Fixing small plastic... things

Tony Duell ard at p850ug1.demon.co.uk
Fri Dec 3 15:31:46 CST 2010


> 
> Hi all --
> 
> Got myself a Friden 1162 desktop calculator.  This is from about 1968-69 
> and has a neat-O keen CRT display and uses a magnetorestrictive delay 
> line memory.  Kinda cool.

Sounds very nice...

[...]

> The 1162 has a rather interesting keyboard encoding mechanism (you can 
> see a decent overview of the device & the keyboard mechanism here: 
> http://www.oldcalculatormuseum.com/friden1162.html)  Close investigation 
> of the keyboard mechanism on my specimen reveals that a few of the 
> plastic "fingers" that are positioned on the rods that move the magnets 
> to/away from the reed switches have snapped off.
> 
> I have a close up picture of what the fingers are supposed to look like at:
> 
> http://yahozna.dyndns.org/scratch/friden/normal-finger.JPG
> 
> And a picture of one of the broken ones at:
> 
> http://yahozna.dyndns.org/scratch/friden/broken-finger.JPG
> 
> The broken fingers no longer make contact with the mechanism, and so a 
> couple of the reed switches do not get activated properly.  I'm guessing 
> that this is at least part of the reason the machine is acting the way 
> that it is (that it's getting unexpected scancodes from the keyboard and 
> going off into the weeds...)
> 
> I need to figure out how to "recap" these fingers.  I don't have a lot 
> of experience repairing plastic stuff like this, anyone have any 
> suggestions?

A number of plastics become brittle with age, and I wonder if this has 
happeend here. 

In any case, I have had very little success in trying to repair things 
like this. I have found the best thing to do (and you won't like it) is 
to make new parts. Do they ahve to be plastic (e.g. for insulation)? I'd 
want to use brass rods and make little 'cams' to fit on them (secured by 
setscrews [1] against flats on the shaft). It wouldn't be oriignal, but 
it would almsocertainly be stronger.

[1] A tirival hint. If you need to use small setscrews (grub screws), I 
suggest getting allen-hex ones. You can put one on the end of the hex key 
and use the latter to get the screw into the hole.

-tony



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