C28 Volume Control, finally bit the bullet.

Discussion in 'McIntosh Audio' started by TT-newbie, Oct 23, 2018.

  1. TT-newbie

    TT-newbie Member

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    After so many years struggling with volume control of C28, I decided to buy Mcintosh after market volume control. It arrived today plus $57.00 brokerage fee from UPS!:(. For the price I paid, the part does not look well made. I am just hoping it will do the job and last so that I can enjoy my C28, which is now sitting idle. I have been using a Denon preamp in its place. Unfortunately I won't be able to install it until early December and report the results once it is done. A couple of photos attached.
    magic___ (1).jpg magic___.jpg
     
    Last edited: Oct 23, 2018
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  2. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    Hello

    I hope for you that the shaft is not too short cuted !
     
  3. TT-newbie

    TT-newbie Member

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    I hope not. But do you think it is a bit too short?
     
  4. c_dk

    c_dk Addicted Member

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    This is the Canadian sourced PEC control......being a generic device for 5 other units, you will have to customize it circuit wise to maximize it's performance.

    I would measure it's resistance through it's ranges and compare to your original to get some ballpark numbers.

    $57.00 for shipping.......did it get served caviar, Chateaubriand, and Dom for it's first class meal?
     
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  5. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    just compare with the original one , and you will have your answer
     
  6. c_dk

    c_dk Addicted Member

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    If my memory serves me, which is iffy nowadays, it was correct length for a MA6100s and C28s and need some cut off for a MAC1900.

    One needed the knob spring clip glue broke free and the knob rotated. I do not remember which one.
     

     

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  7. TT-newbie

    TT-newbie Member

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    You have been too optimistic. Shipping is US$24.00, UPS Canada added carviar of CAD57.00 extra for brokerage!
     
  8. c_dk

    c_dk Addicted Member

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    Sounds like the Canadian revenue service mistook the control as New York cheddar and taxed it as a dairy product........

    There has been a lot of negative postings in this forum about this Canadian sourced control vs. the previous Alps sourced control.

    Originally I believe Mac sourced 5 controls from CTS when these units were first built and consolidated to 3 versions from Alps. Now there is only the one.

    Where the original controls had a 60/40 split for the 4th or loudness tap the new PEC is only available at 50/50 even though it is listed as a "custom" option only available in a quantity order.

    To make the control work well I have needed to add resistance to shift the ratio and since the loudness tap sets the taper of the control this will affect the ratio of gain to the knob rotation.

    I have be fortunate to be able to measure 3 orginal wafers that still had all the carbon resistive material present......way to many were missing much flaked off material and were useless for evaluation. I believe I even posted a picture while measuring them.

    They are no longer just drop in like the ol' days so some reengineering is needed.

    It would be great if Alpha or someone else made a 4th tap control to steal the wafers out of, other than the much smaller one that Radio Shack used to sell.

    At $100 plus shipping and I guess now foreign tarrif there should be enough incentive to find a viable repair option.
     
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  9. TT-newbie

    TT-newbie Member

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    91
    Can you please tell how you add the extra resistor to compensate? I measure two groups on this control, one measured 750K, the other is 680K. It does look great right there!

    I don't think anyone will be interested in producting of repairing parts based on the demand for this hobby. I tried to install this once I go back. I hate seeing C28 ideling there,

    Thanks
     
  10. c_dk

    c_dk Addicted Member

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    Here is a picture of a control wafer. When I measured 3 they ranged from 1.1 to 1.28 meg.

    If you check the schematic you will see that the loudness tap is offset to aprox. 60/40 and is shifted so there is more resistance towards the input side.

    I add resistance to shift the loudness tap to match the 60/40 ratio and increase the resistance to over 1 meg. If that means adding resistance to both the input and ground side so be it. I also try to get the two channels to match better.

    When I have measured the wafers at different spots on the carbon it shows that the control is linear......and you can see from the carbon that is uniform width compared to the long skinny triangle you see when you take apart a audio taper control.

    If you think about how a voltage divider works you can see how the added resistance to ground that is part of the circuit that includes the elements to add bass for loudness and midrange for presence modifies the resistance to alter the taper into a audio taper curve.

    Yes it is much fussing.....it was much easier 25 years ago when there were Alps controls custom made for Mac parts but those three versions are now long gone......

    There is much more to restoration work than repair work.
     

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    Last edited: Oct 30, 2018
  11. TT-newbie

    TT-newbie Member

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    91
    Thank you so much for the info. Yes, it will make sense to add the resistor at least make two channels evenly matched. I don't quite understand how loudness tab work in the volume control. But I figure that is why you add resistor to the mid tab to maintain 40/60 ratio.
     

     

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  12. c_dk

    c_dk Addicted Member

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    Without looking up the schematic, I think the circuit loads the center tap with 10 k to ground, and adds capacitance to create equal loudness compensation when so selected.

    Volume controls need a taper to their response to ramp up volume in a acceptable manor and by having the interaction of say 600k from the loudness tap in one direction, 400k to the other and how those resistances interact with the 10k and the two taps connection to ground that taper is achieved. This is a somewhat complicated voltage divider and I can't help but think of Sid Coderman whipping out his slide rule to plunge into those voltage divider computations. Almost, and I said almost makes me want to find one of my old slipsticks to see if I still can remember how to do logs......

    The challenge nowadays is to replicate that interaction with what is now a generic part that replaces 5 parts.
     
  13. Snappahead

    Snappahead AK Subscriber Subscriber

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    Can I make a suggestion? Take the new volume control, put it back in the box and return it. Then take your old volume control out of your preamp and send it to Ryan at Audio Classics and have them rebuild it. Reinstall it, turn up the volume and enjoy. Trust me, this will save you a lot of aggravation.
     
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  14. 62caddy

    62caddy Trust but verify Subscriber

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    Word of reason here.
     
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  15. c_dk

    c_dk Addicted Member

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    Does anyone know what their rebuilding entails? Have they found generic wafers with a loudness tap? Do they add new carbon to the traces?

    Do their rebuilds actually follow the prescribed circuit expectations or do they just get by?

    Their website just list a Mac part number and a $110.00 price.....how do you know they don't just send you a PEC part same as Mac for $100.00?
     
    Last edited: Nov 3, 2018
  16. Snappahead

    Snappahead AK Subscriber Subscriber

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    I have no clue how they rebuild them or what parts they use. I do know it works perfectly. They returned the "new" Mac volume control to me and wrote on the repair report, "Return to McIntosh for a refund. Junk."
     

     

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  17. twiiii

    twiiii Addicted Member

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    A common occurrence. But the main problem was the two audio sections of the volume control didn't track well together at all. Thats why I owned a C29 for over 30 years. Personally I would trade the C-28 in on the C-30 Audio Classics has right now. Love those graphic tone controls. C-33 and 34 if you want more flexibility and bells and whistles.
     
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  18. Ricktptman

    Ricktptman Great Music deserves Great Gear and Vice-Versa. Subscriber

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    I finally got my C28 volume pot to behave itself! I don't know that it hasn't already been replaced, but it finally got "exercised" into doing what it's supposed to do, more or less the way it's supposed to do it pretty much 100% of the time. (For now?) But it's consistency has shot through the roof since I took it out, carefully cleaned it and reinstalled it snugly with some Scotch 33+ between the face of the pot and the front chassis mounting hole. I hope bragging on it doesn't curse it. So far, so good though. It does seem to reach maximum output a little early in its travel arc. Eleven-ish is about as much as is necessary with my 2155. It might well have been replaced before I bought it. But it works now and I'm happy with it. I have a replacement in storage if it ever loses its marbles again...
     
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  19. twiiii

    twiiii Addicted Member

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    I assume you have your selector switch on the 2155 set at 2.5 volts. Then you turn down the output level controls on top of the unit so that at 12: o'clock with the front volume controls the listening level is just a touch louder than you normally would like. That way the linearity of the loudness control is about right. At lower listening levels. With the very high input sensitivity of the phono input section, you can run into signal to noise problems with turntables if you don't use the top controls to back down the levels. Fortunately my tuner and tape decks all had volume controls so I could balance those line inputs against my Ortofons using external SUT back in the 70's be fore the C-29 . The 29 didn't have the flexibility of the 28, but didn't need it either.

    The volume controls on my C-28 only tracked well from 9:30 to 2:00 so I adjusted the output controls on top and the controls on my amps to give the best performance with my 275's using the mono inputs. And then the twin inputs when I bi-amped. It really wasn't until the C-29 came along that Mac had a really quiet phono section. The c-28 only had 78 db signal to noise below a 10 mv input. todays pre-amps are around 86 or higher below 5 mv and that's a 12 to 15 db difference.

    I guess I should have said during that period of time professional tape decks only had 65 db signal to noise with the ability to capture 10 or so db peaks. So having a phono section with with only 72 db below 5 mv. a more appropriate level was still quieter than the best records. That is until Sheffield came along. That changed things. I guess some credit should be given to Command and Mercury records too.
     
    Last edited: Dec 9, 2018
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  20. Ricktptman

    Ricktptman Great Music deserves Great Gear and Vice-Versa. Subscriber

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    Thanks for that advice, twiiii. I'm really happy with this setup now. I happened across both the amp and the preamp (separately) somewhat by accident and I think they'd both sat for some time in storage unused. Both the amp and the preamp had some quirks and it took some time to figure out which one was doing what. There was a CSJ on the input board of the amp and after that was nulled out, I was able to get a better read on the volume pot's issues. I like these older Classic era models better than the newer ones. Which coincides nicely with my budget, lol...
     

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