Morrow MDT 50 User manual

MDT50
VIDEO DISPLAY TERMINAL
SERVICE
MANUAL

Copyright (C) 1983 by Morrow Inc.
All rights reserved.
No part of this publication may be reproduced, transmitted,
transcribed, stored in a retrieval system, or translated into any
language or computer language, in any form or by any means,
electronic, mechanical, magnetic, optical, chemical, manual or
otherwise, without prior written permission of Morrow Inc.
DISCLAIMER
No representations or warranties, express or implied, are made
with respect to the contents hereof, including, but not limited
to, the implied warranty of merchantability or fitness for a
particular purpose. Further, Morrow Inc., reserves the right to
revise this publication and to make changes from time to time in
the content hereof without obligation to notify any person of
such revision.
Morrow
600 McCormick
St.
San Leandro, CA 94577

TABLE OF CONTENTS
SECTION
o -- INTRODUCTION
Scope of this Manual
WARNING
Components
Block Diagram
Interconnections
Theory of Operation
Terminal Operation
Data Paths
Quick Check-out
1 -- THE VIDEO BOARD
Theory of Operation
Video Signal
Vertical Synch
Horizontal Synch
Miscellaneous Controls
Troubleshooting
Components
Test Points, Typical Waveforms
& Signal Levels
2 -- THE MAIN LOGIC BOARD
Theory of Operation
Test Points, Typical Waveforms
& Signal Levels
3 -- THE KEYBOARD
Theory of Operation
Test Points, Typical Waveforms
& Signal Levels
4 -- THE POWER SUPPLY
Theory of Operation
Troubleshooting
Components
Test Points, Typical Waveforms
& Signal Levels
PAGri
INT-l
INT-l
INT-l
INT-3
INT-4
INT-5
INT-5
INT-6
INT-7
VI})-l
VID-l
VID-l
VID-l
VID-;l
VID-;l
VID-2
VID-6
MLB-l
MLB-l
KYB-l
KYB-l
PWR-l
PWR-l
PWR-l
PWR-3

5 -- MAINTENANCE
The Monitor Unit
General
Replacing the CRT tube
The Keyboard Unit
MTC-l
MTC-l
MT<.;-l
MTC-2

Introduction
Scope of this Manual
IRnODUCTIOB
Page INT-l
This Manual presents details of the circuitry and data paths of the
Morrow MDT50 Video Display Terminal) along with information on typical wave-
forms and signal levels, to allow service personnel to troubleshoot and repair
the terminal, as needed. Familiarity with the Morrow MDT50· User's Manual is
assumed.
While every effort has been made to assure that the information contained
herein is accurate and up to date) Morrow Designs, Inc. reserves the right to
make engineering changes and appropriate parts substitutions without prior
notice in the interests of increased and improved performance.
> > > > WARRING < < < <
CRITICAL COMPORDT WARBlRG:
SERVICEMAR WARRING: This product contains components
which
are critical for X-Radiation Safety. See Service
Manual for proper replacement. Normal 2nd Anode Voltage
is 12 KV at Zero beam current, AC 120V input, and must
lOT exceed 13 KV under any operating conditions. To
measure 2nd Anode Voltage, use High Impedance meter.
Connect (-) to chassis, use aHigh Voltage lead from (+)
to 2nd Anode.
Components
of the Morrow MDTSO Terminal
External
From an outside viewpoint) the Morrow MDTSOconsists of two units: The
Monitor Unit, containing the CRT) power supply) and control circuits; and the
Keyboard Unit, containing the physical keyboard) its decoding circuitry, and
circuits for communicating with the Monitor Unit.
The Keyboard and Monitor Units are connected by a coiled telephone
handset cord with 7,62 mm (0.3 in) male plugs at each end. Power is supplied
via a 3-wire grounded cord, terminated by a U.S. NEMA standard plug (which may
be replaced to suit local power system requirements).
Communications with aHost computer or modem and a printer are provided
via asynchronous RS-232 communication via two female DB-25 receptacles on the
rear of the Monitor Unit. Controls are provided on the Monitor Unit for Power
ON/OFF, Contrast, and setting operating parameters.

Introduction
Internal
Internally, the Morrow MDT50 consists of:
Page INT-2
~ A Transformer and Voltage Selector Switch for converting 115V or 230V
AC mains power to 10.5V AC, 16.7V AC, and 21.3V AC.
~ APower Supply board, with rectifiers and voltage regulators, to pro-
duce +5V, +15V, and -12V DC [regulated to ±IO%].
~ AMain Logic Board, with a68A02 Microprocessor, 16k of EPROM-resident
firmware, 2k of character storage RAM, 4k of attribute and control storage
RAM, a68A45 CRT Controller, a68A21 PIA, one 6850 and two 6851 ACIAs for
Keyboard, Host and Printer communications, respectively, as well as miscel-
laneous control and sequencing logic.
~ AVideo Control board, to convert Horizontal and Vertical synch pulses
to the proper waveforms for controlling CRT raster scan, and circuitry to
control pixel display. Connection is made via cables to a 12" diagonal
CE745129 VRA tube (or equivalent) for actual display.
~ A small Contrast control and connector board, having a rotating poten-
tiometer for contrast control, and a female· RJ-ll receptacle for the Keyboard
connector cord.
~ A Keyboard, with a 93-key switch-matrix keyboard, decoded by an 8035
stand-alone microcomputer, which generates RS-232 signals by toggling one
output line under the control of on-chip software, and receives RS-232 data
via software use of interrupts.

t-f
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tot
o
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....
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POW E RTRANSISTOR
WA007 WA006
VIDEO
8104
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WA003
~ ~ i( SP
8309 g
o
KEY BOARD ~
~ WA009
CONTRAST
-
",,-
~
~8S08
8509
8510~
BVR 501
LOGIC BOARD
8201
c:=::::2 ~ I.· •• ·1
at! 8202 B203~
~~m B~06 ~§~~~§m 8208 ~ :=1--= -=_=-=_= -=_~ ---l
8405 I~ 8404 WA004 8204 : I========~
l34d 7 Vv' A008 L--. ------.J
POWER SUPPLY
WADDS
TRANSFORMER
POWER
UNIT
THIS SCHEMATIC OR SPECIFICATION IS SUBJECT TO CHANGE
WITHOUT PRIOR WRITTEN NOTICE.

1-4
ts
"
CPU DATA BUS
t1
0
ADDRESS IUS
8-
CONT"Oltuf
n
"
....
t0
~
DELAY
VIDEO
&
to--
CKT.
ATTRIBUlE
~
fREFREIH
ATTRIBUTE ADDRESS
RAM
Ll
...............
ADS •
r---
...---
MAIN
~
2 -+1
~
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ATTRIBUTE BIDIRECTION
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PRGM.
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RAM
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CPU
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SELECT
ROW ADDRESS
VIDEO CONTROL
DELAY
CKT.
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SERIAL
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CRT
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TIMING
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AC IA2
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CURRENT lOOP
COMMUNICATION
r--CONTROL
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PIA
SWITCH
T
111
THIS SCHEMATIC OR SPECIFICATION IS SUBJECT TO CHANGE
WITHOUT PRIOR WRITTEN NOTICE.

Introduction
Theory of Operation
Terminal Operation
Page INT-S
The Morrow MDT50 CRT Terminal is designed to be an inexpensive and
convenient data-entry and console device for a wide range of computer and data
applications.
The Morrow MDT50 will normally be connected to a Host computer system,
either directly or via MODEM, by asynchronous RS-232 or 20ma current loop,
through the Host connector on the rear of the Monitor Unit. The terminal's
on-board firmware allows it to communicate with the Host in Full or Half
Duplex, or strictly Local mode (no outside communication). Communication
rates range from 110 to 19,200 baud.
A subsidiary Printer port, also located on the rear of the Monitor Unit,
allows the Morrow MDT50 to be connected to any ASCII printer with an RS-232
serial interface. Baud rates from 100 to 19,200 are available. The terminal
provides two basic modes for printer operation:
Simultaneous Mode -- all characters transmitted from the Host are
displayed on the monitor, and also sent to the Printer
Buffer Mode -- characters from the Host are buffered internally in
the Morrow MDT50) and sent to the Printer without being displayed on
the monitor
The Morrow MDT50"s 16k of on-board firmware provides 93 basic commands, as
well as numerous options, for setting operating parameters. These commands
can be sent by the operator from the Keyboard, or by the Host through the Host
port. The commands and their effects are documented in the Morrow MDT50 User's
Manual.

Introduction
Data Paths
Page INT-6
The Morrow
MDT50"'s Main Logic Board receives serial Asynchronous RS-232C
character data from either the Host Port or the Keyboard. This character data
is stored in a 2k-byte Display RAM. Character data from the display RAM is
sent continuously to the Video display circuitry, where it is processed by a
Character generating ROM, combined with attribute data from a separate 2k-byte
Attribute RAM, and sent to the CRT for display.
If the MorrowMDT50 is in Full Duplex Mode, all character data received
from the keyboard is immediately transmitted to the Host hrough the Host Port,
but is not displayed on the CRT.
If it is in Half Duplex Mode, character data received from the keyboard
is displayed on the CRT in addition to being transmitted to the Host.
In either Mode, XON/XOFF protocols are used in communicating with the
Host.
If it is in Simultaneous (Transparent) Print Mode, character data
received from either the Host or the Keyboard is displayed on the CRT and
transmitted to the Printer Port.
If it is in Buffered Print Mode, character data received from the Host is
transmitted directly to the Printer Port (with XON/XOFF handshaking) without
being displayed on the CRT.

Introduction
Quick Check-out
Page INT-7
To check out the operation of the Morrow MDTSO quickly, and isolate
problems:
1) Check for 11SV AC or 230V AC on the primary (input) leads of the
Power Transformer.
a) If found, go to Step 3.
b) Check the fuse, power switch, and power select switch.
2) Check the secondary (output) leads for 10.6V AC on blue, 2l.3V AC on
white, and 16.7V AC on brown.
a) If OK, go to Step 2.
b) Replace power transformer.
3)
B204) .
Disconnect cable from Power Supply to Main Logic Board (B404 <-->
Check B404 for:
Pin 1
Pin 2
Pin 3
Pins 4&S
-12V
DC ±.O.SV
+1SV DC ±.O. 5V
+SV DC ±.O.2SV
Ground
step 5.
a) If OK, replace cable to Main Logic Board (B404 <--> B204). Go to
4) Disconnect cable to power transistors (B406, 407). Check connectors
for:
B406 Pin
1
IS.IV DC
Pin 229.8V
DC
Pin
329.9V
DC
B407
Pin
1
5.IV DC
Pin 2
I4.3V DC
Pin
3
I4.4V DC
a) If OK, check U40I through U403 and capacitor.
b) Check Bridge Rectifiers BR40I, BR402, BR403, and transistors
Q401 and Q403.
5) Check voltage on B404 again.
a) If OK, go to Step 7.
6) Check Power Transistor voltages:
Q402 Base
Collector -
Emitter
IO.6V DC
5.1 V DC
Il.4V DC

Introduction
Q404 Base
Collector -
Emitter
24.6V DC
1S.IV DC
2S.2V DC
Page INT-8
7)
B104).
a) If OK, check U401 through U403 and capacitor.
b) Replace transistor(s).
Disconnect cable from Main Logic Board to Video Board (B203 <-->
Check signal levels and typical waveforms on Connector B203.
a) If OK, reconnect cable (B203 <--> B104) and run Monitor Check
Subroutine (hit ESC Mon keyboard) for software problems. Check/replace ROMs,
if necessary.
b) Go to Step 8.
8) Disconnect cable to Contrast Board (B208 <--> B508). Check signal
levels and typical waveforms on Connector B208.
a) If OK, Reconnect cable to Contrast Board (B208 <--> B508), go to
Step 9.
b) Check all test points on Main Logic Board for proper signal
levels and typical waveforms, then isolate and replace defective parts.
9)
B309).
Disconnect cable connecting Keyboard to Contrast Board (B509 <-->
Check Contrast Board for defective parts and broken traces.
a) If OK, reconnect cable to Keyboard connector (B509 <--> B309)
and go
to Step 10.
b) Replace Contrast Board.
10) Check signal levels and typical waveforms at B309 on Keyboard PC
board.
a) If OK, check all test points on Keyboard for signal levels and
typical waveforms. Isolate and replace defective components.
b) Replace coiled telephone handset cord.
11) END.

The Video Board
Theory of Operation
Video Signal
The Video Board
Page VID-l
The Video Signal is essentially an ON/OFF signal, which determines
whether a particular pixel position on the face of the CRT will be illuminated
or not by the scanning electron beam. It is generated by the video circuitry
on the Main Logic Board, and received via Pin 3 of Connector BI03 on the Video
Board. It is amplified and pulse-formed by Transistors TRlOl and TRI02, and
fed to the gate of the CRT tube through Pin 1 of Connector BIOI.
Vertical Synch
The Vertical Synchronization signal is generated by the CRT Controller
chip on the Main Logic Board, and reaches the Video Board via Pin 4 of
Connector BI03. It is used by IClOI (muPC 1031 H2) to reset the beam of the
CRT to the top of the screen in preparation for a scan. After pulse-shaping
by ICIOl, it is used to control the Vertical Deflection Yoke (YlOl) to
position the beam correctly.
Horizontal Synch
The Horizontal Synchronization Signal is also generated by the CRT
Controller chip on the Main Logic Board, and reaches the Video Board via Pin 6
of Connector BI03. It controls the rate at which the beam scans the face of
the CRT from side to side. After being amplified and pulse-shaped by TRI03
and TRI04, it is fed to the Horizontal Yoke and the Fly-Back Transformer
(Tl02) to produce scan lines on the face of the CRT tube.
Miscellaneous Controls
Variable Resistors (Potentiometers) are provided to control:
Vertical Hold
Vertical Size
Vertical Linearity
Focus
Brightness

The Video Board
Troubleshooting
Components
Page VID-2
I) Make sure that the power is OFF. Look at the Video Board, and check
for obvious physical problems:
Broken Wires
Loose connectors
Broken connector parts
Dirt or dust
Overheated or burned parts
Correct problems and recheck before going on.
II) Remove the Video Board from the Morrow MDT50 and give it a thorough
look, both front and back.
A) Are you sure the power is off? Turn it OFF.
B) CAREFULLY ground the Anode lead from the CRT tube. A heavy
Jumper from the Anode lead connection at the Flyback Transformer to chassis
ground is the best bet -- the Anode may be at any voltage up to 12kV!
C) Disconnect the Anode lead.
D) Disconnect BI04 (connector to Main Logic Board)
E) Disconnect BIOI (connector to CRT tube)
F) Disconnect BI02 (connector to small PCB on CRT tube)
G) Disconnect BI03 (CRT tube grounding strap)
H) Remove the screws which hold it in the Monitor·Unit.
I) Lift out the board, carefully.
J) Check all of the components, for burns, overheating, leaks, etc.
K) Check for broken or missing components.
L) Check for cracked or broken traces, solder bridges or cold
solder joints.
M) Carefully re-install Video Board, remembering to put the holding
screws back in, and tighten them down.
N) Re-connect BIOI, BI02, BI03, BI04, and the" CRT Anode lead.
0) Turn ON power, wait for the CRT tube to warm up.
p) Enter a random assortment of letters and numbers via the
keyboard. (If nothing happens, check Keyboard and Main Logic Board)

The Video Board
Q) Adjust:
Contrast,
Brightness,
Focus,
Horizontal Hold,
Vertical Hold,
Vertical Linearity, and
Vertical Size
III) Malfunctions
A) Ro Video Signal
Page VID-3
1) Check that the Contrast control knob at the front of the
Monitor Unit is turned up (clockwise).
2) Check for +ISV on Pin 1, Connector BI04, and at base of
TRIOI. If not found, check Main Logic Board and Power Supply.
3) Check for Video Signal at TPlOl (base of TRI02). If not
found, check Pin 3 of Connector BI04.
a) If signal found, check VRlOI (contrast control pot on
small PCB at front of Monitor Unit) and Rl02. Replace as necessary.
b) If not found, check Main Logic Board.
4) Check for Video signal at TPl02. If found, go to Step 9.
S) Check collector of TRl02 for 6.4V, base for 0.4V, and
emitter for 0.3V.
a) If signal found, go to Step S.
b) If not found, isolate collector to check for pin being
pulled down.
c) Check RI03, RI04, Cl03. Replace as necessary.
d) Check/replace TRI02.
6) Check TPI02 (collector of TRlOl) for 6.SV, base for 6.9V,
and emitter for 6.4V.
a) If signal found, go to Step 6.
b) Check RIOl, ClOl, DlOI, CI02. Replace as necessary.
c) Isolate collector to check for pull-down.
signal, check/replace TRIOI.
If no
d) Check LIOl, RIOS, Cl04, Dl02, and Brightness circuitry
connected to Pins 3, 4, &S of Flyback Transformer. Replace as necessary.

The Video Board Page VID-4
7) Check
Pin 1 of Connector BIOI.
a) If signal found, go to step 6).
b) Check RI06. Replace as necessary.
8) Check RI07 and Cl18 for short, open or solder bridges.
Replace/repair as necessary.
9) Check circuitry connected to Pin 2 of Flyback Transformer
-- Dl03, C129, Rl18, Rl19, R122, C132, C133, VRl06. Replace/repair as
necessary.
10) Check CRT tube for open circuit in Cathode. Replace as
necessary.
B) Bo Vertical Deflection
1) Is the power ON? Turn it on.
2) Check connection between RIll and RlOl for +lSV relative to
ground. If not found, check Pin 1, Connector Bl04 for +lSV. If not found,
check Main Logic Board and Power Supply Board.
3) Are all of the connectors from the Video Bqard to the Main
Logic board and the CRT tube actually connected? Wiggle them to make sure.
4)
Main Logic Board.
Check Test Point TPI03 for Vertical Synch signal from the
If no signal, check Main Logic Board.
a) If signal found, go to Step S.
b) Check ClOS, Cl06, and Rl08. Replace as necessary.
S) Check TPl04 for Vertical deflection signal from IClOl. If
no signal, test or replace IelOl.
6) Check Vertical Yoke (YlOl) for broken wires. If found to
be open circuit, replace Yoke.
7) Check cathode of CRT tube for open circuit.
8) If all of the above are OK, check diodes, resistors, poten-
tiometers, and capacitors in the vertical timing circuit for shorts, broken
wires, or internal open circuits.
C) Bo Horizontal Deflection
1) Check connection between RlI6 and Ll02 for +lSV relative to
ground. If not found, check Pin 1, Connector BI04 for +I5V. If not found,
check Main Logic Board and Power Supply Board.
2) Check TPlOS for Horizontal Synch signal from Main Logic

The Video Board Page VID-5
Board. If no signal, check Main Logic Board. If signal does not agree with
illustration of typical signal, check Rl14, and check TRl03 for 0.35V on base,
14V on collector, and Ground on emitter.
3) Check TP106 (collector of TRl03)
a) If signal found, go to Step 3).
b) Check collector of TR103.
c) If signal found, isolate collector of TR103 to see if
signal is being pulled down by Rl16, Rl17, C12l, or C122. Also check T101.
Replace as necessary.
d) Test/replace TRl03
4) Check TP107 (collector of TR104)
a) If signal found, go to Step 5).
b) Check base of TRl04 for -0.1 V, collector for 18V, and
emitter for ground. Also check T101. Replace as necessary.
c) If signal found, isolate collector of TRl04 to see if
signal is being pulled down. Replace components as necessary.
d) Test/replace TR104.
5) Check
for open circuit between Horizontal Yoke and ground.
Check Horizontal Yoke, L103, L104 (adjustable), C127 and C126. Replace as
necessary.
6) If all of above are OK, check all components in Horizontal
Deflection circuitry, including connections to the Flyback Transformer, for
shorts, opens, or corroded wires.
7) Check CRT Tube, replace as necessary.

--
1
TP1021
~ I
I
-.
--
TR101
2 S(2371 L 6.5
6.9
--
R101
R116
--
>-~33- - 1W
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Fig - 1
Fig - 2
o0
5V 100s
VIDEO DIP
TP1"be
0.5V 2ms
VIDEO IIP
v
TP'''LL
1Y 2ms .3I
V. SYN. --F~~_1
TP;04~_ J
2V 5ms
DIP Fig-4_ R114
Y. ~~ - - - - - - TP105 470
TP105
tuu :
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H. SYN. _
TP~r-c- I
50V 10AJS I
Fig -6 COLLECTOR
SPECIFICATION
THIS SCHEM~b~C W~~TTEN NOTICE.
WITHOUT PR

The Main Logic Board
Theory of Operation
The Main Logic Board
Page MLB-1
Serial character data from the Host reaches the Main Logic board via Pin
3 of Connector B201, and is directed to Pin 12 (RXD) of U228 (6551 ACIA).
Serial data from the keyboard reaches the Main Logic Board via Pin 4 of
Connector B208, and is directed to Pin 2 (RxD) of U218 (68A50 ACIA).
The ACIAs convert the serial data to 8-bit parallel data on their output
pins (DO - D7) which are connected to Lines DO - D7 of the CPU Data Bus.
The
68A02 Microprocessor accepts the data and re-transmits it on the Data
Bus to the display memory, U212 (2k x 8 RAM). From the memory, it is later
transmitted via U223 (Octal Transciever) to U224 (Octal Latch). It is then
ready for transmission to U225 (2732 Character Generator EPROM).
If the Freedom 100 is in Full Duplex Mode, the character data is also
sent to U228 (ACIA) for transmission to the Host via Pin 2 of Connector B201.
In Half Duplex, Block and Local Modes, data is not transmitted to the Host.
U209 (68A45 CRT Controller) controls Display refresh, as it withdraws
character data from the Character Generator EPROM (U225). It also takes care
of painting the cursor on the CRT screen.
Output from the Character Generator goes to U236 (74LS166 Shift
Register), where it is converted into a serial bit stream. This serial bit
stream represents the sequence of pixels which must be turned ON to display
one line on the CRT screen. Morrow MDT50 characters are displayed in a 7 x 9
dot block within a 9 x 12 dot matrix.
The serial data from U233 is combined at U229 (74LS74 Dual D Flip-FLop)
with Attribute data from U211 (2k x 8 DRAM) gated through U221 &U222 (Octal
Latches) to produce the Video Data output signal, which is transmitted on Pin
3 of Connector B203 to the Video Board.
Test Points, Typical Waveform.s, !. Signal Levels
See the attached drawings and schematics

TP201B203 Pin 1TP202
B204 Pin 1TP203
B204 Pin 3
+15V Power -12V Power
+5V Po\~er Input
15V + O.5V -12V + O.5V
5V +0.2V
ISV-
ov--------------~-~----------------
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OV----------------l----------------
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-IZV I
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I
5V- i
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OV ----------------t----------------
I
TP204 B212
TP205
U267 Pin 8
T206 U246 Pin 2
Monitor
Dot Frequency CPU Clock
12V +0.5V 4.1V +0.5V 3.8V +0.5V
nNE': ()'05ps/ Df Y
~ 0.265 jJ.S ---+l
I
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OV ----------------i----------------
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TP209 U267 Pin 10
ACIA Clock (RS232 Print Baud Rate)
3.8V +0.5V
TlM£: o.Z p.s JDI V
3.8V-
TIME;: O.Z,u.s/DIV
TI/1£: o.l,us/DIV
TP207 U268 Pin 6TP208
U246
Pin 7
eRTC
Clock Character
Clock
3.8V +0.5V 3.9V +0.5V
--
(J 1IQ ps --+l
O.09p.s
~
----tI I.A P&~
I+- O. '19)J.s
3iV -
3.9V-
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Logic Board Test Points