Operating Voltages; Maximum And Minimum Brightness; Astigmatism Control; Trigger Threshold - Hameg HM304 Manual

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).
The complete instrument (with case closed and POWER
button depressed) is after each intervention undergo 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.

Operating Voltages

All operating voltages ( +12V, -12V, +175V, +144V, -2025V)
are stabilized by the switch mode power supply. The +12V
supply is further stabilized and used as a reference voltage
for -12V stabilisation. 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 no-
minal 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

Two variable resistors (470kΩ), located on the switch mode
power supply PCB, are used for these adjustment procedures.
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 in XY 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. +80V when the trace is in center position)
should firstly be checked, because this voltage will affect
the astigmastism 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 optimised, 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. It is strongly dependent on the
comparator IC. If there are compelling reasons to replace
this comparator, it may be that triggering becomes too sen-
sitive or too insensitive caused by the IC gain tolerances (see
Test Instructions: „Triggering Checks"). In extreme cases,
the hysteresis resistor of the comparator should be changed.
Generally, max. halving or doubling of this resistance value
should 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 display of very small display heights.
26

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, to front
chassis parts, to CRT PCB, to trace rotation coil (inside of
CRTs shielding), and to the control potentiometers and
switches on top of and beneath 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.
3. CRT circuit.
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.
85V and X approx 86V). 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.

Adjustments

As advised in the Operating, Test and Service Instructions,
small corrections and adjustments are easily carried out with
the aid of the Circuit Diagrams and Adjusting Plan. However,
a complete recalibration of the scope should not be attempted
by an inexperienced operator, but only someone with
sufficient expertise. Several precision measuring instruments
with cables and adapters are required, and only then should
the pots and trimmers be readjusted, provided that the result
of each adjustment can be exactly determined. Thus for each
operating mode and switch position, a signal with the
appropriate sine or square waveform, frequency, amplitude,
risetime and duty cycle is required.
2. Horizontal deflection.
4. Power supply.
Subject to change without notice