Noise from MC 225

[Russellc]

Hello, this is my first post here. I have a question about my MC 225. Shortly after powerup, one channel begins to make a sound like e-e-e-e-eeeeeeEEEEEEEE, which gradually gains intensity until I quickly powered down. Tube switching does not affect it.
It appears to be an oscillation of some sort. It was suggested to me that one of the caps in the feedback circuit could be the culprit.
Any thoughts on this would be aapreciated. The cap appears to be a small disc shaped cap. like a ceramic, of 630pf. if it is this part, any suggestions for a replacement would also be appreciated.

Thanks, Russellc

[Rob]

Russell,

It won't hurt anything to lift one side of that suspect disc cap and try the amp. I don't think that is the problem.

Was this amp ever working properly for you before or have you just acquired it?

[Russellc]

Yes, i have had the amp for several years. I went through it when I first got it, replacing all the electro caps in the amp, and a couple on the board. I left all the resistors alone as well as any disc caps. I also left all the caps that sort of look like film caps. the amp worked from the minuet it was fired up until recently.
I also completely retubed the amp at the time of restoration.
Then one day I fired it up, and one channel kind of had this "e-e-e-e-e-e-e-e-e-" noise that steadily gained in frequency/intesity, until turn off. this is not the really loud funcky noises that an unstable amp would make, just its increasing intensity made me believe it was some sort of stability problem. It also happens only in one channel, which basically excuses the power supply. Absent a ground problem around the feedback circuit, i figured as well as another aquaintence, that one of the feedback caps or the associated resister had a problem. The only other thing I can think of would be a leaky coupling cap? Any ideas or suggested diagnosis would be appreciated, I hate to start hacking around inside this amp without being a little more sure of what the problem is. Unfortunately, I do not own an ocilloscope.

thanks, Russellc

[Rob]

OK. I guess then something has changed in the circuit that has caused the FB loop to be making positive FB instead of negative. This could occur if a gain stage has gone from inverting to passing it straight up without gain which might happen if a coupling capacitor shorted. I wish I had a screwmatic to see the circuit to give you better advice.

Try disconnecting the FB loop to the input tube cathode and see if the howl goes away. If it doesn't that might help localize it by then pulling tubes, starting with the input stage.

Just because it occurs only in one channel I wouldn't rule out power supply bypass caps. An open electrolytic, if the channels use separate B+ runs in the low level stages could pass HF feedback and cause oscillation as you describe. To find this culprit just parallel a good 10 to 20uF 450 volt electrolytic (negative tied to the chassis) with a clip lead to each of the bypass electrolytic positive terminals while the amp is running, or shut it down each time to swap the lead. Best not to run the amp steadily with the oscillation going full blast if it is producing considerable power. When (if) you reach an electrolytic that has gone open the oscillation will stop.

[dewickt]

Pulling the 12AX7 first stage kills all FB, 12AU7 is phase inverter it must have a perfect cap on pin 7 to ground (.25uf) any leakage and pins 1 and 6 will not match and all bets are off. Check the 68 ohm screen resistors on 4 or 8 of outputs, and also grid 470 ohm resistors on pin 6 of the outputs. Have a feeling that if the first stage is pulled and it still takes off you have a resistor shifting in value, most are easy in circuit checks, can compare good to bad channel values, power off for resistence checks and make sure that the power supply is at 0V before checking (if not they may give false readings).
If you don't find any problem after all of these checks let me know and I will think of some more hints.

[Russellc]

Thanks for the input, lord knows I need it. I don't believe that it is the power supply for two reasons: One, i totally replaced all the power supply caps. The two larger ones are mallorys, which bolted right in the original locations. There is also a 220 mfd cerafine shoehorned in there as well. second, if it were the power supply, wouldn't it affect both channels? Also, this is not a howl like an oscilating amp can make, it just an e-e-e-e-e-e noise, that really sounds like someone pronouncing the sound of the letter "e", except it slowly rises in frequency. Besides the mallorys and the cerafine, there is also a smaller cap in the power supply area that I replaced with a sprauge atom. On the boards, it looks like I also replaced several .47 mfd caps with sprauge 730p caps (four of them) and also on one end of the board I replaced two caps, right next to each other. I canm't quite see them very well. All of these caps were selected and orderfor me by a friend of mine who repaired mcintosh tube amps (and others) for one of the oldest mcintosh dealers in Kansas City. He suggested that I leave the rather large "film looking" brown caps on the board, and leave the resistor alone. This formula worked for several years, (4-5) until this e-e-e-e-e-e- thing started.

As for pulling tubes while the amp is running and all that I'm not sure (I don't know enough) to know what I am checking for. I understand that the 12ax7a is the first stage, with 1/2 driving each channel, and that the feedback returns to it. Exactly what am I doing by pulling it? Doesn't that make some god awful transients through the old speakers? Obviously with it pulled, there would be no feedback, so can you explain to me how I use this to diagnose? Is the amp stable (normally ) like this?

Sorry to be so dense, I appreciate the help. Russellc

[dewickt]

Might get a pop but not much of one, to be sure pull it with the amp off then turn it on, this amp is very stable and should not do anything strange with any of the tubes pulled.
Has the bias filter been replaced, 10 uF - C30 - it can be most any value safely, I use a 22uF @450V for this cap, they will last forever.
A safe trick to isolate your problem might be to remove tubes one a time to find where the noise starts in the chain.

[Russellc]

Again, sorry to be so dense, but as i understand what you are saying, is to start with the first 12ax7a, pull it out, power up, and see if noise is still there. If it is, problem not with this stage ( which I believe would also excuse the feedback circuit), then power down, replace the 12ax7a, remove the next tube, I believe a 12ua7, power up, and so on untill I hit the place where the noise goes away? Should BOTH 12au7 be removed at same time, or one at a time? Same question as to the 12bh7 tubes?

Thank you very much for your time on this,
My A-7s are crying out for this amp!

Russellc

[Rob]

Yes, that is what I was trying to suggest above and also i'm sure what Dewickt was saying. In my case though I wanted you to open the feedback path in the amp wiring first and try that test before proceding with pulling the tubes. I also suggest that you only pull one tube at a time (i.e. one 12AU7, not both) and then replace it in its socket as you remove each successive tube in the chain. Power down each time before you pull a tube. Give the amp 20 seconds or so after powering down each time before re-applying power. This eases the strain on the power supply seeing hot tube filaments ready to gobble B+ current.

[Russellc]

Thanks for the clarification, I will try all this tonight. When you suggest disconnecting the feedback before pulling tubes, you mean at the 12ax7a, right? I have a schematic somewhere around here, I will search for it as well. many of the components in the mcintosh amps are very securely wrapped around mounting tabs that also have other components "very securely" inter wrapped along with them. i have tried other 12ax7a s in this position just to ascertain that the tube is not the problem. maybe the feedback circuit will more easily lend itself to being dsconnected, but changing out caps on the board was fairly hairy, as securely as the components were wrapped and soldered in place, and the boards as fragile as they are. I wish a new mcintosh board was available, i would painstakingly replace that sucker...the original is fragile. I will let you know how it goes tonight with this feedback disconnection/ tube pulling deal.
Thanks again,
Russellc

[Kamakiri]

Link to schematics:

http://www.berners.ch/McIntosh/en/Frame_McIntosh.htm

[Rob]

Russel,

You can simply open the feedback circuit anywhere convenient, not just at the input tube. The reason I want it opened is that if it doesn't make the howl stop then pulling tubes sequentially after the FB is removed will help identify the tube related to the problem. If for example the problem is in the feedback circuit then by removing any tube will make the howl stop. Leave the FB circuit opened up for all the tube pulling tests.

Thanx Tim for posting a link to the screwmatics. I cannot view them as I have not got a working .pdf viewer in this replacement computer despite trying to install one. I can build apparatus and instrumentation to detect radio signals from a distance of billions of light-years, but I can't do hard stuff like getting computers to work. :withstpd: :dammit:

If this thread were entitled "Noise from NGC 225" maybe I could be of more help! :)

[Russellc]

Good lord!
Now I am really confused. Ijust put the tubes back in, cleaned the dust bunnies out and put amp in system and it no longer makes the noise! I am letting it run to see if it starts up again. When this first happened, I fiddled with tubes, cleaned contacts, etc., and nothing made any difference, upon fire up..e-e-e-e-e-e- would
begin to grow in pitch and loudness! Now, its quiet as only a mcintosh amp can be. There was not that much dust in the amp, but there were some "spider web" looking stuff in a few spots in there, but nothing worse than I have found in other amps. The only other thing is that it has set unused for several weeks. I am letting it burn for a while to see if the escalating whine noise starts again...so far dead quiet..at least on the current 90db efficient speaks which are not as revealing of noise as the much more efficient altec A-7s. Will report back when this stops. I really don't understand this, as on two seperate back to back occasions I tried fiddling with everything, yet the problem persisted like a bad rash, constant and steady. Anybody ever have such a problem just go away?


Heating up the 225,
Russellc

[Russellc]

I would like to thank all of you for your input, any additional would be appreciated! The tube pulling diagnosis was very enlightening, and thanks for the berners site, great mcintosh info. Will keep you all posted about the 225.

Thanks again,
Russellc

[Rob]

Maybe that noise was Mars screaming in for its closest encounter in 6000 years. Chalk this up to unsolved misteries. Yes this kind of thing does happen on occasion. It is when it returns, hope it doesn't, that it can drive you nuts in diagnosis. Keep your fingers crossed and enjoy the tunes.