Point-to-point Construction - Terminal Strip Construction

Terminal Strip Construction

Point-to-point construction uses terminal strips (also called "tag boards"). A terminal strip has stamped tin-plated copper terminals, each with a hole through which wire ends could be pushed, fitted on an insulating strip, usually made of a cheap, heat-resistant material such as synthetic-resin bonded paper (FR-2), or bakelite reinforced with cotton. The insulator has an integral mounting bracket, sometimes electrically connected 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 ends of lengths of wire or wire-ended components such as capacitors and resistors were pushed through the terminals, and usually looped and twisted. When all wires to be connected had been fitted to the terminal, they were soldered together (and to the terminal).

Professional electronics assemblers used to operate from books of photographs and follow an exact assembly sequence to ensure that they did not miss any components. This process is labor-intensive, subject to error and not suitable for automated production. Even after the introduction of printed circuit boards, it did not require laying out and manufacturing circuit boards.

Point-to-point construction continued to be used for some tube equipment even after the introduction 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. American manufacturer Zenith continued to use point-to-point wiring in its tube-based television sets until the early 1970s.

Some audiophile equipment, such as amplifiers, continues to be point-to-point wired using terminal pins, often in very small quantities. Point-to-point wiring is used as a design feature, not due to the economics of very-small-scale production.

Sometimes point-to-point wiring—without terminal strips—with very short connections, is used at very high radio frequencies (in the gigahertz range) to minimise stray capacitance and inductance; the capacitance between a circuit-board trace and some other conductor, and the inductance of a short track, become significant or dominant at high frequencies. In some cases careful PCB layout on a substrate with good high-frequency properties (e.g., ceramic) is sufficient. An example of this design is illustrated in an application note describing an avalanche transistor-based generator of pulses with risetime of a fraction of a nanosecond; the (few) critical components are connected directly to each other and to the output connector with the shortest possible leads.

Particularly in complex equipment, point-to-point wired circuits are often laid out as a "ladder" of side-by-side components, which need connecting to ladders or components by wire links. A good layout minimizes such links and wiring complexity. Amongst complex devices, the pre-PCB Tektronix vacuum-tube oscilloscopes stand out for their very well-designed point-to-point wiring.

If parasitic effects are significant, point-to-point wiring has the disadvantage compared to a PCB of indeterminate parasitic components; while the inductance and capacitance due to a PCB are the same for all samples, values may vary between point-to-point wired units, changing circuit operation.

Placing the completed unit in an enclosure protects the circuit from its environment, and users from electrical hazards.

A few large brand names still use point-to-point boards, but usually for special product lines. Electric guitar amplifier manufacturer Marshall have reissued some of their older models, using point-to-point construction as a design feature, although their standard products have long used PCBs. Thermionic valve equipment usually does not have the valves mounted on the PCB, to avoid heat damage, but uses PCBs for the wiring, achieving the economy of mass-produced PCBs without the heat damage.

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