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Service Instructions
General
The following instructions are intended as an aid for the
electronic technician, who is carrying out readjustments on
the HM 203-7, if the nominal values do not meet the specifi¬
cations. These instructions primarily refer to those faults,
which were found after using the Test Instructions. How¬
ever, this work should only be carried out by properly qual¬
ified personnel.
This concerns the digital storage part in
particuiar.
For any further technical information call or
write to HAMEG. Addresses are provided at the back of the
manual. It is recommended to use only the original packing
material, should the instrument be shipped to HAMEG for
service or repair (see also Warranty, page M2).
Instrument Case Removal
The rear cover can be taken off after unplugging the power
cord's triple-contact connector and after two cross reces¬
sed pan head screws (M4x30mm) with two washers on it
have been removed. While the instrument case is firmly
held, the entire chassis with its front panel can withdrawn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 ail 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 famiiiar with the risk invoived.
When the instrument is set into operation after the case
has been removed, attention must be paid to the accel¬
eration voltage for the CRT — 2000 V — and to the
operating voltages for both final amplifier stages — 150
and 130 V. Potentials of these voltages are on the CRT
socket, on the upper and lower PCBs. Such potentials
are moreover on the checkpoint strips on the upper and
iower horizontal PCBs. They are highly dangerous and
therefore precautions must be taken. It should be noted
furthermore that shorts occuring on different points of
the CRT high voitage and unbianking circuitry will defi¬
nitely damage some semiconductors and the opto-
coupler. 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 recom¬
mended to connect one by one all terminals of the
check strips on the upper PCB across IkQ to ground
(chassis) for a period of 1 second.
Handiing 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
iocal 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 2000 V, 45 to 65 Hz,
between accessible parts to both mains/line supply ter¬
minals. This test is dangerous and requires an
adequately trained specialist.
Operating Voltages
Besides the two
AC voltages
for the CRT heating (6.3V),
Component Tester and line triggering (12 V) there are eight
electronically regulated
DC operating voitages generated
(+12V, -hSVy, -l-5Vx, -12V, -I-135V, +152V, -1900V, and
22V for the unblanking circuit). These different operating
voltages are fixed voltages, except the
-I-
1
2
V, which can be
adjusted. All other voltages are dependent from the accu¬
racy of this voltage (and also from some resistors with close
tolerances). Only 22V in the unblanking circuit is stabilized
with Z-diode. The variation of the fixed voltages greater than
±5% from the nominal value indicates a fault. Excepting
22V, -hi35V, and -1900V, the other DC voltages have no
more than ±2% variation on the average. 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 abso¬
lutely sure that the electric strength of the voltmeter is suf¬
ficiently 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 incom¬
prehensible faults.
Maximum and Minimum Brightness
Two variable resistors of 470 kQ each, located on the upper
PCB, are used for these adjustment procedures (see
Adjusting Plan). They may only be touched by a properly
insulating screwdriver (CautionI High voltage!). The adjust-
Subject to change without notice
SI 203-7
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