PHOTOVOLT TECHNICAL NOTES
Model 577 Brightness (W) Search Unit Repair Instructions For Cable, Photocell and Lamp Replacement, Lamp voltage Adjustment and Linearity Check

EMI/RFI: The Search Unit Cable Assembly, Part No. 1857704 is now provided with a grounding lug attached to a shielding wire that spirals around the wires inside the cable jacket. The lug is installed between the circular housing and the lens holder. It is secured in place by a screw that holds the housing on the lens holder.

The ground circuit may be completed by installing a Grounded Search Unit Interface Cable Part NO. 1857701 on the inside of the Rear Panel Assembly of the model 577 Brightness Meter.

7 Pin Connector Color Code
Pin #     Color
1          Brown
2              Red
3         Orange
4          Green
5             Blue
6          Black
7          White

SEARCH UNIT CABLE REPLACEMEMT:
Remove the following: Three (3) sheet metal screws, circular housing, red/orange and blue/black twisted wire pairs with terminal lugs and cable clamp. See Fig. 1.

Remove four (4) #6-32 x 11/2 inch binding head screws from the lens holder. See Fig. 3. Lift lens holder from cell holder thus exposing the Photocell assembly. See Fig. 2. Remove entire cable assembly.

Install the new cable assembly by reversing the above procedure.

NOTE: Observe the polarity of the Photocell connection and DO NOT OVERTIGHTEN THE NUTS. The green lead is connected to the [+] and the brown lead is connected to the [-]. The

Cable assembly shielding ground lug (white lead with lug) should be bent approximately 90 degrees so that it will sit on the edge of the cell holder over the screw hole located to the right of the cable clamp location. (See Fig. 1) Slide the circular housing over the cell holder and ground lug and secure the assembly with the 3 sheet metal screws.

NOTE: The search unit interface cable inside the 577 Brightness Meter must be grounded to the rear panel assembly to provide additional EMI/RFI shielding.

PHOTOCELL REPLACEMENT:
Without removing the lamp connection, follow the above procedure. Remove one cell clamp and loosen the other just enough to remove the Photocell. If the blue Wratten filter is cracked or badly wrinkled, it should be replaced. See Figures 2 and 3.





LAMP REPLACEMENT:
Remove the following: Three (3) sheet metal screws and circular housing. Disconnect two Lamp Assembly lead wires (green) from terminal block. Remove four (4) screws securing lamp yoke bracket to uprights. Loosen nut on Lamp Assembly and remove Lamp Assembly from yoke.

To install new Lamp Assembly reverse the above procedure.

LAMP and LAMP VOLTAGE ADJUSTMENT:
(for "T", "W", and "Y" search units)

Lamp Voltage Adjustment: After the instrument and the search unit have been on for 30 minutes, remove the cover from the search unit and connect a voltmeter to the lamp connectors. The voltage should be 6.2VDC +/- 0.1VDC.

For the 577, the lamp voltage adjustment potentiometer (R-34 on main PCB) can only be adjusted by removing the cover from the meter.

This lamp adjustment is the same for the 575-meter. However, it has a lamp adjustment switch (SW-2) located on the main PCB.

To obtain approximately 6.2 VDC on SW-2, set the dip switches as follows:

Lamp Adjustment Switch (SW-2)
Dip Switch # Positive (+) Position
1                down
2                    up
3               down
4                    up
5                    up

NOTE: Potentiometer (P-3) can also be adjusted through a 0.5 in diameter hole in the center of Model 570/670 bottom cover

LAMP ADJUSTMENT:
Place a thin piece of paper over the sample opening of the search unit and observe the light spot in the opening. Adjust the lamp bracket until the light spot shows approximately even illumination and appears in the center of the opening. The spot is mostly oval. The lamp should be focused so that the long diameter of the light spot is approximately 0.63 in. To achieve good symmetry, the lamp assembly may need to be rotated. To rotate the lamp assembly, loosen the nut on top of the assembly. Be sure to tighten the nut after the lamp assembly has been positioned.



LINEARITY CHECK:
With search unit warmed up for 20-30 minutes, zero the instrument with the black cavity/zero reflectance standard over the opening of the search unit. Replace zero standard with a high standard (70-90%) plaque. Standardize search unit to that value. Replace high standard with a low standard (35-50%). The instrument should read the low standard value within ± 1.0 reflectance unit.