Dedicated to my
friend, Roger Russell.
History of my amplifiers.
Detail of thermoswitches and wiring.
I purchased my first McIntosh
from "Ramblin' Ted" in Vermont. The
unit was as clean and pristine as one could hope for. I paid a
premium price for the amp, but it is as mint as one could expect for a
unit of that weight and size. A few months later I was contacted
a fellow that had a MC2500 that he wanted to sell. This amp was
saved from the scrap pile. This fellow was an employee of a oil
refining company. This amp is more interesting than a stock
MC2500 as it has a couple of unique features. This amplifier
has a what I call The Tow-Fish
Modification which consists of a locking
toggle switch on the front panel and a secondary
flip down back
panel. The auxilary back panel had two military style
on it as well as a transformer and associated wiring. One
connector was labeled "Tow
Fish" As near as I can tell from the seller's description of his
work and the research that I did, this amp was modified by
McIntosh for use with the oil exploration industry. The switch on
the front panel disables the PowerGuard
circuit. I talked to the fine folks in Binghamton about the
unit. The fellow that answered my questions told me that he did
not advise hooking it up to anything I cared about. He went on to
explain that there were many modifications done to MC2500 amps over the
years and he could not tell me if the amp would work or if it would
turn my speakers into towers of flame and fire. I had already
done preliminary research and determined that the output transformers
were standard production units as was the power transformer as well as
the rest of the circuitry.
From what I can
find, the reason for the PowerGuard disable switch is that
transducers happen to like it if
the excitation signal is a square wave. Put another way,
clipping the amp is preferred in this application. I have no
idea what the
measured output of the amp would be in bridged mode at maximum
power with the PowerGuard off and I really have no need or desire to
find out either. To get the unit ready for normal use, I
removed the auxilary back panel, cleaned out the insect nests, changed
light bulbs, removed the Liquid Paper from the front panel, replaced a
meter op-amp and in general, gave it a good cleaning. BTW,
Liquid Paper is easily removed with "Goof-Off". Be exceedingly
careful with that stuff. It can melt many common plastics and can
types of paint.
After spending a fair amount of time listening to the amp I
that I had some residual 60 cycle buzz that I
could hear from my speakers. I also then started to further
investigate both amps. It was apparant that the left channel of
amplifiers had a 60 Hz 'buzz'. It was quite low in level, but it
was there nonetheless. It was audible 2 feet or more away from my
speakers. I really did not have the time to devote
to investigating the issue, so I put it off as both amps were roughly
the same....at least as far as noise was concerned.
Last month (August 2004) I decided to put my first amp on the bench and
investigate the issue. What I discovered was that the
overheat sensor and fan thermoswitches as well as their associated
wiring are entirely too close to the rear panel mounted input
board. The picture below is opf my second amp but the wiring is
identical. Note the close proximity of the input
PC board to the wiring. I am pointing at the fan switch
wiring. with my pencil/pointer.
Clicking on any image will cause a larger picture to load.
Below is a different view of the
area after I moved the overheat switch. All of the heatsinks have
predrilled for mounting a thermoswitch. I used common computer
with a 1/4 inch hex head. I used a 1/4 inch combination wrench to
install the screws. The wrench is available at Sears. I
think it comes as part of a set only.
O.T. thermoswitch moved
If you notice I have not moved the fan switch wiring yet.
the over temp thermoswitch drops the noise a little in comparison to
the moving of the fan thermoswitch wiring but since it costs so little
to do, why not?
Here is what the fan switch wiring looks like after I am done
it. I had to cut the zip-tie holding the fan wires to the main
contemplated using some magentic shielding but decided to just move the
wiring for now. I am still wondering if something like
would be beneficial. Spira-Shield is available from http://www.magnetic-shield.com/products/cables.html.
In earlier years McIntosh engineers would routinely use a similar if
not the same
product to reduce radiated 60Hz from internal 120VAC wiring.
Fan Wiring moved.