Zenith Data Systems ZVM-124 User manual

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LIMITED RIGHTS LEGEND
Contractor is Zenith Data Systems Corporation of St. Joseph,
Michigan 49085. The entire document is subject to Limited Rights
data provisions.
INITIALS
Copyright 1985 Zenith Data Systems Corporation
St. Joseph, Michigan 49085
Printed in the United
States of America

CONTENTS
Cleaning Procedures ................................................. 7.2
Description and Specifications .......................1.1
Adjustmen ts...................................................................7.2
List of Tools, Test Equipment,
Focus Adjustment ...................................................7.2
and Supplies ................................................. 1.1
B+ Voltage Adjustment ........................................7.2
Tools ............................................................................
1.1
CRT Yoke Adjustment ........................................ 7.3
Test Equipment ...................................................... 1.1 Trou bleshooting ........................................................... 7.3
Supplies ...................................................................... 1.1
Main Circuit Board Troubleshooting ........... 7.4
Specifications ............................................................. 1.2
Main Circuit Board Waveforms ...................... 7.4
CRT Socket Board Troubleshooting .............. 7.5
Installation ...................................................... 2.1
Power Supply Troubleshooting ........................7.6
V
ideo Moni tor ............................................................ 2.1
Vol tage Selection ...................................................... 2.2
Reassembly ........................................................8.1
Connections ................................................................. 2.4 Control Panel Assembly Installation ................ 8.1
Signal Cable Interface ............................................ 2.4 Power Switch Assembly Installation ................ 8.1
Main Circuit Board Installation ......................... 8.1
Operation ...........................................................3.1
Fuse Board Installation ........................................... 8.2
Initial Setup of the Operating Power Transformer Assembly Installation .... 8.3
Controls and Monitor ............................... 3.1 Ca thode- Ra y Tube Installation .......................... 8.3
CRT Socket Board Installation ...........................8.4
Theory of Operation ....................................... 4.1
Back Cover Installation .......................................... 8.4
Theory of Operation .............................................. 4.1
Video and Highlight Inputs ..............................4.1 Parts Lists ..........................................................9.1
Vertical Sync Input ............................................... 4.1
Horizontal Sync Input..........................................4.2 Figures
1.1 The ZVM-124 Video Monitor. ............... l.l
Circuit Description ...........................................5.1
2.1 Video Monitor Front View ..................... 2.1
Power Supply ............................................................... 5.1
2.2 Video Monitor Rear View ...................... 2.1
Video and Highlight 2.3 l20-Volt Wiring ..........................................2.2
Driver Circuitry .......................................... 5.1 2.4 Main Board Transformer Leads ...........2.2
Horizontal Circuitry .................................................5.2 2.5
Power Transformer Replacement ....... 2.3
Vertical Sweep Circuit ........................................... 5.3 2.6 Power Switch Replacement... .................2.3
Schematic .......................................................................5.4 2.7 220-
Volt Wiring............................................ 2.3
2.8 Monitor Cable Connections ....................2.4
Disassembly........................................................6.1
3.1 Front Panel Controls .................................3.2
Back Cover Removal ............................................... 6.1
3.2 Video Monitor Rear View ......................3.2
Power Transformer Assembly Removal ......... 6.1
4.1 ZVM-124 Block Diagram .........................4.2
Fuse Board Removal ................................................ 6.2 5.1 Schematic ........................................................ 5.3
CRT Socket Board RemovaL ..............................6.2 6.1 Back Cover Removal................................. 6.1
Main Circuit Board Removal .............................. 6.3 6.2 Transfer RemovaL .................................... 6.1
Power Switch Assembly RemovaL ................... 6.4 6.3 Transformer Disassembly ....................... 6.2
Control Panel Assembly Removal .....................6.4 6.4 Fuse Board ..................................................... 6.2
Cathode-Ray Tube Removal ................................ 6.4 6.5
Main Board .................................................... 6.2
6.6
Fuse Board Removal .................................6.3
Servicing Instruction ........................................7.1 6.7
CR T Socket Board Removal ..................6.3
Safety and Service Guidelines ............................ 7.1
6.8 Main Board Removal,
AC Leakage Test ..................................................... 7.1
Discharging the CRT ....................... 6.3
Other Precautions ................................................... 7.1 6.9 Power Switch Removal ............................6.4
Page iii

6.10 Control Panel RemovaL ......................... 6.4
Listing
6.11 Preparing the Cathode-Ray
3.1 BASIC Program Used to Fill
Tube for Removal ............................. 6.5
Screen with a Character. ................ 3.1
6.12 CRT Yoke Preparation ............................ 6.5
6.13
CRT Removal ................................................6.5
7.1 AC Leakage Test Arrangement ............ 7.1
7.2 Internal Adjustments ................................ 7.2
7.3
Yoke Adjustment ........................................ 7.3
7.4 Waveform at Base of Q601,
Vertical Oscillator .............................7.4
7.S Waveform at Emitter of Q601,
Vertical Oscillator .............................7.4
7.6
Waveform at Collector of Q601,
Vertical Oscillator .............................7.4
7.7
Waveform at Emitter of Q606,
Vertical Output .................................. 7.5
7.8 Waveform at Emitter of Q607,
Vertical Blanking ............................... 7.5
7.9 Waveform at Collector of Q501,
Horizontal Driver ...............................7.5
7.10
Waveform at Base of Q501,
Horizontal Driver ............................... 7.5
7.11 Waveform at Base of Q20S,
Video Input ...........................................7.5
7.12
Waveform at Pin 2 of VX201,
Cathode of the CRT ......................... 7.5
8.1 Control Panel Assembly
Installation ............................................. 8.1
8.2 Power Switch Assembly
Installa tion .............................................. 8.2
8.3
Main Circuit Board
Installa tion .............................................8.2
8.4
Fuse Board Installation ............................ 8.2
8.5
Power Transformer Assembly
Installa tion .............................................8.3
8.6
Installing the CRT in the
Cabinet ....................................................8.3
8.7
Yoke Leads .................................................... 8.3
8.8
CRT Socket and
High Voltage Anode Lead ............. 8.4
8.9
CRT Socket Installation .......................... 8.4
8.10
Back Cover Installation ........................... 8.4
9.1 Exploded View ............................................. 9.1
9.2 Component Location,
Circuit Boards ......................................9.2
Tables
2.1 Signal Cable Connector J201.. ............... 2.4
7.1 General Troubleshooting ........................ 7.3
7.2
Main Circuit Board
Trou bleshooting ..................................7.4
7.3
Power Supply Troubleshooting ............ 7.6
9.1 Major Assemblies........................................ 9.1
9.2 Component Parts List ............................... 9.2
Page iv
Contents

The Zenith Data Systems ZVM-124 Video
Monitor is fitted with signal and power cables
to interface directly with the IBM PC
computer and includes the following features.
Refer to Figure 1.1.
• Major operating controls behind an access
door on the front panel.
• A
12-inch display, featuring a non-glare,
amber phosphor CRT, capable of
displaying 25 rows of 80 characters per
row.
• A
width adjustment accessible from the
rear panel.
• Focus control and video circuits mounted
on the CRT socket board to aid in
servicing.
• A power switch with a power indicator on
the front panel.
Figure 1.1
The ZVM-124 Monitor
List of Tools, Test
Equipment, and Supplies
The following tools, test equipment, and
supplies are recommended for servicing this
product.
DESCRIPTION
AND SPECIFICATIONS
TOOLS
•
1/4" nut driver
•
Standard screwdriver, 1/4" blade
•
Phillips screwdriver, No.1 tip
•
Phillips screwdriver, No.2 tip
•
Diagonal cutters
• Wire
strippers
• Long-nose
pliers
•
Desoldering tool
•
Soldering iron, 25 to 40 watt
•
Power cord, HE-89-60
TEST EOUIPMENT
• Oscilloscope - DC to 35 MHz, triggered
sweep, with low capacitance (3 pF) probe
• Digital voltmeter - High impedance input,
zero to 1000 volts, zero to 1 megohm,
Heath model SM-2215, or equivalent
• High voltage probe - Zero to 40 KV, Heath
model IM-5210, or equivalent
• Signal source - Quantum 801 A or
equivalent in place of an IBM PC computer
SUPPLIES
• Solder, 60/40, HE-331-13
•
Desoldering braid, HE-490-185
•
Cable ties, HE-354-59
•
Lint-free cloths
Page 1.1

Specifications
Operating Voltage: 120 VAC 60 Hz
(ZVM-124);
220 VAC
50 Hz
(ZVM-124-E)
Operating Current: 0.3 ampere
Nominal Power: 28 watts
Nom. High Voltage: 13.0 KV
Fuse (primary):
Fuse (secondary):
Video
Bandwidth:
Rise Time:
Horiz.
Frequency:
Vert. Frequency:
Input Signal:
CRT:
Character Type:
Characters/Line:
Character Block:
Page 1.2
4.0 amp,
type FX320I
2.25 amp,
type FX70 I
18 MHz
20 nanoseconds
18.432
KHz
50.09 Hz
Four TTL-level inputs
12" diagonal, amber
8 x 10-pixel matrix
80 characters per line
7 x 9 pixels
Description and Specifications

The Zenith Data Systems ZVM-124 Video
Monitor is a self -contained unit and is
designed to be connected to an associated
computer using an attached shielded signal
cable and power cord. The monitor receives
four TTL-level video signals (video input,
highlight input, vertical input, and horizontal
sync input) via the signal cable and D-type
connector and 120 VAC (or 220 VAC on the
ZVM-124-E) power via the power cable and
connector.
The monitor must be located in an area that
will provide proper ventilation. There are
vents on the back and bottom of the cabinet
which permit air flow through the monitor.
Verify that these vents are not blocked.
Video Monitor
The following is a description of the controls
and cables for the video monitor. Refer to
Figure 2.1, position the video monitor so the
front of it is facing you, and open the front
door as shown.
• Contrast - The CaNT. control adjusts the
intensity of the displayed data on the
screen.
•
Black Level - The BLACK LEVEL control
adjusts the intensity of the raster
(background ).
• Vertical Size - The VERT. SIZE control
adjusts the vertical size at the top and the
bottom of video display.
• Vertical Hold - The VERT. HOLD control
adjusts the vertical sweep for a stable
display.
• Horizontal Hold - The HORIZ HOLD
control adjusts the horizontal sweep for a
stable display.
• Power Switch - This push-to-latch, push-to-
release switch turns the monitor on or off.
INSTALLATION
• Power Indicator - This LED lights when
power is applied to the monitor and the
power switch is turned on.
CONT
INDICATOR
POWER
V SWITCH
ERT SIZE HORIZ HOLD
VERT HOLD
Figure 2.1
Video Monitor Front View
Refer to Figure 2.2 and position the video
monitor so the back is facing you.
•
Power Cord - This grounded, AC power
cable connects to the 120-volt (for the
ZVM-124) AC output of the computer, or
with the HE-89-60 power cord, to any
standard (120 VAC) AC power source. (The
ZVM-124-E monitor accepts 220 VAC
through this cord.)
•
Signal Cable - his shielded cable connects
to the output of the monochrome video card
of a computer.
• Width Control - This control adjusts the
horizontal size of the video display.
10
ft.4.-4I!+---t-tr WID TH
CONTROL
o
POWERf ~'J.r.r.r.rrrr.r.rrrr.r,
COR~} SIGNAL
(/ CABLE
a ~
~ii
Figure 2:2
Video Monitor Rear View
Page 2.1

Voltage Selection
Before connecting the monitor to a power
source, make sure it is rated for the voltage
available in your area. The ZVM-124 monitor
is normally wired for 120 volts AC. To change
the voltage rating to 220 volts AC, proceed as
follows:
WARNING: Be
sure the signal cable and power
cord are unplugged from the computer or any
other signal and power source before you
att,empt to change the voltage rating of this
unit.
1. Refer to Section 6, "Detailed
Disassembly," and remove the back cover,
power transformer assembly, fuse board,
main board, and power switch assembly.
2. Refer to Figure 2.3 and unsolder the two
pairs of the l20-volt transformer leads at
A, B, C, and D on the fuse board. (These
leads are normally a pair of light tan (or
white) and a pair of gray leads. One lead
of each pair will normally have a brown
stripe.)
3. Unsolder the two l20-volt power line
leads at G and H on the fuse board.
4. Unsolder the 120-volt power switch (S201
- part number 85-1559) leads at I and J
on the fuse board.
•
, 204-11.7:~C;~
G CX208 U3:eN
• 221)'"
C GIII".lU
II ".,. Tel .......
"... ..v.o B
• ex 209
.
r;s
"o~. :~.'
TRANSFORMER GROUND BLACK
120V WHITE/BROWN (NOT REMOVED)
Figure 2.3
120-Volt Wiring
5. Refer to Figure 2.4 and unsolder the two
120-volt transformer leads at E and F on
the main circuit board. Note how these
Page 2.2
leads are routed in relation to the other
wiring.
Figure 2.4
Main Board Transformer Leads
6. Refer to Figure 2.5 and remove the two
6-20 x .312" screws that hold the
transformer cover in place. Remove the
cover.
7. Remove the two 8-18 x 1.25" screws that
hold the l20-volt transformer to the
power transformer frame. Note how the
leads are routed out of this assembly
8. Replace the l20-volt power transformer
(TX201 - part number 95-3388-04) with a
220-volt transformer (part number 95-
3578-03).
9. Replace the two 8-18 x 1.25" screws that
hold the transf ormer to the power
transformer frame and tighten them.
10. Route the leads of the transformer as
they were for the 120-volt transformer
and use the two 6-20 x .312" screws to
replace the transformer cover. Tighten
the screws.
11.
Refer to Figure 2.6 and remove the two
8-18 x .437" screws at F. Remove and
replace the l20-volt power switch with-a-
220-volt power switch (part number 85-
1646).
Replace the two screws at F.
Installation

12. Refer back to Figure 2.4 and solder the
red and brown transformer leads to E and
F on the main board.
Figure 2.5
Power Transformer Replacement
Figure 2.6
Power Switch Replacement
Installation
13. Refer to Figure 2.7 and solder the two
220-volt transformer leads at A (the gray-
brown lead) and J (the gray lead) on the
fuse board.
14. Solder the power line leads at I (the
brown lead) and L (the blue lead) on the
fuse board.
15. Solder the SX201A power switch leads at
B (the gray/brown lead) and M (the
gray /blue lead) on the fuse board.
16. Solder the SX201B power switch leads at
N (the brown/gray lead) and C (the
brown lead) on the fuse board.
17. Refer to Figure 2.7 and remove and
replace the 4 amp fuse at K on the fuse
board (FX201 - part number 136-114-23)
with a .160 amp fuse (part number 136-
133-07).
Figure 2.7
220-Volt Wiring
220V LINE
18. Refer to Section 8, "Reassembly," and
replace the power switch assembly, main
board, fuse board and power transformer
assembly, and back cover. Verify that all
the power and signal leads are routed as
shown in the figures in that section.
NOTE: The ZVM-124-E monitor is wired for
220 volts AC at the factory. To change the
voltage rating to 120 volts AC, follow the
same procedure, but substitute the 120-volt
parts (95-3388-04, 85-1559, and 136-114-23) for
the equivalent 220-volt parts in the
instructions.
Page 2.3

Connections
To install and connect the monitor, refer to
Figure 2.8 and complete the following
procedure.
1. Place the monitor on a solid work surface
near the computer.
2. Rotate the video monitor until the front
is facing you.
3. Make sure the power switch on the front
of the cabinet is off; also check that the
power switch on the computer is off:
4. Plug the shielded signal cable into the
appropriate video connector on the rear
panel of the computer.
5. Plug the power cord into the power
connector on the rear panel of the
computer.
6. Turn the power on.
Signal Cable Interface
The monitor is designed to be used as a
display device for a computer system. The
shielded cable is used to connect the monitor
to the computer.
CAUTION: A shielded cable is required by
FCC regulations to prevent interference with
other equipment.
Table 2.1 provides information on the signal
cable at the D-type connector.
Table 2.1
Signal Cable Connector J201
PIN
NUMBER SIGNAL FUNCTION
1 Shield ground
2 Signal ground
3 Not used - not connected
4 Not used - not connected
5 Not used - not connected
6 Highlight input
7 Video input
8 Horizontal sync input
9 Vertical input
ZVM-124 MONITOR
tt rrt E: t rcttlC"LtttttEltLLLcLctttLL
POWER
CORD
o
.r'J'J'Jrrr'J'Jrrrr
SIGNAL
CABLE ~--...I
11111111111111111 11111111111111
Figure 2.8
Monitor Cable Connections
Page 2.4 Installation

This monitor uses the latest in solid-state
technology for displaying video signals. The
four TTL-level video signals (video input,
highlight input, vertical input, and horizontal
sync input) are received from the computer
via the signal cable. This section covers the
initial setup of the operating controls.
Initial Setup of the Operating
Controls and Monitor
The following discussion assumes that you will
be
connecting the monitor to a computer, such
as the IBM PC. Since the monitor requires
TTL-level signals to drive it, the normal
composite signals of the Zenith Data Systems
Z-100
and Z-100 PC computers cannot be used
as a
signal source. However, in lieu of a
computer, you may use a Quantum 801 A or a
similar signal source to make your
adjustments.
The location and function of each control is
discussed in Section 2, "Installation."
l. Connect the ZVM-124 to a suitable signal
source, such as an IBM PC.
The power switch is located on the front of
the cabinet and turns the monitor on or off.
The operating controls are located behind a
door below the CRT screen in the front of the
cabinet and on the back cover.
2. Turn on the computer (or test equipment)
to which the ZVM-124 is connected.
3. Locate the power switch on the monitor
and press it to turn it on. Verify that the
power indicator, an LED, lights.
4. Assuming you are using a computer, use
the BASIC program shown in Listing 3.1
to fill the screen with any character as
follows:
a.
Enter the program shown in Listing
3.1 into the computer. You may want
OPERATION
to save this program on a disk for
later use.
b. Run the program by typing RUN and
pressing the RETURN key. The
screen will be filled with the letter
Z or any other character inserted in
line 20.
c.
To end the program, press the CTRL
and BREAK keys at the same time.
Listing 3.1
BASIC Program Used to Fill Screen
with a Character
10 FOR 1=1 TO 2000
20
PRINT "Z"j
30 NEXT I
40
GOTO 40
'replace .the "Z" with the
'character of your choice
With the monitor connected to a computer (or
other signal source) and with a normal video
display on the CRT screen, adjust the front
panel controls. Refer to Figure 3.1 for the
location of the controls during the following
discussion.
l. Adjust the black level control, labeled
BLACK LEVEL, counter-clockwise until
the sweep raster is visible on the screen;
then back off the control until the raster
just disappears. Note that this control
interacts with the contrast control.
2. Adjust the contrast control, labeled
CONT., until the video display is
comfortable to the eye. Note that this
control interacts with the black level
control.
3. Adjust the horizontal hold control,
labeled HORIZ HOLD, as required for a
stable horizontal display on the screen.
4. Adjust the vertical hold control, labeled
VERT. HOLD, as required for a stable
vertical display on the screen. Note that
this control interacts with the vertical
size control.
Page 3.1

5. Adjust the vertical size control, labeled
VERT. SIZE, until the video display is
equal at the top and bottom of the screen.
Note that this control interacts with the
vertical hold control.
6. Locate the width control on the back of
the monitor and use it to adjust the
horizontal size of the video display.
Refer to Figure 3.2 for the location of
this control.
VERTICAL HOLD
Figure 3.1
Front Panel Controls
" /
'-'>.. /
"h='~~'
e
.D
Figure 3.2
Video Monitor Rear View
Page 3.2
Operation

This section provides you with the general
theory of operation for the ZVM-124 video
monitor. Refer to the block diagram in Figure
4.1 (on the next page) while you read the
following material.
Theory of Operation
Four TTL-level input signals - video input,
highlight input, vert sync input, and horz sync
input--are received from the computer via the
signal cable. These signals are applied to the
video, vertical sync, and horizon tal sync
circuits as shown in the block diagram. The
power supply provides +12.7 VDC for the
various circuits.
VIDEO AND HIGHLIGHT INPUTS
The positive video and highlight input signals
are direct-coupled to their respective driver
stages. These signals are amplified, inverted,
and applied to the DC contrast amplifiers 1
and 2. DC contrast amplifier 1 supplies the
contrast signal current (controlled by the front
panel contrast control) to the video output
stage (amplifier).
The video output amplifier drives the signal
to a level necessary to feed the cathode of the
CRT. The CRT raster intensity is determined
by the front panel black level control.
Adjusting this control supplies a bias voltage
to the control grid of the CRT.
VERTICAL SYNC INPUT
The positive-going vertical sync signal is
coupled to the vertical oscillator so that the
internally generated vertical frequency of the
monitor may be synchronized with the
incoming sync pulse signal. The lock-on point
of the signal is determined by the front panel
vertical hold control, which is adjusted to
stabilize the vertical sweep.
THEORY OF OPERATION
The synchronized output of the oscillator is
applied to vertical amplifier 1, which
functions as a differential amplifier. The
difference signal is fed to vertical amplifier 2,
which drives the vertical output stages 1 and
2. These stages make up a complimentary
push-pull class B power amplifier that drives
the vertical deflection yoke of the CRT. The
vertical blanking stage provides retrace "kick-
up"
during the retrace period of the sweep
output.
HORIZONT AL SYNC INPUT
The positive-going horizontal sync signal is
coupled to the horizontal sync amplifier,
which amplifies and inverts the signal to
produce a negative-going pulse. This pulse is
applied to the horizontal processor, where it
synchronizes the horizontal oscillator to the
incoming frequency. The lock-on point of the
pulse with the internal oscillator is determined
by the front panel horizontal hold control. It
is adjusted to stabilize the horizontal sweep.
The output from the horizontal processor is
coupled through a transformer to the
horizontal output driver. The flyback
transformer is used to develop the ramp
voltage for the horizontal deflection yoke of
the CRT. This circuit also is used to develop
the high voltage for the anode of the CRT.
Page 4.1

VIDEO
INPUT (TTL)
HIGHLIGHT
INPUT (TTL)
VERTlCA,L SYNC
INPUT (TTL)
HORIZONTAL SYNC
INPUT (TTL)
Page 4.2
VERTICAL
OSCILLATOR
HORIZONTAL
SYNC
AMPLIFIER
HIGHLIGHT
DRIVER
VERTICAL
AMPLIFIER 1
HORIZONTAL
PROCESSOR
DC CONTRAST
AMPLIFIER 1
DC
CONTRAST
AMPLIFIER 2
VERTICAL
AMPLIFIER 2
HORIZONTAL
OUTPUT
DRIVER
VIDEO
OUTPUT
AMPLIFIER
DEFLECTION
YOKE
FLYBACK
TRANSFORMER
B+12.7VDC
120 VAC
Figure 4.1
ZVM-124 Block Diagram
Theory of Operation

This section provides you with a detailed
circuit description of the ZVM-124 video
monitor. Refer to the schematic (a foldout at
the end of this section) while you read the
following circuit descriptions.
Power Supply
The AC line voltage enters the equipment via
a special power cable that is permanently
attached to the monitor and is designed to be
attached directly to the rear panel of an IBM
Pc.
The AC line voltage is fed to the power
transf ormer, TX20 1, and the bridge diodes
CRX701, CRX702, CRX703, and CRX704.
Approximately +17.1 volts DC output is
developed across the bridge rectifier and
capacitor CX707. Ripple at this point is
limited to 1.7 volts rms on the DC voltage. The
+17.1 VDC is fed to the regulator stages of the
power supply to produce the B+ voltage of
+12.7 VDC and provide power to light the
power-on indicator, LED CR202.
Regulator QX703 serves as a variable series
element, thereby dropping more or less voltage
from base to collector in order to maintain the
output voltage at a constant +12.7 volts DC.
As
the output voltage attempts to increase or
decrease due to input line voltage fluctuations
or load requirements, the voltage change will
start to appear at the base of error amplifier
Q701 via the voltage divider network
composed of resistors R704, R706, R707, R708,
and RX709, and capacitor C709. Control
RX709 is used to adjust the output voltage so
that it remains at +12.7 VDC even if there are
variations introduced into the circuit values
because of component tolerances or drift in
component values due to age or operating
temperature.
Capacitor C709 is used to couple the AC ripple
to the base of Q701. This voltage is compared
against a reference voltage generated by the
,CIRCUIT DESCRIPTION
zener diode CR 706 (+4.7 volts) and capacitor
C708. Any voltage difference will cause the
collector current to increase or decrease
proportionally. This current is amplified by
the error amplifier Q702 and fed to the
regulator QX703. If the output voltage starts
to increase, Q701 will attempt to turn off,
causing Q702 and QX703 to start to turn off.
If the output voltage starts to decrease, Q701
will turn on "harder" (the current will
increase), causing Q702 and QX703 to conduct
more, which will raise the output voltage back
to normal (+12.7 volts).
The regulator circuit also is designed to switch
from a regulator to an active filter at low line
voltages. The voltage at the collector of Q702
is
proportional to the available input voltage
to the regulator. When the input voltage is too
low to
maintain +12.7 volts output, the voltage
at the collector of Q702 drops below the zener
diode (CR 706) reference voltage, causing diode
CR 705 to conduct.
This additional current, sinking through
resistor R701, disables the zener diode
reference voltage, causing the DC output
voltage to drop and no longer be regulated for
DC variations. However, the AC reference
remains in control; thus the AC ripple
regulation continues, in effect providing an
active filter which suppresses the ripple from
the bridge rectifier.
Video and Highlight Driver
Circuitry
The video input signal is fed directly to the
base of Q205, the video driver amplifier. The
highlight input signal is fed directly to the
base of Q204, the highlight driver amplifier.
These are positive polarity input signals and
may be driven by TTL-level circuits.
The two input signals are amplified and
inverted by Q205 and Q204, respectively,
combined, and applied to the junction of R209
Page 5.1

and R2IO which are the emitter resistors for
DC contrast amplifiers Q202 (1) and Q203 (2).
R8IS is connected to the base of Q202 to
provide an adjustment of the bias voltage,
which controls the signal current through the
transistor.
Q203 is used as a current sink, supplying a
constant voltage reference to Q202 through
R209 and R2IO. When R8IS is rotated
clockwise, the bias of Q202 will increase,
allowing more
signal current to flow through
the transistor. Because Q203 is supplying a
constant voltage reference to the network, less
current will flow through Q203. Any video
signal
at the collector of Q203 is passed to
signal ground through capacitor C2I1. The
adjustment range of R8lS will take care of
any normal variations in the values of the
circuit components.
Q201, the video output amplifier, drives the
cathode of the CRT display. The combined
video/highlight signals are connected from the
collector of Q202 directly to the emitter of
Q201. The signal is amplified, but not
inverted, by Q201 and is applied directly to
the CRT cathode. R838, the black level
control, is used to adjust the raster intensity to
just below the point of visibility. The control
supplies the control voltage to grid 1 of the
CRT. The +24 and -40 VDC are obtained from
the flyback transformer in the horizontal
output circuit.
Horizontal Circuitry
The horizontal processor is an integrated
circuit (ICSOI) that contains a phase detector,
an oscillator, a regulator, and a predriver.
The horizontal synchronization pulse from the
computer is amplified and inverted by Q802.
It is coupled through CS02 to pin 3 of the
horizontal processor, the phase detector. The
phase detector consists of a differential
amplifier and a gated current source, the
inverted horizontal sync pulse. The current
division between the two transistors of the
differential amplifier is determined by the
phase relationship of the sync pulse and
sawtooth waveform at pin 4 of ICS01. The
sawtooth waveform is derived from the
negative horizontal flyback pulse coupled
Page
?2
from one of the outputs of transformer
TXS02.
When the sync and sawtooth signals are in
phase, the current division between the two
transistors in the differential amplifier will be
equa1. When
there is a phase difference, the
current will either flow into or out of pin S of
ICSO 1. This pin is connected by a low-pass
filter to pin 7 of ICSOI, the control point for
the oscillator. This current controls the
oscillator.
The oscillator, an RC type, is controlled by
the current applied to pin 7 of ICSOl. The
timing capacitor is charged up by an external
resistor to a trip voltage set in the integrated
circuit. When the trip voltage is reached, the
capacitor is discharged to a new trip value.
This process is repeated, producing a sawtooth
waveform.
The output (pin S of ICSOI) of the phase
detector controls the oscillator through RSIO
to
pin 7 of ICS01. The horizontal hold control
RSI9 is connected to pin 7 of ICSO l. The two
resistors, RSOS
and RS18 in the horizontal
hold circuit, center the control's range. An
internal diode in series with the hold control
is used for oscillator temperature
compensation.
The voltage set by the regulator in ICSOI is
between 8
and 9 volts. The regulator is
temperature compensated internally by two
high current diodes in series with a zener
diode via pin 6 of ICS01. The zener current is
determined by an external resistor RS28,
which is connected to +12.7 VDC.
The predriver in ICSOI is composed of a four-
transistor circuit which receives the sawtooth
waveform formed at pin 7 of ICS01. It
produces a variable duty cycle waveform at
pin 1. This output is used to drive the
horizontal driver QS01. The period of the
output waveform is determined by the bias
voltage
on pin 8 of ICSO 1. This voltage is
derived from a series of clipping resistors
which match the integrated circuit to the
video monitor.
The output of horizontal processor (ICS01) is
fed to the horizontal driver QS01. Transformer
TSOI couples the signal from the horizontal
driver to the horizontal output transistor
QXS02. Resistor RSZ2 and capacitor CSII are
used to prevent transformer ringing.
Detailed Circuit Description

Transistor QX502 functions as an electronic
switch, which is turned on for approximately
60 to 70 percent of the horizontal scan period.
Capacitors C516 and C520 provide a pseudo
B+ voltage for the horizontal deflection yoke
TX202B. When QX502 is on, current flows out
of C516jC520, through TX202B, LX505A,
LX508, LX503, and QX502 to ground. This
action produces the right side of the scan. Coil
LX503 provides left to right linearity.
Transformer LX505 sets the horizontal scan
width. Thc value of C520 determines the shape
of the parabolic waveform on the pseudo B+
voltage which controls the center to edge of
scan linearity. A network, composed of CR510
and R531, provides suppression of spurious
ringing (a cause of black vertical lines that
sometimes appear at the left side of the
raster ).
When QX502 turns off, the resonant circuit
consisting of CX514 and LX503, LX505, and
TX202B, continues to ring for one-half cycle
equal to the LC time constant of the resonant
circuit CX514 and LX505. This is the retrace
pulse that moves the scanning beam from the
right to the left side of the CRT and also
produces a high amplitude pulse for use in
developing the auxiliary power supplies. The
fly back transformer, TX502, steps up the pulse
to develop the 13 KY high voltage used by the
CR T anode. TX502 also inverts and steps
down the high voltage pulse. The pulse is
rectified by CR504 to provide the video B+
voltage and is clamped by CR509 and C525 to
provide a negative supply voltage for the
CR T. The pulse is also rectified by CR506 to
provide the B+ focus voltage for the CRT.
Diode CR505 and capacitor C522 also rectify
the pulse to provide the B+ voltage for the
black level control.
At the end of the retrace, the sine wave goes
negative, biasing the damper diode CR503 on.
Current flowing through the yoke circuit into
the damper diode provides reference point for
the left side of the raster screen. The above
horizontal output cycle is then repeated. The
+ 12 volts DC for the horizon tal scan is fed to
diodes CR507 and CR508 via a parasitic
suppressor choke, LX504. The diodes are tied
to a tap on TX502 where transformer action
increases the voltage to +24 volts and provide
the pseudo B+ voltage necessary for the
vertical deflection circuits.
Detailed Circuit Description
The posItIve pulse from QX502 saturates the
flyback transformer TX502 and charges C5l6
and C520 through the width transformer
LX505 and horizontal deflection yoke
TX202B. When
QX502 turns off, the magnetic
field of TX502 collapses, generating the
acceleration voltage need by the CRT through
an internal rectifier diode. Diode CR506
rectifies the focus voltage, charging C205 to a
static level. Diode CR502 and CX514 form a
damper network to eliminate the ringing
effect of flyback transformer TX502. The
width transformer LX505 and linearity coil
LX503 modify the sawtooth waveform
generated by TX502 to provide a linear
horizontal scan.
Vertical Sweep Circuit
The vertical sweep circuit is a DC-coupled,
ramp-genera ting oscillator circuit.
In general, the vertical sync signal is coupled
through C601, R602, R603, and C606 to the
base of the vertical oscillator, Q601. The
oscillator output is fed to the base of
differential amplifier Q602. The difference
signal is applied to vertical amplifier driver
Q603, which drives the complimentary push-
pull class B amplifier, composed of transistors
Q604 and Q606. Transistor Q607 provides
vertical blanking for the sweep circuit output.
The output from the emitters of vertical
output stages, Q604 and Q606, are fed back to
oscillator Q60I through resistor R618 and
capacitor C613. This differentiated, positive-
going flyback pulse from the Q604 and Q606
charges capacitors C606 and C608 through
Q601. The capacitors discharge through their
respective resistor networks.
Capacitor C606 discharges during the trace
interval to 0.6 volts below the emitter voltage
of Q601. At this point, Q601 conducts and
turns off the amplifier stages. This causes the
yoke voltage to fly up and repeat the cycle.
The presence of a vertical sync signal on the
base of Q60I causes it to start conducting just
before the voltage on C606 discharges to 0.6
volts
below the emitter voltage of Q601. This
synchronizes the oscillator with the vertical
sync signal.
Page 5.3
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Figure 5.1
Schematic

Capacitor C608 discharges in a linear manner
through its resistor network to the yoke
current-sensing resistor R624. Here, the ramp
voltage appears at the same amplitude as the
ramp voltage across C608. Because a constant
voltage appears across the discharge resistor
R613, it maintains a constant discharge
current from C608. Capacitor C608 provides,
to the base of Q602, a linear, negative-going
ramp voltage of an average DC value, which is
established by R608 and R609. The signal to
the emitter of the differential amplifier Q602
comes
from the yoke return circuit.
The ramp voltage across R624 receives an
inverse S-correction signal to produce a linear
display on the CRT. The correction signal is
injected through capacitor C609 and is derived
through the shaping network composed of
resistors R620 and R621. This adds to the
ramp-plus-S-correction signal appearing across
R624 to
produce a linear ramp at the emitter
of Q602. This linear ramp is compared by
Q602 to the linear ramp across C608. The
difference between the two ramps is coupled
to the succeeding amplifier stages which
restores the yoke current to the desired levels
for producing a linear display on the CRT.
Schematic
In some instances, redundant circuitry is
incorpora ted for additional circuit protection
and X-radiation safety. Special circuits are
also used to prevent shock and fire hazard.
The letter X in the schematic (a foldout from
this section), parts list (see Section 9), and
component location illustration (Figure 9.2 in
Section 9), designates special critical safety
components. These should be replaced only
with types that are identical to those in the
parts list and on the schematic.
All areas of the schematic that contain critical
circuit components are outlined using a "dot
and dash" line.
The schematic may occasionally differ from
the actual circuit used. This way,
implementation of the latest safety and
performance improvement changes into the
equipment is not delayed until new service
literature is printed.
Page 5.4
NOTE: When servicing this equipment, under
no circumstances should the original design be
modified or altered without permission of
Zenith Electronics Corporation.
Detailed Circuit Description

585-77
SERVICE MANUAL
Monochrome Video Monitors
ZVl\1-124 and ZVl\1-124-E
7'''I'H I data
systems
860-65

This section provides instructions
to
disassemble
the ZVM-124 video monitor.
WARNING: Be sure the signal cable and power
cord are unplugged from the computer or
other signal and power sources before
disassembling this monitor.
Normally, the disassembly sequence will be:
1. Remove the back cover.
2. Remove the power transformer assembly.
a.
Remove the fuse board.
3. Remove the CRT socket board.
a. Remove the cathode-ray tube.
4. Remove the main circuit board.
5. Remove the power switch assembly.
6. Remove the control panel assembly.
Note,
however, that some of the assemblies can
be removed independently of the others; see
each procedure.
D
Figure 6.1
Back Cover Removal
DISASSEMBL Y
Back Cover Removal
1. Refer to Figure 6.1 and position the
monitor as shown. Remove the three 8-18 x
.625" screws
at A from the back top and
rear of the cabinet.
2. Remove the two 8-18 x .625" screws at B
from the rear bottom of the cabinet.
3. Remove the 6-20 x .500" screw at C that
secures the width control panel to the back
cover.
4. Turn and slide the width control board and
panel through the cutout in the back cover
into the monitor.
5. Carefully remove the back cover from the
cabinet.
Power Transformer Assembly
Removal
1. Remove the back cover.
2. Refer to Figure 6.2 and remove the two 8-
18 x .625" hex head screws at Q that secure
the power transformer assembly to the
bottom of the cabinet; then remove the
assembly from the cabinet.
NOTE: If the power transformer is to be
replaced, then proceed to step 3.
Figure 6.2
Transformer Removal
Page 6.1

3. Refer to Figure 6.3 and remove the two 6-
20 x .312" screws
at R that secur.e the cover
to the transformer; then remove the cover.
Figure 6.3
Transformer Disassembly
4. Refer to Figure 6.4 and unsolder
transformer leads A, B, C, and D from fuse
board.
NOTE: If you are servicing a 220-volt monitor,
unsolder leads A and J, instead of those
indicated previously in this step.
Figure 6.4
Fuse Board
5. Refer to Figure 6.5 and unsolder
transformer leads E and F from the main
board.
6. Refer to Figure 6.3 and remove the two 8-
18 x 1.25" transformer screws at S; remove
the transformer.
Page 6.2
Figure 6.5
Main Board
Fuse Board Removal
1. Remove the back cover and power
transformer assembly.
2. Refer to Figure 6.6 and remove the 8-18 x
.312"
hex head screw at H that secures the
board to the bracket; then remove the fuse
board.
3. If the board is being replaced, prepare a
chart, showing all connections, similar to
Figure 2.3 and Figure 2.7 in Section 2.
4. Once the chart is prepared, unsolder the
various connections and remove the board.
CRT Socket Board Removal
CAUTION: The CRT socket board plugs onto
the pins on the neck of the cathode-ray tube.
Use extreme care when you remove or replace
this board to a void damaging the pins on the
cathode-ray tube.
1. Remove the back cover.
2. Refer to Figure 6.7 and carefully unplug
the CRT socket board from the pins on the
neck of the cathode-ray tube.
Disassembly