[time-nuts] Racal-Dana tactile switches

Philip Pemberton lists at philpem.me.uk
Mon Apr 14 06:04:09 UTC 2014

On 05/04/14 21:53, Jim wrote:
> I finally had to dive into my Racal Dana 1992 counter, after it had 
> developed several bad switches. I found that through-hole 6x6mm tactile 
> switches with four leads will fit in the counter pcb holes diagonally. That 
> is, if you trim off the leads from two opposite corners, the other two leads 
> are very close to the required distance to go into the existing pcb holes. I 
> just ordered some of these switches from ebay with a long plunger that looks 
> to be perhaps 2mm round. I'm going to drill holes into the back of the 
> existing keycaps to be a press-fit to the plunger on the new switches. I 
> think it will all hold together. If this works, it is a cosmetically and 
> functionally perfect solution. I'll report back after I've tried it.

I've seen this done on one of Ian Scott Johnston's Youtube videos --
https://www.youtube.com/watch?v=RMYR2_TJOu4 ).

I think Gerry Sweeney might have done something similar --
https://www.youtube.com/watch?v=EstQVVmX-oo possibly?

To be honest though, if I was doing this repair I'd make a jig and a
heated cutting die, then modify the switches. Ruining a switch is less
of a problem than ruining a keycap, although admittedly the latter
problem can still be solved with a blank keycap and casting equipment or
a 3D printer and some ingenuity with the CAD software.

Heck, I replaced the broken keypad in a Solartron 7150plus with a
3D-printed front piece and a custom 0.8mm-thick PCB. Sadly I botched the
PCB (I think it was scratched during photo-etch development or etching)
and now the Ohms switch has become non-functional.

Sooner or later I'll have to de-bond the front panel again and replace
the keypad... frankly, I'd rather get a front panel from a broken meter
as this one certainly looks like it's had a hard life :(

It still works over GPIB though, and I rarely use ohms or diode check on
the 7150s, so "sooner or later" might turn out to be "the day after the
Vogons arrive to build that hyperspace expressway"... unless someone
offers me a spare 7150Plus panel with or without the LCD board ;)

I'd kinda like to do a replacement 7150Plus LCD, some of the Electronic
Assembly "DOG" graphic displays look to be about the right size. That
depends if I ever find an S7150plus with a broken display ;)


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