Operating Voltages
Besides the two AC voltages for the CRT heating (6.3V)
and Component Tester and line triggering (12V) there
are seven electronically regulated DC operating voltages
generated ( + 24V, + 5V, —12V, + 1 41 V , + 260V,
— 1 900V, and 22V for the unblanking circuit). These dif
ferent operating voltages are fixed voltages, except the
+ 1 41 V (vertical final stage) and the high voltage, which
can be adjusted. The variation of the fixed voltages
greater than ± 5 % from the nominal value indicates a
fault. Both adjustable voltages have to be set precisely for
+ 1 41 V and — 1 900V respectively by the adjustments of
two potentiometers 2.5 kQ. These voltages are measured
on the Checkpoint strip with reference to ground (see
Adjusting Plan). Measurements of the high voltage may
only be accomplished by the use of a sufficient highly
resistive voltmeter (> 10MQ). You must make absolutely
sure that the electric strength of the voltmeter is suffi
ciently high. The 22V for the unblanking circuit can be
measured as the difference between two high voltages
with reference to ground. It is recommended to check the
ripple and also the interaction from other possible
sources. Excessive values might be very often the reason
for incomprehensible faults. The maximum ratings are
specified on the circuit diagrams. For the measurement
of the high voltage ripple, a probe capable of withstand
ing 2000V (e.g. X I 00 probe HZ37) is needed. But also a
normal probe X10 is sufficient with a capacitor (10 ...
22nF 2000V) connected in series to the probe tip.
Maximum and Minimum Brightness
Two variable resistors of 500kQ each, located on the
upper X-PCB, are used for these adjustment procedures
(see Adjusting Plan). They may only be touched by a
properly insulating screwdriver (Caution! High voltage!).
The adjustments may possibly have to be repeated,
because the functions of both variable resistors are
dependent on each other. Correct adjustment is achieved,
when the trace can be blanked while
HOR. EXT.
pushbutton is depressed and, in addition, when the
requirements described in the Test Instructions are met.
Astigmatism Correction
The ratio of vertical and horizontal sharpness can be
adjusted by the variable resistor of 50kQ, located on the
lower Y-PCB (see Adjusting Plan). As a precaution
however, the voltage for the vertical deflecting plates
(approx. + 85V ) should firstly be checked, because this
voltage will affect the astigmatism correction. While the
adjustment is being carried out (with pressed
HOR. EXT.
button and medium brightness), the
FOCUS
control knob
has to be repeatedly turned to and fro until the shape of
the luminous spot, whether round or oval or rectangular,
stays the same to the right and left of the optimum focus
ing. The interaction of focus adjustment and astigmatism
correction should be noted. After this adjustment, a
square-wave signal should be displayed and verified once
more in accordance with the Test Instructions. The final
adjustment has always to be the
FOCUS
control.
Trigger Threshold
The internal trigger threshold should be in the range 3 to
5mm display height. It is strongly dependent on the
71OCN comparator 1C. If there are compelling reasons to
replase this comparator, it may be that triggering
becomes too sensitive or too insensitive caused by 1C
gain tolerances (see Test Instructions: "Triggering
Checks", page T3). In most cases, these tolerances can
be eliminated by adjusting the "Trig, threshold" pot (see
Adjusting Plan). In extreme cases, the series resistor to
this pot should be changed. Generally, halving or dou
bling of its resistance value should normally be sufficient.
A too small trigger threshold cause double-triggering or
premature trigger action due to interference pulses or
random noise. A too high trigger threshold prevents the
representation of very small display heights.
Trouble-Shooting the Instrument
For this job, at least an isolating variable mains/line
transformer (protection class II), a signal generator, an
adequate precise multimeter, and, if possible, an
oscilloscope are needed. This last item is required for
complex faults, which can be traced by the display of
signal or ripple voltages. As noted before, the regulated
high voltage and the supply voltage for the final stages
(approx. 260V) are highly dangerous. Therefore it is
recommended to use
totally insulated extended probe
tips,
when trouble-shooting the instrument. Accidental
contact with dangerous voltage potentials is then unlike
ly. Of course, these instructions cannot thoroughly cover
all kinds of faults. Some common-sense will certainly be
required, when a complex fault has to be investigated.
If trouble is suspected, visually inspect the instrument
thoroughly after removal of the case. Look for loose or
badly contacted or discolored components (caused by
overheating). Check to see that all circuit board connec
tions are making good contact and are not shorting to an
adjacent circuit. Especially inspect the connections be
tween the PCBs, to the power transformer, to front
chassis parts, to CRT socket, to trace rotation coil (inside
of the CRT's shielding), to the 3 BNC connectors at the
rear chassis, and to the control potentiometers and
S2 20 3-4
Please note Change Information,
if any, on last page.
