General:The McIntosh MC250 is a relatively rare solid-state power amplifier. In fact, this is the first one we've ever had in our shop! It's essentially an MC2505 - which is one of our favorite 50-watt amplifiers ever made -without the front-panel meters, housed in a tube amp chassis.An added bonus is that this MC250 is switchable between 120V and 240V operation, making it easy to use in regions where 240V mains power is standard. We believe this is a factory-original configuration from McIntosh. This is definitely a rare option!The MC250 has all the hallmarks of a classic vintage McIntosh amplifier. Build quality is exceptional, with premium components and a thoughtful layout that makes servicing straightforward. During testing, it sounded wonderful, delivering the rich, full-bodied vocals and warm, effortless presentation that McIntosh amplifiers from this era are known for.Like many classic McIntosh amplifiers, the MC250 features output autoformers, allowing it to deliver its full 50 watts per channel into 4, 8, or 16-ohm speakers. This means it is a true 50-watt amplifier regardless of speaker impedance. It makes an excellent pairing with most medium- to high-efficiency speakers and is a fantastic way to experience classic McIntosh performance.This particular amplifier was serviced in-house at SkyFi, where all necessary age-related components were replaced to ensure reliable operation for years to come. This type of maintenance is essential on vintage amplifiers that are 50 years old like this one. Included will be a replacement McIntosh box so it arrives safely at its new home.SkyFi Cosmetic Notes:This amplifier is in very good cosmetic condition, showing minimal chrome wear. Light marks are preset on the transformers and other metal parts. One output barrier strip was replaced with a period current part during service.SkyFi Technical Notes:The amplifier was serviced in house here at SkyFi. We replaced the aging electrolytic capacitors on the driver and input boards along with select transistors. Bias adjustment controls were added, controls cleaned, and proper operation verified. The unit arrived with one damaged barrier strip which we replaced as well. NOTE: A 120V/240V switch has been added to the control panel in place of the normal convenience outlet. This may have been a special order device from McIntosh. The serial number 3091 is only 4 digits which does not match the normal ranges outlined in the service manuals of 10L01 through 91L43. This is a late version of the unit evidenced by the version of the driver board installed. The switch is labeled 117V / 234VBrand Background:McIntosh Laboratory is an American manufacturer of handcrafted high-end audio equipment based in Binghamton, New York. The company was founded in 1949 by Frank McIntosh. The company designs and produces audio amplifiers, stereo tuners and other consumer electronics products.Ownership:Single OwnerConnections:RCA inputs, Speaker Screw Down terminals.General Sound:Smooth, uncolored, undistorted natural and cleanWorking Condition:Working perfectly and tested in our lab and listening room.Included:Unit, box, and reprinted manual on heavyweight paper.Packing:Original Manufacturers PackingSpecs:Power output: 50 watts per channel into 8Ω (stereo), 100 watts into 8Ω (mono)Frequency response: 20Hz to 20kHzTotal harmonic distortion: 0.25%Damping factor: 38Input sensitivity: 0.5VSignal to noise ratio: 90dBSpeaker load impedance: 4Ω to 16ΩSemiconductors: 24 x silicon transistors, 18 x diodesDimensions:179 x 267 x 397mm (7.05 × 10.51 × 15.63 in)Weight:35 lbs.Approximate Age:1974Link to Manual:Click HereRecommended Cables:Kimber Kable - RCA Interconnects - BetterKimber Kable - RCA Interconnects - BestKimber Kable - Speaker Cables - BetterTesting Process:We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCBs for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR.If the amplifier passes visual inspection, we move on to a controlled power on sequence using a Sencore safety analyzer to monitor current draw in real time. Once the amplifier is determined to be safe to operate, we connect it to full AC mains for function and power testing. We connect the speaker outputs of the amplifier to a Sencore PA81 Power Analyzer which acts as a dummy load, DC offset monitor, and oscilloscope interface. We start with a low level 1 KHz test signal at the amplifier's input and slowly increase its amplitude while monitoring the output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. If the device under test has both balanced and single ended inputs they are both tested at this time.We finish off the bench evaluation with a 1 KHz square wave check and a 20 Hz to 20 KHz sine sweep to assess the amplifier's frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention.Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours minimum. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when the unit has fully come up to temperature.
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