Description Of A2-100 Mhz Version - Fluke PM3082 Service Manual

UNIT DESCRIPTIONS
5.2.7

Description of A2-100 MHz version.

5.2.7.1 Final Y amplifier.
Diagram 1.
The output signal from the delay line is applied to the input pins 6 and 9 of amplifier IC D1001. This
IC and surrounding components comprise the MF and HF square wave compensations. Also the
vertical CRT text signal is applied to D1001 (pin 15 and 16). The switching between text/cursors and
signal is done via control signals that are applied to pin 17 and 18 of D1001. Text is written if the
voltage at pin 18 is higher than at pin 17. This switching signal is derived from the collector of the
transistors V1201 and V1202. The control signals XYSW1 and XYSWO come from the text/cursor
generator IC on microprocessor unit A3. Via the transistors V1203 and V1204 similar switching
signals XSW1 and XSWO are routed to the X-deflection section for switching between text and signal.
The output signals from pin 21 and 22 of D1001 are routed via the common base circuits V1026 / V1027
to the final Y-amplifier. The final amplifier is located on the small printed circuit board at the CRT socket.
It drives the vertical deflection plates of the CRT. This final stage is indicated on diagram 2.
The delay line leads are terminated into 75Q. The impedance is composed by 300£2 inside D1001
parallelled by external impedances of 60£2 formed by the external resistors R1024/R1026 and
R1027/R1028. All termination resistors are applied to a voltage controlled by D1001/pin 13. Pin 13
determines the common mode voltage level of the D1001 inputs. The HF square wave compensation
is achieved by the two dual varicap diodes V1009 and V1011 together with associated resistors.
Varicap V1009 is controlled by the output signal at pin 20 of octal DAC N1001. This output voltage
can be in the range 0 to 10 V and controls varicap V1009 via operational amplifier N1002/1,2,3. This
operational amplifier converts the N1001 output voltage (that is related to 0 V) into a voltage related
to the voltage at D1001/pin 13.
Varicap V1011 is controlled by the output signal at pin 18of octal DAC N1001. This output voltage
can be in the range 0 to 10 V and controls varicap V1011 via operational amplifier N1002/5,6,7. This
operational amplifier converts the N1001 output voltage (that is related to 0 V) into a voltage related
to the voltage at D1001/pin 13.
LF square wave compensation is also done inside D1001. This is controlled via currents applied to
pin 27 and 28. These currents are derived from the N1001 DAC voltage outputs 13 and 11 via the
transistors V1014 and V1016. The actual compensating components are situated outside D1001 and
are R1037/C1011/R1038 and R1029/C1009/R1031. V1023 and V1024 form a 20 mA current source
to supply the bias current for the final stage inside D1001. V1024 is switched as a diode and gives
temperature compensation.
Offset is adjusted via the currents from the collectors of V1021 and V1022. These currents are
controlled by the DAC output pins 17 (coarse) and 16 (fine). Two parallel DAC outputs are used to
obtain sufficient resolution. V1018 is a current source and V1019 is for temperature compensation.
Gain control occurs via an adjustable current applied to D1001/pin 26. This is done via N1001/pin 14
and R1051
DAC output N1001/15 applies via operational amplifier N1003/1,2,3 the signal YFSHF to the final
stage on diagram 2. YFSHF is an adjustable dc voltage that is applied to varicap diodes V4113 that
control the HF square wave response of the output stage.
The circuit with operational amplifier N1004/2,3,6 is used for automatic calibration of the vertical
deflection. It measures the voltage at the vertical deflection plates of the CRT. This value is applied
as signal YCAL to the microprocessor on unit A3.
5.2 - 35