As a point of followup, I received Al's MX114 today and did a few quick checks to provide a basic assessment of the preamp section's operation. The first observation was apparent right off the bat: It was dead as a door nail.
Oops, wait a minute. Now it's on. But now it's off. The internal connection of the original molded AC plug had clearly given up the ghost. A quick snip and a temporary plug attached that's way more vintage than even many vacuum tube pieces, and the unit now had proper motivation. On to some quick scope shots which tell the tale quickly enough:
Below: A temporary replacement power plug resolved power issue.
Below: With the Tone and Balance controls centered, Loudness and Filter switches turned off, and the volume control at max, a 250 mV 1 kHz sine wave signal into the Aux inputs produced a very clean, well balanced 2.50 Vac output from both channels. This pic actually shows the output of both channels superimposed on top of each other with the scope in dual trace mode. Hard to argue with this! All the following shots are in superimposed dual trace mode as well.
Below: Since the proper display of a square wave requires a flat circuit response from 1/10th the fundamental frequency to 10X the fundamental, a 2 kHz square wave sent through the unit as set above will quickly check for response issues from 200 Hz to 20 kHz. With 1 kHz referencing 0 db, actual response at 20 kHz is R= -0.35 db, and L= -0.50 db, with both channels within specification. The slight rounding at the top of the leading edge indicates amplitude loss between the waveform presented, and that of an ideal square wave. The balance between the channels at 20 kHz is actually quite good.
Below: Carrying on from above, a 200Hz square wave will quickly check for any response from 20 Hz to 2 kHz. Here, some tilting of the wave is to be expected, since the signal is experiencing phase delay from the RC circuits of the tone controls. The important point is that the wave top is still reasonably flat, indicating a uniform amplitude within the band. Again, with 1 kHz referencing 0 db, actual response at 20 Hz is R = -0.40 db, and L= -1.10 db. While the left channel is out of specification, even the most trained Golden Ear would have trouble detecting this discrepancy. Again, as high quality equipment of this vintage goes, this performance is still very good.
Below: But now, we come to the crux of the matter. Here, with a 1 kHz sine wave presented to the Aux inputs, the scope sensitivity is increased to 50 mV/div, and the MX114's volume control is set at 11:00. The Left Channel output of the MX114 in this shot (0.15 vac) would be driving an 8Ω speaker connected to the output of a 50 watt amplifier with an input sensitivity of 1.0 vac to a power level of just over 1 watt, or to about 200 mW for a 50 watt amplifier with an input sensitivity of 2.50 vac. The former could be too loud in an apartment setting with sensitive vintage Klipsch speakers, while the latter still might produce banging on the walls. This level of imbalance would be very apparent, and destroy the stereo image and sound stage!
Below: Apparently, somebody has been here at the volume control before. Further proof is the pink sticky note seen in the first pic found inside the unit that clearly notes how to reconnect the wires to the volume control after it was removed at some point. The control currently installed is beyond bad. It is specified as being an 800K unit, with the Left Channel section measuring ~ 950K, and the Right Channel section measuring 1.6 megohm. The loudness tap readings were equally bad.
A call to Brian at McIntosh parts found that a new volume control complete with AC switch is still available as new part number 134-369 for the MX114 for $100. Hopefully, it will be here the first of next week. Once everything is first repaired on the unit (or determined to be corrected by way of a typical restoration), then a clear plan for actual restoration can be developed with Al for the unit going forward.
Dave
AL
