Fluke PM3082 Service Manual page 364

UNIT DESCRIPTIONS
Illumination control
The illumination of the graticule must be variable. For this reason the illumination voltage can be
varied between about 2 and 28 V. Control of the illumination amplifier V1148 takes place by means
of the signal DAC2, level about 1.7 to 4 V. The output voltage from the amplifier is -16 to +10 V.
EHT converter
The EHT converter supplies three voltages.
- An AC voltage of 6,3 V (F1, F2), to supply
the filament of the CRT.
- The cathode voltage to the CRT, a DC
voltage of -2200 V.
- The post acceleration voltage to the CRT,
a DC voltage of +14 kV.
These voltages are made in a separate
converter, equipped with a separate
transformer. The EHT converter is a resonant
flyback converter, the output voltages of the
transformer are sinusoidal.
Basically, the converter consists of a resonant
LC circuit formed by the transformer with its
parasitic capacitances. This resonance circuit
defines the converter frequency which is
about 80 kHz. Energy is supplied to this LC
circuit by injecting current to it from the supply
voltage, the +58 V, by switching ON V1109.
Most of the time, V1109 is OFF. The primary
peak to peak amplitude is about 200 V, the
negative peak about -40 V. During the
positive half of the sine-wave, capacitor
C1111 is discharged very little via R1114.
When the sine-wave reaches its most negative value, a small current will pass through C l 111 and
V1106 and this acts as base current for V1102. Due to this, the thyristor configuration V1102-1109 will
be switched ON and energy is supplied to the resonant circuit. The ON time of V1109 can be
controlled by the operational amplifier N1101 pin 7.
To protect against too high voltages, e.g. caused by a defective N1101, the circuit is provided with an
over-voltage protection. This circuit consists of V1103 and V1104. The maximum output voltage is
defined by this circuit as it will overrule the control circuit at too high voltage.
The AC voltage at T1002 pins 4 and 5 is used to supply the CRT filament voltage of 6.3 V.
The voltage at pins 11 and 3 is rectified and this -2200 V is used as CRT cathode voltage. The voltage
at pins 11 and 2 is rectified and multiplied in a cascade circuit. The output, +14 kV, is used as post
acceleration voltage to the CRT.
Diagram 3
Diagram 3 comprises the following circuit parts:
- secondary output circuitry
- over- and under-voltage protection
- power fail circuit
- temperature protection
- 10 V reference circuit
BV
) T1002
I LP
V1108
/
V1109
igure 5.7 High tension generator
5. 6- 3