Technical Support Document 050 - 5/97

VC-CLS TUNING RIG OPERATION INSTRUCTIONS

INTRODUCTION

The VC-CLS is a cold-light lightsource. The nature of the mercury-vapor cold light fluorescent tubes that cold light heads use is that they are unstable as their temperature changes. The VC-CLS is unique in that its closed-loop system measures the light output and makes automatic corrections as the light changes.

For the closed-loop system to function properly, the internal color sensor which measures the light output must be calibrated for operation with the lamps in the head. This is done using an external sensor positioned to read the head's light output at the diffuser.

The VC-CLS tuning rig measures this light output using a factory-calibrated sensor. This sensor is designed for easy operation with any VC-CLS lightsource, and is mounted in a base upon which the VC-CLS is placed. This assures that the sensor is in the correct position for light output measurement.

This rig has been made available to dealers and service representatives of the VC-CLS so that VC-CLS units that have been serviced do not need to be sent to ZBE for factory tuning.

WHEN WILL A VC-CLS NEED RE-TUNING?

The VC-CLS closed-loop computer board (VC-CLS CPU) includes the head's internal light sensor, which governs the output of the unit. The internal sensor in each VC-CLS is adjusted for operation with the particular lamps in that unit. The lamps in one VC-CLS may be brighter or dimmer than those in another. The VC-CLS Tuning Rig will allow all VC-CLS units to be tuned to a standard so that VC grades will be the same for all units. The difference in output from unit to unit will then only be in density, not in grades.

When a VC-CLS unit is serviced and a part is replaced, the head may or may not require re-tuning. The parts in the VC-CLS which will affect the head's internal balance, and thus require re-tuning, are listed below:

Green Gridlamp

Blue Gridlamp

VC-CLS CPU Circuit board

Either High-Voltage Lamp-Drive Transformer

It is a good idea to check the tuning of any VC-CLS received for service, regardless of the problem, in order to avoid future problems with that unit. Some problems are symptoms of a unit being out of tune, and may be fixed by simply re-tuning the head. A few problems that may be fixed by re-tuning are:

Inconsistent density of prints, particularly if this occurs from beginning to end of a long print run.

Inconsistent contrast of prints, particularly if this occurs from beginning to end of a long print run.

Flickering light output.

The following will outline the actual use of this Tuning Rig.

USING THE RIG TO CHECK VC-CLS TUNE

For a VC-CLS head that is to be serviced, checking the output for proper tuning is a good first step. This is a simple process using the Tuning Rig. The steps are as follows:

Place the VC-CLS on top of the tuning rig. The round output aperture of the head should seat into the recessed portion of the rig. The front of the VC-CLS should be oriented to the front of the rig. This is the side of the rig that has the white button and red LED indicator on it.
Unplug all of the accessories from the rear panel I/O connectors.
Plug the tuning rig connector into either one of the two connectors in the rear of the VC-CLS. Turn the VC-CLS power on in the normal manner. When the rig is plugged in and the power turned on, the red LED indicator light on the rig will illuminate.
In the "PREF" menu, set the focus light to the high power setting. Spin the knob to the "FOCS," press ENTER, then spin the knob to get "HI" in the right hand display. Press ENTER to return to the main screen.
Use the FOCUS key to turn the lamps on at full power.
Leave the lamps on until the lamp temperature is at least 120 degrees. To check the lamp temperature, press the OPTIONS key, spin the knob to get the TEST menu, and press ENTER. In the TEST menu, spin the knob until "LTP" appears in the left-hand display. The number in the right-hand display is the current lamp temperature. The unit may be left in this mode for continuous monitoring of the lamp temperature.
Once the lamps have sufficiently warmed up, the balance of green versus blue may be read. To do this, an exposure at the highest grade will be made. Press the GRADE button and spin the knob clockwise until the dash "-" appears. Press ENTER. This grade is equivalent to grade 2-1/2, with equal amounts of green and blue light output, but at the maximum possible lamp power.
Set the exposure time to a high value; anything above 50 seconds will suffice. Press the TIME key and spin the knob clockwise. Press ENTER to lock in the new long exposure time.
While holding down the TIME button, press the FOCUS button. This will start the exposure.
While the time is counting down, press and hold the white button on the tuning rig. This will cause the display of two numbers on the display screen. The number on the left represents the green light output, while the one on the right is the blue light output. These numbers are in standard log density units (30 points = 1 f-stop) with higher numbers representing lower levels of light.
Note the two numbers. They should ideally be the same, but will usually be a couple of points apart. If the difference is more than four (4) points, the VC-CLS will need to be re-tuned.

If it is determined that the VC-CLS will need a re-tune, unplug the unit and allow it to cool for at least two hours. This will ensure proper tuning. The procedure for a full re-tuning of the VC-CLS starts on the next page.

TUNING THE VC-CLS USING THE TUNING RIG

The tuning of a VC-CLS is a two-part process. First the light output is measured at maximum lamp power to determine the dimmest lamp, whether green or blue. The head is then tuned so that the output at its brightest closed-loop setting (Grade "-") will be ten (10) points dimmer than the dimmest lamp output measured in the first step. This will allow enough "head room" so that the VC-CLS output will remain consistent in the future.

NOTE: Before performing this procedure, make sure that the unit being tuned has been unplugged for at least two hours. It is important that the unit be completely cool to assure that the proper amount of headroom is "tuned into" the head.

Place the VC-CLS on top of the tuning rig. Position it such that the round output aperture sits completely into the recessed area on top of the rig.

Remove all accessories from the rear panel connectors. The footswitch may be left installed.
Connect the tuning rig sensor to the VC-CLS by plugging it's connector into the rear of the VC-CLS unit.
Plug the VC-CLS into the mains outlet. Turn the main power on.
At this time the unit should be cold. For the purpose of tuning, it will need to warm up for a period of one hour. Leave the unit turned on and plugged in with the lamps turned off for about one hour.
Once the head has reached operating temperature, it may be tuned. This can be checked by pressing the footswitch. If the unit beeps and reads out "too cld," it still needs to warn up. If the exposure starts, immediately stop it by pressing the CANCEL key, and proceed to the next step.
The initial light output reading will now be made. This is done by pressing and holding the white "TEST" button on the tuning rig. This will turn on the lamps at full power and cause two numbers to appear in the VC-CLS display. The number on the left is the green light output, and the one on the right is blue. Watch both of these numbers carefully as soon as the rig button is pressed.
While watching both of the numbers, keep a mental note on the highest number read out in either position. This represents the dimmest light level output when the lamps are lit at full power. Either of the two numbers may be the one with the highest value. Keep in mind that a higher number means dimmer light.
Now that the lowest light level (the highest number) is known, the head may now be tuned so that it's brightest grade will be below the dimmest lamp output. Add ten (10) points to the highest number attained in step (8), above, and make a note of this value. This will be the value to which both lamps will be tuned for closed-loop operation at the brightest grade. This will allow 10 points of "headroom" for comfortable operation of the closed-loop system.
Press the GRADE key and spin the knob all the way clockwise. When the Grade display moves past "5" and displays a dash ("-"), press ENTER. This grade selection tells the VC-CLS to run at its brightest light output, with equal amounts of green and blue light.
Enter an exposure time of 99.0 seconds. This will allow time for tuning while the exposure is counting down.
Locate the two small holes on the bottom of the VC-CLS. They are located just under the front panel, toward the right side. These holes allow access to the adjustment screws. Position a small, flat-blade screwdriver, into one of the holes, such that its blade mates with the adjustment screw inside. The front panel may be removed to view the adjustment screw itself, but leave the panel connected to the base unit. Start with either screw. The one on the left will adjust the Green light output, the right-hand screw adjusts the blue.
Start an exposure at grade "-" by pressing the TIME and FOCUS keys together (or by pressing the footswitch). Press and hold the Test button on the tuning rig. The numbers displayed by the VC-CLS are the light levels that are output for this grade. Keep holding the button while performing steps (14) and (15).
The adjustment screw, watching how its corresponding number in the display changes. Adjust the screw until its number is equal to the value noted in step (9).
Repeat step (14) for the other color. When finished, both numbers should be equal. If the lamp shuts off while tuning the VC-CLS, release the rig Test button, and re-start the exposure.
After the tuning is completed, start another exposure (at grade "-") and double check the tuning by pressing the rig Test button. The two numbers displayed should be equal within one point.
Stop the exposure by pressing the CANCEL key.
Press the rig Test button once more to display the maximum lamp power light values. The highest of these numbers should be at least 10 points less than the values to which the head was just tuned.

The VC-CLS is now tuned to factory specification and will operate in a stable manner.