Conclusiones Parciales

Most of us are familiar with the traditional or `hard' tools such as screw drivers, soldering irons, wrenches, pliers, etc. However, some chemicals or `soft tools' are available which are as important as the screw driver and other hard tools in the servicing field. They are `soft' tools because they are consumable, i.e. part of the tools is used up when it performs its function, unlike the hard tools which have indefinite life when handled with care. The chemical tools which are generally employed include:

• Solvents;

Spray solvents used in servicing usually contain either chlorinated hydrocarbons or fluorocarbons.

Chlorinated hydrocarbons are compounds of hydrogen, carbon and chlorine. The best known example is carbon tetrachloride.

Fluorocarbons are compounds of hydrogen, carbon and fluorine. The most popular compound is freon. Freon TF is one form of this compound, which can be compounded with a variety of hydrocarbons or other solvents to make many specialised cleaning agents. Spray cleaners can be divided into the following groups.

Non-lubricating Cleaners (no-residue cleaners): They are used to remove grit and dust from circuit boards, mechanical assemblies and parts like magnetic tape heads and ganged tuning capacitors (such as used in radio receivers). Lubricating cleaners should not be used for such applications in which the lubricant is likely to cause more dust to accumulate quickly and would do harm.

Lubricating Cleaners: Lubricating cleaners are preferred for such items as potentiometers and switches, that is because those cleaners leave behind a coating of

silicone that provides protection from oxidation and the resulting contact degradation.

Foam-type Cleaners: Foam-type cleaners are specially formulated viscous compounds (spray-cleaners) which come out of the can thick. These compounds contain mild abrasives such as jeweller's rouge, in a viscous base. They are generally used to clean and restore most wiping contacts, including those on wafer and slide switches. What those products do is to adhere to the metal contacts and continually clean and burnish the switch parts. However, be sure to remove the abrasive completely, using a no-residue solvent, once the cleaning and burnishing is done, otherwise the switch action will suffer.

Most spray solvents are safe when used inside a unit, but they should be kept off any of the exterior parts such as cabinets, dial scales, clear plastic window, etc. Generally, plastic such as nylon, delrin and similar substances will not be harmed by most solvents.

Bottled Solvents: For cleaning the rubber drive surface in tape recorders, turntables and the like, bottled rather than spray type solvents are generally used. Most of the people use alcohol-type cleaners. They are not recommended because they tend to leach out the stabilizers in the rubber, making it soft and sticky. The ketones such as acetone or methylethyl ketone (MEK) are preferred because they evaporate very quickly and thus have little time in which to damage the good rubber.

However, acetone and MEK are flammable. They should not be used near sparks or open flames. Secondly, they are notorious for dissolving anything made of plastic and therefore, one should be careful when using them. They are also potent solvents and will destroy the rubber if too much is used. The best way to use these chemicals is to apply them sparingly to the rubber's surface with a cotton swab. That will allow them to take off the top layer of rubber and residue, without harming the good rubber beneath it.

To summarize, the following cleaners are commonly used:

• Contact cleaner (in spray can): used for switches and relays;

• Control cleaner (in spray can): used for potentiometers; includes some type of nondrying lubricant;

• WD40: For cleaning or freeing rusted screws; for cleaning, removing labels and label gum; coating tools to prevent rust etc.;

• Flax Remover: Isopropyl alcohol available in spray can;

• Isopropyl alcohol: for general cleaning; (i) used with Q-tip swabs (cotton buds) for cleaning of everything, but for video heads on VCRs and other helical scan tape transport (ii) chamois covered cleaning sticks for video heads;

• Tuner cleaner (spray can); and

• De-greaser (spray can).

4.2.2 Adhe sive s

Adhesives are used to fasten two pieces of almost anything together reliably and quickly. They can be useful if parts or the broken pieces are available. This often happens with such things as cabinet assemblies, decorative parts and other plastic pieces, especially if the unit being repaired is old. Although it is generally advisable to replace a broken part with a new one, there are situations when that is not possible and use of adhesives to bond broken parts is the only solution. The various types of adhesives that are commonly encountered in practice are:

Epoxy: Expoxies are universal adhesives that have a variety of setting times and viscosity. The viscosity and the setting time depend on the particular formulation used.

They are two-component adhesives that must be mixed before application. At room temperature, heat or pressure is not required for setting.

Epoxy will bind most materials, including metals, glass, ceramics, cardboard, wood, rubber, fibre and most plastics (except nylon and similar compounds), with a bond strength which is generally greater than the strength of the bonded materials themselves.

Viscosity tends to be related to setting time. Quick setting epoxy is thinner than the slower setting ones. The choice of an epoxy for a particular application is determined by what is to be joined as well as how many items are involved. If only one item is to be bonded and their surfaces closely match, one of the thin, quick-setting types epoxy is preferred. If there are gaps in the two surfaces to be joined and possibly more working time is needed, the slower-setting epoxies are better in such situations.

Epoxies are generally used as electronic sealers and potting compounds to insulate a high voltage connection or to encapsulate a circuit for reliability. Hysol epoxy is an example of a potting compound that is normally used to totally encapsulate the circuits.

De-potting an electronic assembly is pretty easy. All that is needed is the judicious application of a small welding torch flame to locally heat the epoxy above its glass temperature and a little patience.

Solvent-Release Adhesives: Solvent-release adhesives include a wide variety of resins and polymers that harden when the solvent either evaporates or is absorbed. The tan-coloured compounds will provide a flexible, waterproof bond between virtually any two materials. The adhesives are used by coating each piece to be joined with a thin layer, waiting for the thermoplastic to become tacky and then pressing the pieces together. Drying time ranges from 15 to 30 minutes.

Silicone-Rubber Adhesive: It is best used to seal against air or moisture or when a flexible bond is needed. It will adhere to almost any clean surface and dries when it is exposed to the moisture in the air. This adhesive can be easily removed with a scraper if necessary, which gives it an advantage over other adhesives. Also, it offers non-running consistency, one-component convenience and resilient bonding. It is a handy-compound for potting components and sub-assemblies. However, its tensile strength is lower than that of either epoxy or other adhesives.

Cyanoacrylate Adhesives: They are single component, solvent-free glues that have the property to set very quickly. They will bond any substance provided good surface contact can be maintained. The biggest advantage these adhesives have over epoxy is that they can be used to join such hard-to-glue materials as polyethylene, teflon, vinyl and silicon rubber. They have an excellent tensile strength. Although their setting time is specified as a few seconds, it is better not to stress the joint for 10 to 15 minutes.

4.2.3 Lubricants

Parts that slide against each other do require occasional lubrication. However, when lubricating small mechanisms, it may be remembered that the lesser the quantity of lubricant used, the better.

For gear trains, bearings and the like, a good grade of light-bodied greases made of polymerised oil provides long-lasting lubrication. This grease will not oxidise or become gummy for several years under normal conditions. For parts that slide against each other, use a good molybdenum di-sulfide or lithium-based grease. Silicon lubricant, which comes in the form of spray is ideal for lubricating such items as hinges and sliding plastic parts. Those sprays are especially useful for restoring proper operation to a slide potentiometer. To do so, the pot is first cleaned with a very small

amount of tuner cleaner or Freon TF. This should be done until the cracking sound disappears. Next, an ample amount of silicone lubricant is applied and the control is worked until slider moves smoothly.

Seized bearings can often be freed by a good penetrating lubricant under pressure.

These are sold under many trade names like LPSI and WD 40.

4.2.4 Fre e ze Sprays

`Freeze sprays' are used to cool suspected noisy or intermittent components. They are of several types but in composition all are either fluorocarbons or chlorinated hydrocarbons.

The sprays which evaporate quickly are best for components with good thermal conductivity, such as metal-cased transistors, small electroytic capacitors and metal film resistor. On the other hand, a long evaporating time (20 to 30 seconds) is more suitable for plastic or epoxy encased semiconductors, mylar insulated capacitors or other parts with a heavy insulation. Special anti-static freeze sprays are also available for use with MOSFET and other static charge sensitive components.

Freeze sprays do not harm the plastic used on circuit boards. However, their use should be avoided around clear plastic or cabinet plastic or any painted finish.

Another use of freeze sprays is to assemble or dis-assemble tight fitting metal parts, as cooling the inside piece will reduce its size temporarily.

Soft tools are among the most useful items in the workshop. You may take time to learn their proper use, but it will be observed that in the long run, repairs undertaken with soft tools are faster, easier and less expensive.

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