[time-nuts] Austron 2100 switch repair

[GandalfG8 at aol.com]

Given the current Loran situation I know this might have come a bit late  
but thought it worth sharing in case it's of any benefit to others. If it's 
old  news then apologies for that but I've not seen it documented elsewhere.
 
Along with other equipment, the 2100 is known to suffer in the  long term 
from keybounce problems with the front panel tactile switches.
A common "cure" is to increase the keybounce time constant on the  74C923 
keyboard encoders, the 1uF fitted to the 2100 is quite low and I've found  
that this can be increased to at least 10uF without adding any additional  
problems.
 
However, there is likely to come a time when this is no  longer an adequate 
fix and switches can become severely intermittent or  even totally open 
circuit.
 
Replacements are available from Mouser or Digikey, the current manufacturer 
 is E-Switch, but with the numbered key caps not being a stock item I  
checked my "faulty" switches to see if the caps could be  removed and the 
original numbered caps transferred to new  switches.
 
Having found that, with care, the caps can be removed, I also discovered  
that these switches are mechanically straightforward and physical failure  is 
highly unlikely.
Other than the cap itself, the only moving part in the switch is a metal  
"clicker dome" which when depressed shorts across the contacts formed by  the 
internal ends of the solder pins used for mounting the switch.
One of these pins is in the centre of the switch, directly below the centre 
 of the dome, and the other is in one corner with a corner of the dome 
plate  resting on it.
I'm not sure what the plating is on these pins but, in my switches anyway,  
the contact surfaces were heavily tarnished and this was the source of all  
the problems.
Prior to cleaning the contact surfaces were dark brown to black  and looked 
very much like tarnished silver, albeit in this  case seemingly 
non-conducting.
This tarnishing also seems to have a knock on effect on the gold plating on 
 the contact surface of the dome so this will need cleaning too. Given that 
the  dome plate has four distinct corners it's easy enough to ensure that a 
 fresh corner is used as this contact.
 
A small drop of isopropyl alcohol with a folded offcut of cartridge or  
printer paper used for burnishing seems to do a good job of removing the  
tarnish and polishing the contact areas, I certainly wouldn't recommend anything  
more severe.
After cleaning and reassembly all my switches have a contact resistance of  
0.1 Ohms or less and the unit is fully functional again. I could probably 
now  reduce the 10uF debounce capacitors but all is working ok so have left 
well  alone.
 
Practical tips....
 
It isn't necessary to remove the switches from the circuit board in order  
to remove the caps but it will need a very fine blade or hook to get in  
alongside the cap and lever it out.
 
What I used has been in my tool kit for years and I'm not even sure what it 
 is, some kind of sewing implement I think, with a plastic handle and a  
bent bent end with the bent section around 3/8 inch long, very hard and  
pointed, and not much thicker than a reasonably fine sewing needle.
 
The black caps of the flat faced switches used on the numeric keypad and  
for the back light switch have a lug approx 1/8" wide protruding downwards 
from  the centre of each side. These seem to be fairly rugged and careful  
leverage alongside the lugs should remove the caps without  damage.
 
The white caps with a raised tapered section, on the switches along the  
bottom of the front panel, have only two lugs, top and bottom, and these seem  
more fragile.
I found it best to lever out the top of the cap first, top being defined as 
 when viewed in the panel, having broken the end off a bottom lug when  
levering one out bottom first, thanks be for superglue:-), but offer no  
guarantees that what worked for me will be the best solution for anyone  else.
 
Given the cost of replacement switches, plus delivery charges, a few  hours 
work has saved me over 50GBP, so again I hope this information may be of  
benefit to others.
 
Unfortunately though, from this point on, you're on your own, you break it, 
 you fix it:-)
 
Regards
 
Nigel
GM8PZR