Fluke PM3082 Service Manual page 199

UNIT DESCRIPTIONS
The final X amplifier half that drives the left deflection plate is explained now. Its function is identical
to that of the other amplifier half. The balanced input signals XDRIL1 and XDRILO are applied to input
stage V2101. V2104 is a current source. XDRILO is not used and connected to +5 V. In the collector
circuit of V2101 are the zener diodes V2102/V2103 that function as level shifters. The emitter
followers V2132 and V2111 supply the output stage V2134/V2113 with signal. V2113 functions as an
amplifier stage and V2134 mainly as a current source. V2134 receives signal via C2131 and mainly
functions as amplifier stage in the fastest sweep speeds. Via R2133/R2134 however some dc drive
is also applied to V2134.
The circuit with operational amplifier N2003 is used to measure the output voltage that is applied to
the X-deflection plates of the CRT. This measurement is done via high-ohmic resistors R2301 and
R2302. The analog output voltage XCAL of N2003 is applied to an ADC incorporated in the
microprocessor on unit A3. This enables the microprocessor to exactly measure the horizontal output
voltage. This is used for automatic calibration of the horizontal deflection.
Diagram 4
This diagram shows the final intensity (Z) and focusing amplifiers. These amplifiers drive the intensity
electrode G1 and the focusing electrode G3 of the CRT. The input circuit is formed by IC D3001.
Input pin 5 and 6 of D3001 receive the Z-pulses ZLTRAO and ZLTRA1 that determine the intensity
during signal display. Input pin 9 and 10 receive the Z-pulse ZTXTO and ZTXT1 that determine the
intensity during text display. The selection between signal intensity and text intensity is done via the
signals ZSWO and ZSW1 that are applied to pin 7 and 8 of D3001. These signals originate from the
text generator IC on microprocessor unit A3. Text is displayed with ZSWO at 0 mA and ZSW1 at 1
mA. Trace is displayed with ZSWO at 1 mA and ZSW1 at 0 mA.
The final Z-amplifier and the focusing amplifier are identical halves that receive input signals that are
in opposite phase. These signals originate from the D3001 outputs pin 24/25 (ZDRIF1/ZDRIF0) and
pin 18/19 (ZDRII0/ZDRII1).
Via V3002 part of the ripple on the negative CRT cathode voltage (ZRIPPLE) is applied to pin 15 of
D3001. This compensates for unwanted intensity modulation due to this ripple.
The diodes V3236, V3136, V3237 and V3137 protect D3001 against possible high voltage surges.
V3301 supplies the intensity determining signal INTEN-DA. This signal adjusts the "dark level" of the
CRT. V3301 is controlled by DAC N2002 output signal DARK. V3311 supplies signal ASTIG that
determines the astigmatism of the CRT. V3311 is controlled by DAC N2002 output signal ASTDR.
The upper amplifier half that is used for the dynamic focus control is explained now. Its function is
identical to that of the lower amplifier half that is used for intensity (Z) control. The balanced input
signals ZDRIF1 and ZDRIFO are applied to input stage V3101. V3104 is a current source. ZDRIFO is
not used and connects to +5 V. In the collector circuit of V3101 are the zener diodes V3102/V3103
that function as level shifters. The emitter followers V3132 and V3111 supply the output stage
V3134/V3113 with signal. V3113 functions as an amplifier stage and V3134 mainly as a current
source. V3134 receives signal via C3131 and mainly functions as amplifier stage for fast input
signals. Via R3133/R3134 however some dc drive is also applied to V3134.
The circuit with V3282/V3212 suppresses the light on the screen at power down. V3282 becomes
conductive via the fast falling +12 V and the slowly falling charge in C3281.
Diagram 5
In this diagram the circuit is shown that forms an interface between the signals that determine the
intensity/focusing and the CRT electrodes G1/G3 that control intensity/focusing. These electrodes,
the cathode and filament (heater) of the CRT are at a high negative voltage. This voltage is separated
from the other oscilloscope circuits via the 3kV capacitors C4004, C4061, C4036 and C4037.
5.2- 3