I know what you guys are thinking- no, this isn't for a keyboard collection or some modern build or some other nonsense like that. I have a friend who is restoring a fairly interesting and historically significant vintage computer- The correct SD-series replacement switch would be the 4B3A- *** These can allegedly be found on some of the Diablo printing terminals.*** Subject to what /appears/ to be a batch-related encapsulation failure in the glue on the proprietary hall effect sensors, around a little over half of the switches on the current keyboard are bad. It is possible other switches ending in ***A could be made to work with a bit of labor and disassembly (swapping the fairly brittle sensors). I am not a keyboard expert but I have learned that you can remove a key on some of these microswitch keyboards and read the model fairly easily on each switch. Please let me know if you have a lead on a donor for these switches. They will be put to good use, and you can reply to me off-list for more details.

> On Jul 9, 2020, at 21:58, Brent Hilpert <bhilpert at shaw.ca> wrote: > ?On 2020-Jul-09, at 9:39 PM, Ian Finder via cctalk wrote: > >> I know what you guys are thinking- no, this isn't for a keyboard collection >> or some modern build or some other nonsense like that. >> >> I have a friend who is restoring a fairly interesting and historically >> significant vintage computer- >> The correct SD-series replacement switch would be the 4B3A- >> >> *** These can allegedly be found on some of the Diablo printing >> terminals.*** >> >> Subject to what /appears/ to be a batch-related encapsulation failure in >> the glue on the proprietary hall effect sensors, around a little over half >> of the switches on the current keyboard are bad. >> >> It is possible other switches ending in ***A could be made to work with a >> bit of labor and disassembly (swapping the fairly brittle sensors). >> >> I am not a keyboard expert but I have learned that you can remove a key on >> some of these microswitch keyboards and read the model fairly easily on >> each switch. >> >> Please let me know if you have a lead on a donor for these switches. They >> will be put to good use, and you can reply to me off-list for more details. > > Are these the key-switch model which snap into a thin-gauge springy stainless-steel U-channel to form the rows of the keyboard?

No, as I understand it that is the predecessor

4B3A is a non-multiplexed contact closure type which is extremely difficult to find.

On Fri, Jul 10, 2020 at 11:06 AM Al Kossow via cctalk <cctalk at classiccmp.org> wrote: > > Since it isn't very fancy, I wonder if you could make a new board with a > modern Hall > effect sensor? You just need a contact closure.

Yeah Al, that's a great suggestion. I was looking at the AH1815 last night. The challenge is getting the whole assembly as thin as the original, which presents a not insignificant challenge- Even using the SOT553 package of this: https://cdn.sparkfun.com/assets/4/4/8/2/a/AH1815.pdf - I'd have to do some trickery, like have it mount into a hole into the center of the board.

A few responses- On Fri, Jul 10, 2020 at 12:29 PM Brent Hilpert <bhilpert at shaw.ca> wrote: Great catch, Brent. I'll keep looking at other options. I must have skimmed the data sheet a bit too hard. On Fri, Jul 10, 2020 at 01:19 PM Santo Nucifora wrote: this is a home made... I have many of the keyboards with these 3-pin switches. As mentioned previously, what the sensors need to be electrically is the ***A sensor, which is harder to find by a considerable margin. Sink level, vs sink pulse, vs scan. In Al's words- non-multiplexed contact closure type. http://telcontar.net/KBK/Micro_Switch/SD The machine is a Xerox system. On Fri, Jul 10, 2020 at 02:01 PM Al Kossow wrote: I agree. What is interesting to me is this- If I look at other hall effect keyboards, and even other examples of the same sensor: http://telcontar.net/KBK/Micro_Switch/images/SD/Micro_Switch_SD_Hall_sensor… They appear to be encased in a hard, black epoxy of some kind. Some of the other keyboards I have with the same SD switches- but alas the wrong sensor output type- appear to exhibit this as well, and have not failed. My failed sensors have something with the gelatinous consistency and transparency of RTV silicone. It did not appear to have become conductive, but who knows. There were three distinct failure modes: * 1) Output stuck at Vcc (+5v) - By the spec sheet, this should never happen. Vo should be a fraction of a volt when not actuated, dropping to 0v when actuated. * 2) Output stuck at 0v - Self explanatory. * 3) Output appears to be correct-ish Vo, but does not actuate to 0v with magnet. This was the rarest failure. Based on this, my best guess is that there are two failure modes: 1) RTV silicone decomposes over time, leaching something into the die that destroys the IC's passivization layer per Al's comment 2) The RTV silicone (or similar) is not robust enough to shield the delicate sensor die to ceramic substrate connections from vibration and force-based trauma as it decomposes. My expectation is that most of these switches have the black epoxy style of package for a reason- this is the correct design, hopefully should last a long time, and the ones that use this transparent goop will all inevitably die. I have never seen any other semiconductor die encased in what appears to be silicone... This is, incidentally, more about keyboards than I have ever cared to know. The fact that this is an entire hobby for some people is all the more shocking to me. I'd far rather be debugging some logic... Cheers- I

I have a number of keyboards with Micro Switch ST switches, but no SD. Does anyone still use old Memorex or Telex keyboards? -----Original Message----- From: cctech [mailto:cctech-bounces at classiccmp.org] On Behalf Of Ian Finder via cctech Sent: Thursday, July 09, 2020 11:34 PM To: classiccmp at classiccmp.org Subject: In search of 4B3A Microswitch Keyswitches (for a restoration, not a keyboard collector!) I know what you guys are thinking- no, this isn't for a keyboard collection or some modern build or some other nonsense like that. I have a friend who is restoring a fairly interesting and historically significant vintage computer- a and subject to what appears to be a batch-related encapsulation failure on the proprietry hall effect sensors, but could have been some other trauma, around a little over half of them are bad. The correct SD-series replacement switch would be the 4B3A- *** These can allegedly be found on some of the Diablo printing terminals.** It is possible other switches ending in ***A could be made to work with a bit of labor and disassembly (swapping the fairly brittle sensors). I am not a keyboard expert but I have learned that you can remove a key on some of these microswitch keyboards and read the model fairly easily. Please let me know if you have a lead on a donor for these switches. They will be put to good use, and you can reply to me off list for more details. Thanks, - I -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus

There are a couple of serious researchers out there who do useful work, here is what exists for the SD switch https://deskthority.net/wiki/Micro_Switch_SD_Series