Main Cpu Assembly (A9); Power Supplies; Outguard Supplies; Inguard Supplies - Fluke 5520A Service Manual

5520A
Service Manual

2-7. Main CPU Assembly (A9)

The Main CPU (A9) attached to the rear panel assembly communicates with the
following assemblies:
The main CPU memory is Flash ROM. There is a real-time clock with a battery backup.
Each analog assembly has the same bus structure:
Signals to the front panel jacks are routed by output relays on the motherboard.

2-8. Power Supplies

AC line voltage is applied through a line filter to a power module in the rear panel that
provides switching for four line voltages. The outputs of the power module are wired
directly to the primaries of the mains transformer. The safety ground wire is attached
from the power module to the rear panel.
Major internal grounds are SCOM, which is tied to OUTPUT LO and the guard shell,
ICOM, which is the internal ground for the current function, and GCOM, which is the
outguard common and is tied to earth ground.
2-9.

Outguard Supplies

The motherboard generates the outguard power supplies: +12VG, -12VG, and +5VG. All
the transformer connections for the outguard supplies come through one bundle of wires
connected to the motherboard with P1. A row of test points is provided in front of the fan
for the raw and regulated supplies. The outguard supplies are used only by the CPU
assembly (A9) and Encoder (A2) assemblies.

2-10. Inguard Supplies

The inguard supplies are located on the Voltage assembly (A8). The mains transformer
connections (inguard SCOM referenced) are connected to the Motherboard (A3). Current
protection devices for each of the supplies are located on the Motherboard. It is unlikely
these devices will blow unless there is another fault since the regulators will limit current
below the device ratings.
Filter capacitors for the high-current supply for the Current assembly (A7) are located on
the Filter assembly (A12).
The inguard SCOM referenced supplies are +15 V, -15 V, +5 V, -5 V, and +5RLH. The
+5 V and +5RLH supplies share the same raw supply. The +5RLH supply is used
exclusively as a relay driver and is nominally about 6.3 V. Test points for these supplies
are located in a row across the top of the Voltage assembly. The 65 V supplies are
rectified and filtered on the Motherboard but regulated on the Voltage assembly (A8).
2-8
Inguard CPU on the DDS assembly (A6)
Display assembly CPU
Serial and IEEE interfaces
External amplifier (5725A)
One or more Chip Select lines
Common data bus that connects to the motherboard, latched in by latches
A Fault line that sets all modules to a safe state in case of malfunction