New (NOS) switches with contact problems -how to fix?

[TTL]

I recently came across several NOS (New Old Stock) tactile pushbutton switches which I bought as they're no longer made or available elsewhere, in order to replace worn out buttons in some gear from the mid 90s. The switches looked new as there was no sign of solder on them, but I was shocked to find out that almost none of them worked!
I used a digital multimeter with its continuity mode (buzzer), but it seems I was able to "ressurect" some of them though I don't know if repeated button-presses did the trick or if they worked to begin with and I was just not touching the contacts properly with the multimeter probes.

With the switches being perhaps over 20 years old, is there a possibility that corrosion or something might have developed and they can be made to fix again with many button-presses (so as to clean it internally), or are tactile switches known to fail after a while, even when they're brand new?

 

[davenn]

The switches looked new as there was no sign of solder on them, but I was shocked to find out that almost none of them worked!
I used a digital multimeter with its continuity mode (buzzer),

you probably weren't testing between the correct pins

show a pic of the switches

 

[Alec_t]

The internal contacts have likely oxidised/tarnished. Any chance you could squirt a drop of switch-cleaner inside?

 

[(*steve*)]

Beware that there are some tact switches out there which will be destroyed by some types of contact cleaner.

(I may shortly be posting a thread asking if anyone has seen similar tact switches :-( )

My recommendation is to use contact cleaner, but to carefully try it on one tact switch before you spray the rest of them.

 

[TTL]

I was hoping I had mistaken the pins when testing, but alas not. Attached are photos of the switch and the pin layout.
So internal contacts can oxidize even if they're not used? I don't know if there's a way to access the internals but I'll take a look. Testing one switch with a contact cleaner first is a good idea. What kind of contact cleaner would be suitable for this sort of thing? I believe I already have a spraycan of CRC's "Electronic cleaner". Will that do?

 

[Audioguru]

Switches for electricity have silver plated contacts. The silver tarnishes (oxidizes and turns black) but the high voltage and current of electricity blasts the tarnish away. Low level signals use a switch with gold plated contacts that does not cost more but works "forever" with low level signals.

The switches that do not work probably have high voltage and high current silver contacts that might be able to be cleaned, again and again and again.

 

[Minder]

You could sacrifice one just to determine the cause.
This also may indicate if they actually were in use at one time.
M.

 

[TTL]

Switches for electricity have silver plated contacts. The silver tarnishes (oxidizes and turns black) but the high voltage and current of electricity blasts the tarnish away. Low level signals use a switch with gold plated contacts that does not cost more but works "forever" with low level signals.

The switches that do not work probably have high voltage and high current silver contacts that might be able to be cleaned, again and again and again.

I don't quite follow.
Are you saying that once I put the device I'm repairing (a music synthesizer) into operation again and push the (currently not working) buttons a few times they'll get "cleaned" by the electricity which flows through them?
I don't know if they have silver or gold plated contacts as I can't find any datasheets on them, but the model number is EVQ-Q8R13K (I believe they were made by Panasonic, or rather Matsushita -which Panasonic of course is a part of).

 

[(*steve*)]

Yes, for moderate currents, the small amount of sparking which occurs as the switch closes actually cleans the contacts.

For very low currents you need material that doesn't tarnish (gold)

"Need" is a relative term.

 

[Audioguru]

I said high voltage (120VAC) and high current (1A or more) makes a spark which zaps the tarnish away. The high voltage and high current will zap the gold away if the contacts have gold plating. Low levels don't work for long with silver contacts.

 

[TTL]

Having replaced the switches it appears I've also solved the contact issues mentioned above.

At first many switches didn't work, so I repeatedly pushed them (power up the synthesizer which this was for), and while some switches started to work after a few presses, others needed some more persuasion (pressing them harder, holding down longer), but it appears that the more I pressed them the better they worked.

I don't know if it's the repeated mechanical button-pressing which has made them work, the electrical current blasting away the oxydization or a combination of both, but it works