HP 85662A Troubleshooting And Repair Manual page 117

The time constant of the filter capacitor is about 25 seconds. Before servicing,
allow adequate time for discharge after line power cable is removed.
+23 V UNREG
The +23 V unregulated supply, bridge rectifier CR9 through CR12, includes an overvoltage
protection circuit. If line power from a 220 V/240 V source is applied when the Line Voltage
Selector is set to 100 V/120 V, the overvoltage causes CR6 to conduct, turning on CR5 to
blow the fuse in the Line Module.
+5 V UNREG
Bridge rectifier CR1 through CR4 provides unregulated +5 V and -15 V to A1A6 ±15 V
Regulator.
A1A9 Bus Transition, Circuit Description
A1A9 Bus Transition provides jack J2 (inside front panel) for the 50-wire Instrument Bus,
which is routed through the IF-Display Section through a flat ribbon cable W8.
HPON, the IF-Display Section power-on control line (W8 wire 22), is sensed by amplifier Ql
and Q2 to activate fan relay Kl, which supplies line power to the cooling fan Bl.
The i-5.2 V supply is connected to the LSTP line (W8 wire 43) through CR1 to halt
operation of Al5 Controller if there is a supply failure.
Warning
0
2 A1A8/A1A9