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From: N_Cook on 1 May 2010 09:20
That age-old problem of conflicting requirements.
Someone's cherished domestic amp from 1983 uses slideway ribbons from the
front panel to pcb mounted slide switches. I last saw this amp 5 years ago,
loss of some function switch function.
Same problem this time. Then it was a build up of black copper sulphide/
silver sulhphide? forming a black insulated layer , eventually too tough for
the sliding phosphor bronze contacts to break through. The structure of the
switch with 1mm gaps between the static contacts means it needs lubricant to
overcome the sliding contacts slipping in and out of these gaps or I imagine
they are likely to buckle and jam. Sliding contacts seem fine, they retract
fully on removing from the static pins, no distortion. No trace of
corrossion this time, good to see that worked, but the lubricant I used
probably did the same as the original sulphide and blocked contact ,
thickening over time perhaps. Previously I cleaned out all the corrossion
and ex-lubricant goo and then used a mixture of silicone paste and graphite,
only signal levels, so not too concerned about stray graphite conductive
paths, >100Kohm or so. Anyone else any ideas?
I was thinking this time to totally clean out , rotate contacts 180 degrees
and use a liberal amount of graphite only in the recesses as a dry
lubricant. Then when switch is resoldered into place surrounding the whole
switch with a membrane to keep the graphite inside. Or much the same as
before (5 years is not too bad any way) but less silicone grease in the mix
proportion. Too little grease and it will not stick to the contacts. A
different sort of grease perhaps with the graphite .
The conflicts are
Keep corrossion at bay
lubricate sliding contacts
retain electrical conductivity in static posistions
sliding contact pressures cannot be increased or they would score into the
silver plating or jam and then buckle
long term material stability


From: William Sommerwerck on 1 May 2010 09:31
Complex problem, which I don't fully understand. But I'll respond anyway...

Why isn't a reducing agent -- such as TarnX -- sufficient to remove the
sulphides? And once removed, wouldn't the sliders return to their original
state, with or without a bit of lubricant?

You needn't reply. I'm just asking questions to provoke thought.


From: N_Cook on 1 May 2010 12:39
William Sommerwerck <grizzledgeezer(a)comcast.net> wrote in message
news:hrhaf7$5f7$1(a)news.eternal-september.org...
> Complex problem, which I don't fully understand. But I'll respond
anyway...
>
> Why isn't a reducing agent -- such as TarnX -- sufficient to remove the
> sulphides? And once removed, wouldn't the sliders return to their original
> state, with or without a bit of lubricant?
>
> You needn't reply. I'm just asking questions to provoke thought.
>
>

Normally the gaps between the dual-in-line pins 2x13 here would only have
0.2mm or so spacing to maintain electrical isolation and unobstruced slide
action but these ones have large gaps and consequently have more requirement
of lubrication I would have thought.
The stop/go resistance to movement is much more than normal "wave-band
change" multiway multipole slide switches



From: N_Cook on 1 May 2010 13:09
I just thought of another factor in the conflict. You could do away with
lubricant if you filed in the gaps but then there would be no cleaning of
the sliding contacts on each slide movement.


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