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Mcintosh C28 restoration

Discussion in 'McIntosh Audio' started by rg3117, Jan 18, 2018.

  1. rg3117

    rg3117 New Member

    Messages:
    34
    I recently bought a Mcintosh C28 preamplifier on Ebay. It looks pretty original from the pictures (original caps, etc.). I like restoring vintage gear, and would like to upgrade it while keeping it as original as possible.

    I can see a lot of information on the C28 on Audiokarma, but in many different posts. My goal is to present the work being done on this amplifier, get some help from the community, and insert some links to the best resources I can find as I progressively upgrade this old gear.

    My plan is to:
    - replace the capacitors, starting with the power supply - hopefully restuff the large cans if I can -, then move on to other electrolytics, and finally mylar caps.
    - replace power transistors.
    - clean and lubricate all knobs and pot
    - replace the broken glass
    - replace the light bulbs with LED
    - repaint the scratched top cover
    - replace all RCAs with gold-plated ones

    The C28 is on its way somewhere on a Fedex truck, it should arrive in the next couple of days.

    In the meantime, here are some pictures from the Ebay auction.
     

    Attached Files:

    Last edited: Jan 19, 2018

     

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  2. MACKIE1975

    MACKIE1975 Well-Known Member

    Messages:
    666
    Listen to the unit before do anything.
     
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  3. rg3117

    rg3117 New Member

    Messages:
    34
    Will do, thanks for the advice!
     
  4. showett

    showett AK Subscriber Subscriber

    Messages:
    145
    Location:
    NW burbs IL
    I also just recently bought a C28 off eBay for a steal that was listed as not working and after cleaning all the pots and rca jacks found it works perfectly. It does need all new lamps and I can't decide between led or original style. I will be watching your progress as I plan to do some upgrades in the future as well but I need to get the 2 mc2505 amps I also just bought working first.
     
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  5. rg3117

    rg3117 New Member

    Messages:
    34
    Good plan. I just did a MC2505 too, it is waiting for his new friend.
     
    Last edited: Jan 19, 2018
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  6. CDinMB

    CDinMB AK Subscriber Subscriber

    Messages:
    130
    Location:
    Melbourne, FL, USA
    I bought an “as is” C28 from eBay a month ago. One of things I was told was to check and resolder all grounds. I found many were totally clean and not a resistance problem, but worth checking because some were corroded especially at the input/output jacks. I found fixing them, troubleshooting a bad solder joint, and a bit of cleaning made the unit sound great. Relaxing and listening.

    Interested to read about your journey.
     
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  7. rg3117

    rg3117 New Member

    Messages:
    34
    Yes, grounding and problems with the volume control pot seem to be common issues with C28s. This is on my list of things to check. Mine has serial number 76X??, which makes it one of the ‘early’ ones, probably made around 1973. I am expecting the same issue as yours. If I replace the RCA connectors this will have to be done anyway.
     
  8. Hunted11

    Hunted11 Active Member

    Messages:
    150
    Location:
    Western Massachusettes, USA
    rg3117, yours might be newer,, my C28 was purchased new in 1973 and the serial is 67X39.
     
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  9. rg3117

    rg3117 New Member

    Messages:
    34
    Good to know. I was going from this thread which placed 99X99 in February 1974:
    http://audiokarma.org/forums/index.php?threads/mc2100-c28-cornwall-wow.300636/

    Well the FedEx truck came to our house today, but there was nobody there to answer the door, so they are coming back tomorrow.

    In the meantime I purchased:
    - a can of DeoxIT FaderLube F5. This seems to be the most recommended product on the market, and with the volume control pot being the Achilles' heel of C28s, I thought that I should only settle for the best:
    http://hosatech.com/product/f5/

    - New Old Stock Motorola transistors to replace the TO-66 transistors (one 2N3738 for Q301, and three 2N4231 for Q302, Q209 and Q210). Going from that thread:
    http://audiokarma.org/forums/index.php?threads/mcintosh-c28-transistor-replacements-and-substitutes.352998/
     
    Last edited: Jan 19, 2018
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  10. c_dk

    c_dk Addicted Member

    Messages:
    6,067
    Location:
    West Michigan
    Q301 and 302 are used for voltage regulation and 209 and 210 are for the 2 watt class A headphone amp.

    In the first case I would check the zener reference diode if your voltage are off and as for the headphone amp, well it is for headphone listening so has little to with the phono EQ or other functions that might be noisy.

    I have found that the "charge up" time of the headphone amp power supply can add some turn of "racket" to the main preamp turning turn on.
     
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  11. rg3117

    rg3117 New Member

    Messages:
    34
    Thanks c_dk. I do use headphones, and would like to connect my turntable to the C28. I also have a tube preamp that I built some time ago, in case I don’t like the sound from the C28 preamp. I read elsewhere that the preamp section was not great.

    Are some of the other transistors either known to fail, or worth replacing as part of an upgrade? I have to admit that I went for the TO-66s first as they just snap in and are super easy to replace.
     

     

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  12. rg3117

    rg3117 New Member

    Messages:
    34
    Restuffing the large filter capacitors

    I found this website showing that someone was able to do it. Maybe he is a AK member?
    https://www.flickr.com/photos/77313494@N03/sets/72157650207402927

    I am attaching a picture showing the cans open with the replacement caps. It seems that he used:
    - Nichicon KG(M) 3,300uF 50V for C231 and C232 (instead of 3,000uF, 20V)
    - Nichicon PW(M) 2,200uF 25V for C301B and C301C (instead of 2,000uF 25/20V respectively)
    - Some Nichicon PW(M) for C301A, which is supposed to be 50uF 200V. Looks like the closest on Mouser in the Nichicon UPW 47uF 200V.
    - A Nichicon 220uF 100V for C302B (instead of 200uF, 80V), probably a PW too.

    The other two I am not sure, but looks like Nichicon to me also. These are:
    - C302A, a 500uF 150V according to the service manual. This could have been replaced with a Nichicon UPW 470uF 160V, slightly less than 500uF but the tolerance is 20% anyway.
    - C302C, a 50uF 80V according to the service manual (must be the smallest one on the right). This could be replaced with a Nichicon UPW 47uF 100V following the same logic.

    Any thoughts on which of these values should or can be increased? Some are suggesting here that the values could be doubled:
    http://audiokarma.org/forums/index.php?threads/c-26-re-cap.42829/

    Will be happy to do that if it helps with the sound and can fit into existing cans.
     

    Attached Files:

    Last edited: Jan 19, 2018
  13. c_dk

    c_dk Addicted Member

    Messages:
    6,067
    Location:
    West Michigan
    Years agoTerry posted a spectral analysis of the noise spectra of a C28. The power supply fundementals were the limiting factor and should be the focal point of any restoration.

    Unsheilded cabling that pickup noise, that was acceptable by 1973 standards can be improved to approach today's expectations.

    The S/N of a new C28, the accuracy of it's match to the RIAA phono equalization standard was a magnitude better than the earlier preamps. They are over 40 years old though and because they are a complicated design, with a lot of features, there is more to go out of spec.

    Basic, documented, tech work goes a long way to bringing these back to better than new specs......no silver bullets that I know of.
     
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  14. rg3117

    rg3117 New Member

    Messages:
    34
    Alright, some I am guessing that a reasonable increase of capacitance in the power supply section would not hurt.

    Anybody looked into upgrading diode bridges D301 and D302? Mcintosh part numbers are 070-044 and 070-045.
     
    Last edited: Jan 19, 2018
  15. c_dk

    c_dk Addicted Member

    Messages:
    6,067
    Location:
    West Michigan
    What makes you think that adding capacitance will lower the 60 and 120 Hz. noise?

    To each his own but I found adding capacitance created more turn on noise and had little effect on the supply rail noise.

    I did find that addressing both circuit interconnection cabling and rethinking and correcting the grounding scheme, greatly affected induced noise.

    My project C28 was "confiscated" by a local who I loaned it too while his C26 was on the bench so I did not have the chance to document it but on other Mac preamps adding board level bypass caps will suppress higher frequency noise.
     
  16. rg3117

    rg3117 New Member

    Messages:
    34
    Well, it looks like in some cases I will have to choose between a slight decrease or increase of the values, based on what is available now. I will not be able to do a significant increase anyway, if I want to restuff the original cans. I misunderstood what you meant by "limiting factor", I thought you meant from a power delivery point of view, not noise. Maybe the quality of the caps will be more important than the values.
     

     

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  17. c_dk

    c_dk Addicted Member

    Messages:
    6,067
    Location:
    West Michigan
    I have used the "pencil"caps from Nichicon (UCS2D101MND9) for example to restuff the cans but also have found that a neat job with new tag strips underneath the deck offers other advantages.
     
  18. rg3117

    rg3117 New Member

    Messages:
    34
    Goods news, the C28 has arrived!

    On the bright side:
    - it powered on, and I am getting some sound!
    - on the Aux position, I cannot hear any level difference betweel the left and right channels, which gives me hope for the vlume pot.

    On the not-so-bright side:
    - the turntable preamp section is dead. I am only getting sound from the left channel on Phono 1, and only the right channel on Phono 2.
    - the red light on the stereo mode switch does not work
    - the Tape 1 and Tape 2 Monitor push buttons do not work. They won’t stay in when I push them.
    - the rubber feet are different from the ones I have on the MC2505, and so the two units are not at the same level.

    Adding a picture of my temporary set-up.
     

    Attached Files:

    Last edited: Jan 20, 2018
  19. Hunted11

    Hunted11 Active Member

    Messages:
    150
    Location:
    Western Massachusettes, USA
    I'd start by cleaning all the switches, pots, and the mechanism for the pushbutton tape switches. The audio connectors on the phono also have a habit of corroding.
     
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  20. rg3117

    rg3117 New Member

    Messages:
    34
    Yes, very corroded indeed!
     

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