Mcintosh MC30s Restoration (Looking for correct schematic)

Discussion in 'McIntosh Audio' started by Tysen, Jan 9, 2017.

  1. Tysen

    Tysen AK Subscriber Subscriber

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    Well. Set 110v, my variac is putting out 125v. Wow. I feel kinda stupid, but I assumed variacs put out the setting that they're on....so set at 110v would be out 110v.


    Overall, my AC line appears to run between 120-130v.
     
  2. BillWojo

    BillWojo AK Subscriber Subscriber

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    Unless there is a flat on the shaft for the setscrew in the knob, that knob could be pointing anywhere. Always verify with a meter.

    BillWojo
     
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  3. x3workshop

    x3workshop AK Subscriber Subscriber

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    Yeah, I have had (bad) experience with that high LA area voltage.
     
  4. Pio1980

    Pio1980 AK Member Subscriber

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    Does the variac data state the reference input voltage?
    If 110 VAC, the ratio is 1/1 at that setting regardless of dial markings.
    One of my variacs has a dial on which I've inked the 1/1 position, the other a voltmeter that I've corrected for age drift. The 2nd one has been mod'ed for a series lamp current limiting test with a bypass switch for the lamp.
     
    Last edited: Jan 31, 2017
  5. Tysen

    Tysen AK Subscriber Subscriber

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    Yep....lesson learned!



    What was your bad experience??


    I unfortunately don't have the data sheet. What do you mean ratio is 1/1?
     
  6. Pio1980

    Pio1980 AK Member Subscriber

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    At one point on the dial, the in/out voltage ratio is one to one. Measure the socket voltage, then measure the variac output voltage and match the socket voltage. That set point is a 1/1 ratio. The variac is an adjustable AC auto transformer that varies output to input by varying it's transformation ratio with a dial knob via a sliding tap that can go from zero to above the input tap. When the sliding tap is at the point of the input tap, the ratio is 1/1.
     
    Last edited: Jan 31, 2017
  7. dshoaf

    dshoaf That high voltage buzz Subscriber

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    Agreed with PIO1980, my variac has a scale but it is meaningless for measurement purposes. I _always_ measure output voltage of the variac with my Fluke meter to really know what the input voltage of the amp really is.

    In engineering school labs, my wonderful old prof beat into me, "Trust your meters but verify them every time you take a measurement". In my past 40 years, he's been right more times than I can count.

    Cheers,

    David
     
  8. x3workshop

    x3workshop AK Subscriber Subscriber

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    Lighting crew plugged into house mains, not the show genny. When the voltage spiked it took out the ballasts on a pair HMI's of dubious repair. They don't go quietly.
     
  9. x3workshop

    x3workshop AK Subscriber Subscriber

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    This was one of the best investments I ever made. You can find them for $100-$150 depending on condition. Of course, I verified the scale with a multimeter in the output socket.:thumbsup:

    [​IMG]
     
  10. Tysen

    Tysen AK Subscriber Subscriber

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    Thanks for the tips guy. I'll save my pennies for the Sencore!


    Dang. Sorry about your HMIs. That's rough!!



    I need to invest in a Fluke. Is the 77 fine?
     
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  11. Pio1980

    Pio1980 AK Member Subscriber

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    Any variac is fine, but I'd just run them direct at 125 tap and call it good.
     
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  12. Tysen

    Tysen AK Subscriber Subscriber

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    Great!



    Well, I've been running them for the first time as a pair, and with my Altecs for the last hour. Dang. Wow. I've never heard such a difference between amps. Gives way more depth and dimension than my Knight KB-85!





    I'm having some hum in the background. The amps came with power cords, of which the ground is connected to the chassis. I'm going to install cords next week. Could that be the source of my light hum? The AC Ground?
     
  13. x3workshop

    x3workshop AK Subscriber Subscriber

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    I believe the consensus is to not us a three prong plug on these units as it can induce ground loops.
    Is it in both amps to the same degree?

    A few things to check to see if you can reduce the hum:

    - Check your interconnects
    - Adjust the hum trim pot(s)
    - Swap the first 12AX7's out with the second one. Sometimes a noisy tube in this position can be the problem.
    - If it is on both amps disconnect one from your preamp and see if the hum stops. If it does, you have a ground loop issue between the gear.
    - Try a ground lift cheater plug on one or both amps.
    - Or, run a single wire between the two amp chassis and the preamp chassis. I had an issue with mine because of this when running off of a specific preamp.
    - A low noise driver tube like the Sovtek 12AX7LPS is pretty good in that position. It's sonics have been compared favorably to vintage tele's and RCA's. It's cheap too.

    If your speakers are uber efficient you may always hear a very slight hum, but it should not be audible from say 3 ' away. If it is it needs to be addressed.
     
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  14. pantoramasan

    pantoramasan AK Subscriber Subscriber

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    My recent purchase, a pair of MC30's pretty much all original. I used a variac to bring both up although the previous owner stated it had been in use a while ago. I took some photos of the inside and I noticed this goo on one of the capacitors. Should I not use this amp until I get it repaired? Or is it safe to hook up to pre amp and speakers? DSC_0646.JPG
     
  15. x3workshop

    x3workshop AK Subscriber Subscriber

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    Regardless of what that goo is, you'll need to do a rebuild. Electrolytic caps are surely shot and the other caps are likely leaking current. Resistors out of spec and so on. I say don't risk running it without a full rebuild.

    You can bring it up very very slowly on a variac with some audio input to confirm that the OPT's are good. Only so much voltage applied until you hear signal, then turn it off. Again, my 2¢ Always with speakers connected.
     
  16. pantoramasan

    pantoramasan AK Subscriber Subscriber

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    Thanks for the input...I have the rebuild scheduled already but I am fighting it hard not to listen to these as original. But you are right. They are more than likely way out of spec (better not risk it) and I would not be enjoying the sound they should be giving.
     
  17. Tysen

    Tysen AK Subscriber Subscriber

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    Good luck with your re-build! Share photos. I'd love to see how you do it.




    The problem has to be a ground loop—it's audible more than 3' away with no sound playing. I have altecs, which are like 99db, but this is different than just normal amp noise. It also doesn't get louder with more volume.



    I'm going to install some new power cords today. In-rush goes between AC Line and transformer—not in between line and fuse, correct?
     
  18. x3workshop

    x3workshop AK Subscriber Subscriber

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    Go through the process I wrote above and you'll find where it's coming from. My money is on the three prong plugs.
     
  19. Tysen

    Tysen AK Subscriber Subscriber

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    Okay, removed the ground from the chassis. NO more hum. Thank goodness!



    These sound so freaking good. It's insane. Again, I've never had an amp change the sound so dramatically.
     
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  20. x3workshop

    x3workshop AK Subscriber Subscriber

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    Excellent!
     

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