McIntosh MC2500

Dedicated to my friend, Roger Russell.

History of my amplifiers.

I purchased my first McIntosh MC2500 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 by 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 exploration and 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 miniature toggle switch on the front panel and a secondary flip down back panel.  The auxilary back panel had two military style connectors mounted 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 remove many types of paint. 

Latent Issues?

After spending a fair amount of time listening to the amp I dicovered 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 both 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 XRT22 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 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.

Fan thermoswitch wiring
Detail of thermoswitches and wiring.

Below is a different view of the same area after I moved the overheat switch.  All of the heatsinks have holes predrilled for mounting a thermoswitch.  I used common computer case screws 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.

switch detail

O.T. thermoswitch moved

If you notice I have not moved the fan switch wiring yet.  Moving 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 redressing it.  I had to cut the zip-tie holding the fan wires to the main loom.  I contemplated using some magentic shielding but decided to just move the wiring for now.  I am still wondering if something like Spira-Shield would be beneficial.  Spira-Shield is available from  In earlier years McIntosh engineers would routinely use a similar if not the same product to reduce radiated 60Hz from internal 120VAC wiring.

wiring detail
Fan Wiring moved.

Next:  The RTA results.

MC2500 Part Two
Circuit mods

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