HP 85662A, Troubleshooting And Repair Manual

Page: 85 / 594

V Regulator
High Voltage Power Supply
V, V Regulator
V Regulator Circuit Description
(Serial Number Prefixes:
Warning
Unregulated voltages as high as V are present in the IF-Display Section
whenever the power cord is connected to a power source. This condition is
annunciated by LED indicator
Remove the power cord when setting
up service procedures.
The IF-Display section is activated by the logic signal HPON (power on), which is connected
to the RF Section via wire number 22 of the Instrument Bus. A logic “0”
V)
holds the instrument in a standby condition with all unregulated supplies active and the
regulated supplies turned off. A logic “1”
V) t urns on the regulators and activates the
instrument.
V Regulator contains the V Regulator, to which all other supplies are
referenced, the -15 V Regulator, a thermal shutdown circuit, and active components
necessary for
High Voltage Regulator to activate the CRT.
Note
I
Reference is made in the following paragraphs to components in other
assemblies:
High Voltage Regulator and $100 V, V
Regulator. These components are identified by complete reference
designations; for example,
V Regulator (A)
and Q2 form a current source from the unregulated V typical) supply. Under active
conditions, this current drives Darlington regulator on until amplifier senses an
voltage condition between voltage reference
pin 3) and the sense input pin 2).
then holds that regulated condition by shunting the drive current through R4.
current limiting is provided by Q4 and the voltage divider string on its base. This
circuit senses an overcurrent condition (higher voltage across R18) and limits the current
to a value proportional to the supply voltage.
Overvoltage crowbar circuit CR4 and associated circuitry shorts the output when an
overvoltage condition of approximately V is reached. Zener diode CR3 conducts,
turning on CR4, which conducts and blows fuse