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The X Series ( as well as some of the prior M Series ) used the Crossfield Head Technique, where an additional head placed on the rear side of the tape, and aimed at the center of the record head, would separately infuse just the bias signal, a high-frequency supersonic Bias waveform, to make the audio capabilities of the magnetic tape medium operate in a more linear region. This supposedly for less self-erasure and to lower distortion. Crossfield Technique, has been the subject of much speculation and supposition over time, the results remain, to this day unresolved. Some of the X-100 and X-200 series sets, used the same single motor chassis as other Akai's, the 1700 series and the 1800 / 1900 multi-purpose sets.
Some of the X-300 Series sets, used a more sophisticated three motor chassis, in which case, the Capstan motors were likely to be the Synchronous-Hysteresis type, with a belt drive, and the capstan mounted on a large flywheel. This type of drive often using a precision bearing or two. Those bearings could be inside the motor, or in the capstan assembly, or used in both. I've seen/heard those types of bearings get noisy. I had one bearing - in another brand of tape deck that was so bad after thirty years, it sounded like the type of machine that grinds coffee beans... it was easily audible more than ten feet off. After replacing that bearing ( and luckily it was of a standard type designation ) the set was so quiet, you had to have your ear pressed on the front plate to even hear the motor purr... I've never heard an Akai's bearing go bad and become audible like that, but some of these older sets do have two or three of them in there, so it could happen.
The audio circuitry used in the X-Series was very early solid state. What that means is that some sets used early Germanium Transistors, which tended to be noisy. Perhaps in those early days, the Crossfield Head technique had an advantage, in reducing noise and improving overall distortion, or audible levels of distortion, and self-erasure of high frequencies.
The Crossfield head technique, was eventually abandoned by Akai, when they developed and used their Glass and Crystal Ferrite heads, in the later GX Series which began to appear around 1969. This may have started with the Model GX-365D. By the late 1960's low-noise silicon transistors were available and used, so, noise and distortion levels, being either vastly different, or much reduced compared to early Germanium parts, then, perhaps the additional cost of also implementing the Crossfield Head technique, no longer produced either measurable or audible improvements. Also, perhaps the focused field of the Ferrite Gapped heads could be made to tighter tolerances, with smaller gaps for better high frequency response. Still, early on, the GX heads must have been very expensive to produce, and the GX-365D, and even much later sets ( like the GX-210D ) used the mechanical moving head mechanism, where a solonoid moves the entire Playback Head Assembly up and down to select which set of tracks ( forward, or reverse ) is being played. Some later low-end GX Series sets still used this mechanical moving head mechanism, but in the later GX Series separate playback heads were used and after 1975, I think the solonoid/mechanical method was never used again.