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Oak Series 400



Series 400 from Oak comprises tall mechanical keyboard switches. These switches are very distinctive, with their clear plastic shell and plain, flat metal keystem. Although stamped metal keystems were fairly common at the time (as with IBM’s beam spring, Alps “SCK”, Fujitsu FES-1/2/3 and high-profile SMK reed) although rather than being bifurcated, the Series 400 keystem has plain parallel sides. The switch body is stated to be 0.5″ square by 1.25″ tall, if the poor-quality Google Books image can be read correctly. Manufacturing of keyboard switches was reported in September 1973 to be automated.

Details on Series 400 are fairly scarce. Presently, the earliest known mentions of the series are from 1969, when they were advertised in Machine Design and Computer Design magazines (as shown by Google Books). Series 400 was described in several advertisements as “feather-touch”, a term that would be applied also to Series 415. As advertised in 1969, switch lifetime was given as 20 million operations, a figure that would remain given in 1971 and 1973.

Although at least one of the 1969 advertisements appears to depict the switch, Google Books refuses access to the illustration. The oldest accessible depiction of Series 400 is from a 1971 Oak advertisement in Electronic Design magazine. All Series 400 switches seem to be Oak-branded.

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

Pricing was given as under 40 cents per piece in production quantity orders of SPST-NO switches 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.

The current owner of Oak Industries’ switch line reports that no details remain on the keyboard switches.


Little is known about Oak mechanical switches. The 1969, 1971 and 1973 advertisements give (albeit not all at the same time) the following details:

Bounce time Less than 10 ms (1969)
Less than 3 ms (1971–3)
Rated lifetime 20 million cycles
Operating force 100–150 grams (1969)
3 oz (approx. 85 grams) (1971–3)
Travel 316″ (equiv. 4.8 mm)

A “keyboard switch” advertisement in Electronic Engineer magazine in July 1971 notes that you can “specify any std. operating force from a min. of 3 to a max. of 16 oz ±0.50 oz (85, ± 15 g) with this switch designed for PC board applications.” The model of switch is not stated, but from the date and the price (40 cents each in production quantities), Series 400 seems a reasonable assumption.

A 1969 advertisement noted that contact materials “range from silver-plated copper alloy to precious metal”. Jacob Alexander obtained switches matching the Series 400 design marked as carrying 1 A at 125 V AC. Confusingly, Jacob’s examples—albeit in terrible condition—appear to be gold prism contact just as with Cherry’s also then-new 261/262 (later M61/M62) switches, which seems incongruent with the power rating, precious metal self-cleaning contacts are intended for milliamp currents where contact resistance must be minimised.


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.


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.

Plate-mounted Series 400 switches can also be found in a Quindar QuinType 80, built around an IBM Selectric. Here the patented plate mount adapters can be clearly seen.


All material was scanned by Bitsavers unless otherwise noted.