LORAD AFFINITY Series, Service Manual

Page: 121 / 214

P/N 9-500-0255 5-5
Service Manual
Chapter 5: Maintenance—General
Troubleshooting
Filament Bias
There is a negative bias voltage applied between filament common and the x-ray
tube grid. This voltage is derived from a dc-to-dc converter to two potentiometers,
one for the small and one for the large filaments, which are factory adjusted to a pre-
determined value. This bias voltage is utilized to highly reduce the off-focus
radiation and to improve the focal spot distribution characteristics.
Stator Control
The stator drive is derived using the high-voltage inverter. During the ROTOR
BOOST period, a 180 Hz signal is applied to the HV inverter and its output is
applied to the stator coil through relay K1. This stator has two windings, main and
phase, with a common return. The common return is sensed for current flow during
the boost period and after a specified period of time, reports a "Rotor Ready" signal
to the Host microprocessor. There is now drive voltage to the stator coil during the
exposure. A 90 VDC signal is applied at the end of an exposure to "brake" the rotor
below its mechanical resonance frequency. This function resides on both the Power
Control PCB and the Inverter PCB.
High Voltage Control
The high voltage (HV) is derived using the PWM servo control in the inverter. The
output of the inverter is applied through relay K1 to the primary of the HV
transformer. The output of the transformer is rectified and multiplied in the multiplier
section and applied to the anode of the x-ray tube. There is a sensing coil at the
output of the multiplier to detect any large current spikes due to tube arcing and is
used to shut down the HV drive. There is also a HV divider within the multiplier that
returns the voltage value as a feedback signal to control the HV during the exposure.
An additional HV connector is available for external measurement of the actual HV.
An analog signal is applied to the Power Control PCB from the Host microprocessor
which represents the kV value selected by the technique. This value becomes the
input to the PWM to control the actual output kV value. The feedback signal is fed to
the Host microprocessor and compared with the desired value to provide servo
control during the exposure of the actual kV value.
3.1.6 Mechanical Functions
Compression Mechanism
The compression mechanism is comprised of a motor, clutch, and brake. Electrical
control resides on the Power Control PCB. Compression motion, up and down, is
motorized or may be manually operated. The motorized motions are activated by
either switches located on the C-arm or by foot-switches. The downward motorized
force is limited to 200 N (45 lb) and may be set to a value less than that utilizing the
User Defaults. Manual operation is designed to afford fine control of final
compression. The compression brake is used to prevent back-drive of the
mechanism. There is a function, if selected, which will automatically release the
mechanism, or drive it upward, at the end of an exposure. This function is inhibited
upon the sensing of a localization paddle. If power is removed while a patient is
under compression, you may manually back drive the mechanism to release the
compression force to safely remove the patient.
There is a strain gauge and a potentiometer associated with the compression
mechanism to measure pressure applied to the compression paddle and to measure
the distance compression paddle from the attached breast support (Bucky or