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Timer/Luxmeter

The Jobotronic 200 (out of production) groups the functions of a timer and luxmeter/densitometer.

Here are its caracteristics:

• Digital display.
• Enlarger lightning manual switch.
• Exposure times from 0.1 to 999.9 seconds, counts down and is interruptible.
• Manual start by the red button or with the foot pedal (grey cable).
• Add/substract time by half stops (green buttons +/-).
• Very small cell (<5mm).
• Spot reading directly on the baseboard.
• Integrated reading with the cell mounted close to the lens pointing to the baseboard or with the probe on the baseboard and a diffuser mounted under the lens.
• Independent sensivity adjustment between spot/average reading (left/right black buttons).
• Time memorisation (green button MEM).

Manual

PDF version

JPG version: Controls - Probe mounting - Pages 1, 2, 3, 4, 5, 6

Speed adjustment

As there are no direct ISO(P) adjustment, you have to evaluate it by yourself.
Pay attention that I'm not sure at all that the following informations are consistent from sample to sample.

• selective speed on Jobo  = 4100 / paper sensivity (ISO(P))         0.60 grey (25% reflectance)
• average speed = selective speed * 3.9

Keep in mind that the Jobo cell nor the bulb of your enlarger have the same spectral balance as your paper.
So, for multigrade papers, you should make test strips with each grade and obtain the same final density in order to calculate each grade real jobo speed.

One word about which density to use, 0.60 is a good choice as paper speeds are given for such density and many papers have their sensitometric curves crossing on that point.

Example for the Ilford Multigrade IV RC and *my* configuration:

Another interesting method for simpler analyzers by Paul Butzi.

Here are some useful and very cheap tools that could help you in testing films/papers:

Left column:

• Kodak Reflection Density Guide: allows opaque densities measures by visual comparison
• Kodak Projection Print Scale: replaces the proof sheet

Right column:

• Stouffer Transmission Projection Step Wedge 35mm (TP35): grey scale to be used in the negative holder and projected onto the paper
• Stouffer Reflection Scale (R3705): allows opaque densities measures by visual comparison
• Stouffer Transparent Step Wedge (T4105): allows films and papers tests by contact printing

12V high current switching

This modification lets you drive directly high currents like with a 12V stabilized power supply.
The positive impulse is taken between the ground and the relay.
As the timer has not enough energy to power directly the relay, a transistor acting as buffer is inserted, the 1K resistor limits the current drawn from the Jobotronic.
There is not so much place inside the timer to insert this small circuit, so it was inserted inside the regulated supply itself.
The impulse arrives from the timer through a cable and 3.5mm jacks.

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