HP 85662A Troubleshooting And Repair Manual page 85

(Serial Number Prefixes: 2403A-3001A)
Warning Unregulated voltages as high as +150 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 AlA8DSl. 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" (<+0.8 V)
holds the instrument in a standby condition with all unregulated supplies active and the
regulated supplies turned off. A logic "1" (>+2.0 V) t urns on the regulators and activates the
instrument.
referenced, the -15 V Regulator, a thermal shutdown circuit, and active components
necessary for AlA High Voltage Regulator to activate the CRT.
Reference is made in the following paragraphs to components in other
Note
assemblies: AlA High Voltage Regulator and AlA $100 V, +5.2 V
I
Regulator. These components are identified by complete reference
designations; for example, AlA6Q7.
conditions, this current drives Darlington regulator Q5 on until amplifier Ul senses an equal-
voltage condition between voltage reference VRl (Ul pin 3) and the sense input (Ul pin 2).
circuit senses an overcurrent condition (higher voltage across Rll, 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 +18.5 V is reached. Zener diode CR3 conducts,
turning on CR4, which conducts and blows fuse Fl.