Philips PM3208 Service Manual page 27

4-6 4 CIRCUIT DESCRIPTION
4.3.2 CRT contro!
The CRT has several electrodes.
~ The filament, pins 1 and 14, is supplied with 6,3 V (AC) from the line-transformer.
- The cathode, pin 2, is at a fixed potential of — 2000 V.
- G1, pin 3, has already been described. The potential is about — 2000 V. Intensity
control is made by RV3033, "INTENSITY".
- G2, an anode, pin 12, is at a fixed potential of about +100 V, by means of the
voltage-divider R2016-2017.
— G3, pin 4, has already been described. The potential is about — 700 V.
Focus control is made by RV3035, "FOCUS".
- G4 and G5, pin 6, are used for astigmatism control, by means of VR2018. The
potential can be adjusted between 0 and +160 V.
- The vertical deflection plates, pins 7 and 9, are driven by the vertical output amplifier,
as already described.
- The horizontal deflection plates, pins 11 and 13, are driven by the horizontal output
amplifier, as already described.
- The trace rotation coil L2001, resistance about 500 2, has been placed around the
CRT and is intended to align the trace with the horizontal graticule-lines. The current
through the coil can be adjusted between — 8 and +12 mA by means of RV3036,
"TRACE ROTATION".
- The illumination of the graticule can be controlled by RV3037, "SCALE ILLUM".
4.3.3 HV supply
The ~— 2000 V high-voltage for the CRT is made by means of a DC-DC converter
consisting of Q2001, 1C2001 and the transformer T2001. The converter is supplied with
about +18 V. Voltage control is obtained by means of 1C2001. The output of the
converter is a sinusoidal AC voltage, available at the secondary winding of T2001. The
peak-value of this voltage is about 1000 V. The voltage is rectified and multiplied by
voltage-doubler C2013-D2011-D2010-C2011 and smoothed with C2013.
4.4 CALIBRATOR
In the calibrator circuit, a square-wave voltage for checking purposes is produced.
Frequency of the square-wave is about 1 kHz. The pulse duty-cycle is 50 % and the peak
to peak amplitude 1 V. The square-wave is generated in a free-running multivibrator,
consisting of two gates of 1C4001, pins 4 and 3. The other two gates, pins 11 and 10, are
for pulse-shaping. Via the voltage divider R4010-4011-4012 the signal is fed to the probe
adjustment output terminal at the front of the instrument.
4.4 CALIBRATOR