McIntosh Modifications

Discussion in 'McIntosh Audio' started by Mike Gibson, Oct 28, 2012.

  1. c_dk

    c_dk Addicted Member

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    Audio shops back in the day ran on coffee...still have my 3 mast sailing ship mug from back in 1978 right here today.

    However this thread is about McIntosh updates and mods so......

    Terry,

    Back a number of months ago you posted a pdf of a noise spectrum of a C28. I find this interesting and informative. Only 24 times did anyone bother to look at it. It is a shame because it shows how much can be done to improve the C28 preamp.

    I have reposted because I believe we all can learn much from it if we bother to study it.

    Can you highlight the test equipment you used for those of us still stuck in back in the day?

    Do you remember what you did to this unit?

    The 60 Hz noise still seems high...I have found shield issues on my project C28, mostly in the filter circuits and the convoluted ground drains....how about you.
     

    Attached Files:

  2. c_dk

    c_dk Addicted Member

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    Looks like no one is interested in this sticky beyond coffee mugs.

    Anyone have an opiniun/experience with the Rightmark analyser program?
     
    Last edited: Nov 6, 2012
  3. dewickt

    dewickt Will fix about Anything

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    The noise measurements were made using Right Mark Audio Analyzer and a Delta 66 sound card with the Omni IO interface in my computer. I have since changed to a newer computer and am in the process of tweaking a new sound board with interface to get reliable data, currently I can loop back on the Xonar Essence STX board and get a base line of 102dB for my SNR. This number is acceptable though I think it can be improved closer to the board specs of 124dB by tweaking the interface.
     
  4. c_dk

    c_dk Addicted Member

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    Does the Rightmark program have the abillity to"correct" for soundcard issues like some FFT speaker analyser programs do?
     
  5. jdwk

    jdwk AK Subscriber Subscriber

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    I am interested in this thread and I am fine with the coffee mug I use currently....although I do like the one Terry has. :)
     
  6. c_dk

    c_dk Addicted Member

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    It would be nice if a common format freeware program would allow all to publish their mod's measurements, other than just "listening" impressions. This would open up some of these updates for discussion.Those with traditional old school testing measures would then also have a way to display what we discover as we work through repairs and such on these vintage units.
     
  7. AudioClassics1

    AudioClassics1 Super Member

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    Here is our famous Mod-a-mod that is done to all non digital McIntosh tuners by Richard Modafferi, designer of MR78.


    Modafferi Mod
    Install Hysteresis Modification
    Modify IF amplifier output stage to reduce IF noise radiation at 9th and 10th harmonics of IF (96.3 MHz and 107.0 MHz)
    Replace all audio coupling and bypass capacitors in the feedback circuits with High Quality Metalized polypropylene capacitors
    Install bypass caps in the power supply (Tube Units)
    Modify the Stereo Decoder to improve Stereo Separation
    Modify the Stereo Decoder for higher Slew rate and Stereo Separation
    Optimize pilot carrier limiter circuit
    Change muting and 19 KHz circuits for more reliable muting and stability of the stereo decoder (Tube Units)
    Re-route IF cable from IF chassis to detector to eliminate tailbiting at 96.3 and 107.0
    Modify Narrow Filter to accept over modulating stations.
    Front end modified for higher RF selectivity
    Adjust IF gain
    Perform complete RF, IF and MPX alignment
    Convert to 220 Volt operation - if required
    Change de-emphasis to European Standard - if required
    Listening check
    Install Cardas output connectors
    Unit will now exceed original factory specifications
    Run performance graphs
    Clean chassis, controls, panel and knobs
    Unit signed and dated (inside) by Richard Modafferi
    Selectivity modification

* * Investment: Tuner Approx. $700, Tuner-Preamps Approx. $900
* * Turnaround Time: Approx. 3 weeks
* * All changes are not applicable to every unit
     
  8. Rich Andrews

    Rich Andrews Well-Known Member

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    There is a program available at a modest cost. The problem with an audio analysis program is it takes some time to learn it. I did take some measurements and published them with my MC2500 mods.
     
  9. SteveJewels

    SteveJewels TunerHeaven.com Subscriber

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    What program did you use? What audio interface? Where did you publish the results?

    Thanks,

    Steve
     
  10. calman46

    calman46 AK Subscriber Subscriber

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    What about updating the rectifier diodes ? Aren't there faster ones available now ? How fast is the 1N4820?
     
  11. dewickt

    dewickt Will fix about Anything

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    Watch out for diode changes, a fast diode can introduce noise into the system, especially in a preamp or tuner. Most McIntosh supplies are regulated and well filtered, a fast diode will not have any effect on this type of supply. It can have an effect on a power amplifier by helping stiffen the voltage, but again watch out as a fast diode can cause the peak current draw from the power transformer to increase. I have seen this in a 240 where the peak current increased causing solder joints in the power transformer to over heat and crystallize, not a fun job to unpot and re-solder.
     
  12. Rich Andrews

    Rich Andrews Well-Known Member

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    I am with Terry. Fast diodes are not a good idea for that application. Put another way. when McIntosh designed the transformer and circuitry, Selenium diodes (rectifiers) were the norm. A slower diode than that would be hard to find. Fast diodes switch hard. Slow diodes switch slowly....just like the AC line.
     
    Last edited: Nov 25, 2012
  13. Rich Andrews

    Rich Andrews Well-Known Member

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    Steve,

    I published the results on my webserver at http://rich.homeunix.com/mc2500/mc2500mod.html.

    I was able to use a borrowed commercial audio analyzer. It was Spectralab.

    The audio card was whatever was built in to the computer. Not a great audio card, but good enough.
     
  14. calman46

    calman46 AK Subscriber Subscriber

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    I was reading the C34v love post by Ron-C and that was one of the first thing he did was put new fast diodes in power supply . I have a C33. Is your suggestion for tube preamps or SS or both ?
     
  15. c_dk

    c_dk Addicted Member

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    Did not Echowars answer this for you already?

    It seems that RonC was repairing this part of the preamp, someone had already replaced a single diode and one was burnt. Most likely from a lightning event
     
  16. calman46

    calman46 AK Subscriber Subscriber

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    Yes he did but it was a general explanation I thought ( I forgot about that ) and soft might be a quieter choice . I read a lot posts etc.. and "upgrading" the rectifier diodes is in a lot them . Ron_C was repairing or upgrading he used fast diodes and that was good enough for me . The UF5404 are in now . I'll let you know , thanks for the memory jolt.
     
  17. ron-c

    ron-c AK Subscriber Subscriber

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    That C34v is still running perfectly so I guess it is happy!

    Thanks,
    Ron-C
     
  18. dusty-rhodes

    dusty-rhodes AK Subscriber Subscriber

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  19. Rich Andrews

    Rich Andrews Well-Known Member

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    What he did was to electrically break out the receiver into it's basic parts and provided connections on the back panel for each function. While it sounds easy, the devil is in the details.
     
  20. dusty-rhodes

    dusty-rhodes AK Subscriber Subscriber

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    MAC 1700 mod

    Thanks for the info, Rich. It sounds like this isn't something I should attempt.

    Sam
     

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