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RFT TSS

Overview

TSS—“Tastenschalter mit Schutzrohrkontakt”—is a series of reed keyboard switches from East German brand RFT. The name translates to “pushbutton switch with protective tube contact”, referring to the glass capsule that encloses the metal reeds, rather than the more expected “Reed-kontakt”. These rare switches can be found in the Robotron K7609.51 keyboard used with the PRG600, PRG700 and PRG710 computers. Other use includes the Poly-Computer 880 (or POLY880), Bulgarian ELKA calculators and as front panel buttons in various radios. The Hall effect equivalent is TSH 19 (“Tastenschalter mit Hallelement”).

Catalogue description

The description in the Passive elektronische Bauelemente ’79|80 catalogue (helpfully uploaded by Holm Tiffe) is as follows:

Der Tastenschalter mit Schutzrohrkontakt ist vorwiegend für den Einsatz in Tastaturen der Datenverarbeitung, Steuer- und Regelungstechnik bestimmt.

Er wird in den Ausführungsformen rastend, nichtrastend, Goldkontakt oder Rhodiumkontakt gefertigt. Das Frontflächentraster beträgt 17,5 × 17,5 mm (Montageplattenbefestigung) bzw. 19 mm × 19 mm (Schienenbefestigung).

Zur Beleuchtung wird die Signalkleinlampe MSKF nach TGL 10 449 eingesetzt.

Das Schutzrohrkontaktsystem zeichnet sich durch hohe Zuverlässigkeit, Schutz vor Umwelteinflüssen, geringe Prellneigung und TTL Kompatibilität aus.

Broadly speaking, this indicates:

The reed pushbutton switch is mainly intended for use in data processing keyboards and control engineering.

It is manufactured in momentary and alternate action configurations, with a choice of gold and rhodium contact surfaces. The switches are designed for a 17.5 × 17.5 mm mounting plate and 19 mm × 19 mm rail mounting.

The switches can be illuminated using an MSKF miniature bulb (per standard TGL 10 449).

The reed capsule system is characterized by high reliability, protection against environmental influences, low bounce time and TTL compatibility.

The catalogue does not illustrate either the mounting plate or rail mount options. The switches have no sign of any facility to secure them into anything. The rail mount system can be seen in the this K7609 keyboard but there is no sign of what attaches the switches to the rails. (This is in contrast to Micro Switch SW and SN series and clones, with large attaching loops in the metal rails.) While covered Cherry M8 switches do not clip into the faceplate, they do have a ledge around the outside to allow the faceplate to rest on the switches and hold them laterally steady. There is nothing of the sort with TSS switches.

Variants

There are multiple variants of these switches.

Short version

In the 1979–80 catalogue, the upper portion of the shell is 15 mm square and 7.5 mm tall, and the shell is 15 mm tall overall. These switches were available in upright (“stehend”) and horizontal (“liegend”) versions; the latter has the switch contacts bent through 90° to allow the switch to lie on the PCB with the contacts facing into the PCB. In practice, the horizontal versions also use a metal cover with its own solder terminals to secure the switch.

Tall version

The tall version is covered by East German standard TGL 34716, from October 1979. These have a shell that is 22.8 mm tall. These are supplied in two versions: TSS 17,5 designed for a mounting plate with 17.5 mm spacing (which strangely, the switches fit into from above), and TSS 19, which uses a rail mounting system and a standard 19 mm spacing. The upper shell is also enlarged to 17 mm square at the top, tapering down to 16.7 mm square at the bottom of the upper portion of the shell.

TSS 17,5 is the common version that you will find for sale on eBay.

I received some TSS 17,5 switches from 10ko on Deskthority (from Bulgaria), and these are almost but not quite the same as in TGL 34716. The standard depicts 4.0±0.5 mm of travel, and mine are quite clearly 4.5 mm travel, and 0.4 mm taller.

Specifications

The specification table is confusingly formatted; it appears that rhodium contacts offered a shorter life but higher current and voltage capacity (see the materials page).

The catalogue gives the following specifications, where RKR 13 is the rhodium-contact version and RKG 13 is the gold-contact version:

Characteristic RKR 13 RKG 13
Switching current, min. 0.01 mA 0.001 mA
Switching current, max. 300 mA 200 mA
Maximum voltage 42 V
Switching capacity with resistive load 5 W 4 W
Average lifespan at 5 V 5×10⁵ (“m.R.”) 1×10⁶ (“o.R.”)
Contact resistance ≤ 1 Ω
Operating force, momentary ≤ 1.3 N
Operating force, alternate action ≤ 2 N

For clarification, the complete set of specifications, and to verify my understanding of the page, please consult the RFT catalogue! There may be some gross errors in the above table owing to my lack of understanding of electrical terminology, East German or otherwise!

The operating force is a little on the high side, being a little heavier than Cherry MX Linear Grey: the latter is rated at 80±25 cN, with a terminal force of around 110 cN, which at the same tolerance would be 85–135 cN, a fraction above that of TSS.

The lifetime specifications are unclear, as RKR 13 (rhodium contact) only has an “m.R.” figure, and RKG 13 (gold contact) only has an “o.R.” figure. Even the lifetime of one million cycles for a reed switch is surprisingly low considering that it is well below what an exposed contact switch typically provides.