commgrp@silver.ucs.indiana.edu (BACS Data Communications Group) (04/10/91)
to sci.electronics and comp.sys.ibm.ps2.hardware
[Originally posted on sci.electronics, August 1990.]
IBM MONITOR REPAIR NOTES: Models 8512, 8513, 5154, 5175 and 3192
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14 August 1990. rev: Aug/Nov/Dec 1990, Jan/Apr 1991.
8512, 8513
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IBM's "PS/2" VGA color monitors, models 8512 and 8513, have had
frequent maintenance problems here and reportedly elsewhere. There
are at least two different models of each monitor. They are
manufactured in Korea, probably by Samsung or Tatung.
Schematic diagrams are apparently unavailable. The following is a
summary of many frustrating hours at the repair bench:
Opening the cases:
The top of the 8512 or 8513 case is fastened either by two internal
plastic clips (see below), or "tamperproof" Torx T-20 screws with
center pins. The Torx driver must have a long, slender shaft
(preferably magnetic) to reach the screw heads.
S/N 23-xxxxx: Plastic clips hold top front of case. The internal
catches can be released by inserting a pair of special tools made of
sheet-metal, shaped approximately like:
_____ /|
______________________________| | |
\_____________________________ | |
|_____|/
The angle at the end is more acute (25 degrees) than shown in
this ASCII diagram.
Alternatively, insert long, thin screwdriver into rectangular holes
and push clips downward. Use Apple Macintosh case-separation tool to
help remove back cover. It is best to release both clips
simultaneously.
NOTE: Cut the silicone glue at the CRT base and unplug the tube before
working on the main board! The glass neck is easily broken. A
monitor with broken CRT is not economically repairable. The voice of
experience speaks. :-)
Symptom: No power or intermittent power.
Some models (including those whose serial numbers begin with 23) have
the AC power connector soldered directly to the main circuit board.
The wave-soldered connections are too thin for heavy components or
those subject to flexure: They crack, opening the circuit. Resolder
them, using lots of solder. Also use a magnifying lens to inspect the
solder joints of the flyback transformer, the deflection-yoke
connector and (rarely) the CRT socket. These often fail in the same
manner as the power connector. I always resolder all the above-
mentioned connections on general principle.
Symptom: High voltage present but screen dark.
Inspect solder joints on CRT socket, especially if serial number
begins with 23-. (See above.)
Symptom: Horizontal line across screen, remainder dark.
Failed component: Vertical amplifier (large IC attached to heat sink;
TDA-1670A manufactured by SGS, whose quality control is infamous).
People at IBM have told us that this chip is a weak point. Use heat-
sink compound when installing new IC. The replacement IC may last
indefinitely, or may fail in a few days. Sylvania's ECG 1862 is a
replacement for TDA1670A.
When the IC fails, a 2.2 ohm 1/4-watt resistor in series with its
power supply lead (diode and capacitor leading to a flyback pin) burns
beyond recognition. This resistor is used as a fuse. The value is
not critical (may be up to 20 ohms) but be sure the replacement
resistor is a metal-film type which burns quiescently, instead of a
carbon resistor which will start a fire. Leave the resistor leads
long, mounting the resistor one inch [2cm] mabove the circuit board.
Before replacing IC, inspect for cracked solder connections on the
male deflection-yoke connector (mounted on the main circuit board).
Symptoms:
1) Immediate power supply shutdown.
2) Power supply shutdown/restart about once per second.
3) Squeal from power supply, wavy left and right edges of
picture; may become normal after warmup.
Bad component: C120 (10 uf 35v). This capacitor is common to most
models of 8512/8513, and has the same identifying number on their
circuit boards. It is located near the front of the power supply
area, in the corner with the green LED pilot light, sometimes near a
large green resistor. Replace this capacitor on general principle
anytime you open an 8512 or 8513. This capacitor fails from heat;
dissection of bad capacitors reveals no moisture within. The
replacement should have a temperature rating of 105 deg. C. (typified
by end plug made of epoxy instead of rubber). Heat-induced capacitor
failure is a common problem in switching power-supplies. If high-
temp. capacitors are unavailable, leave the leads long so that the new
capacitor can be placed further from the abovementioned resistor which
gets hot enough to discolor the circuit board and adjacent plastic
standoff.
If capacitor is good, check diode CR105. Rarely, we have found that
thermal problems with this diode cause symptoms as above. Replace
with a fast-recovery diode such as 1N4937; leave leads long for
cooling. (We use 1N4937 as a generic replacement for high-frequency
rectifiers in monitors, both in switching power supplies and flyback-
derived voltage sources. Common rectifier diodes such as 1N4004 will
overheat when used at the frequencies of switching power supplies or
of flyback transformers.
Symptom: Blown fuse
Bad component: De-gaussing thermistor (rectangular black box with 3
leads, near de-gauss coil connector).
If new thermistor is unavailable: Remove the shorted
device, use a handheld de-gaussing coil after the monitor has been
returned to its operating position.
Replacement thermistors are not available from IBM. A suitable
replacement for the thermistor in IBM 8512, 8513 and 3192 is the one
used in NEC 1401 monitors. It is NEC part number 38112026, "positive
thermistor degaussing control," $2.95 from:
NEC Home Electronics
1255 Michael Dr.
Wood Dale IL 60191-1094.
The NEC device is slightly larger and perhaps more robust than the
original. The middle leg must be bent to fit the hole in the printed-
circuit board.
(Rare): Blown fuse may result from shorted diodes which rectify 115
VAC power, or from shorted power-supply switching transistor. The
transistor base-emitter junction is transformer-coupled, so normally
looks like a DC short when you attempt to test the base-emitter
junction in-circuit with an ohmmeter.
Symptom: Bad power supply (in model where power supply is a
separate unit in gold-colored metal cage), verifiable by swapping
for known-good power supply:
Replace two small electrolytic capacitors located under a rectangular
ceramic power resistor which stands about 1" above the circuit board.
8512 s/n 72-xxxxxxx (7 digits after 72-) This model has extensive
metal shielding inside. The power supply is a separate unit in gray
metal housing. Power cord permanently attached, cover has Torx
screws:
Symptom: Squealing sound from power supply; supply operates
normally when 2-wire (red/white) power connector is removed from
CRT board:
Failure: One or more shorted video driver transistors: 2SC3502,
2SA1370. (No ECG equivalents are listed.)
IBM has agreed to replace some 8512 and 8513 monitors under certain
conditions: See IBM documents 8512 ECA026 and 8513 ECA017.
8512's: Serial number with "72" prefix, s/n between 72-0120000 AND
72-0405000: Display is out of focus and/or excessively bright.
8513's: Serial number with "72" prefix, s/n below 72-0640000:
Display out of focus.
Those series appear no less reliable than others. Some replacement
units are new, others are used/reworked and have new serial numbers
attached. These appear to have new flyback transformers. Some have
subsequently failed for other reasons.
Many of our 8512 and 8513 monitors (including picture tubes) have
failed from long service-- Since they have integral power switches,
and the screens are black when the computer is turned off, users tend
to leave them on forever. Life will be significantly extended by
turning them off overnight and on weekends.
Symptom: No picture, no high voltage; pilot lamp on or blinking:
Replace flyback transformer. We have had many flyback failures, and
have located a good source of replacements:
Component Resource Corp.
15316 East Valley Blvd.
City of Industry, Ca. 91746
1-800-366-1272
This company also has replacement flybacks for DEC VT-240 color
monitors (actually manufactured by Amdek).
Before replacing flyback: Look for broken solder connection at flyback
pins and deflection yoke connector.
The horizontal-output transistor appears robust; we have never
replaced one, even after flyback transformer failure.
Symptom: One color absent or displayed improperly.
The large circuit board attached to the CRT socket contains the
cathode drivers for each color. The large output transistors and
their driver transistors often fail. Apply a test pattern (e.g.,
white-on-black text) and use an oscilloscope to compare the three
color channels, to locate the failed component(s).
Before we discovered the most common problems of 8512 and 8513 and
located a source of replacement flybacks, we had several repaired
(with good results) by:
Circuit Test, Inc.
12479 W. Hillsborough Ave.
Tampa, Florida 33635
Problem: Bad video cable. Wires often break inside the connector.
Correspondents report that new cables are available from IBM for about
$50, but that delivery time may be extremely long. Also, there are
several variations of the inner end of the cable.
New connectors are available from electronics parts distributors, but
installing them is very difficult because the inner conductors of the
coaxial cables are small and brittle. We have spliced a few cables
recovered from junked monitors but that is a laborious process.
Connector pinout of IBM color VGA connector (model 8513):
1 Red video 9 Reserved
2 Green video 10 Ground
3 Blue video 11 Monitor sense (ground)
4 Reserved (absent) 12 Monitor sense (open)
5 Self test 13 Horizontal synch
6 Red video return (coax shield) 14 Vertical synch
7 Green " " " " 15 Reserved (absent)
8 Blue " " " "
The "self test" line causes a white raster on the screen when the
cable is disconnected. The video signals are terminated at 75 ohms.
IBM 5154 AND 5175 MONITORS
--------------------------
EGA (5154) and PGA (5175) monitors are nearly identical except for the
video electronics contained in a metal box on the left side (when
facing the screen). Schematics ARE available from IBM, in _Technical
Reference, Options and Adapters, Volume 3_ of the "Personal Computer
Hardware Reference Library." Introduced in the mid 1980's, many 5154
and 5175 monitors are nearing the end of their service lifetimes,
i.e., tubes have dim picture and poor focus, and numerous capacitors'
values have decreased. In severe cases, we advise users to scrap the
monitors and convert to VGA.
The most common failure is in the power supply: Symptoms include
chirping noise, repetitive startup/shutdown (blinking LED pilot lamp)
or jagged edges on left and right of picture.
Replace three small electrolytic capacitors in the power supply:
C11 (47uF 40v), C13 (10uF 35v) and C14 (1uF 35v). These capacitors
fail from heat after long service.
The power-supply unit is removable. You must drill-out three pop-
rivets to open the metal box; replace them with sheet-metal screws.
IBM 3192 CRT Terminal
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My employer, an enormous state university, has found it very
economical to discontinue maintenance contracts on about eight hundred
IBM 3192 terminals and repair them in-house. 3192 repairs are
generally trivial, however, the assumed importance of the
administrative functions for which they are used makes it extremely
expedient to keep an adequate supply of spares available for instant
replacement.
The 3192's color monitor has insides somewhat reminiscent of the 8512
and 8513 VGA monitors (see above).
Case Removal:
The back cover of the monitor is held by two screws on the bottom and
two plastic spring-clips on the upper left and right sides (visible
with flashlight through rear ventilation slots). Use two L-shaped
tools (e.g., back-plates from IBM PC), inserted through upper
ventilation slots on the sides, to depress both clips simultaneously.
Then lift the cover off.
Symptom: No power, blown fuse.
The de-gaussing thermistor has probably shorted. This is the most
common failure we have encoutnered in 3192's. The thermistor is
identical to those used in 8512 and 8513; see above. Before replacing
fuse, use analog ohmmeter, or diode-test function on DVM, to check the
junctions of the two large power transistors in the power supply
section. Alternatively, measure resistance across the line-voltage
filter capacitor (C107) with power OFF and capacitor discharged. The
resistance should be greater than 500 ohms.
An intermittent "flutter" of the display can indicate incipient
failure of the de-gaussing thermistor; it is supposed to conduct
current through the de-gaussing coil for a few seconds during initial
power-on. Prior to failure, it may begin to conduct during normal
operation after warmup.
Symptom: No picture; repetitive clicking or beeping sound.
Bad part: High-voltage divider (the assembly between the flyback
transformer and the CRT anode), IBM part number 6405282 (about $20).
Replacement is easy: Unclip and unplug the ends going to CRT and
circuit-board. Pry the red plastic retaining ring out of the flyback
transformer; save it for the new divider assembly.
Voltage-divider failure causes additional damage in about 50% of
3192's:
Symptom: Vertically-rolling picture (no vertical synch) but
video is present over entire face of tube.
This symptom often appears after the HV divider is replaced. The bad
component is IC303, which is a 7406 (TTL open-collector hex inverter).
Installing a 14-pin socket at IC303 may be advisable.
Horizontal-synch problems may also result from failed IC303 (7406).
A severely-shorted voltage divider may pull power-supply voltages to
low levels, resulting in the unit appearing dead (no pilot-light)
although the fuse is intact. Disconnect the red HV wire from the
flyback transformer, then apply power. If pilot light illuminates,
replace voltage-divider.
A 3192 left on for a long time with shorted high-voltage divider will
burn-out two large transistors in the switching power-supply (Q101 and
Q201, types 2SD836A and 2SD1441, respectively). Look for physically
burned transistors, oxidized solder-connections, or darkened circuit-
board. Another 2SD1441 is used for the horizontal output transistor.
Symptom: No vertical synch, horizontal line in center of tube,
possibly with deflection above or below line.
Bad part: Vertical amplifier (TDA2653A: large IC on heat sink).
Sometimes fails along with power supply, voltage divider and IC303, as
described above.
Most 3192's have proprietary-numbered integrated circuits. By
inspecting numerous units, we have compiled a list of industry-
standard numbers:
IC301 TDA2582 (rare failure makes power supply inoperative.)
IC302 74LS221
IC303 7406
IC800 LM320T-12 (voltage regulator)
IC801 ?
Vertical amplifier: TDA2653A
Connect terminal to the appropriate communications system and adjust
the picture as needed, before replacing the back cover. The upper
knob on the flyback transformer is CRT focus, the lower knob is screen
voltage (a.k.a. "sub-brightness").
Power switches commonly fail in 3192's. The switch (part number
8233403) is available from IBM. Since 3192's are rarely turned off,
it is practicable to remove a broken switch and solder wires between
the circuit-board pads in order to return a unit to service
temporarily if new switches are not in stock.
--
Frank Reid reid@ucs.indiana.edu