MC 250 Oscillation

Discussion in 'McIntosh Audio' started by basil13, Sep 13, 2017.

  1. basil13

    basil13 AK Subscriber Subscriber

    Messages:
    101
    Location:
    Ontario, Canada
    Hi All,

    I have been working on a stubborn MC 250 with some help from c_dk and thought I was successfully done with it. This has been my first MC 250 to work on and has been an adventure.

    After a few days of play testing, I noticed some distortion on one channel. I put it on the scope and sure enough there is some oscillation on the positive half cycle (see picture).

    It only happens under load, it is clean otherwise.

    The channel is using the original drivers and output transistors. The class A board still has its original transistor as well. However, I had to essentially rebuild the class A board with many new parts.

    The 0.56 ohm 5 watt wirewound resistor was replaced previously with a NTE, I have read that Mac does use NTE parts, is this a Mac replacement (see picture)? If not, could it be the source of my problem?

    Any other parts I should look at troubleshooting?

    An oscillation problem is a new experience, so any other advice would be appreciated.

    Regards
    Bob
     

    Attached Files:

    Last edited: Sep 13, 2017
  2. c_dk

    c_dk Addicted Member

    Messages:
    5,498
    Location:
    West Michigan
    Is that R56? Mac used Rockwood or later Cober wirewounds that were inductive. Not much but just enough to keep the circuit stable.

    When Rockwood changed their manufacturing process for .15s as used in the MC2205 Mac had to change the circuit to work with the lower inductance.

    PM me again
     
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  3. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,754
    Location:
    France
    I already get this trouble and the answer is not really good .
    Can you try to disconnect the grey wire (output of power amplifier who goes to the input of output transformer ) and connect it to a oscilloscope input . See if the signal stay clean or not and this at all levels .
    If yes , good news !
    Can you make high precision measurement of this resistor ? Is it inductive or not ?
    Waiting to know more
     
  4. c_dk

    c_dk Addicted Member

    Messages:
    5,498
    Location:
    West Michigan
    I still have a stash from 25 years ago and will send him one to confirm if needed.
     
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  5. basil13

    basil13 AK Subscriber Subscriber

    Messages:
    101
    Location:
    Ontario, Canada
    Hi Patrice

    Just to clarify, I assume you want me to check the output transformer by feeding the signal into it by connecting my signal generator to the gray wire?

    I did that with the dummy load connected and got a clean signal measured across the load. 2 volts is the max signal it will send which should be fine because I was seeing the oscillation at low gain.

    Unfortunately I don't have a precision tester to measure the resistor, I will take c_dk up on his offer though.
     

    Attached Files:

  6. c_dk

    c_dk Addicted Member

    Messages:
    5,498
    Location:
    West Michigan
    Will the Mounties let a poor old wirewound into the country or will I have to get it a passport?

    The last time I went into Canada they were concerned I wanted to stay.....of coarse that was in the early 70s and well you know there was a small emigration of draft age men......

    PM me a address and I will mail you one and we can settle this issue.....
     
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  7. arts

    arts Super Member

    Messages:
    2,480
    Location:
    Qc, Canada
    Considering how many ''refugees'' are illegally flooding across the Canadian border (especially in my neck of the woods) from the US presently due to your governments policies,perhaps WE need to build a wall and send Trump the bill.

    Don't wise-ass my country,I really don't appreciate it or tolerate it. And as far as your draft-dodging hippies go,I would have had them returned to you. No amnesty.In my world,your deserters and traitors would have been shot,not welcomed.

    Best regards,

    Art
     
  8. MACKIE1975

    MACKIE1975 Well-Known Member

    Messages:
    521
    Lighten up.
     
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  9. c_dk

    c_dk Addicted Member

    Messages:
    5,498
    Location:
    West Michigan
    I would say that those young men would have called themselves resistors also......'twas nothing but a play on words....

    We were going up to Agawa Bay to go camping and rock climbing.....the young Canadian border official took issue with how much food we brought with us for a 4 night stay. The seasoned veteran official looked in our cooler and predicated we being 19 years old would be out of food in two days.....so sent us on our way.

    We had a great time and went up a few times more.
     
  10. basil13

    basil13 AK Subscriber Subscriber

    Messages:
    101
    Location:
    Ontario, Canada
    I received the resistor from c_dk yesterday and installed it today. The resistor has taken care of the oscillation issue so far.

    The amp has provided so many twists and turns I am apprehensive to say its resolved. I will monitor it next week and report back if the oscillation returns.
     
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  11. c_dk

    c_dk Addicted Member

    Messages:
    5,498
    Location:
    West Michigan
    I am glad that you have so far been able to confirm my memory from 30 years ago......I am sure this tidbit was shared by DOB during a amp clinic.

    I am sure there are WWs currently available from suppliers with the needed inductance, can't be much, but without a scope and test bench facilities for a repaired amp how would you know......
     

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