24
Subject to change without notice
2.5 CRT Heater
Look between the CRT mu-metal shielding and the CR board
to see whether the heater (fi lament) glows.
STOP
Attention:
The heater voltage is superimposed on -2 kV.
Note:
If all measurements and tests have been completed success-
fully except this item, switch the oscilloscope off.
Locate and identify Molex connector J1009 (4 wires) connecting
the power supply board with the CR board. See photo 2.8
– Disconnect the ribbon cable from J1009.
– Use an Ohm meter to measure approx. 15 Ohm between
wire 1 and 2 leading to pin 1 and 14 (heater) of the CRT.
– If the heater seems to be defective, remove the CRT socket
and measure the CRT heater resistance between pin 1 and
14. to eliminate the CRT socket.
2.6 Measurements
If any of these voltages are missing or incorrect, the following
procedure is recommended:
– Disconnect the instrument from mains/line.
– Remove the power supply shielding.
– Unsolder the wire soldered to the protective earth (PE)
connection at the inner side of the rear chassis, marked
by an earth symbol.
– Remove all cables and wires connected to the power sup-
ply.
– Unfasten the power supply mounting screws.
– Remove the power supply and replace it by a new power
supply.
Proceed in reverse order to mount the new power supply.
STOP
Attention!
Special care must be taken for the protective earth
connection (PE). It must be fi xed and thereafter sol-
dered in such a way that even accidental contact with a
soldering iron does not open the connection.
Security check!
Check that after PS board replacement the protective earth
connection is re established.
2.7 Adjustment after power supply change.
The following procedures are required after changing a power
supply due to tolerances.
– Check and 146 V and +12V as described under item
1 and 2 in the Adjustment Procedure.
– Follow the instructions of the Per formance Check
HM303-6.
H M 3 0 3 - 6 T r o u b l e s h o o t i n g
2.8
