【NEC】 N7761 and N7765 Hard Disk Units

In November 1982, NEC marketed the N7761 high-capacity hard disk unit with a capacity of 2.68GB. It was first shipped in December 1983.

With progress in magnetic recording media, this unit achieved higher recording density by, among other things, using the first Japanese-made thin-film magnetic head, and employing 2-7RLL (Run Length Limited) code, which has a minimum magnetization reversal 1.5 times that of MFM (Modified Frequency Modulation). As a result, the same memory capacity per spindle (672MB) was achieved while reducing the number of 14-inch disk media from the 8 disks of the previous model to just 5 disks, and a magnetic disk unit with a total memory capacity of 2.68GB was configured using 4 of these HDAs (Hard Disk Assemblies).
An average seek time of 16msec was achieved by reducing vibration with a dual-end bearing structure, and improving the servo system with a variable corner frequency differentiator, feedforward speed control and other innovations. A data transfer speed of 3.0MB/sec was achieved by increasing media density, using a thin-film head, and improving electronic circuits by, for example, using ECL circuits.

In September 1985, NEC announced the N7765 hard disk unit as an even higher capacity version of the N7761. The N7765 had an increased memory capacity of 5.37GB, and was first shipped in September 1986.

This unit doubled the overall memory capacity of the N7761 by slightly increasing the number of disks, and increasing track density by 1.5 times using a thin-film head, coated recording media and improvements to the positioning mechanism and other components.
The appearance of the N7761 and N7765 was almost the same.

Specifications of N7761 and N7765 Magnetic Disk Units (per HDA)
Model name N7761 N7765
Memory capacity(MB) 672 1344
Average seek time(msec) 16 17
Average rotational delay(msec) 8.3
Data transfer speed(MB/sec) 3.0
Disk diameter(inch) 14
Number of disks 5 6
Number of heads(information recording part) 16 19
Number of cylinders (figure in parentheses is for alternates) 885+(2) 1491+(4)
Recording density(bpi) 15300 16100
Track density(tpi) 800 1200
Recording system 2-7RLL
Rotation speed(rpm) 3620
Total memory capacity(GB)(entire unit) 2.68 5.37
Unit dimensions(mm) (entire unit) 1455(W)×800(D)×1790(H)

N7765 Hard Disk UnitThe N7761 Magnetic Disk Unit had the same appearance.