Point-to-point construction

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Point-to-point construction is the way most electronics circuits were constructed before the 1950s. Point-to-point construction is still used to construct prototype equipment with few or heavy electronic components.

Before point-to-point connection, electrical assemblies used screws or wire nuts to hold wires to an insulating wooden or ceramic board. The resulting devices were prone to fail from corroded contacts, or mechanical loosening of the connections. Early premium marine radios, especially from Marconi, sometimes used welded copper in the bus-bar circuits, but this was expensive.

Point-to-point construction uses terminal strips (sometimes called 'tag boards') or turret boards. The crucial invention was to apply soldering to electrical assembly. In soldering, an alloy of tin and lead, or later bismuth and tin, is melted and adheres to other, nonmolten metals, such as copper or tinned steel. Solder makes a strong electrical and mechanical connection.

Contents

Terminal strip construction

Point-to-point construction uses terminal strips (also called 'tag boards'). A terminal strip is a stamped strip of tin-plated loops of copper. It is mounted in a way that electrically insulates each loop from the others. The metal loops are mounted on a cheap, heat-resistant material, usually synthetic-resin bonded paper (FR-2), or bakelite reinforced with cotton, or sometimes paxolin. The insulator has an integral mounting bracket, sometimes shorted to one or more of the stamped loops to ground them to the chassis.

The chassis was constructed first, from sheet metal or wood. Insulated terminal strips were then riveted, nailed or screwed to the underside or interior of the chassis. Transformers, large capacitors, Tube sockets and other large components were mounted to the top of the chassis. Their wires were led through holes to the underside or interior. The wires of electronic components were physically looped through the terminals and soldered to them. Small electronic components were mounted by twisting their wires around terminals and soldering.

Professional electronics assemblers used to operate from books of photographs and follow an exact assembly sequence to assure that they did not miss any components. Although this process is error-prone and nearly impossible to automate, it is quite good for building small numbers of units when labor costs are low.

Point-to-point construction continued to be used for high quality tube electronics even after the invention of printed circuit boards. The heat of the tubes can degrade the circuit boards and cause them to become brittle and break. Circuit board degradation is often seen on inexpensive tube radios produced in the 1960s, especially around the hot output and rectifier tubes. The American manufacturer Zenith continued to use point-to-point wiring in its tube based television sets until the early 1970s.

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