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Service Instructions
panel can withdrawn forward. When the chassis is inserted
into the case later on, it should be noticed that the case has to
fit under the flange of the front panel. The same applies for the
rear of the case, on which the rear cover is put.
Caution !
During opening or closing of the case, the instrument
must be disconnected from all power sources for
maintenance work or a change of parts or components.
If a measurement, trouble-shooting, or an adjustment
is unavoidable, this work must be done by a specialist,
who is familiar with the risk involved.
When the instrument is set into operation after the case has been
removed, attention must be paid to the acceleration voltage for
the CRT approx. -2025V and to the operating voltages for both
final amplifier stages 175V and 146V. Potentials of these voltages
are on the PS-Board, the CRT-PCB and on the main PCB. They are
highly dangerous and therefore precautions must be taken. It
should be noted furthermore that shorts occurring on different
points of the CRT high voltage and unblanking circuitry will
definitely damage some semiconductors. For the same reason it
is very risky to connect capacitors to these points while the
instrument is on.
Capacitors in the instrument may still be charged, even when the
instrument is disconnected from all voltage sources. Normally,
the capacitors are discharged approx. 6 seconds after switching
off. However, with a defective instrument an interruption of the
load is not impossible. Therefore, after switching off, it is
recommended to connect one by one all terminals across 1kΩ to
ground (chassis) for a period of 1 second.
Caution
Handling of the CRT needs utmost caution. The glass bulb
must not be allowed under any circumstances to come into
contact with hardened tools, nor should it undergo local
superheating (e.g. by soldering iron) or local undercooling (e.g.
by cryogenic-spray). We recommend the wearing of safety
goggles (implosion danger). If any work has been carried out,
the complete instrument (with case closed and POWER
button depressed) must undergo a safety test, including a
voltage test with 2200V DC, between accessible parts to both
mains/line supply terminals. This test is dangerous and requires
an adequately trained specialist. The maximum impedance
between each accessible metallic part and the safety earth
connector at the 3-pole power connector must be less than
0.1Ω.
Operating Voltages
The operating voltages ( +12V, -6V, -2025V) are stabilized by
the switch mode power supply. The +12V supply is further
stabilized and used as a reference voltage for -6V stabilization.
These different operating voltages are fixed voltages, except
the +12V, which can be adjusted. The variation of the fixed
voltages greater than 5% from the nominal value indicates a
fault. Measurements of the high voltage may only be
accomplished by the use of a sufficient highly resistive voltmeter
(>10MΩ). You must make absolutely sure that the electric
strength of the voltmeter is sufficiently high. 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.
Maximum and Minimum Brightness
A variable resistor (100kΩ), located on the CRT PCB, is used
for this adjustment procedure. It may only be touched by a
properly insulating screwdriver (Caution! High voltage!). Correct
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adjustment is achieved, when the trace can be blanked in XY
analog mode and, in addition, when the requirement described
in the Test Instructions are met.
Astigmatism control
The ratio of vertical and horizontal sharpness can be adjusted
by the variable resistor of 47kΩ, located on the CRT PCB. As
a precaution however, the voltage for the vertical deflecting
plates (approx. +71V when the trace is in center position)
should firstly be checked, because this voltage will affect the
astigmatism correction. While the adjustment is being carried
out (with medium brightness and a 1MHz square-wave signal),
the upper horizontal square-wave tops are firstly focussed
with the FOCUS control. Then the sharpness of the vertical
lines are corrected with the 47kΩ Astigm. pot. The correction
should be repeated several times in this sequence. The
adjustment is optimized, when the FOCUS knob exclusively
brings no improvement of the sharpness in both directions.
Trigger Threshold
The internal trigger threshold should be in the range 0.3 to
0.5div display height.
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 voltages for the final
stages 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 unlikely. 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 connections are making good
contact and are not shorting to an adjacent circuit. Especially
inspect the connections between the PCBs. This visual
inspection can lead to success much more quickly than a
systematic fault location using measuring instruments. Prior
to any extensive trouble-shooting, also check the external
power source.
If the instrument fails completely, the first and important step
- after checking the power fuses - will be to measure the
deflecting plate voltages of the CRT. In almost any case, the
faulty section can be located. The sections represent:
1. Vertical deflection.
2. Horizontal deflection.
3. CRT circuit.
4. Power supply.
While the measurement takes place, the position controls of
both deflection devices must be in mid-position. When the
deflection devices are operating properly, the separate voltages
of each plate pair are almost equal then (Y approx. 71V and X
approx 90V). If the separate voltages of a plate pair are very
different, the associated circuit must be faulty. An absent
trace in spite of correct plate voltages means a fault in the CRT
circuit. Missing deflection plate voltages is probably caused by
a defect in the power supply.
Subject to change without notice
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