The following modifications are not about increasing the number of fun experimental games you can play with a contact printer. Rather, the nature of all of these modifications is simply to either make a non working contact printer function again, or else make a simple contact printer function in a similar way to a more advanced one.

Very often the main printing light of contact printers (at least continuous contact printers that run at moderate to fast speeds) was intended to be powered by DC (direct current) rather than AC (alternating current). This was to avoid the supposed problem of the frequency of the alternating current (in some countries 50hz in others 60hz) creating a exposure variation from frame to frame or even within frames. The faster a printer is running, the more likely such a problem will be. Thus with any printer that was intended to run at all fast, there was usually a large and very heavy DC power supply. These old power supplies are however often lost, or very noisy due to decay of the transformer insulation, or indeed deemed too ugly looking to be considered safe to use. The simplest modification is to just power the lamp with AC, and if there is a choice of running speeds for the printer, always choose the slowest. Another modification: when running the lamp on AC, it is possible to incorporate a so-called “variac” (a variable voltage AC transformer) to set the lamp voltage at a standard value.

The “Model C” era of printers are probably the best continuous contact printers for ECA. They are pretty much the first generation of so-called “additive” colour contact printers yet function for all intents and purposes exactly like the most recent additive printers. Built in the 1960s, they are still almost entirely mechanical or electro-mechanical. This means that, short of physical damage to the machine (which is very unlikely given their extraordinarily heavy construction) they will remain repairable indefinitely (which is quite the opposite for the latest generation of 'panel' style colour printers which are doomed because of their digital components). The main development with the Model C type additive printers over earlier models was the additive light control system. This involved dividing the incoming white light into red, green and blue components and then using three separate 'light valves' to control the amount of each of these light primaries separately, before adding the colours back together for printing. In this way, not just the brightness of the printer light, but also the exact colour of the light, could be precisely controlled.

The main problem with renovating one of these printers is in controlling the electro-mechanical solenoids that operate each of these light valves. Originally this was done with the printer controller box, also known as the 'tape reader' because they typically used old-fashioned paper punch tapes to record and instruct the light changes. The tape readers are the one part of these printers that are full of electronics – albeit simpler 1960s electronics – and are often either absent or defective. One thus often needs to find a solution for sending the required 150 volts DC to operate the up-to 9 solenoids on each of these valves.

The easiest option is just to control sending the required DC volts to the one solenoid on each light valve that actuates the change in brightness. This is the 'fire' solenoid. It doesn't control the extent of movement of the light valve opening, but merely makes whatever change might have been some-how pre-loaded into each valve actually happen. Typically you want a light valve change to happen right on a shot change when printing your film. Activating the fire solenoid at the right time will do exactly this. Whatever has been pre-loaded into the valve will happen the instant the fire solenoid is activated. Ordinarily it is all the other solenoids on each valve that are responsible for pre-loading the value or extent of each light valve change over a range of five stops. In the absence of this control, pre-loading a small change of up to two stops of light is still possible simply by turning the red, green and blue 'trim' knobs by hand. Whatever changes you pre-load manually by rotating the trim knobs will happen instantly as soon as the fire solenoid is activated.

Most contact printers will already be fitted with a so-called 'notch detector'. This is a simple micro-switch fitted to the printer that feels the edge of the negative being printed. A small notch is cut in the side of the negative at the point a light change is desired. As this notch passes the micro-switch, the switch will close very briefly, sending a signal to the light valve controller to 'fire'. However some printer heads, notably the Schmitzer wet-gate head for the Model C printer, were not intended for printing changes and thus lack a notch detector. Mounting a new micro-switch with 3d printed feeler arm is a simple and very powerful modification.

The trim settings are intended to only be a secondary way of changing the openings of the light valves. The primary way was the set of solenoids on each valve that pre-load changes by activating mechanical cams. The trim knobs themselves were only intended for making simple manual changes to the amount of light to account for subtle differences at the lab on a daily basis or differences between one printer, or one lab from another. The trim knobs are connected to the back of the light valves themselves by a series of mechanical linkages and cogs. A useful modification to these printers for labs that don't have full control of the solenoids is to 3d print adaptors that allow the fitting of small stepper motors to these linkage components. These steppers can then be controlled by a computer to effectively 'robotise' the turning of the trim knobs, once again giving automated light value changes. (This modification also looks amazing as the red, green and blue trim knobs appear to turn all by themselves while you are making a print).

This is a fairly major modification. It involves finding a method to send the required 150 volts DC to each of the 8 or 9 solenoids on each of the three light valves. Basically it is a complete replacement of the old tape reader. While it is not that difficult in theory to control 24 different relays using an arduino (a little programmable controller about the size of a credit card), it is difficult to switch large DC voltages. This is because unlike AC (alternating current), DC (direct current) will generate a large spark as the two metal contacting parts of a switch are separated. This spark quickly burns out these metal contacts, destroying the switch or relay. The original tape readers used what were called 'mercury relays' to do this switching. The metal contacts inside each relay would be covered in liquid mercury. This mercury was a sacraficial material in that it would be burnt off by the spark after each operation of the relay. The metal contacts would then return to a mercury bath inside the relay to be re-coated before the relay's next use. A less expensive method for making a DIY controller is to use arduino controlled relays to switch AC power, and then convert the AC to DC after it has been switched. In practice, a simple bridge rectifier after each relay will make an unfiltered DC that is nontheless adequate to fire the solenoids. The whole arduino powered system can be programmed and controlled from an external computer.

The traditional additive colour printer takes the light from a white lamp and divides it into component red, green and blue colours using dichroic mirrors. After then controlling the colours separately, the three colours are re-combined for printing. While this is very clever, it is not something one can easily replicate. These days however it is possible to replace the original white lamp of a simple printer with a controllable red, green, blue light source, thus making the printer into an additive colour printer.

Using an Arduino or other programmable controller it is possible to build a simple but effective programmable red, green and blue light source using an RGB LED lamp. This makes a subractive printer into an additive colour printer. However it has to be pointed out that the method of controlling the brightness of an LED is to flicker it (called 'pulse width modulation'). When used as the light source for a continuous contact printer you again run the risk of variations in brightness within parts of each frame. Sometimes the only solution is to slow the printer down which is not an easy modification. This is not an issue with LEDs and step contact printers however.

Written by Richard Tuohy.