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Oak Series 400

Contents

Overview

Series 400 from Oak comprises tall mechanical keyboard switches. These switches are very distinctive, with their clear plastic shell and plain, flat metal keystem. Stamped metal keystems were fairly common at the time (as with IBM’s beam spring, Alps SCK, Fujitsu FES-1/2/3 and SMK reed) but unlike the curved and stepped shape of SCK and bifurcated profile of the other types, the Series 400 keystem has straight sides.

Details on Series 400 are fairly scarce. One of the oldest switch series (being introduced somewhere around 1971), it seems to have fallen out of favour over time. The current owner of Oak Industries’ switch line reports that no details remain on the keyboard switches.

Series 400 is depicted in a 1973 Oak advertisement in Electronic Design magazine. A switch type appearing to be the same design appears in the Electronic Engineers Master catalogue for 1971–72. All Series 400 switches seem to be branded. At present, these two advertisements are the only official documentation obtained. Series 400 is also named in US patent 3924722 (but not in Google’s transcript at the time of writing, which is erroneous: download the PDF to see this reference), an electronic typewriter patent from CPT Corp, filed in 1973, the same year as the advertisement.

Series 400 is covered in US patent 3708635 “Multiple switch assembly with improved reciprocating leaf spring contact cam actuator”, filed in June 1971.

Pricing was given as under 40 cents per piece in quantity orders in 1971, and under 30 cents per piece by November 1972 (around $2.55 and $1.84 in today’s figures). In Electronic Design magazine in 1972, where Series 400 is briefly mentioned, the quantity figure associated with the 30-cent price is 500,000.

Series 475 appears to be a lower-profile derivative.

Specification

Very little is known about Oak mechanical switches. The 1973 advertisement gives the following details:

Bounce time Less than 3 ms
Rated lifetime 20 million cycles

The switches are described as having self-cleaning precious metal contacts. Jacob Alexander obtained switches matching the Series 400 design marked as carrying 1 A at 125 V AC. It is not clear whether these are Series 400 (as its capacity is unknown) or an equivalent series rated for mains operation. Precious metal self-cleaning contacts are intended for milliamp currents where contact resistance must be minimised: as keyboards operate at tiny currents, they require special low-energy switches. Jacob’s examples are in terrible condition, but they do appear to be gold prism contact just as with Cherry’s also then-new 261/262 (later M61/M62) switches, as incongruent as this seems with the much higher power rating indicated.

Features

Wang 2200 service literature gives the plunger length as ½″. They also used a slightly shorter ⁷⁄16″ plunger, which could have been a customisation for Wang. The plunger could be upright or angled, for sloped and stepped keyboards respectively. In Electronic Design magazine in 1972 [ED1972-FOK], they state that Oak will offer any spring pressure or key stem height (sloped or stepped) as well as alternate action. The alternate action design has yet to be discovered.

The switch is formed from two identical halves, in the same manner as Datanetics DC-50 and is thus designed for double-pole operation. The switch contacts can be normally open or normally closed, as illustrated in the patent. The 1971 advertisement lists the following contact configurations:

The basic switch appears to be PCB-mount only. The dummy option may be a way to gain additional solder points for securing the switch. The patent also includes a plate-mount facility, in the form of two sprung metal pieces that fit to either side of the switch. This sits on the plate from above, and presses against it from below. The use of a separate metal piece for the design echoes that of the contemporary Fujitsu FES-1/2/3.

Keyboards

At least two Wang 2200 models used keyboards with Oak switches that appear to be Series 400, going back to 1973 with the original 2200. Photographs of the Wang 2200 PCS-II keyboard (introduced in 1977) show switches that appear to be Series 400. These have the same Wang part number as the switches in the original 2200 keyboard. The types used in these keyboards are Wang parts 325-2405 (SPST with ½″ plunger), 325-2407 (“SPDT” with ½″ plunger) and 325-2413 (SPST with ⁷⁄16″ plunger, used on the thin top row keys). The type described as “SPDT” is shown in a diagram as being DPST-NO+NC. The Oak part numbers are not given in the service manuals.

Similar if not identical switches to those of the 2200 are used in the Wang 462 Statistical Programmable Calculator.

There are Oak switches in an unidentified Facit keyboard; these use the metal plate attachment depicted in figure 13 of the patent.

Very similar switches can be found in the Honeywell 80-02 keyboard, from the Honeywell 5921 terminal. These have pairs of ridges on them that sit above the plate, a design not confirmed from official illustrations. A single switch can be seen with the plate mount design shown in the patent.