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RAFI RS 74 M SPDT latching

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One switch not present in recent RAFI catalogues is the RS 74 M SPDT latching. It has a common terminal, a normally closed terminal, and a normally open terminal. The switch latches between these two states. RS 74 was not intended for keyboards (at least, not after RS 76 was introduced), so this switch is not actually a keyboard switch, but from what I can tell, the latch mechanism is the same as that used in RS 76. Until such time that I ever get my hands on the whole RS 76 series, I cannot say for certain.

View full-size image Latched state
View full-size image Bottom, showing RAFI branding

I found these switches by way of a Google image search, and I must say that they are possibly the most evil switch I have ever encoutered. My number 1 evil switch until now was always SMK second generation, owing to their being a headache to open and in many cases absolutely impossible to reassemble (without devoting a week of time to a monastery to contemplate on the process).

Enter RAFI’s latching mechanism! Fully open-top RS 74 switches are only 6 mm tall, from the top of the PCB to the top of the shell (around half as tall as Cherry MX); the shell height is 5.6 mm if you discount the standoff rails. Putting a latching mechanism into such a diminutive switch requires a design that is extremely compact. The heart-shaped cam track is still there: Cherry are the only company I have encountered who took an alternative approach. However, the cam track is in a slide block. Slide blocks themselves are rare; Alps SKFL Lock uses a slide block, but the cam track is still in the slider. With RAFI switches, the slider contains a horizontal hole, into which is fitted a brass spring-loaded follower pin. As the slider is depressed, the slide block moves laterally to allow the pin to follow the track.

I bought ten switches. I have nine springs. Take a look at the spring:

View full-size image Spring length
View full-size image Spring size
View full-size image Spot the small spring in this silhouette view of the parts

The spring is around 2.7 mm long, and 0.9 mm in diameter! At one point I had to start a fresh spring hunt because the latent magnetism in either the spring or my tweezers accelerated the spring fast enough to overshoot the tweezers and ping off onto the floor.

Initially I counted a total of nine parts, but after taking all the photographs I realised that the track that the slide block slides in is itself a removable part, as evidenced by the mismatch in plastic texture. I reshot one of the full disassembly photos as a result, but not the other.

Design

The switch uses the same flat loop of metal as other RS 74 M switches, but this time it’s vertically deformed in its resting state. The switch is not gold crosspoint like the RS 74 M momentary variety; instead, there are flat surfaces on the stationary contacts, and gold-plated hemispheres on the movable contact, one above and one below.

Slide block

View full-size image Slide block fully left
View full-size image Slide block partially removed
View full-size image Slide block fully removed, revealing the spring-loaded pin
View full-size image Slide block front
View full-size image Slide block rear

The slide block has, as can be seen above, a ridge running across its rear. This ridge runs inside a groove within an insert that can itself be removed:

View full-size image Slide block track block removed

The scuffing of the top of the guides for the slot that holds the block itself was probably caused by trying to reinsert the track block. Removing the track block required pliers for grip, and the block won’t go back in.

Bestiary

In all seriousness, the RAFI RS 74 latching mechanism is exquisite, and beautifully smooth, with the right amount of tactility when latching. As a collector of latching switches, it’s right up there at the top for the best designs of latching mechanism.

View full-size image Disassembly: bottom views, with the track block not yet removed
View full-size image Disassembly: top views (with side view of the slider assembly)