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Text and illustrations courtesy of Electronics Australia
I
f you're reading this, then chances are that you're a TV and/or computer mon- itor repair technician - who doesn't need to be told that horizontal output stage faults cause more than their fair share of headaches! Operating at high voltages, frequencies and power levels, many components in this part of the circuit are highly stressed, and failures are not only common but their cause is often hard to identify. The usual symptom of a major hori- zontal output stage fault is a serious overload of the DC power supply feeding the primary winding of the line output transformer, or LOPT' for short (called theflyback' transformer or FBT' in North America). This is often accompa- nied by a collector-to-emitter short cir- cuit in the horizontal output transistor orHOT'. (For consistency, we'll be referring to the line output transformer as the LOPT' throughout this article - North American readers please mentally substitutefly- back' for this term!) Any of quite a few possible compo- nents could be the cause of such a failure, the more common being one of the high- speed rectifier diodes fed by the LOPT's secondary windings, including the diode stack(s) which produce the extra-high-
tension (EHT) supply of around 25 kilo- volts for the final anode circuit of the cathode ray tube. It's also possible the HOT has failed simply from old age or overheating due to unevenly- applied/solidified heatsink compound. Another occasional culprit is an insula- tion breakdown in the deflection yoke's horizontal winding.
However the failure which service technicians dread is a shorted winding in the LOPT itself. Unfortunately LOPTs tend to be specifically designed for the make and model of the TV or monitor they are used in, which can mean a lot of hunting around for a replacement. In addition they are hardly ever cheap, and not always physically easy to replace. In short the LOPT is not a component which is easy to test by substitution, and a service technician needs to be as cer- tain as possible that the LOPT really is defective, before tracking down a replacement! Identifying faults
Several techniques have been devel- oped over the years for identifying faults in horizontal output stages, and testing LOPTs in particular for the presence of shorted winding turns. The components in the horizontal out- put transistor's collector circuit, includ- ing the LOPT's primary winding, deflec- tion yoke horizontal winding, and tuning capacitors form a reasonably low loss (high Q) resonant circuit, especially at low voltage levels. Most testing techniques, including the one used in this design, are based on the fact that nearly all serious faults in the
Assembly Manual
ACN 000 908 716
KK II TT
In-circuit LOPT/FBT
Tester
K 7205
Please read Disclaimer carefully as we can only guarantee parts and not the labour content you provide.
Cat No.
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.
WEBSITE: www.electronicsaustralia.com.au E-MAIL: electaus@magna.com.au
PROJECT INFORMATION SUPPLIED BY ELECTRONICS AUSTRALIA - August 1998 Issue
by Bob Parker
horizontal output stage will greatly increase the losses in the LOPT's primary circuit. That is, they lower the Q. We chose the principle of ring' testing as the basis for this instru- ment because it's easy to implement with relatively simple circuitry and common components, and produces predictable results with no need for calibration.Ring' testing gets its name from the fact that when a fast pulse is applied to the primary winding of the LOPT, the total inductance and capacitance in the circuit will produce an electrical ring' - a decaying AC voltage which can have a duration of a dozen or more cycles before it reaches a low value. It's the electrical equivalent of tap- ping an empty glass; in each case, an energy impulse generates damped oscil- lations. WaveformA' in Fig.1 shows the HOT collector voltage waveform in a typical fault-free TV (a General Electric TC63L1 in this case), in response to a pulse from this tester. However if the losses in the horizontal output circuit are increased, the amplitude of the ringing' waveform will decay much more quick- ly. WaveformB' shows the effect of a shorted rectifier diode on one LOPT sec- ondary winding of the same TV, but note that a shorted LOPT winding or several other faults would have a similar effect. A collector-emitter short in the HOT or a shorted tuning capacitor will result in no ringing at all, indicating a really major fault. So to do an initial check of a horizon- tal output stage, with this tester, you first make sure the TV or monitor is de-ener- gised(!). Then you simply switch the tester on, connect the ground lead to the
chassis and the HOT Collector' lead to the horizontal output transistor's collec- tor. One LED will illuminate for eachring' cycle above about 15% of the ini- tial pulse value, and in general if four or more LEDs are glowing, the horizontal output stage is OK. We'll talk more about using the tester later, after the circuit description. For the moment though, it's worth mentioning that because the tester uses a low-voltage testing pulse, it is suitable for testing LOPTs `in circuit' - i.e., without having to disconnect the yoke or other connec- tions. Circuit description
At first glance the circuit in Fig. might look a bit complicated, but it real- ly consists of three quite simple sections. These are the low frequency pulse gener- ator, the ring amplitude comparator and the LED bar-graph display. We'll now look at these in turn.
1. The low frequency pulse genera- tor: Voltage comparator IC1a is set up as a low frequency oscillator, whose output on pin 7 is normally pulled up to essen- tially the positive supply rail by R6 and R7. Due to the time constants produced
by C2, R4 and R5/D1, pin 7 pulses down to ground potential for about 2ms every 100ms, and it's during these low-going 2ms pulses that each ring test occurs. When IC1 pin 7 drops low, Q1 is driven into saturation by its base current flowing in R7, and its col- lector voltage jumps to the +6V supply, which makes two things happen. First, C6 in collaboration with R16 sends a positive pulse of about 5us duration to the reset pins of four-bit shift registers IC2a and IC2b, which drives all their outputs to a low state - switching off all the LEDs, in readiness for a new ring test. At the same time, about 20mA flows through R8, driving D2 into a low impedance state and dropping about 650mV across it. The voltage step across D2 is coupled via C3 to the test leads and the LOPT primary winding, causing this circuit to `ring' a bit below its natural res- onant frequency due to the presence of C3 (which functions as the resonating capacitor when testing an LOPT on its own).
2. The ring amplitude comparator: The `ringing' waveform is coupled by C to the inverting input of comparator IC1b, which is DC biased to about +490mV by the junction of R11 and R12. D3 is constantly forward-biased by about 1mA flowing through R10, and its entire voltage drop of about 600mV is applied to IC1b's non-inverting input as a refer- ence voltage, via R13. R14 produces a small amount of positive feedback around IC1b, ensuring that its output switches cleanly between its low and high voltage levels. The result of all this is that an inverted and squared-up version of the ringing waveform appears at the output of IC1b,
Page 2^ Text and illustrations courtesy of Electronics Australia
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Fig.2: The circuit is simple, but elegant. IC2 shows clearly how many rings are supported by the inductor under test.
FIg.1: Ringing waveforms from ‘good’ (top) and ‘shorted winding’ line output transformers, in response to the tester’s pulse.
collector, or excessively high EHT result- ing in HV shut-down. Because this tester uses impulses of only 650mV to minimize the forward biasing of semiconductors, such defects will not be reflected in the ring count. Under these circumstances, I check for measurable leakage resistance between the EHT cap and the other LOPT pins. It should be unmeasurable, otherwise the LOPT is defective. If I have gone through the above tests and have these symptoms and a normal ring count on the tester, the diagnosis can usually be confirmed only by substi- tuting a known-good identical LOPT, or by testing with a chopper similar to the one described in Sam Goldwasser's Electronics Repair FAQ, located on the Internet at http://pacwest.net/byron13/ sam/flytest.htm. Something else I do when testing a LOPT is to supply it with a reduced B+ to enable scoping the HOT and measur- ing EHT (in situations where the monitor goes into HV shutdown). To reduce the B+, I use two light bulbs in series, one end to B+ supply, centre-tap to LOPT B+ connection, other end to ground. One bulb is 60 watts, the other is 100, so I can reverse the end leads and increase or decrease the B+ value used in testing. At the outset, when I have power sup- ply cycling but have confirmed there are no shorts from HOT-C to ground, I sub- stitute a dummy load (60W bulb) for the LOPT where the B+ enters, to see if the power supply works with the LOPT out of the equation.
Overall, the LOPT tester can identify about 80% of LOPT failures. When try- ing to solve a puzzle, if someone offers information that is right 80% of the time, it's a lot better than having to guess 100% of the time, especially if the ante is the price of a LOPT and wasted, valu- able time. Michael Caplan does general electron- ic servicing in Ottawa, and added the fol- lowing useful points in relation to TVs: It's pretty straightforward to use, with the usual precautions of ensuring that the under-test unit power is off and any caps are discharged. When testing an LOPT in circuit, it might be necessary to disconnect some of the LOPT terminals, and/or yoke plugs that could load it down and upset the readings. The tester will often not detect bad HV diodes in integrated split-diode LOPT units, nor shorts/arcing that is voltage dependent - but then no other passive tester does either. I have found it useful for checking TV deflection yokes, both horizontal and vertical. A good yoke lights at least five and typically the full eight LEDs. However, many yokes have built-in par- allel or series damping resistors, and
these must be temporarily disconnected. Otherwise the reading will be low, even though the winding itself is fine. The tester can be used for checking high-Q transformers such as those used in SMPS's. However, my experience has shown that it will not provide more than a two or three LED indication for good TV horizontal drive transformers. It can be used for these, however - to indicate shorts (no LEDs lit). On the other hand the ESR Meter (Dick Smith catalog num- ber K-7204) can do much the same with these low resistance transformers. Wayne Scicluna services TVs in Sydney, and is the technician who talked me into developing the tester in the first place. Here are his hints: If you've already checked for the more obvious leaky and shorted semiconduc- tors and capacitors etc., and are still get- ting a low reading on the tester, there are some other traps to avoid. You need to get a good connection with the test leads, because contact resistance can cause a low reading. The same applies to defective solder joints in the horizontal output stage, especially on the LOPT itself and HOT. In fact connecting the tester with clip leads, flexing the board and wiggling components is a good way to show up bad solder joints in this area. Body conductivity can also cause a lower than normal reading if you're touching the test leads and your skin is damp. Low readings can also be caused by having the test leads reversed, i.e., connecting 'HOT Collector' to chassis, and by faults in an external voltage tripler. How to build it
Before soldering anything to the print- ed circuit board, hold it up to a bright light and examine the copper side care- fully for fine track breaks and especially whiskers or bridges - particularly where tracks pass close to component solder pads. Referring to the board overlay in Fig.3, begin installing the components, starting with the resistors and diodes and work- ing your way up to the tall ones including the four PCB pins for GND',HOT' and `+6V' terminal connections- but leaving
Page 4^ Text and illustrations courtesy of Electronics Australia
Value 4 Band (1%) 5 Band (1%)
270R Red-Vio-Brn-Brn Red-Vio-Blk-Blk-Brn 1K Brn-Blk-Red-Brn Brn-Blk-Blk-Brn-Brn 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 47K Yel-Vio-Org-Brn Yel-Vio-Blk-Red-Brn 150K Brn-Grn-Yel-Brn Brn-Grn-Blk-Org-Brn 1M Brn-Blk-Grn-Brn Brn-Blk-Blk-Yel-Brn 2.2M Red-Red-Grn-Brn Red-Red-Blk-Yel-Brn
Resistor Colour Codes
Printed Circuit Board to Spacers 4 x Screw M3 x 6mm (zinc plated)
Front Panel to Spacers 4 x Screw Countersunk M3 x 6mm (Blk)
Front Panel To Case 4 x Screw Countersunk No4 x 6mm (Blk)
Screw Size and Allocation
Guide
Value IEC Code EIA Code
100pF 100p 101K 0.01uF 10n 103K 0.047uF 47n 473K
Capacitor Codes
The assembled PCB, which supports virtually all of the circuitry.
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 the LEDs into their cutouts in the front panel, and push each LED down slightly to ensure its face is level with the front of the panel. In the unlikely event that a LED won't fit, use a small file or similar to remove the excess powder coating inside the hole. Now solder all the LEDs into place, then connect the test lead sockets and the closest terminal of the power switch to their respective wires from the board, and finally the red battery snap wire to the free switch contact (Ref. to Fig.4.
wiring diagram). Snip off the battery holder's PCB mounting pins, then install four AAA' cells into it. Connect the battery snap to the terminals, and switch the unit on. If everything's OK then the bottom red (1') LED will illuminate, and shorting the test leads will cause it to go off. An effective way to test the unit is to
connect the test leads to the primary winding of a known good LOPT out of circuit, which should bring all eight LEDs on. Then thread a loop of solder around the ferrite core of the LOPT (simulating a single shorted turn), and the LED count should drop to 1-3 as the loop is closed. If everything's OK, use double-sided
Text and illustrations courtesy of Electronics Australia (^) Page 5
Fig.4: Shows how the battery snap (positive lead) is wired through the switch to the printed circuit board. Note, as the component overlay shown is viewed from the copper side of the PCB, wiring terminations for the Power and Hot Collector/GND should be made to the PCB pins on the component side of the board.
