mc 250 with hum

Discussion in 'McIntosh Audio' started by muscmp, Jan 26, 2019.

  1. muscmp

    muscmp Active Member

    Messages:
    173
    Location:
    socal
    i broke down one of my two mc250 amps due to a hum. i have not run any voltages yet. i'm enclosing some pics. though old, 1967, the can caps and axials all seem fine. i don't see any burn spots anywhere.

    any recommendations?
    thanks,
    mikeB

    mc1.jpg mc2.jpg mc3.jpg mc4.jpg mc5.jpg mc6.jpg mc7.jpg mc8.jpg
     

     

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  2. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Second pic down on the left, looks like a broken lead off the cap that looks like a resistor........
     
  3. Pio1980

    Pio1980 AK Member Subscriber

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    A DVM on AC could show the effectiveness and balance of the power supply elyticaps for starters. If the amp is passing a signal, but with him, check for hum with the level controls fully down to find whether fault is internal or external to the amp.
     
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  4. muscmp

    muscmp Active Member

    Messages:
    173
    Location:
    socal
    this is 2nd pic down. i chopsticked but didn't find anything loose.
    thanks for your eyes.
    mikeB mc3.jpg
     
  5. muscmp

    muscmp Active Member

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    173
    Location:
    socal
    great info, thanks
     
  6. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Third pic down then..have a look..cap on the lower left...looks like a resister....lead on the right hand side...
     

     

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  7. Pio1980

    Pio1980 AK Member Subscriber

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    You are looking for the for a balanced reading between the large elyticap can and neutral. If not, one may be going out.
     
  8. Pio1980

    Pio1980 AK Member Subscriber

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    The caps further downstream should absorb any residual ripple if all else is good. This is a simple but non standard procedure, so readings are relative.
     
  9. Pio1980

    Pio1980 AK Member Subscriber

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    A DVM, Oscilloscope, and in-circuit ESR checker are handy for basic checks, many hobbyists might have only a DVM.
     
  10. muscmp

    muscmp Active Member

    Messages:
    173
    Location:
    socal
    the top orange one is just a bent right hand lead. it is connected very well.
    thanks,
     
  11. muscmp

    muscmp Active Member

    Messages:
    173
    Location:
    socal
    i have a dvm. i'm used to working on tube guitar amps so this is a different animal to me. i did just recently buy a used heathkit oscope but i have not done any beginners practicing. i believe it is a 40ms scope and i have a 10x probe.
    i hope to do my testing tomorrow. i've built a load box with 4 and 8 ohm resistors and cheapo 6" speakers when i need to hear something.
    thanks,
     

     

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  12. Pio1980

    Pio1980 AK Member Subscriber

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    The o'scope can show you if the power supply is sufficiency "clean", and the residual hum and noise level.
     
  13. SA7I

    SA7I AK Subscriber Subscriber

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    Have you called AudioClassics?
     
  14. mcgjohn

    mcgjohn Active Member

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    Location:
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    yep, agree with Pio1980, you need to measure the power supply caps. If you don't have an ESR meter,. you can use a DVM set to AC mv and look for AC ripple on the meter. Anything over a few mv would suggest the caps are shot.
     
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  15. Pio1980

    Pio1980 AK Member Subscriber

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    DC imbalance as well, check both.
     
  16. muscmp

    muscmp Active Member

    Messages:
    173
    Location:
    socal
    i've been out of it with a cold so i'm finally getting some voltage readings. i've enclosed pdf file that includes the schematic

    mc 250 voltages

    Line voltage 113

    C3 and C4, 100uf/12v input section both 15.68v

    C11 and 12, 330uf/3v left and right output sections, .009v left and .058v right

    C27 and 28, 9300uf/45v power supply section 41.4 and 41.6v

    C29A, B, C and D 80/200,80/200,150/150 and 50/150 power supply section A105.3, B112.3,

    C84.5, D98.2

    C31A200/100 and B 500/75, power supply section, A-42.2, B-57.8

    C32 and C33, 500/16, power supply section, C32 15.64, C33 -11.44

    thanks,
    mikeB
     

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  17. LectroLinear

    LectroLinear New Member

    Messages:
    12
    Me2
    Early McIntosh MC250.
    Okay, this thing is messing with my mind. First year model. Bring it up on a variac get a loud 120Hz buzz through both channels on stereo setting at low voltage with vol controls at min which increases to serious jagged waveform at about 30V AC. All wiring to caps appears correct and grounds test as ground. Removed and tested filter caps on nice LC101 tester. Replaced both multisection caps with new as test bad, large 9300uF caps test only 400uA leakage and ESR of .18 ohm and within 5% of each other so using them. After reforming caps, installed and same problem. Scope confirms serious 120Hz on positive rails at bridges (negative rails are smooth). Tried swapping big caps, no difference scope is showing all hum on positive rail. Check bridge diodes with Fluke 77 says OK. Remove power from all but power amp, no change. Remove power from each channel independantly, no change. All four heatsinks have 2 insulators (nylon and paper) each side of main chassis.
     
  18. LectroLinear

    LectroLinear New Member

    Messages:
    12
    can you elaborate on what you mean by a DC imbalance - between positive and negative rails with reference to ground? As I bring mine up on variac the offsets are not the same even if I switch the 9300uF cans.
     
  19. Pio1980

    Pio1980 AK Member Subscriber

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    Look for corroded or lifted chassis/ common ground somewhere.
     
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  20. Pio1980

    Pio1980 AK Member Subscriber

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    DC and AC voltages should be essentially identical between them, a balanced condition.
     
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