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KBK update for Wednesday, 20th November

Oops.

Back in 2016, I came across a couple of Korean websites (ePartsHub and ECPlaza) that provided specifications and photographs for Futaba/Sejin switch subseries (MD-4P, MD-7, MR-6C, ML-88, MA41/42 and MA71/72). This allowed most series to be given names that are meaningful even if not fully accurate. This data is all gone from ECPlaza but it remains available on ePartsHub under Push Button Type Switches. (Repeated attempts to contact both sites for further details turned up lots of dead and ignored e-mail addresses.)

What surprised me the most was the rated lifetimes: a mere three million for the little unreliable switches in the BBC Master, ten million for the high-end switches, and yet 30 million for the cheap switches in the BBC Micro. This always seemed implausible, but there was a clear photograph to prove it.

With the discovery of limited amount of official documentation, it is now clear just why that 30 million figure seemed so unlikely: the photograph of MR-6C shows the wrong switch. Comparison of the diagram of MR-6 in the catalogue against the diagram of KBR in the GRI catalogue shows that the two are a close match, while the diagram of ML-41 is a close match for the switches we have been calling MR-6C. The specifications for MR-6/6C also match up with those of KBR. This means that MR-6 is the missing reed type from US patent 4041427 (MR-6C may just be the type with a cruciform mount, but this is not certain).

The catalogue is still sloppy, inaccurate, incomplete and confusing, but it does clear up a major misunderstanding. It also shows that ML switches are not even defined in terms of size, and suggests that instead they are defined in terms of switch contact design, with both sizes being rated for three million cycles each. This single low-rated lifetime makes sense from the perspective of how they are designed, but not specifically why the 2.5 mm version seems to have much lower reliability than the 3.1 mm version. One would also like to believe that MR being reed would be signficant enough to warrant special mention, and it may well be that MR is not reed, but all indications are that it is.

The classification now seems to be as follows:

MR
Reed, 30 million cycles lifetime, rated for 24 V DC at 5 mA; standard height with 3.1 mm travel
MD
Sealed metal contact, 10 million cycles lifetime (for momentary non-illuminated; there are strange discrepancies with the other types), rated for 24 V DC at 1 mA; standard height with 3.1 mm travel
ML
Metal foil contact, 3 million cycles lifetime, rated for 24 V DC at 1 mA; available in standard height with 3.1 mm travel and low profile with 2.5 mm travel

The sealed type and a newly-reported tag terminal type remain without series names. Since ML-81 seems to be a replacement for ML-3, one could argue that the type found in the BBC Micro is going to have a subseries such as ML-1 or ML-2.

View within the updates for 2019

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