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  #1  
Old 03-07-2012, 10:10 AM
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Recapping/Updating a C26

I've got a c26 preamp that I use along with a rebuilt Dynaco ST-70, a Sansui tuner, and a couple of turntables in the basement studio ( Sonny #3, 17 is a keyboard player, and down there are a Hammond M3, Leslie, drum kit ( Boy #4)). I've learned to do these things myself over the last couple of years, without spilling too much blood....The Dynaco has a new VTA driver board, upgraded power caps, running Genelex Gold Lion KT-66s. The Hammond and Leslie 145 have been gone through, with some updates as well. My point here is that I've some experience bringing some old, barely working stuff back to life, but I'm no electrical engineer or circuit designer. I enjoy the challenge of making these devices sound better, and learning some fun skills too.
Note: This post cut off and timed out before I finished, so I'll go back in and re-create again......

The C26 works fine. , but it was built between '68 and '77.
I have limited time, resources, but would like to attack areas that will give me most bang for the buck.
Electrolytics? coupling caps? where should I spend my energy and $, since I will not be replacing 100% of the caps on this foray.
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Last edited by 4MarkNY; 03-07-2012 at 10:56 AM.
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  #2  
Old 03-07-2012, 11:02 AM
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That high voltage buzz
 
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Suggested approach:

- Rebuild power supply - electrolytic caps, etc, then confirm it is developing regulated voltages to spec.

- Next, clean all the controls but you've already discovered how that helps.

- Then move on to coupling caps. I'd suggest you start with the line stage first and work backwards from the output. Priorities for replacement are: 1) electrolytics, 2) any caps directly in the audio chain, then 3) film caps in the feedback loops.

- If you're into vinyl, then follow the same approach for the line stage but now in the phono preamp.

- Lastly, pay attention to the grounding of the RCA jacks, wire runs and drain wires used to connect all the parts. These are known to break while working on them. Similar to the C28, they are also known to cause hum and noise problems. Search around here for discussions on the C28 wiring issues and how to address them.

Hope that helps.

Cheers,

David
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Old 03-07-2012, 11:40 AM
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Bought mine a few years ago as a non-working unit. Filter caps were toast and a shorted transistor in the phono stage. Replaced the transistor and filter caps (new snap-in caps on top with the smaller values underneath) and got it up and running.

Murray
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Old 03-07-2012, 12:24 PM
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Quote:
Originally Posted by skippy_ps View Post
Bought mine a few years ago as a non-working unit. Filter caps were toast and a shorted transistor in the phono stage. Replaced the transistor and filter caps (new snap-in caps on top with the smaller values underneath) and got it up and running.
Could you describe the snap-in caps on top more? Thanks


-Gregory
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Old 03-07-2012, 12:27 PM
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Quote:
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Could you describe the snap-in caps on top more? Thanks


-Gregory
There's an old thread here but I doubt that I can find it. Should have the pics at home and I'll dig them up.

Murray
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Old 03-07-2012, 02:00 PM
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Recapping/Updating a C26

Thanks, DS. That appears to be a sound strategy.
I forgot to ask during my re-write of the disappearo portion of the post...Are there mods that are popular when re-conditioning these? Increased capacitance if the filter caps? any known and reliable modifications? (I'm not a purist....)

Thanks

Mark
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Old 03-07-2012, 02:39 PM
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That high voltage buzz
 
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I'd recommend getting it running first in stock form. You can try tweeks on it, of course, but I know of no well-accepted or well-proven mods to many of the Mac pieces. They were well-engineered to begin with!

Besides, you can't really know if sort of mod does any good unless the piece is back up to spec.

Cheers,

David
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Old 03-07-2012, 02:44 PM
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First pic shows the snap-in caps from the bottom including the two brown caps tacked on underneath.

Second pic shows the new filter caps glued on top. Used hot glue but it didn't really stick very well to the sleeve over the caps.

Murray
Attached Images
File Type: jpg c-26 caps bttm sml-1l.jpg (131.7 KB, 95 views)
File Type: jpg c-26 caps top sm-1.jpg (115.7 KB, 94 views)
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Old 03-07-2012, 02:58 PM
c_dk c_dk is offline
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I found that taking the volume control apart and burnishing the wiper inner cotact surface (NOT THE CARBON) makes a world of difference.
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Old 03-07-2012, 04:11 PM
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Quote:
Originally Posted by skippy_ps View Post
First pic shows the snap-in caps from the bottom including the two brown caps tacked on underneath.

Second pic shows the new filter caps glued on top. Used hot glue but it didn't really stick very well to the sleeve over the caps.
NICE!! where do/(did) you get those caps?

Are the lower caps held in place by adhesive pad tie-wraps? Maybe the OP can be located? Search C26 Murray?


-Gregory
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Old 03-07-2012, 04:27 PM
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Quote:
Originally Posted by Gregory View Post
NICE!! where do/(did) you get those caps?

Are the lower caps held in place by adhesive pad tie-wraps? Maybe the OP can be located? Search C26 Murray?


-Gregory
I'm sure they came from Mouser. The ones on top are something like 450v so they'd be big enough to fit on the fiber thingie on top of the chassis. The ones underneath are tie wrapped to a one of those stick on pads and of course are much lower voltage. I tried to pick something pretty close to the original capacitance of the original twist-tab caps.

As far as using the snap-in caps, it's actually pretty easy to wrap the wire around and solder. Makes a pretty neat looking connection. I did have some trouble getting all of the connections tied to the negative lead on one of the "on top caps." It might have worked better to use a solder lug in between the cap and wires.

Murray
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Old 03-08-2012, 08:27 AM
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Quote:
Originally Posted by skippy_ps View Post
I'm sure they came from Mouser. The ones on top are something like 450v so they'd be big enough to fit on the fiber thingie on top of the chassis. The ones underneath are tie wrapped to a one of those stick on pads and of course are much lower voltage. I tried to pick something pretty close to the original capacitance of the original twist-tab caps.

As far as using the snap-in caps, it's actually pretty easy to wrap the wire around and solder. Makes a pretty neat looking connection. I did have some trouble getting all of the connections tied to the negative lead on one of the "on top caps." It might have worked better to use a solder lug in between the cap and wires.
Sorry I'm not too clear on "snap in" caps and what that involves.

How did you mount the 450V caps on top to the dielectric mounting rings/washers?


-Gregory
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  #13  
Old 03-08-2012, 10:19 AM
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Quote:
Originally Posted by Gregory View Post
Sorry I'm not too clear on "snap in" caps and what that involves.

How did you mount the 450V caps on top to the dielectric mounting rings/washers?


-Gregory
Snap-in caps have bent legs, probably made from flat, zinc plated steel, and are made to be pushed into a pcb and they snap in place. The bent leg then holds them in.

I just glued the big caps down but it didn't work very well. About the only other thing to do would be to drill and tap the chassis and use a clamp.

Murray
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Old 03-09-2012, 10:26 AM
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Recapping a C26

Well, this is a fine kettle of fish, but I suppose that you guys knew this all along....This being my first adventure to McIntosh land...
Starting in the power section, I identified a pair of multi section electrolytics that would likely succumb to age, etc in this preamp. There are two of them C304 & C305, with odd values....both are 160/200/200/150V multisection caps, from their description in the parts list. Of course I can't find a google hit for anyone supplying these. At mouser, I can find 161uf electrolytics, but $28.00 apiece at low volumes ( several of the "161s" have minumums of 40 units....)
CE Manufacturing, who apparently has the old Mallory Machines, doesn't have anything that comes close.
I'm a bit surprised, since I would think that there is a business opportunity to supply these odd ball multisections to the McIntosh community, but I can't find them. Looks like the standard procedure with this preamp is to await a failure, and then re-stuff the can if you can find correct valued componentry.
Or perhaps there are other solutions someone can suggest. It looks like a pre-emptive strike to put this device up to snuff is a bit more adventuress than I would have thought.
Ideas?
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Old 03-09-2012, 10:50 AM
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Once you get past the fact that there are no twist tab cans to use, there are a variety of ways to take care of this. Axial or radial caps underneath, or clean out the old cans and insert maybe one cap inside each can and two more under the chassis. Note that C305 can be 100v, which would be a smaller cap called for in the schematic.

Murray
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