Mcintosh MC30s Restoration (Looking for correct schematic)

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

  1. Tysen

    Tysen Active Member

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    Hey folks,

    I recently acquired a pair of MC30s off of this great site! I'm going to restore them, and I understand there is the "Hints & Kinks" thread, but I'm trying to consolidate to one thread because I'm very new to this.


    The first question I have is in regards to the schematic. Both amps have A-152-D and A-150-C transformers.

    The only clear schematics I can find is the A-152-B schematic. Can anyone help with this? I assume I can't use that schematic.


    Serial numbers are in the 150xxxx range.
     
  2. dshoaf

    dshoaf That high voltage buzz Subscriber

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    There's not that much difference in the circuit topologies to make that much of a difference on MC-30s - especially for restoration purposes. Your priorities are:

    - Power supply rebuild - caps primarily

    - Interstage coupling caps - they all gotta go

    - Bias circuit - replace selenium rectifier and electrolytics. Then you'll need to adjust the bias voltage resistors to achieve proper bias voltage on the grids of the output tubes.
    That's where the Hints and Kinks thread helps - discusses how to use a variac to adjust it.

    - Lastly, some us (me included) have discovered that most of the resistors will change value after the above items are done and it has been run for a while.

    There are plenty of threads here at AK so 'nuff said here. Done up properly, these amps will sing again!

    Cheers,

    David
     
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  3. x3workshop

    x3workshop AK Subscriber Subscriber

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    Here's the schematic for that version. There are just a few differences in this latest iteration of the amp - a couple of extra caps. The hand written value notes on the schematic are not mine and I haven't checked if they're accurate.

    PM me if this isn't large enough and I can email it to you.
     

    Attached Files:

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  4. Tysen

    Tysen Active Member

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    David, thanks for the advise. I'm ordering authenticaps for the power supply, sprague atoms for the other three electrolytic caps, and then Russian PIOs for the remaining. I'll also get the diode and resistor to replace the selenium rectifier.

    I'm stuck on resistors—based on the chatter that surrounds Carbon Comps, I'm tempted to use NOS because of the "tone". However, I've read a lot of unreliability.... Any experience or thoughts on Ohmite's carbon comps? I'm nervous if I don't use Carbon Comps, I'll have that stupid "what-if" thought running through my head.




    And THANK YOU for the schematic. The size is perfect—thank you, thank you, thank you!
     
  5. SilverTpt999

    SilverTpt999 Member

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    Lots of chatter based on no real data and precious few anecdotes. The main reason people don't do a top-to-bottom replacement including all the resistors, I'm convinced, is because it's harder and takes longer.

    I heard my amps when they had all original resistors. They sounded OK; I didn't audition them extensively in that configuration. I've now replaced literally every original resistor (see my restoration thread) with Dale metal films, with one exception: the 12k close-tolerance resistors, for which I used Amtrans carbon films.

    No regrets whatsoever. The amps sound amazing, and I know they are performing at spec. And they will continue to do so for another 50+ years. Resistor tech is just better today; there is no 'magic' peculiar to carbon films.

    When doing my restore, I also noted that almost none of the carbon films could be properly tested in the circuit - and when testing them as I removed them, over half were outside spec (some grossly). Furthermore, keep in mind carbon comps are notorious for drifting with temperature and/or at performance voltage so even the ones you think are ok... So if you're going to properly test these by lifting at least one end, just do yourself the favor of refreshing the resistors. I'd go for modern Dale metal films; a full set per amp costs under $50.
     
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  6. x3workshop

    x3workshop AK Subscriber Subscriber

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    I would agree and recommend a full on refurb using all new, modern carbon film, or metal film resistors. I have 4 of these amps with an identical pair to the pair you picked up. Rebuilt with authenticaps, K40 PIO's and metal films. They're amazing performers.

    Don't get caught up in the romance of carbon comp. IMO the sound quality is better with modern components and reliability is exponentially greater. Again, my opinion.
     
    Last edited: Jan 11, 2017
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  7. dshoaf

    dshoaf That high voltage buzz Subscriber

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    Agree with X3workshop on the carbon comps. Go with Metal Film.

    Cheers,

    David
     
  8. Tcomisky

    Tcomisky AK Subscriber Subscriber

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    I really like the Carbon film and metal film Takmans sold by Sonicraft, who is a sponsor here.
     
  9. Tysen

    Tysen Active Member

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    This site is the best.


    Thanks for all the responses—not gonna use CCs. Should I use Metal Film throughout? Or are there tasteful places for the Carbon Films?


    And the Takmans look perfect—looks like they'd fit my $50 budget for resistors.
     
  10. Tysen

    Tysen Active Member

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    also,

    are wire round resistors okay to use for R28 & 29?
     
  11. x3workshop

    x3workshop AK Subscriber Subscriber

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    Per the schematic I sent, R27 & R28 are wirewound resistors. R29 is the gain pot.
     
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  12. cjwebber

    cjwebber AK Subscriber Subscriber

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  13. dondoucette

    dondoucette AK Subscriber Subscriber

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    I have a massive supply of carbon comp resistors, I use them on guitar amps when the owner insists on using them. I have thrown away many because they're out of spec just sitting in my parts bins. Don't waste your time use a quality modern resistor.
     
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  14. Tysen

    Tysen Active Member

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    Okay, great.


    I'm thinking about using Takman's carbon films throughout—based on their description, it kinda seems like the best of both worlds.

    Any thoughts?
     
  15. dshoaf

    dshoaf That high voltage buzz Subscriber

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    ^^^ This ^^^

    Cheers,

    David
     
  16. x3workshop

    x3workshop AK Subscriber Subscriber

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  17. Tysen

    Tysen Active Member

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    Perfect.

    Okay, bought the Can caps last night—going to order the resistors and remaining caps today, plus the diode.
     
  18. Tysen

    Tysen Active Member

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    Before I order the resistors tonight, 1/2 watt, unless otherwise noted, right?
     
  19. x3workshop

    x3workshop AK Subscriber Subscriber

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    Correct.
     
  20. Tysen

    Tysen Active Member

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    Thanks.

    Here are some photos of the insides... I know I need to replace the bumble bees, but if the black beauties test okay, are those fine to keep?


    Also, where is the selenium rectifier??

    IMG_1297.JPG IMG_1298.JPG IMG_1300.JPG IMG_1301.JPG IMG_1302.JPG
     

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