MC 40 Complete Restoration
The MC 40 is one of a series of mono amplifiers produced and sold by
McIntosh Laboratories. Back in the day, when things were
mono... you only needed one. No problem, for stereo, simply get two
of them, use a stereo preamp, and there you are. These are
particularly wonderful amplifiers. They have a very good sound, very
smooth and very warm. They are quite simply designed and very well
constructed. The tube complement is not at all exotic, so you will
never have any trouble finding tubes for it. They are not very full
of surprises. The work that they require is very consistent, making
it easy to predict what will be found when you get into one. There
isn't a whole lot of need in this write-up for me to tell you how to
evaluate whether yours should receive a restoration or not. In fact,
there are only two types of MC 40's: those which have recently been
completely restored, and those which need it. Note that hiring some
clown to replace two or three caps and leave the rest is no overhaul.
Capacitors do not after all enjoy being 40 years old. They become
cranky and disagreeable and forget who they are, they become paranoid
and think that the electrons are out to get them. So don't sweat
checking it out yourself. If you have just purchased an MC 40, send it
in. If you found one in the closet, send it in. If Uncle Otis gives
you one, send it in. If you have been using one (two) for a while and
you do not wish to walk in and find that one of the priceless
transformers has been damaged because of a stupid bad capacitor... you
have it... SEND IT IN!!!!! Hmmm, I wonder if I made that point with
sufficient emphasis.
In all seriousness, this is extremely important because the
only parts that are nearly impossible to replace are the
transformers. Since a cranky capacitor can destroy these
priceless transformers, a thorough restoration is the only way to
go.
Since I have now convinced you that you benefit greatly from buying
the complete restoration for your MC 40, you are undoubtedly champing
at the bit to find out what that consists of. You already know that
capacitor replacement is the name of the game. So you may find
yourself thinking that it is simply out with the old capacitors and in
with the new. Furthermore, you may think anyone can do it, so you
don't need a wise acre like me to do it... right? Wrong! This would
fall into the category of "don't try this at home". Here is the deal:
in the power supply, you will find rather large can caps mounted
horizontally. One of them has three different taps, and the other two
are one each. Those particular caps as unavailable as it gets... you
absolutely cannot get them. So what to do? The solution is
relatively straightforward. Caps of the same value must be found and
then they must be mounted in some very tidy way so that all of the
electrical connections which were present in the original design and
set up are duplicated. Caps of the proper values can be found. In
amps of this sort, they are usually not extremely high in their
capacity or mfd values, but they are often up in the 500 volt range.
No problem, in fact new caps are usually smaller in size which is very
helpful when it comes to the mounting question.
The preceding picture will give you a sense of what you are up against
on the cap replacement front. All of the caps removed from this MC 40
are laid out in front of it. As you can see, some of them are fairly
large. The filter caps from the power supply are the largest
ones. These were mounted on a bracket under the chassis. Since new
caps will be physically very different, the same mounting is
impossible. I chose to locate caps of the appropriate value and to
epoxy them in. The original bracket was a perfectly fine place to put
them. So check it out:
In the preceding picture, you can see the bracket on which the
original cans were mounted. This bracket was the obvious place to
epoxy in the new caps. It was actually a very good point to run
various ground connections. After all, in the original design, many
grounds went to the negative lugs of the cans, and these were mounted
in the bracket. It struck me that using this bracket for the grounds
was in keeping with the spirit of the original design. So, simply put,
the key to a successful repair of a piece like this one is to locate
the new caps in a very stable place, and then to wire in all of the
original connections. It isn't rocket science, but you have to be very
deliberate and precise. The mark of doing it well is in the testing.
If, after several hours of wiring, you can plug it in and it works on
the first try, no corrections required, then the job has been well
done. To the left of the bracket and the new filter caps, you will see
the pc board which holds all of the coupling caps. These are replaced
as a group. After this has been accomplished, the unit will have no
original capacitors in the circuit at all. After this has been done,
it is necessary to remove the selenium rectifier and replace it with a
silicon diode.
I have so far not mentioned the tubes. Of course the tubes get
checked during this process. If you have early or original tubes in
the unit, they may still be good. They were rather well made so they
are worth using if they still test well. If not, or if you wish to
upgrade, there are a world of tubes out there to choose from. Once the
unit has been recapped, it needs finally to be cleaned up, both
cosmetically, and the tube sockets should be cleaned with a solvent,
and then be deoxidized. After that is done, the unit needs only to be
checked for proper performance and set up. At that point, it is ready
for additional years of service as a very sweet and beautiful
performer. Enjoy!
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