4.2.1
Power Supply PC Assembly (A2)
(See schematic diagram #16996 in Chapter 8.
The Power Supply PC board generates the +5 Vdc and +15 Vdc power sources required by the
instrument. Since both voltages are produced in a similar manner, this discussion will cover only the
+5 V circuitry.
The +5 Vdc produced on the board is the master voltage regulator for the system. It takes its reference
from CR21 and compares it with the output voltage between the +5 V Sense and +5 V Common lines.
Sensing is in the circuit composed of R41, R43, R66, and adjustable at R42. Q7 and Q5 are regulating
transistors with the regulation voltage available at TP9 for checking.
-5 V is slaved from the +5 V and regulated through Q9 and Q10. Both the plus and minus voltages are
compared with ground, and both have current limiting. The +5 V circuit is sensitive to the voltage drop
across R29, which is controlled in parallel through Q6. U2A is the current limiter for +5 V, and U3A is
the current limiter for -5 V. U4 compares the unreg5 V with the reference from U5. If its output
is too low, the AC Fail signal will tell the CPU that the power is about to fail. R61, CR19, and C21 will
keep the voltage supply at U4 up long enough for the +5 V to drop to a logic zero threshold after the
instrument goes to a power-off state.
A2 Test Points
The test points in Table 4-1 can be checked for the correct signals on the A2 Power Supply board:
Test Point
Description
TP1
Reads the current in the +15 V drive circuitry
TP2
Reads the current in the -15 V drive circuitry
TP3
+15 V
TP4
+15 V Common Sense
TP5
-15 V
TP6
+15 V Common
TP7
Reads the current in the +5 V drive circuitry
TP8
Reads the current in the -5 V drive circuitry
TP9
+5 V Sense
TP10
-5 V Sense
TP11
5 V Common Sense
Table 4-1. A2 Test Points
Theory of Operation
Manual No. 20790, Rev C, November 1998
4-3
Summary of Contents for 8501A
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