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Assembly Manual
In-circuit LOPT/FBT Cat No. K 7205
Tester
by Bob Parker
PROJECT INFORMATION SUPPLIED BY
ELECTRONICS AUSTRALIA - August 1998 Issue
ACN 000 908 716
WEBSITE:
E-MAIL:
www.electronicsaustralia.com.au
[email protected]
Please read Disclaimer carefully as we
can only guarantee parts and not the
labour content you provide. K I T
Here's the design for a low cost, easy to build and use battery operated `shorted turns' tester for
line-output or `flyback' transformers, and other HF wound components like deflection yoke wind-
ings and SMPS transformers. Tests have shown it capable of finding at least 80% of LOPT/FBT
faults, so it can save a lot of time and trouble. Small and rugged, it's well worth a place in the toolk-
it of anyone involved in servicing TV receivers, video monitors and computer power supplies.
f you're reading this, then chances are tension (EHT) supply of around 25 kilo- However the failure which service
I that you're a TV and/or computer mon-
itor repair technician - who doesn't need
volts for the final anode circuit of the
cathode ray tube. It's also possible the
technicians dread is a shorted winding in
the LOPT itself. Unfortunately LOPTs
to be told that horizontal output stage HOT has failed simply from old age or tend to be specifically designed for the
faults cause more than their fair share of overheating due to unevenly- make and model of the TV or monitor
headaches! Operating at high voltages, applied/solidified heatsink compound. they are used in, which can mean a lot of
frequencies and power levels, many Another occasional culprit is an insula- hunting around for a replacement. In
components in this part of the circuit are tion breakdown in the deflection yoke's addition they are hardly ever cheap, and
highly stressed, and failures are not only horizontal winding. not always physically easy to replace.
common but their cause is often hard to In short the LOPT is not a component
identify. which is easy to test by substitution, and
The usual symptom of a major hori- a service technician needs to be as cer-
zontal output stage fault is a serious tain as possible that the LOPT really is
overload of the DC power supply feeding defective, before tracking down a
the primary winding of the line output replacement!
transformer, or `LOPT' for short (called Identifying faults
the `flyback' transformer or `FBT' in
North America). This is often accompa- Several techniques have been devel-
nied by a collector-to-emitter short cir- oped over the years for identifying faults
cuit in the horizontal output transistor or in horizontal output stages, and testing
`HOT'. LOPTs in particular for the presence of
(For consistency, we'll be referring to shorted winding turns.
the line output transformer as the `LOPT' The components in the horizontal out-
throughout this article - North American put transistor's collector circuit, includ-
readers please mentally substitute `fly- ing the LOPT's primary winding, deflec-
back' for this term!) tion yoke horizontal winding, and tuning
Any of quite a few possible compo- capacitors form a reasonably low loss
nents could be the cause of such a failure, (high Q) resonant circuit, especially at
the more common being one of the high- low voltage levels.
speed rectifier diodes fed by the LOPT's Most testing techniques, including the
secondary windings, including the diode one used in this design, are based on the
stack(s) which produce the extra-high- fact that nearly all serious faults in the
Text and illustrations courtesy of Electronics Australia
horizontal output stage will greatly by C2, R4 and R5/D1, pin 7 pulses
increase the losses in the LOPT's down to ground potential for about
primary circuit. That is, they lower 2ms every 100ms, and it's during
the Q. these low-going 2ms pulses that
We chose the principle of `ring' each ring test occurs.
testing as the basis for this instru- When IC1 pin 7 drops low, Q1 is
ment because it's easy to implement driven into saturation by its base
with relatively simple circuitry and current flowing in R7, and its col-
common components, and produces lector voltage jumps to the +6V
predictable results with no need for supply, which makes two things
calibration. happen. First, C6 in collaboration
`Ring' testing gets its name from with R16 sends a positive pulse of
the fact that when a fast pulse is FIg.1: Ringing waveforms from `good' (top) and about 5us duration to the reset pins
`shorted winding' line output transformers, in
applied to the primary winding of response to the tester's pulse. of four-bit shift registers IC2a and
the LOPT, the total inductance and IC2b, which drives all their outputs
capacitance in the circuit will produce an chassis and the `HOT Collector' lead to to a low state - switching off all the
electrical `ring' - a decaying AC voltage the horizontal output transistor's collec- LEDs, in readiness for a new ring test.
which can have a duration of a dozen or tor. One LED will illuminate for each At the same time, about 20mA flows
more cycles before it reaches a low `ring' cycle above about 15% of the ini- through R8, driving D2 into a low
value. It's the electrical equivalent of tap- tial pulse value, and in general if four or impedance state and dropping about
ping an empty glass; in each case, an more LEDs are glowing, the horizontal 650mV across it. The voltage step across
energy impulse generates damped oscil- output stage is OK. D2 is coupled via C3 to the test leads and
lations. We'll talk more about using the tester the LOPT primary winding, causing this
Waveform `A' in Fig.1 shows the HOT later, after the circuit description. For the circuit to `ring' a bit below its natural res-
collector voltage waveform in a typical moment though, it's worth mentioning onant frequency due to the presence of
fault-free TV (a General Electric that because the tester uses a low-voltage C3 (which functions as the resonating
TC63L1 in this case), in response to a testing pulse, it is suitable for testing capacitor when testing an LOPT on its
pulse from this tester. However if the LOPTs `in circuit' - i.e., without having own).
losses in the horizontal output circuit are to disconnect the yoke or other connec- 2. The ring amplitude comparator:
increased, the amplitude of the `ringing' tions. The `ringing' waveform is coupled by C4
waveform will decay much more quick- Circuit description to the inverting input of comparator
ly. Waveform `B' shows the effect of a IC1b, which is DC biased to about
shorted rectifier diode on one LOPT sec- At first glance the circuit in Fig.2 +490mV by the junction of R11 and R12.
ondary winding of the same TV, but note might look a bit complicated, but it real- D3 is constantly forward-biased by about
that a shorted LOPT winding or several ly consists of three quite simple sections. 1mA flowing through R10, and its entire
other faults would have a similar effect. These are the low frequency pulse gener- voltage drop of about 600mV is applied
A collector-emitter short in the HOT ator, the ring amplitude comparator and to IC1b's non-inverting input as a refer-
or a shorted tuning capacitor will result the LED bar-graph display. We'll now ence voltage, via R13. R14 produces a
in no ringing at all, indicating a really look at these in turn. small amount of positive feedback
major fault. 1. The low frequency pulse genera- around IC1b, ensuring that its output
So to do an initial check of a horizon- tor: Voltage comparator IC1a is set up as switches cleanly between its low and
tal output stage, with this tester, you first a low frequency oscillator, whose output high voltage levels.
make sure the TV or monitor is de-ener- on pin 7 is normally pulled up to essen- The result of all this is that an inverted
gised(!). Then you simply switch the tially the positive supply rail by R6 and and squared-up version of the ringing
tester on, connect the ground lead to the R7. Due to the time constants produced waveform appears at the output of IC1b,
BATT
+
R6
1k R10 15
Q1 R15
R1 BC328 4.7k 4.7k
1M DATA
C6 14 16
RESET A A Vcc
R7 100pF
R3
1k 6
1M RESET B IC2
SW1 R14 1M
POWER R13 4015 C7
10k 1 0.047uF
3 CLK A
5 8
1 9 8
C1 7 IC1b CLK B DATA Vss
IC1a R8 2
+ 100uF 6 270W 4 IC1 Q0A Q1A Q2A Q3A B Q0B Q1B Q2B Q3B
LM393 13 12 11 2 7 5 4 3 10
R4 2.2M
+
R11
R5 C3 C4 33k R16 R17 R18 R19 R20 R21 R22 R23 R24
6V R2 0.047uF 0.01uF 47k 1k 1k 1k 1k 1k 1k 1k 1k
(4xAAA) 1M
47k D1 C5
- 1N4148 D3 0.047uF
1N4148
R9 D2 R12
C2 150k LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8
1k 1N4148 RED RED RED YEL YEL GRN GRN GRN
0.047uF
BATT
-
EARTH HOT
(Collector)
Fig.2: The circuit is simple, but elegant. IC2 shows clearly how many rings are supported by the inductor under test.
Page 2 Text and illustrations courtesy of Electronics Australia
until the ringing amplitude has decayed put, connected to both shift registers' a monitor is connect the tester between
down to about 15% of its initial value. clock inputs, to a low (logic 0) level - the HOT collector and ground. If no or
This square wave is connected straight to unless the test leads are shorted. only a few LEDs light, I check the HOT,
the clock inputs of shift registers IC2a If the LOPT primary circuit is OK, it damper diodes and tuning caps for
and IC2b. will ring during the next several hundred shorts using a DMM. If these are OK, I
3. The LED bargraph display: IC2 microseconds. For each ring above about check for an open fusible resistor in the
consists of a pair of identical four-bit 15% of its initial value, it will cause a circuit feeding B+ to the LOPT, and for
serial-in/parallel-out shift registers, con- high-going pulse to be applied to the shorts/leakage in diodes on the LOPT
nected to form a single eight-bit unit, shift register clock inputs, resulting in secondaries. I also check the bypass
with each output driving one LED in the the logic 1 on IC2 pin 15 being moved capacitor on the DC supply to the LOPT
`bargraph' display via resistors R17 to one shift register stage further along. It primary for excessive ESR.
R24. The serial data input of the first doesn't matter if the LOPT rings more If these check OK, I ring the horizontal
stage (pin 15) is permanently connected than eight times - all LEDs will still yoke with its connector unplugged. It will
to the positive supply, or logic 1. remain illuminated. normally ring seven times on its own. If
One measurement So the overall result is that one LED the yoke rings OK, I unsolder all but the
illuminates for each LOPT ring cycle LOPT primary winding and ground pins,
For the first 5us after the commence- above 15% of the initial level, and this and ring the primary. If the primary still
ment of a new 2ms measuring pulse, condition remains until the start of the rings low with everything else discon-
both shift registers are reset to zeroes on next 2ms measuring pulse. Phew! nected, the LOPT is probably defective.
all outputs, as described earlier. At the Usage & limitations Most LOPTs on their own will ring 8+
same time the initial positive pulse times, but some ring only four or five,
applied to the LOPT drives IC1b's out- In order to assess the usefulness of this even when they are perfectly normal. So
design, we gave several prototype it is prudent to confirm the diagnosis by
Parts List LOPT/FBT testers to technician friends ringing an identical known-good LOPT,
to evaluate for many months, then asked if at all possible.
Resistors for their comments and thoughts on how Sometimes an LOPT is defective, but
(All 5% 0.25W carbon) to put the tester to best use. still rings normally with the tester, e.g.
R1,2,3,14 1M The first response is from Larry Sabo, due to leakage or arcing that only occurs
R4 2.2M an experienced monitor technician in at full operating voltage. The problem
R5,16 47k
Ottawa, Canada who also suggested the will sometimes be manifest by heavy
R6,7,9,
R17-24 1k front panel layout: loading of the B+ supply, spurious ring-
R8 270 ohms One of the first things I do to check out ing and/or reduced voltages on the HOT
R10,15 4.7k
R11 33k
R12 150k
R13 10k
Capacitors
C1 100uF 16/25VW
RB electrolytic
C2,3,5,7 0.047uF MKT
C4 0.01uF MKT
C6 100pF disc ceramic
Semiconductors
D1,2,3 1N914 / 1N4148 silicon
diode
IC1 LM393 dual comparator
IC2 4015 / MC14015 / CD4015
dual 4-bit shift register
LED1,2,3 Rectangular red LED
LED4,5 Rectangular yellow LED
LED6,7,8 Rectangular green LED
Q1 BC328 / 2N5819 PNP
silicon transistor
Miscellaneous
PCB, ZA1137 51 x 76mm; small (UB3)
plastic case, 130 x 68 x 41mm (DSE H-
2853); front panel; battery holder for 4 x
AAA cells; battery snap ; power switch,
push on/off; one DIP8 IC socket, one
DIP16; 4 x tapped spacers; screws,
nuts and washers (see Screw size and
allocation guide); 1 x red, 1 x black 4mm
banana sockets; test leads with 4mm
banana plugs; double-sided adhesive
tape; wire, PCB pins, solder and instruc- Fig.3: Use this PCB overlay and the facing photo as a guide in assembling the
tions. tester.
Text and illustrations courtesy of Electronics Australia Page 3
collector, or excessively high EHT result- these must be temporarily disconnected.
ing in HV shut-down. Otherwise the reading will be low, even
Because this tester uses impulses of though the winding itself is fine.
only 650mV to minimize the forward The tester can be used for checking
biasing of semiconductors, such defects high-Q transformers such as those used
will not be reflected in the ring count. in SMPS's. However, my experience has
Under these circumstances, I check for shown that it will not provide more than
measurable leakage resistance between a two or three LED indication for good
the EHT cap and the other LOPT pins. It TV horizontal drive transformers. It can
should be unmeasurable, otherwise the be used for these, however - to indicate
LOPT is defective. shorts (no LEDs lit). On the other hand
If I have gone through the above tests the ESR Meter (Dick Smith catalog num-
and have these symptoms and a normal ber K-7204) can do much the same with
ring count on the tester, the diagnosis these low resistance transformers.
can usually be confirmed only by substi- Wayne Scicluna services TVs in
tuting a known-good identical LOPT, or Sydney, and is the technician who talked
by testing with a chopper similar to the me into developing the tester in the first
one described in Sam Goldwasser's place. Here are his hints:
Electronics Repair FAQ, located on the The assembled PCB, which supports If you've already checked for the more
Internet at http://pacwest.net/byron13/ virtually all of the circuitry. obvious leaky and shorted semiconduc-
sam/flytest.htm. tors and capacitors etc., and are still get-
Something else I do when testing a Overall, the LOPT tester can identify ting a low reading on the tester, there are
LOPT is to supply it with a reduced B+ about 80% of LOPT failures. When try- some other traps to avoid.
to enable scoping the HOT and measur- ing to solve a puzzle, if someone offers You need to get a good connection with
ing EHT (in situations where the monitor information that is right 80% of the time, the test leads, because contact resistance
goes into HV shutdown). To reduce the it's a lot better than having to guess can cause a low reading. The same
B+, I use two light bulbs in series, one 100% of the time, especially if the ante is applies to defective solder joints in the
end to B+ supply, centre-tap to LOPT B+ the price of a LOPT and wasted, valu- horizontal output stage, especially on the
connection, other end to ground. One able time. LOPT itself and HOT. In fact connecting
bulb is 60 watts, the other is 100, so I can Michael Caplan does general electron- the tester with clip leads, flexing the
reverse the end leads and increase or ic servicing in Ottawa, and added the fol- board and wiggling components is a
decrease the B+ value used in testing. lowing useful points in relation to TVs: good way to show up bad solder joints in
At the outset, when I have power sup- It's pretty straightforward to use, with this area.
ply cycling but have confirmed there are the usual precautions of ensuring that Body conductivity can also cause a
no shorts from HOT-C to ground, I sub- the under-test unit power is off and any lower than normal reading if you're
stitute a dummy load (60W bulb) for the caps are discharged. touching the test leads and your skin is
LOPT where the B+ enters, to see if the When testing an LOPT in circuit, it damp. Low readings can also be caused
power supply works with the LOPT out might be necessary to disconnect some of by having the test leads reversed, i.e.,
of the equation. the LOPT terminals, and/or yoke plugs connecting 'HOT Collector' to chassis,
that could load it down and upset the and by faults in an external voltage
Screw Size and Allocation readings. The tester will often not detect tripler.
Guide bad HV diodes in integrated split-diode How to build it
LOPT units, nor shorts/arcing that is
Printed Circuit Board to Spacers voltage dependent - but then no other Before soldering anything to the print-
4 x Screw M3 x 6mm (zinc plated) passive tester does either. ed circuit board, hold it up to a bright
Front Panel to Spacers
I have found it useful for checking TV light and examine the copper side care-
4 x Screw Countersunk M3 x 6mm (Blk) deflection yokes, both horizontal and fully for fine track breaks and especially
vertical. A good yoke lights at least five whiskers or bridges - particularly where
Front Panel To Case and typically the full eight LEDs. tracks pass close to component solder
4 x Screw Countersunk No4 x 6mm (Blk) However, many yokes have built-in par- pads.
allel or series damping resistors, and Referring to the board overlay in Fig.3,
begin installing the components, starting
Resistor Colour Codes with the resistors and diodes and work-
ing your way up to the tall ones including
Value 4 Band (1%) 5 Band (1%) the four PCB pins for `GND', `HOT' and
`+6V' terminal connections- but leaving
270R Red-Vio-Brn-Brn Red-Vio-Blk-Blk-Brn
1K Brn-Blk-Red-Brn Brn-Blk-Blk-Brn-Brn Capacitor Codes
4.7K Yel-Vio-Red-Brn Yel-Vio-Blk-Brn-Brn
10K Brn-Blk-Org-Brn Brn-Blk-Blk-Red-Brn
33K Org-Org-Org-Brn Org-Org-Blk-Red-Brn Value IEC Code EIA Code
47K Yel-Vio-Org-Brn Yel-Vio-Blk-Red-Brn
150K Brn-Grn-Yel-Brn Brn-Grn-Blk-Org-Brn 100pF 100p 101K
1M Brn-Blk-Grn-Brn Brn-Blk-Blk-Yel-Brn 0.01uF 10n 103K
2.2M Red-Red-Grn-Brn Red-Red-Blk-Yel-Brn 0.047uF 47n 473K
Page 4 Text and illustrations courtesy of Electronics Australia
the LEDs off the board for now. Take
care with the orientation of the polarised
components, including the IC sockets.
With everything but the LEDs installed
on the PCB, once again illuminate it
from the top, and check for and correct
any solder bridges or other problems.
Now turn your attention to the front
panel, mounting the banana sockets and
the power switch in their respective
holes.
Attach the tapped spacers to the cor-
ners of the board using plain 3mm
screws, and solder long component lead
offcuts to the `GND', `HOT Collector'
and `+' solder pads, followed by the bat-
tery snap's black wire to the `-' pad.
Then, without soldering them, poke the
leads of all the LEDs through their
respective holes in the board. Make sure
the coloured LEDs are in their correct
places, and that all the (long) anode and
(short) cathode leads are correctly orient-
ed as shown in Fig.3.
Using black countersunk 3mm screws,
attach the front panel to the board assem-
bly and place the whole thing face-down
on a soft flat surface. Manoeuvre all of
Fig.4: Shows how the battery snap (positive lead) is wired through the switch to
the LEDs into their cutouts in the front
the printed circuit board. Note, as the component overlay shown is viewed from
panel, and push each LED down slightly the copper side of the PCB, wiring terminations for the Power and Hot
to ensure its face is level with the front of Collector/GND should be made to the PCB pins on the component side of the
the panel. In the unlikely event that a board.
LED won't fit, use a small file or similar wiring diagram). connect the test leads to the primary
to remove the excess powder coating Snip off the battery holder's PCB winding of a known good LOPT out of
inside the hole. mounting pins, then install four `AAA' circuit, which should bring all eight
Now solder all the LEDs into place, cells into it. Connect the battery snap to LEDs on. Then thread a loop of solder
then connect the test lead sockets and the the terminals, and switch the unit on. If around the ferrite core of the LOPT
closest terminal of the power switch to everything's OK then the bottom red (`1') (simulating a single shorted turn), and
their respective wires from the board, LED will illuminate, and shorting the the LED count should drop to 1-3 as the
and finally the red battery snap wire to test leads will cause it to go off. loop is closed.
the free switch contact (Ref. to Fig.4. An effective way to test the unit is to If everything's OK, use double-sided
Text and illustrations courtesy of Electronics Australia Page 5
No4 x 6mm
Countersunk M3 x 6mm
S/T screw Countersunk
screw
Case pillar Spacer Banana
socket Front panel
Case
PCB
M3 x 6mm
Pan head
screw
Battery holder
Double-sided
tape
Fig.5: Shows how the PCB with LEDs is mounted to the front
panel using 19mm hex tapped spacers. The battery holder is
fixed to the bottom of the case by two pieces of double-sized
tape.
adhesive tape to stick the battery holder done now is to screw the front panel into Sabo, Michael Caplan and Wayne
into the bottom of the case, with the cells place and try out your tester on some Scicluna for their assistance in complet-
aligned in a `north-south' direction for LOPTs and their associated circuitry. ing this project. We couldn't have done it
easiest access. All that remains to be Finally, our sincere thanks to Larry without you!
Winding a Test Coil
In order for constructors to test the unit once assembled we have provided details
and parts to construct a simple transformer coil which enables the circuit to ring all
'8' LEDs.
Please refer to the following for coil details.
1.Using the balun core provided, wind around 45 turns (tightly wound) through
the two centre holes as shown in the accompanied photo.
2.Once completed trim lead length to approximately 50mm and clean the enamel
from each lead end so that a positive connection can be made.
3.Now test the coil, the unit should display and ring all '8' LEDs. By simply feed-
ing through an additional winding and shorting the ends will reduce the rings to
either 1 or 2 LEDs giving a good indication that the unit is working correctly.
Parts Supplied
1 x Balun core (R 5440)
1 x Enamel copper wire (30B&S or 0.25mm dia x 2 metres)
Assembly Notes
ACN 000 908 716
CNR Lane Cove & Waterloo Roads
North Ryde NSW 2113
PH: (02) (lnt 612) 9937 3200 Fax: (02) 9888 3631
Text and illustrations courtesy of Electronics Australia Dick Smith Electronics ◦ Jabse Service Manual Search 2024 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo