith8605 said:
No don't mess with wrong tubes.



Maybe a different tech . . .
Click to expand...
 
I've decided that before I bring the amp back to the shop (a new shop), I'm going to open her up and take a look for myself. I really feel this must be a grounding issue inside the amp, due to the fact that the tubes react to my touch. It seems like the whole channel is very susceptible to any RF and EMF around. If anyone has worked in this amp or modern McIntosh tube amps in general and can lend a suggestion as to what to look for in regards to internal grounding, I very much appreciate it.

Thanks,
Matt
 
Call Ryan at Audio Classics. He will be straight with you. He will want to see it for sure, but he may have an idea or have seen the issue before. They sold quite a few of them.
 
Matt The Cat said:
Thanks for the warnings, guys, but how could there possibly be "lethal" voltage inside an amp that not even plugged in. Are you referring to the caps holding a charge?

Matt
Click to expand...
Not going to join the argument about how much electrical shock is OK. Knock yourself out though. Figuratively, not literally.

-Geoff
 
Hi guys,

OK, so I finally got the time to open up the 2275 and take a look. All in all, it was a good experience and since I don't have much experience with circuit boards and all, I was only going inside to see if anything seem out of whack, grounding-wise. I was very careful and checked all the ribbon wire connections to make sure they were secure. I looked at the phono stage, the underside of the tube sockets, the ground and all looked fine and felt secure. There were no electric shocks or anything like that. I had unplugged it overnight, so I'm sure all the caps had lost their charge by then. Below are some pics from inside the amp. I put her back together, hooked her up and the noise was still present on the phono stage.

I'm still stumped.

It makes me wonder, though. Is it possible that a cap, resistor or transistor has blown on the phono stage somewhere that might invite it to become ungrounded and allow it to pick up all this extra noise?

Last week, I got a rare opportunity as I woke up to not only my cable/internet being out, but also my call phone tower was down. Figuring there would be reduced RF and EMF in my area, I booted up the 2275 to see if this affected the phono channel noise at all. It did. Big time. The noise was much, much lower. But this test doesn't tell us anything we don't already know. We know the phone stage is picking up a lot of RF and/or EMF, but the question is, why? I've lived here for almost six years and the 2275 didn't pick these frequencies up six months ago, before it went into the shop for an unrelated issue.

If anyone has any thoughts or insight, please let me know. Besides the added noise, the phone stage sounds GREAT! I'm still enjoying my records, BUT in-between sides, I can pick up the RF noise from my listening place and then that brings me back down to Earth again. I'd love to get this figured out. The tech I took the amp to six months ago was unable to replicate the problem in his shop as I guess RF is not bad down there. He's been trying to help me figure this out over the phone, but is stumped as well.

Matt
 

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You say it does not pickup the noise in other repair shop, correct?

Can you hear it is a station or is it just a buzz, squeal or some such noise.

I would suspect a cold solder joint or lead dress......
 
Thanks for the reply.

The noise is constant and gets greater when I touch the 12ax7s. When touching one, a hum is introduced and when I tough the other, static. The static almost reaches radio levels, but doesn't quite bring in any stations or anything. Here is a link to a short video I made of the phono stage noise.
http://www.mattthecat.com/MacPhonoVideo.mp4

Matt

c_dk said:
You say it does not pickup the noise in other repair shop, correct?

Can you hear it is a station or is it just a buzz, squeal or some such noise.

I would suspect a cold solder joint or lead dress......
Click to expand...
 
Last edited:
So, it's been over a year and I still haven't figured out the solution to the RF that my phono stage is picking up. I have cleaned the tube sockets, since I thought they had something to do with it. When you touch the 12ax7s in the phono stage, the RF distortion is amplified a bit. Cleaned sockets and replaced NOS tubes with new production Gold Lion ax7s. They sound great, but on mid to high volume listening, you can pick up the noise floor from my listening place. Annoying, sure, but the sound quality is still there and it doesn't ruin the entire experience. The noise floor can be heard even at very low level listening if you seek it out. I have a sound file of the RF distortion listed above. If anyone has any thoughts, please let me know.

Thanks so much.
Matt
 
Matt The Cat said:
So, it's been over a year and I still haven't figured out the solution to the RF that my phono stage is picking up. I have cleaned the tube sockets, since I thought they had something to do with it. When you touch the 12ax7s in the phono stage, the RF distortion is amplified a bit. Cleaned sockets and replaced NOS tubes with new production Gold Lion ax7s. They sound great, but on mid to high volume listening, you can pick up the noise floor from my listening place. Annoying, sure, but the sound quality is still there and it doesn't ruin the entire experience. The noise floor can be heard even at very low level listening if you seek it out. I have a sound file of the RF distortion listed above. If anyone has any thoughts, please let me know.

Thanks so much.
Matt
Click to expand...

I just listened to to your video. Did you have the volume control all the way up for that?

Can you hear the noise with headphones? If so, maybe physically move the unit somewhere else on the house or to another house to see if it changes (like moving an antenna to change reception). If you are near some sort of radio noise source, moving the unit somewhere, like into the basement, might change or eliminate the issue. If you do find a close source of RF noise, you can maybe complain to the people responsible or the FCC or something like. Note we all live close to cell towers and we all don't have that issue.

If there is no nearby RF noise source, you should send it to Audio Classics for repair.
 
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