HP 5501A Operating And Service Manual page 214

6.4 Laser Head Troubleshooting
The laser head has diagnostic indicators and corresponding signal levels available on the rear
panel. Most common malfunctions of the laser head are evidenced by one of the rear-panel
diagnostic indicators beinr
':*
-%en-- of the REFERENC'
"'"
dAL, or by absence of the
laser beam itself. If the ALL indicator is lit. the RETUNE pushbutton.
After approximately 9 seconds, the RETUNE FAILURE and indicators should go out.
If either indicator remains o n or if any of the previously described symptoms of failure exist,
refer to the separate laser head operating and service manual for specific trouble isolation
procedures.
6.5
Receiver Troubleshooting
The system receivers supply the measurement signals (MEAS) for each measurement axis in
the system. When a sufficient laser beam signal is received, an LED indicator on the receiver
will light and the dc voltage at the external receiver test point will be between approximately
0.6 and 1.5V dc. Additionally, presence of a measurement signal can be verified by observing
a lit LED indicator on +'-n 10760A Counter card associated with the suspected receiver. The
10760A contains two indicators. The indicator farthest from the front edge of the card,
when lit, signifies the presence of a measurement signal. The indicator closest to the front
edge of the card signifies presence or absence of the reference signal from the laser head.
Improperly aligned optical devices can also cause a receiver to appear bad. Check for this by
either placing the receiver directly in the laser beam path from the laser head, or by reflecting
the laser beam onto the receiver's photodetector using only a retroreflector. This isolates all
other optical devices from the system. Most systems contain more than one axis and, con-
sequently, more than one receiver. If trouble is suspected with one receiver, exchange it with
another receiver to verify the suspected malfunction.
If the receiver or the optical devices are rotated around the laser beam axis, the receiver
'
--
indicator may remain on even
if
the beam between the interferometer and retroreflectcl,
, ,
blocked. This can also occur occasionally with correct optical alignment
if
the measurement
path
is
very short and few optical devices are used in the measurement path. If this situation
occurs, refer to the operating and service manual supplied with the receiver for information
regarding the "overload" adjustment.
6.6 Optical Devices Troubleshooting
Problems with the optical devices usually consist of misalignment of the devices. Refer to the
alignment procedures in Section II for further information. Air turbulence caused by ventila-
tion equipment or temperature gradients near the laser beam path can also cause measurement
problems. If this is suspected, shield the area around the laser beam and optical devices with
cardboard tubing, plastic sheet, or other suitable material. Some problems with sporadic
counting and drift can be traced to air turbulence around the measurement path. This should
be considered as a possibility before troubleshooting other parts of the system. Section I 1
contains extensive information o n optical problems and can be used as a basis for additional
optics troubleshooting.
6.7 System Controller Troubleshooting
A wide variety of system controllers can be used with a laser measurement system. Diagnos-
tic programs are available for all of the Hewlett-Packard controllers that can be used with
the system.
If a controller malfunction i s suspected, run the diagnostic programs to determine
if
the con-
troller operates properly.