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Manufacturer Model Symptom Try This
ACORN 28-AKF17 and 28-AKF30 14 " No display and whistling noise from PSU
Check Horz O/P transistor for S/C. If OK suspect EHT/LOPTX assembly.
Test by substitution.
A less common fault is a problem with the over voltage protection
circuit. This is a crowbar type across the 28V supply and diodes
6452/6454 type PHF15 (15V 300mW Zeners) and thyristor 7452 type BT151
can fail causing a short circuit. These can fail for no apparent reason
or a fault in the regulator circuit causing excessive voltage. If these
have failed remove them from the circuit and disconnect the following.
Collector of the horizontal output transistor to avoid the possibility
of excessive EHT, R3563 feed to frame O/P and R3512 feed to horizontal
drive circuits. Connect a meter across the 128V rail and switch on the
unit. If the voltage is excessive and does not respond to adjustments
with R3414 switch off immediately and examine the regulator circuitry.
Transistor 3470 BC558B can also fail and is best tested out of circuit
using a transistor tester. If in doubt replace it.
No Horz drive but PSU working normally Check R3512 28V feed to
Horz drive circuit for O/C. (locate by following track from R3563 near
front of board)
Frame collapse just visible at max Brill Check R3563 28V feed to
Frame output circuit. (near front of PCB)
AKF50 and Microvitec 1438 Dead The following components can cause
problems. IC1 = UC3844 8 pin PWM controller. TR1 FET BUZ80AFI. C8, D2
and D3, R20 2.2 Ohm and IC2 12V regulator. Another common failure is R6
15K going O/C. This is in series with R3 and appears to be mains sync
feed to PWM chip IC1.
AKF52 Microvitec Chassis Intermittent tearing of vertical lines on
screen Several examples have had this fault. It is normally caused by
poor joints on C18 in the PSU.
AKF60 Microvitec Chassis Power light comes on with no video input. If
video is connected the power light flashes. The video signal is
switching the monitor out of power save mode and the flashing power
light is caused by the PSU starting and shutting down due to an overload
usually caused by a fault in the Horz O/P stage. Check the Horz output
transistor BUH515D. If faulty and a replacement is not available a
BU508DFI has been successfully used.
NOTE:- On some examples of AKF60 substituting a BU508DFI can cause
horizontal tearing for approximately 2 minutes after switch on. It is
therefore recommended that a BUH515D is used. If this proves difficult
to obtain it may be ordered from TANDY mail order department, or through
a local store.
ARCH 514AS 14 " Very poor height and buzzing noise The buzzing was
coming from the scan assembly and L402 near the LOPTX was overheating.
Cause was C406 0.68J capacitor O/C.
COMMODORE 1084 and 1084S No display and faint whining noise This
problem is normally caused by the failure of the EHT/LOPTX unit. This is
best confirmed by substitution.
1084S-D1 No display and faint whining noise Check D224 ERC06-15
flyback damper diode for S/C. If it has failed and this type is not
available it may be substituted with a BY228 which is common in TV
chassis.
1084ST No display and noise from PSU Disconnect the collector of the
line output transistor and check PSU voltages. If OK check the line O/P
transistor for S/C. If transistor is OK suspect the LOPTX. This is
normally a Sampo FEA188 which can be substituted with a HR transformer
type number TFHR7495.
COMPAQ 460-P Power light comes on then goes out When power is applied
the PSU starts up and then shuts down. Check the Horz output transistor
for S/C. This example used a 2SC3482. This was replaced with a BU2508F
and gave no problem during long soak test.
DAEWOO Crystal 14S No display or power light If PSU appears to be
working OK check Horz driver transistor 2SC3502-E for S/C. If this
device is difficult to locate it can be replaced with 2SC2271 which is
popular in many Televisions.
After several minutes the display starts to jitter and the height and
width reduces eventually resulting in complete loss of display.
Suspect STR59041 IC in PSU circuit. This can often be confirmed by
cooling the component with freezer to see if the fault clears for a few
minutes.
ELONEX CD1442VGA Dead Check voltage across smoother. If OK check R2
120K resistor on PSU board for high resistance or O/C.
SV14LR 14 " PSU and EHT start up OK but no display Check CRT heater
supply circuit from PSU TX D818 cathode for +7.5 Volts. If voltage is
low check C823 for low value/leakage. Should be 470uF but faulty unit
measured 340uF on C tester.
E.H.T fires up then shuts down and power light remains on Common
component failures on this chassis are C818, C820, C821, C424, C817,
Q403 and R807. This particular symptom was caused by C424 33uF 160V
capacitor located near the LOPTX.
MN009/1 14 " No display or power light If frame output and EHT appears
to be working check for failure of the 7808 regulator mounted on the
plate at the rear of the main PCB.
EMC EF836 14 " Screen at Max Brill and displaying flyback lines Check
for approx. 90V across C230 47uF 160V on tube base PCB. If very low or
0V check R530 4R7 fusible resistor toward rear of main PCB for O/C.
ESCOM ESCOM / HIT KT81-144C/8M Low height and poor or no Horz sync
Using a CRO and X10 probe check for strong oscillation on 3.58MHz
ceramic resonator X201 connected to IC201 near front of board. Approx.
40mV PtoP on one leg and 10mV PtoP on the other. If signal is missing or
amplitude is poor replace the resonator.
GOLDSTAR 1460 14 " No Display PSU was working but no EHT. No line drive
due to no voltage on IC701. Fault D701 1N4001 diode near D711 high
resistance.
HANTEREX F1428SE 14 " Could be TATUNG Chassis Dead Check voltage across
smoother. If OK check R104 120K Resistor near the smoother for O/C.
ICPI 72G9140 14 " Dead PSU not working but voltage across smoother OK.
Check R603 and R604 33K 3W resistors in PSU for O/C.
MICROVITEC Microvitec / Link / Commodore / Wyse LN15E 15 " OSD No
Display or Power light Check for faint ticking from PSU circuit. If so
check Horz O/P transistor BU2520AF for leakage or S/C. If suspect
replace with correct type.
If after switching off the ticking noise gradually gets slower and dies
away check Q603 2SC2655 in cold side of PSU circuit.
Excessive width and pin cushion adjustment not working Check D212
UF5408 soldered on heatsink near LOPTX.
CUB (Metal Case) Dead If the mains input fuses are blown check the
bridge recy for S/C and smoother C11 100uF or 220uF for leak or S/C.
Failure of the PTC feeding the degauss coils can also cause the fuses to
blow. A quick check can be made by simply disconnecting the degauss coil
assembly. If the DC fuse F3 has failed check the chopper transistor for
S/C.
No display and the PSU can be heard tripping The most common cause
is excessive current being drawn by the Horz output stage. Check the EHT
tripler/recy assembly by disconnecting it from the secondary of the Horz
output TX. If the PSU runs normally replace the tripler assembly. Also
check the Horz output transistor TR202 BU500 for S/C.
Poor width and height or no display Check HT rail on secondary of
chopper TX i.e. across C26. If this voltage is low and the Horz output
stage checks OK suspect one or more of the following in the PSU circuit.
D18 BZX79 C7V5, D20 BZX79 C33V, TR1 BC307, C23 and C12 4u7 35V.
Poor height or frame collapse Most symptoms of this type are
normally caused by the output chip IC301 TDA1170S.
MITAC L1766PD 17 " PC Monitor With OSD Red Green or Blue video missing
but colours OK while displaying built in menu screen. Check the
transistors type number 2SC945 on CRT PCB. Each transistor has a 68 Ohm
resistor feeding the top leg. Check for approx. 1 volt at this junction
of each device. If this voltage is incorrect suspect the relevant
transistor.
Intermittent loss of display and mode relay constantly switches on
and off. This was caused by a poor joint on inductor L906 in the PSU
where the copper wire from the coil was wound round and soldered to the
pin of the component not the PCB.
Power and Aux. lights on EHT and PSU OK but no display Increase A1
voltage to see if a display appears. If so try adjusting the Brill
control. If this has no effect on Brill level check the following.
Negative supply from anode of D509 near LOPTX. approx. -200V
Voltages on Brill control connector J502 with control central. Black =
32V, RED = 43V, Yellow = 55V. All negative values.
Check voltage swing on Brill control Red lead with control on Min (-55V)
and Max (-32V) settings.
If errors are found check the semiconductors in the ABL and associated
circuits paying particular attention to Q525 2SC945 which can go leaky.
This will normally only show if tested out of circuit.
T1770HD 17 " With OSD Power light on but no display The power light
confirms voltage across the smoothers but check for expected voltage
with a meter. If the voltage is OK check the feed resistors to the
startup circuit R903 and R943 38K for O/C.
1450PD 14 " PC Monitor With OSD 1450PD 14 " PC Monitor With OSD Change
Q201 2SC945 this should cure the problem. For improved reliability it is
recommended that this device is substituted for 2SD669AC.
Lack of width on early examples. Change R907 1Meg for 1.5Meg 2W
and R912 550 Ohm for 1K. Replace links JP950 and JP931 for 10 Ohm 0.25W
resistors and remove C949.
Check +B for 87Volts on loop near LOPTX. If necessary adjust with VR601
under CRT.
Check for +145Volts on R924. If necessary adjust with ?
No Display, but power and Aux. LEDs on and EHT OK Slowly increase
A1 control on EHT assembly and see if a display appears. If so adjust
the Brill control. If it does not function check R507 56K resistor
located near the Brill control connector for O/C. Adjust A1 back to
correct level. Note: In some examples R507 is 33K. This will reduce the
voltages on the Brill control by approx. 20V.
Brill at Max and flyback lines are visible. Brill or Contrast have
little effect Check R505 100K near Brill control connector for O/C.
Below are typical voltages on Brill and Contrast control connectors on a
working chassis with controls at center. The feed to the Brill circuit
is derived from the Horz output TX via D504 and smoothed by C511 10uF
250V. The voltage across C511 is normally 150V.
Brill connector. Black = -50V, Orange = -65V, Yellow = -75V (see note
above)
Contrast connector. Yellow = +9V, White = +6V, Black = +2V
Power light on but no display and AUX light not working With video
input signal connected check if a faint ticking noise can be heard from
the PSU approx. twice a second. If so disconnect video signal and check
if ticking stops. If it does check Q501 Horz O/P transistor on heat sink
type BU2508DF for collector to emitter short. If this device is not
available then use a substitute to verify fault cure. Types tested
include BU2508D (not recommended as a permanent substitute as it will
require an insulating kit) and 2SD1877. Both are popular in Televisions.
When fault is confirmed it is recommended that the correct device is
fitted.
Power light on but no display Check PSU, if it is not running
check voltage across smoother. If OK check resistors R904 and R905 47K
2W for O/C.
AM4050PD, similar to chassis 1450 Various As this chassis is similar
to the model 1450 many of the faults are common. The following is data
that may be useful during fault finding. Any voltages were measured with
a standard 20K OPV multimeter.
Junction of D501 and D502 from pin 10 of LOPTX. 1000V AC or +50V DC.
Anode of D502 65V AC or +9.4V DC.
Anode of D504 from pin 4 of LOPTX neg 180V.
Cathode of zener diode ZD502 to ground 25V.
ZD601 160V, D609 90V, D914 95V, D915 160V, D918 95V, D919 25V, D920 7.2V
U501 LM1391N Horz Osc Chip.
Pin 1 = 0.75V & #9; & #9;30uSec square wave 1to2 ratio 2V P to P.
Pin 2 = 0V
Pin 3 = 2V & #9; & #9;Inverse of 1 3.5V P to P.
Pin 4 = 2V & #9; & #9;Poor sawtooth 1V P to P.
Pin 5 = 3.2V
Pin 6 = 9V
Pin 7 = 4V Sawtooth 3.5V P to P.
Pin 8 = 3.4V & #9; & #9;Clipped sawtooth 1.5V P to P.
Q511 Line driver transistor Base 0.75V P to P Collector Square wave 30V
P to P with 20V overshoot.
L1564PDM 15 " WITH OSD Intermittent loss of picture and picture
sometimes fades slowly. This is typical of an intermittent heater supply
fault. It is normally caused by poor soldered joints in the heater
supply circuit. The supply is derived from D811 from the secondary of
the chopper transformer and is supplied to the CRT base via an orange
lead on a mutiway connector.
Power light on but AUX light not working Power light on but AUX
light not working
MTC EM1427 14 " Frame collapse and partial Horz scan Check for supply
voltage on frame chip IC301 TDA1170N. If OK in most cases the fault will
be caused by failure of the IC.
Note: If you have no data on this chip the schematics of early
Microvitec monitors may aid fault diagnosis as they used the same
device.
PEACOCK PM14P48LR Dead no display or power light Check voltage on
smoother. If OK PSU is probably not attempting to start. Check 27K
resistors R603 and R604 for high resistance or O/C.
PHILIPS 4CM 4770/25T Intermittent Horz Sync problems May be connected
with power save circuits. Try replacing link 3804 with a 100R resistor
and changing transistors 7317 and 7320. All on tube base PCB.
CM11342/CM11362/
CM8832/33/52 No display and faint wining noise Very common problem on
these models see COMMODORE 1084 for details.
Power light on but no display Check 125V rail from PSU board. If
this is low disconnect output power connector and check if voltage is
OK. If so it is likely that the line O/P stage is drawing to much
current. Check the output transistor for leaks, if all checks out
disconnect the EHT cap from the CRT. If this results in a substantial
increase in supply voltage suspect the Recy in the LOPTX assembly.
7CM5279/20T Power light on but no display If PSU is working check for
voltage on transistor 7512 (BC640). If voltages are missing suspect the
transistor and resistor 3512 47R. Approximate voltages on a working unit
are as follows. Base 90V, Collector 90V, Emitter 95V. In several units a
2N5401 has been used as a substitute but note the leadout is different.
SAMSUNG CVM 4967T or Wearnes XGM-1415W 14 " No display but power light
comes on Check for 135V at adjust point near LOPTX. If voltage OK check
L605 next to C614 for O/C. Should have approx. 25V on both ends. Colours
on inductor RED, RED, BROWN, SILVER.
SAMTRON SC-428PSL Pale white background and text is green or other
primary colour. A slight burning smell may also be noted Check the 70V
supply to CRT base PCB. If this is low it normally indicates that
excessive current is being drawn by the video driver circuits. Any smell
is usually caused by R600 70V feed resistor overheating on the main PCB
and/or diodes overheating on the CRT base PCB. Check all the diodes on
the tube base PCB they often suffer a high resistance reverse leakage,
which does not always show on an ohm meter. If any diodes are suspect it
is best to substitute them, or remove them and test for reverse leakage
with a bench PSU set to around 30V. Connect the diode under test in
series with a 1K resistor across the supply with the cathode to
positive. If there is a voltage drop across the resistor, this indicates
a reverse leakage and the diode should be replaced. The 1N4148 diode
faults normally show on an ohm meter but the BAV21 diodes at locations
D104R, D104B and D104G can have a reverse leak which will only show on a
PSU test. These diodes have a 250V reverse voltage rating and must be
replaced by a similar type. Voltages on the video output chip IC102 type
LM2406T while displaying a DOS screen, starting from the top pin, are as
follows. 72.2V, 2.7V, 72.2V, 2.9V, 71.8V, 74.4V, 0V, 1.4V, 1.4V, 11.6V,
1.4V. Also check for 72V across each BAV21 diode. We have had a few
examples like this, the last had 6 duff diodes and a duff chip, this may
have been caused by a flash over in the CRT.
TATUNG 3401 and AMSTRAD PC14M28LR/PC14M39 Works OK for a few minutes
then shuts off These symptoms are normally caused by the +B voltage rail
drifting causing the EHT to increase and the overvoltage circuit to
trip. Check resistor R504 220K in series with the +B adjust pot RV503.
If the value is not correct replace with close tolerance high stab
component.
Powers up then the PSU shuts down and the power light goes out.
EHT build up and collapse can also be heard which would normally
indicate that the line output stage and EHT circuit main components are
OK. This symptom is common on this chassis and can be caused by a number
of problems. If you have no service information then the following
details should be useful. The shutdown of the PSU is often caused by
triggering of the crowbar thyristor D418 across the 21V supply. This
crowbar circuit monitors several conditions including EHT voltage.
Excessive EHT is normally caused by a fault in the B+ voltage regulator
circuit consisting of TR501 to TR505 and associated components. If the
supply voltage is to high the EHT rises and trips the crowbar. EHT is
monitored by a pulse taken from pin 3 of the LOPTX and is virtually
instantaneous making diagnostics difficult. If the B+ regulator circuit
is suspect a quick test can be carried out by disconnecting the emitter
of TR504. As the circuit is a shunt regulator a supply voltage will be
maintained Via the 150Ohm resistor R509. If the monitor runs check the
following. The output/source voltage of the PSU which should measure
110V across C819 of the PSU circuit. The voltage drop across R509 which
should be approx. 30V indicating that the Horz output stage is drawing
the correct current. If all is well then it is most likely that the
fault is in the regulator circuit. If the unit still shuts down, then as
in most Horz output stages, failure of the tuning or damping components
can also result in excessive EHT. Check D402, D403, C420 and C421. A
problem with C420 is most common. When the unit is repaired adjust the
B+ voltage at the emitter of TR504 to 84V with a Horz scan freq. of
31.5KHz using RV503.
Some revisions of this unit incorporate a power saving circuit which
removes power from the Horz output stage during the absence of video
sync pulses. Failure of this circuit can result in similar symptoms to
the above or no EHT at all due to no supply to the Horz output stage,
but the power light remains on and the PSU runs normally. If fitted the
circuit board is located on the heatsink near the LOPTX and is mounted
with two screws. The circuit contains two distinct parts one
incorporating IC701 74LS123 and the other transistors TR701/2 and
Q701/2. If the circuit is suspect the fault can be quickly isolated to
either section by moving the jumper PL704. If the monitor works then
suspect IC701 and associated components or the wiring to the module
resulting in the absence of sync pulses. If it does not work then
suspect the section containing the transistors. It is unusual for a
semiconductor to fail on this module and most faults are caused by
faulty resistors particularly the higher values.
TM5501 Power light on but no display IF EHT etcetera are OK, check if
CRT heaters are working. If not check supply (6.4V) from PSU board to
power save control module. Two white wires from SK401 on PSU board Via
ferrite ring to PL709. If voltage is missing suspect D828 on PSU board.
If voltage OK check for broken joints on Q702 and PL908 on power save
control PCB. If voltage is present on PL908 check for broken joints on
tube base PCB.
TAXAN SU775EV 14 " Intermittent loss of picture. Sometimes slowly fades
while working This is normally caused by broken soldered joints in the
CRT heater circuit. Check Q906 and associated components (near front of
main PCB). Also check C913 for intermittent O/C. If in doubt replace.
No display and strange noises from area of PSU circuits Many PSU
problems are caused by the main input smoothers failing, causing
excessive ripple. Remove them and test with a capacitance meter before
proceeding with lengthy fault finding. Occasionally visible signs of
leakage are apparent after they have been removed.
SV787LR Dead. Input fuse blown. Before replacing the fuse check the
following. The four diodes that form the input bridge and the chopper
FET 2SK727. If the FET is S/C also check R825 0R39 ohm and D806 18V
500mW Zener diode. The failure of the FET can sometimes be cause by
arking joints on the smoothing capacitor. Resolder these in all
examples.
TRUST CM2231 14 " Dead Check fuse for O/C. If so make the usual basic
cold checks for switch mode supply problems, but pay special attention
to C821 560pF and D814 1N4936. In several examples of this model both
these components have been S/C. Note: 1N4936 is a 1Amp fast recovery
device.
No display but power light showing green PSU running OK. Poor line
drive due to short circuit C415 104Z. This causes a high voltage drop
across R431 resulting in low voltage to line driver stage. Voltage
across C415 should be approx. 26V. Drive waveform to base of LOP
transistor (across secondary of drive TX) should be approx. 2V PtoP.
TYSTAR TY1415 No display and a whistling noise which dies away when
switched off. Check the Horz output transistor BU508A
WESCOM GW500-E Dead This chassis has a separate power PCB which is
often problematic. If the fuse is blown check the following. Bridge recy
and Q3 2SC3460. If Q3 is S/C it may have also damaged the following. Q4
2SC1384, Q5 and Q6 2N2907A or 2N5401, IC3 4N35 Opto isolator and
resistor R14 0.68 Ohm 3W. When this resistor was out of stock a 1 Ohm
was used and operation appeared normal. & lt; /p & gt;
& lt; p & gt; Positive output voltage to Horz output stage is marked on PCB as
45-135V. During correct operation in a DOS screen +V = approx. 80V and
+B = approx. 27V. With no video signal +V drops to approx. 53V.
Brill control not working No negative voltage to Brill control due
to R449 56K O/C. Also check negative voltage across C425 should be
approx. 175V.
The Brill and Contrast controls connect in a group of six wires near the
edge of the PCB. The typical voltages on these wires are listed below
working from left to right looking from the edge of the PCB.
1 = Min Contrast 2.5V, Max Contrast 10.3V
2 = 11V Small changes with Contrast & Brill
3 = Min Brill -14.3V, Max Brill +11.9V
4 = 12V
5 = 5V
6 = 0V
Brill voltage on R450 Min -42V, Max -20.8V
UNBRANDED 15 " Model SRC1502L No display but power light shows orange
and faint whine from PSU Check Horz output transistor Q802 2SC3886A for
short circuit. May be substituted with BU2508AF. If after replacing the
PSU functions OK but the Horz output stage is dead check R422 1R5
resistor for O/C.
Display rotate intermittent Check the trace rotate POT on the
front panel. This should be a 1K high quality preset POT. On some
examples this is 2K2 and the track suffers damage at the point where the
wiper makes contact. The problem usually occurs the first time the pot
is adjusted.