Gilbert's McIntosh C22 Rebuild
This McIntosh C22 stereo preamplifier was sent to me by
Klipsch forum member Gilbert. Gilbert has owned the
preamp for a year, and it had been serviced just two months
before he purchased it. The technician who worked on it
had given it a clean bill of health. Even so, Gilbert noticed
a few obvious issues with its operation, and sent it to
me for the NOSValves treatment.
Just thought I would share the experience of rebuilding
this excellently built piece of audio equipment. I really
like doing work for Gilbert because he and I have the same
way of looking at this old gear. He just tells me to do
what I would do if it were mine. I'm a firm believer that
45 year old capacitors, whether they are PS filters or coupling
caps, can't possibly be doing the job McIntosh, Scott, Fisher,
or ____ designers intended them to do. It’s important to
realize that these old tube components may indeed be working
to some extent, but it's almost a given that they are not
working up to their full potential.
McIntosh tube gear is by far the easiest tube audio gear
to work on, at least from my point of view. The perception
that a technician has to be a McIntosh specialist to work
on McIntosh gear really gets under my skin! Tube gear is
tube gear: It all works on the same principles, especially
when dealing with preamps and amplifiers (tuners are another
The first thing to be done, that I'm not going to be able
to post pictures of, is to disassemble the entire face plate
(I did this portion of the work before I decided to do this
write-up). This is required for cleaning the controls and
switches on the C22; in most gear you don't have to take
this step but McIntosh glass is really delicate and it's
not worth the chance of damaging the finish or lettering
with accidental overspray from contact cleaner. I'm a firm
believer in drowning the controls with Caig
D5 to really get them clean! After this is done I clean
out any residue with compressed air and re-assemble the
So let’s get on to the fun stuff!
Here are some before pictures of the top and bottom innards
of the C22:
Okay, time to get to the main work. In testing the electrical
condition of the circuit, the first thing I noted was that
the voltages at the three reference points that McIntosh
provides for the power supply in the service manual,
that these voltages were down from 14 to 27 volts.
First reference point was 296V should be 310V
Second reference point was 272V should be 290V
Third reference point was 241V should be 268V
The next thing I did was to replace all the diodes. For
the B+ I used Fairchild brand Stealth Soft, fast recovery
diodes (what the heck, they really don't add much to
the cost). For the heater supply I used a basic bridge rectifier
rated at 4A 400Piv. This did nothing to correct the voltages,
not that I expected it to, since the original were SS type
diode that don't degrade over time--they either work or
don't work, no in-betweens.
The next step is to rebuild the power supply filtering.
For this I ordered in the original type, twist lock cans
from CE Manufacturing. These are built on the original Mallory
tooling and are of great quality. Plus they look original
and aesthetics count, especially with classic equipment.
With the C22 or C11 there are three cans in the power supply.
Two of these cans are no problem replacing, but the last
can is a problem since it’s no longer available.
This last can has three connections: two are filters
for the heater supply DC voltage and the third is used merely
as a connection point for various components (no filter
connection). For this can I use separate radial lead capacitors,
which you can see in the photos, and leave the can in place--but
disconnected--to keep the original look intact. There is
one more section that is part of one of the other cans that
is also for the heater supply; this is also replaced with
a radial lead under the chassis. For the two B+ filter
cans I used a 4-section 50uF each @ 350V. This is a substantial
upgrade over the factory filtering, which never hurts. The
original McIntosh design has 210uF of effective filtering,
while the new upgraded design has 375uF. Does this make
a huge difference? Nope, but why not beef it up when you
can at no real extra cost to the customer? And it actually
will help stiffen the bass and quiet the preamp to some
Here is a photo before replacement
Power supply after rebuild
(I did reuse one axial lead cap since it had already
been replaced recently and tested as perfect on a cap analyzer.)
Now it’s time to fire up and test the voltages to see what
good I have done.
First ref. point now 305V should be 310V
Second ref. point now 288V should be 290V
Third ref. point now 262V should be 268V
Much better! I tested the cans after removal (which is required
for testing; in-circuit is worthless) on my Sprague cap
analyzer. I found five of the six sections for the B+ to
be leaking some voltage to ground above the minimum suggested
tolerance. The sixth section was completely dead.
Now let’s move
on to the rest of the circuit. One
the problems that Gilbert experienced with the preamp was
the rumble filter would cut a channel completely out when
engaged. Not a big deal since the switch in this day and
age should never be used anyway, at least if your turntable
is any good at all. If you look at the shielded wire in
this picture you will see it has obviously been tampered
with, and under investigation I found the shielding only
traveling an inch or so into the jacket. The solder joints
on the surrounding components were obviously worked on at
one time also. I went through and ran a new shielded
wire and replaced the coupling caps that the signal
runs through when the switch is in the off position. The
rumble circuit, like all the filters, loudness control, and
tone controls on the C22 are handled with PEC devices (Packaged
Electronic Circuit). Fortunately there was no problem with
the actual PEC, since a replacement is not available. When
I fired up the preamp, the problem was gone. I am not sure
which part of what I did actually fixed the problem, but
it is fixed nonetheless.
Rumble filter area before
Rumble filter area after (Rick, does that
shielded wire look familiar?)
I call these next
four capacitors “little buggers” since they
are in one heck of a fun spot to get to. They should be
replaced since they perform an important function. Two
had already been replaced but for the sake of making them
match, I replaced them all. I don't care for the 25-cent
yellow jobs anyway!
Little buggers before
Little buggers after
Here are all the main coupling caps, before and after. These
caps were leaking some voltage and had become resistive,
which creates a similar effect to having the High and Low
filters in the “on” position.
Coupling caps before
Coupling caps after
So what do those tell-tale voltages measure now?
First reference point is 309V should be 310V
Second reference point is 292V should be 290V
Third reference point is 267V should be 268V
All voltages were taken with the unit powered at 117V with
my Variac, which is the spec noted in the McIntosh schematic
and service manual.
I also ran sine wave and square waves through the preamp--they
look equally good.
Thanks for reading and I hope you enjoyed it. This may help
a few DIY'ers out there also.