Background:
Because of their modular design, ease of use, and
reasonable cost, the Schadow type FE switches have found extensive use
both in home and professional audio applications. However several problems
with them have shown up on older equipment. The silver plating tends to
tarnish in some locations, there was a potential for flux entry into some
versions of the switch, and the lubricant used seemed to deteriorate in
time. While newer versions have been designed and are being manufactured
by ITT-Schadow, there are still thousands of the newer type in use, and
thousands of the previous units in use at this time. Servicing of the switches
usually involves laborious disassembly followed by cleaning of the switch
housing and floating contacts on the slide assembly. The number of very
small parts and springs makes this operation rather difficult when large
number of switches are involved.
Other solutions that have been tried:
Dayton Wright Associates Ltd. started to use the
Schadow type FE switches in the early 1970's in the production of preamplifiers.
Problems with the switches led the to use such production precautions such
as hand soldering of the switch to the circuit board after all other components
were soldered in place, the use of flux-sealants hand-applied around the
projecting PC-mount pins, as well as the substitution of other lubricants
for the one supplied by the manufacturer. While all these things did increase
the life of the switches, the major improvement came with the development
of Stabilant 22. Over several years, The Dayton Wright Group Ltd.,
developed a procedure for treating the Schadow type FE switches without
disassembly that has cut down on the amount of service work and has proven
quite satisfactory in extended use. While the procedure is quite simple,
for proper results all the contaminants have to be removed and only a minimal
film of Stabilant 22A should be applied.
Contaminant problems:
There are two basic contaminant problems we have
found with these switches. Contamination during manufacture of the circuit
board assembly, and atmospheric contamination.
Unless the switches are of the type having a sealant
applied to the fixed contact (those fitted to the case body) the possibility
exists for flux to wick up into the interior of the switch assembly during
the soldering operation. Methods used to combat this, have ranged from
adding the switch units after the other components have been wave soldered
in place, to the use of solder preforms as a method of limiting the amount
of flux that can be applied to each pin. Sometimes, a slight amount of
tarnish on the PC contact can make soldering difficult and this, in turn,
leads to an excessive use of solder (and flux).
Because the plastics used did not withstand some
of the vapor phase defluxing systems and were damaged by other solvent
defluxers, some manufacturers relied on a two phase system, a somewhat
less aggressive defluxers followed by a general wash off of the liquefied
flux. The potential problem with the latter was that it could leave a minuscule
coating of flux over all of the board's components. While this was rarely
enough to cause problems on exposed wiring and card-edge connectors, the
semi-enclosed interiors of the switches were more vulnerable as here the
build-up was much greater.
Closed rear push-button switches exhibit one trait
in common, when they are released, they tend to "inhale" some of the atmospheric
contaminants from the front panel area, holding them inside the rear of
the switch assembly where they often precipitated onto the contact assemblies.
Where electronic equipment is used by smokers, there will be a gradual
accumulation of tar and nicotine within the switch, in concentrations much
greater than found on the other components. For the switch to function
properly, both residual flux and other contaminants have to be removed
along with any spent lubricants.
Cleaning out the residual contaminants:
The procedure starts with flushing out the contaminants
& lubricant residue with isopropyl alcohol. We recommend the use of
a 50 cc syringe with a 2 inch long needle. The needle can usually be inserted
at the rear of the switch between the stamped brown-phenolic wafer and
the black molded body. Insert the needle so that the hole is towards the
body of the switch. Working the switch, flush it out with about 10 to 20
cc's of isopropyl alcohol. Depending upon the age of the switch, this may
have to be done a second time within a week to ten days as the hardened
contaminants dissolve in the newly applied Stabilant film and distribute
themselves on the contact surfaces. In any event all of these contaminants
have to be removed for the switches to work reliably once more, and solvents
that would remove all of the contaminants at the first shot could damage
the plastic parts. Remember that isopropyl alcohol is flammable and work
should be done in a well ventilated location.
Applying the Stabilant 22A:
Using a 1 cc disposable insulin type of syringe
inject about 1/8 TO 1/6 cc (for a 2 or 4 pole) to 1/4 TO 1/3 cc (for a
6 to 1 0 pole) of Stabilant 22A into the rear of the switch. If this can
be done with the switch in an approximately horizontal position, it will
minimize the loss of the
Stabilant 22A from the front of the switch
while the alcohol evaporates. Obviously, where a great number of switches
are involved it is better to flush them out all at once and then proceed
with the addition of the Stabilant 22A Immediately, upon injection
of the Stabilant 22A, vigorously cycle the switch to distribute
the material before the isopropyl alcohol evaporates. Only a very thin
film is needed on the contacts. If the film is too thick, a phenomenon
can occur called hydroplaning. This is where motion of the switch pushes
a wave of lubricant ahead of it gradually thickening the film and widening
the gap. This could delay the switch on of the contact when instantaneous
operation is needed. Generally, hydroplaning should be suspect when it
is the switches that are most often used that cause problems, rather than
the least used switches. The slight detergency action of the Stabilant
will lift much of the existing tarnish over a period of time, holding it
in solution. Normally this does not degrade the operation of the switch.
Some use prefer to repeat the procedure six to twelve months later to remove
the suspended tarnish.
Precautions and re-applications:
As noted, on equipment that has been in service
some time, the detergency action of the Stabilants may well loosen up so
much detritus, and even residual flux inside the switch that it might be
necessary to re-flush-out the switch and re-apply the Stabilants.
The need for this will be shown up if the treated switch becomes noisy
once again within two to three weeks. Remember, a# traces of the existing
grease MUST have been removed as well!
Results obtained:
Measurements on treated switches have shown a marked
improvement of the signal-to-noise ratio on signals, as well as a significant
reduction in total harmonic distortion. As an example, although the material
has been used on preamplifier switches. since the early 1980's no repeat
lubrication has proven necessary for units treated in that time indicating
a useful life in excess of five years..
CAUTION: A virtual copy of the ITT-Schadow switch is being manufactured
by TONELUCK(TM) and is now being used by Sony both in new equipment and
as replacements. The sealant used on these switches does not appear to
meet the usual standards for conformal coatings and may be damaged by many
of the solvents used in electronic service. We would strongly advise against
using any cleaner or contact treatment on these switches. They can be distinguished
by the name TONELUCK underneath the switch, their gray body and clear plastic
(rather than phenolic) top plate.
Stabilants are a product of Dayton Wright research &
development and are made in Canada
NATO Supply Code 38948
15 mL of S22A has NATO Part # 5999-21-900-6937
The Stabilants are patented in Canada
- 1987; US Patent number 4696832. World-wide patents pending. Because the
patents cover contacts treated with the material, a Point-of-sale License
is granted with each sale of the material.
MATERIAL SAFETY DATA SHEETS ARE AVAILABLE ON REQUEST.
Stabilant, Stabilant 22, and product
type variations thereof are Trade Marks of D.W. Electrochemicals Ltd.
© Copyright 1987, '88. '89, '90
- D.W. Electrochemicals Ltd. This note may be reproduced or copied, provided
its content is not altered. The term "contact enhancer", © 1983 Wright
Electroacoustics.
NOTICE: This Application Note
is based on customer-supplied information, and D.W. Electrochemicals is
publishing it for information purposes only. In the event of a conflict
between the instructions supplied by the manufacturer of the equipment
on which the Stabilant material was used, and the service procedure employed
by our customer, we recommend that the manufacturer be contacted to make
sure that warranties will not be voided by the procedures.
While to our knowledge the information
is accurate, prospective users of the material should determine the suitability
of the Stabilant materials for their application by running their own tests.
Neither D.W. Electrochemicals Ltd., their distributors, or their dealers
assume any responsibility or liability for damages to equipment and/or
any consequent damages, howsoever caused, based on the use of this information.
Stabilant, Stabilant 22, and product
type variations thereof are Trade Marks of D.W. Electrochemicals Ltd.
HOME PAGE
COMPANY & CREDO
LIST OF NOTES
BEGINNING OF APPLICATION NOTE 3