Rohde & Schwarz ZVB 14, Service Manual

Page: 130 / 198

Instrument Construction and Function Description R&S ZVB
1145.1078.82 3.62 E-4
Interpreting the Results of the Selftest
Negative voltages are transformed into positive voltages by means of a positive voltage and a resistor
network because the A/D converters that are used can handle only positive voltages. This is why an
acceptable negative voltage may elicit a FAIL because the associated positive voltage is out of
tolerance.
The voltages supplied by the power supply are not checked directly. The failure of a power supply
voltage can, however, be deduced from FAILs of certain voltages on several boards. The following
Table shows how the board-oriented voltages checked during the selftest are derived from the power
supply voltages.
Power
supply +3.4 V +5.2 V +6.5 V +8.25 V +12.25 V -6.5 V -12.25 V
Fr +5V_A
+5 V_REF +10 V_A
+12V_STB
-5 V_A -10 V_A
Nd +2.5 VD_MDD1
+2.5 VD_MDD2
+1.5 VD_FCON +5 VA_ADC -5 VA_ADC
Sy +5 V_A
+5 V_REF +7 V_A
+10 V_A
Rm +5 V +10.5 VA
+10.5 VB
+12 V
+12V FAN -12 V
If the voltages listed in a column are all FAIL, the corresponding power supply voltage has failed or is
out-of-tolerance. The power supply must then be replaced.
On the motherboard, there is a separate fuse for each board and for each of the power supply voltages
used on the board. If a FAIL message is output, the first action to take is to check the fuse. The LO
divider board must be removed before the fuses on the motherboard can be checked (See Chapter
Board Replacement).
If an internal board voltage is out-of-tolerance, even though the power supply voltages used on the
board are OK, the board must be replaced.