Distorted MC275 vi

[tdelahanty]

Originally Posted by FreddieFerric

I don't come to these forums to argue with other members, but I do try to help others whenever I can. You go on and believe anything you wish to. I'll do the same and we'll leave it at that.

I chose my profession from my mother's love of music, we spent many hours listening together, the old Mo-Town stuff. As a fourteen year old I spent countless hours trying to burn down the house while working on old Crosley push-pull amps with 6L6 output tubes. During my career reality and expediency taught me my job was to fix things not to do detailed circuit analysis on every problem (which is exactly what I wanted to do). I fully understand the laboratory environment, which is fantastic for those qualified.

Sometimes I regret mixing profession and hobby. I think we can agree the quickest and safest thing to do is replace the tubes.

[sandbites]

I appreciate all the comments/suggestions and the different approaches put forward here has made me feel comfortable that all is not lost with the amp.

As much as i am challenged to open up the amp and test the voltages, my past electrocution experiences as a kid still haunt me. In the interest of safety, specifically mine, I will tube roll all the tubes when i can get my hands on them. If that does not work then i will open up the beast and see what's wrong.

If i do muster the courage or i am led astray by boredom I might open up the amp. Boredom does make one irrational, we are on our 19th day of quarantine with 10 days more to go.

Question can a blown resistor/ capacitor cause this grittiness in the sound?
If i do open up the amp, would blown resistors/ capacitors manifest themselves by sight or smell? Where should i look?

If it's not detectable by sight or smell, where and what do i check first before i decide to throw in the towel and send it in for repairs.

[tdelahanty]

Originally Posted by sandbites

I appreciate all the comments/suggestions and the different approaches put forward here has made me feel comfortable that all is not lost with the amp.

As much as i am challenged to open up the amp and test the voltages, my past electrocution experiences as a kid still haunt me. In the interest of safety, specifically mine, I will tube roll all the tubes when i can get my hands on them. If that does not work then i will open up the beast and see what's wrong.

If i do muster the courage or i am led astray by boredom I might open up the amp. Boredom does make one irrational, we are on our 19th day of quarantine with 10 days more to go.

Question can a blown resistor/ capacitor cause this grittiness in the sound?
If i do open up the amp, would blown resistors/ capacitors manifest themselves by sight or smell? Where should i look?

If it's not detectable by sight or smell, where and what do i check first before i decide to throw in the towel and send it in for repairs.

I have a story which may ease your mind and make you feel better about your MC275. Early in my career I worked on guided missile telemetry which used analog signals. There were two test stations that used MC275 amps to inject analog signal simulating telemetry commands. The amps were used 16hrs a day and left on 24hrs a day and never failed except for an occasional tube replacement. Just tremendous!!!

I strongly suggest changing the tubes and not opening the amp if you have no formal training.

[tdelahanty]

Originally Posted by sandbites

I appreciate all the comments/suggestions and the different approaches put forward here has made me feel comfortable that all is not lost with the amp.

As much as i am challenged to open up the amp and test the voltages, my past electrocution experiences as a kid still haunt me. In the interest of safety, specifically mine, I will tube roll all the tubes when i can get my hands on them. If that does not work then i will open up the beast and see what's wrong.

If i do muster the courage or i am led astray by boredom I might open up the amp. Boredom does make one irrational, we are on our 19th day of quarantine with 10 days more to go.

Question can a blown resistor/ capacitor cause this grittiness in the sound?
If i do open up the amp, would blown resistors/ capacitors manifest themselves by sight or smell? Where should i look?

If it's not detectable by sight or smell, where and what do i check first before i decide to throw in the towel and send it in for repairs.

I have a story which may ease your mind and make you feel better about your MC275. Early in my career I worked on guided missile telemetry which used analog signals. There were two test stations that used MC275 amps to inject analog signal simulating telemetry commands. The amps were used 16hrs a day and left on 24hrs a day and never failed except for an occasional tube replacement. Just tremendous!!!

I strongly suggest changing the tubes and not opening the amp if you have no formal training.

[tdelahanty]

Sorry somehow I double punched the submit, please delete one.

[W9TR]

Originally Posted by sandbites

I appreciate all the comments/suggestions and the different approaches put forward here has made me feel comfortable that all is not lost with the amp.



As much as i am challenged to open up the amp and test the voltages, my past electrocution experiences as a kid still haunt me. In the interest of safety, specifically mine, I will tube roll all the tubes when i can get my hands on them. If that does not work then i will open up the beast and see what's wrong.



If i do muster the courage or i am led astray by boredom I might open up the amp. Boredom does make one irrational, we are on our 19th day of quarantine with 10 days more to go.



Question can a blown resistor/ capacitor cause this grittiness in the sound?

If i do open up the amp, would blown resistors/ capacitors manifest themselves by sight or smell? Where should i look?



If it's not detectable by sight or smell, where and what do i check first before i decide to throw in the towel and send it in for repairs.

Yes - a blown resistor or cap can make the amp sound bad. Is it happening on both channels?

Yes - sometimes blown components are clearly visible, but sometimes not.

There are four large rectangular resistors, one for each power tube. If a tube fails, these resistors will sometimes overheat and fail to protect the output transformer.

Before you get your new tubes, remove the bottom cover after the amp has been off for an hour or so. This will allow plenty of time for any high voltages present to dissipate. A coupe of minutes will actually do it, but, hey, we’ve all got some time now, right?

Take a look at all the parts - sometimes damage is visible. Anything that smells burnt or is discolored should be investigated.

If you have an Ohm meter or can borrow one, pull the power tubes and measure the value of the four large rectangular resistors. They should be around 220 ohms. Anyway they should match the value printed on the side of the resistor. A blown resistor will be an open circuit or will measure something way off of 220 ohms. These resistors are the most common failure when a power tube goes “tango uniform”.

If you don’t see any obvious damage, and the four big resistors are ok, go ahead and install all the new tubes and fire that baby up.

If the noise persists, it is unlikely that you will be able to find the issue via simple voltage measurements so it would be best at that point to send it in for service to avoid having to deal with the lethal voltages inside the amp.

Tom

[FreddieFerric]

I took a peak under the hood of my MA8000 when stray voltage from an unexpected lightening storm zapped the digital audio board. It was out of warranty, so no harm with that. What I learned? There are no user serviceable parts inside.

If changing the tubes doesn't cure the problem, I'd suggest a trip to your local McIntosh service center.

[sandbites]

The grainy sound got worse since my original post. The voice became more raspy and grainy and the sound was more distorted and weaker. Out of boredom I used my LINN as a preamp on the 275 and the sound that came out was smooth making me realize that the culprit all along was the Mcintosh C220 preamp. I ordered and installed Mullard 12AX7 reissues. The problem was cured. Gone were the defects. The music is once again clear and sharp. Guitar strings are being plucked cleanly. Bass and mids are round and solid. Only difference is I seem to have lost some top that i had to dial up the treble a little.
The system is still 2% raspy so i might roll the signal tubes for the Mc275 anyway.
I already contacted Jim McShane for the replacement tubes but if my shipper cant ship soon i might get locally even at a higher price. I will let you know then.

My choices for the new issue small tubes are JJ, Electro Harmonix, Sovtek, TUngsol and Mullard. Which one gives the highest treble or at least does not cut it off that much.

picture of the new tube and old tubes
[IMG]Mullard 12AX7 by sandbites, on Flickr[/IMG]

[miner]

For future reference, if you swap L/R cables between the amp and preamp, and the issue moves to the other channel you know the problem lies within the preamp. If it does not switch channels, the amp has the issue.

[sandbites]

Originally Posted by miner

For future reference, if you swap L/R cables between the amp and preamp, and the issue moves to the other channel you know the problem lies within the preamp. If it does not switch channels, the amp has the issue.

You are correct. Initially the left channel became weaker that the right Andi suspected the speaker for a long time. It was only when the distortion progressively got worse that I suspected the pre-amp.