Hi, I hope this will clear up some of the questions that have been raised about the Otari MX-50/55/5050 Master Recorder Series.
The basics can be found at
http://www.otari.com/company/index.html.
The MX-55 predates the MX-5050 Series by three years. The build quality is generally the same on all of these fine units and is quite good, essentially military tank quality.
The MX-55 was introduced in 1987 as a 1/4", three speed (two speeds available front panel), two track plus timecode track 'Master Recorder' and can handle tape reels from 3 inch to NAB 10 1/2 inch reels.
It was followed by the MX-50 in 1988 (no timecode) and the venerable MX-5050 in 1990.
The MX-5050 was shipped in various minor model configurations (BII, BIII, BIV) depending on features, but is basically an updated MX-55/50 - It is a 1/4", two track 'Master Recorder'
without timecode (and brother to the venerable MX-5050 MK III 1/2" 8 track recorder), capable of recording speeds of 3 3/4, 7 1/2, and 15 IPS, with both NAB and IEC EQ.
In addition, most of the MX-5050 machines contain a fourth head mounted in the headstack which allows play (but not recording) of 1/4" stereo (four track stereo) tapes of the era.
All machines featured direct drive motors (no belts) and some of the smoothest transport servo technology around (really nice for handling old tapes for restoration or transfer to digital).
I hope this answers some of the questions I've seen around the internet regarding these machines, which are frequently found for sale at very reasonable prices. I wouldn't class the electronics to be up there with the Ampex mastering recorders of the same ilk, but their cost and maintainability are certainly attractive in comparison. (Otari also manufactures the MTR-12 Mastering Recorder and it's brother the MTR-90 Multitrack.)
I hope this helps some of you who have tried to find this information.
...JB