Text and illustrations courtesy of Electronics Australia

If you're reading this, then chances arethat you're a TV and/or computer mon-

itor repair technician - who doesn't needto be told that horizontal output stagefaults cause more than their fair share ofheadaches! Operating at high voltages,frequencies and power levels, manycomponents in this part of the circuit arehighly stressed, and failures are not onlycommon but their cause is often hard toidentify.

The usual symptom of a major hori-zontal output stage fault is a seriousoverload of the DC power supply feedingthe primary winding of the line outputtransformer, or `LOPT' for short (calledthe `flyback' transformer or `FBT' inNorth America). This is often accompa-nied by a collector-to-emitter short cir-cuit in the horizontal output transistor or`HOT'.

(For consistency, we'll be referring tothe line output transformer as the `LOPT'throughout this article - North Americanreaders please mentally substitute `fly-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'ssecondary windings, including the diodestack(s) which produce the extra-high-

tension (EHT) supply of around 25 kilo-volts for the final anode circuit of thecathode ray tube. It's also possible theHOT has failed simply from old age oroverheating due to unevenly-applied/solidified heatsink compound.Another occasional culprit is an insula-tion breakdown in the deflection yoke'shorizontal winding.

However the failure which servicetechnicians dread is a shorted winding inthe LOPT itself. Unfortunately LOPTstend to be specifically designed for themake and model of the TV or monitorthey are used in, which can mean a lot ofhunting around for a replacement. Inaddition they are hardly ever cheap, andnot always physically easy to replace.

In short the LOPT is not a componentwhich is easy to test by substitution, anda service technician needs to be as cer-tain as possible that the LOPT really isdefective, before tracking down areplacement!

Identifying faults

Several techniques have been devel-oped over the years for identifying faultsin horizontal output stages, and testingLOPTs in particular for the presence ofshorted 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 tuningcapacitors form a reasonably low loss(high Q) resonant circuit, especially atlow voltage levels.

Most testing techniques, including theone used in this design, are based on thefact that nearly all serious faults in the

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ACN 000 908 716

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In-circuit LOPT/FBTTester

K 7205

Please read Disclaimer carefully as wecan only guarantee parts and not thelabour content you provide.

Cat No.

Here’s the design for a low cost, easy to build and use battery operated ‘shorted turns’ tester forline-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/FBTfaults, 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.auE-MAIL: electaus@magna.com.au

PROJECT INFORMATION SUPPLIED BYELECTRONICS AUSTRALIA - August 1998 Issue

by Bob Parker

horizontal output stage will greatlyincrease the losses in the LOPT'sprimary circuit. That is, they lowerthe Q.

We chose the principle of `ring'testing as the basis for this instru-ment because it's easy to implementwith relatively simple circuitry andcommon components, and producespredictable results with no need forcalibration.

`Ring' testing gets its name fromthe fact that when a fast pulse isapplied to the primary winding ofthe LOPT, the total inductance andcapacitance in the circuit will produce anelectrical `ring' - a decaying AC voltagewhich can have a duration of a dozen ormore cycles before it reaches a lowvalue. It's the electrical equivalent of tap-ping an empty glass; in each case, anenergy impulse generates damped oscil-lations.

Waveform `A' in Fig.1 shows the HOTcollector voltage waveform in a typicalfault-free TV (a General ElectricTC63L1 in this case), in response to apulse from this tester. However if thelosses in the horizontal output circuit areincreased, the amplitude of the `ringing'waveform will decay much more quick-ly. Waveform `B' shows the effect of ashorted rectifier diode on one LOPT sec-ondary winding of the same TV, but notethat a shorted LOPT winding or severalother faults would have a similar effect.

A collector-emitter short in the HOTor a shorted tuning capacitor will resultin no ringing at all, indicating a reallymajor fault.

So to do an initial check of a horizon-tal output stage, with this tester, you firstmake sure the TV or monitor is de-ener-gised(!). Then you simply switch thetester on, connect the ground lead to the

chassis and the `HOT Collector' lead tothe horizontal output transistor's collec-tor. One LED will illuminate for each`ring' cycle above about 15% of the ini-tial pulse value, and in general if four ormore LEDs are glowing, the horizontaloutput stage is OK.

We'll talk more about using the testerlater, after the circuit description. For themoment though, it's worth mentioningthat because the tester uses a low-voltagetesting pulse, it is suitable for testingLOPTs `in circuit' - i.e., without havingto disconnect the yoke or other connec-tions.

Circuit description

At first glance the circuit in Fig.2might 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 andthe LED bar-graph display. We'll nowlook at these in turn.

1. The low frequency pulse genera-tor: Voltage comparator IC1a is set up asa low frequency oscillator, whose outputon pin 7 is normally pulled up to essen-tially the positive supply rail by R6 andR7. Due to the time constants produced

by C2, R4 and R5/D1, pin 7 pulsesdown to ground potential for about2ms every 100ms, and it's duringthese low-going 2ms pulses thateach ring test occurs.

When IC1 pin 7 drops low, Q1 isdriven into saturation by its basecurrent flowing in R7, and its col-lector voltage jumps to the +6Vsupply, which makes two thingshappen. First, C6 in collaborationwith R16 sends a positive pulse ofabout 5us duration to the reset pinsof four-bit shift registers IC2a andIC2b, which drives all their outputs

to a low state - switching off all theLEDs, in readiness for a new ring test.

At the same time, about 20mA flowsthrough R8, driving D2 into a lowimpedance state and dropping about650mV across it. The voltage step acrossD2 is coupled via C3 to the test leads andthe LOPT primary winding, causing thiscircuit to `ring' a bit below its natural res-onant frequency due to the presence ofC3 (which functions as the resonatingcapacitor when testing an LOPT on itsown).

2. The ring amplitude comparator:The `ringing' waveform is coupled by C4to the inverting input of comparatorIC1b, which is DC biased to about+490mV by the junction of R11 and R12.D3 is constantly forward-biased by about1mA flowing through R10, and its entirevoltage drop of about 600mV is appliedto IC1b's non-inverting input as a refer-ence voltage, via R13. R14 produces asmall amount of positive feedbackaround IC1b, ensuring that its outputswitches cleanly between its low andhigh voltage levels.

The result of all this is that an invertedand squared-up version of the ringingwaveform appears at the output of IC1b,

Text and illustrations courtesy of Electronics AustraliaPage 2

LED4YEL

LED5YEL

LED1RED

R181k

R191k

R201k

R211k

R221k

R231k

R241k

R171k

R14 1M

D31N4148

R12150k

+

-

6V(4xAAA)

SW1POWER

C1100uF

BATT+

BATT-

R21M

R11M

R31M

6

5

7

8

IC1a

R4 2.2M

IC1

LM393

IC2

4015

R5

D11N4148

C20.047uF

47k

D21N4148

R91k

C30.047uF

C40.01uF

R71k

R61k

Q1BC328

C6100pF

R1133k

R8

270�

C50.047uF

R1310k

3

2

1

4

IC1b

R104.7k

R154.7k

R1647k

HOT(Collector)

EARTH

LED2RED

LED3RED

LED6GRN

LED7GRN

LED8GRN

13 12 11 2 7 5 4 3 10

15

14

6

1

9

16

8

C70.047uF

RESET A

RESET B

CLK A

CLK B

DATAA

Q0A Q1A Q2A Q3ADATA

B Q0B Q1B Q2B Q3B

Vss

Vcc

+

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, inresponse to the tester’s pulse.

until the ringing amplitude has decayeddown to about 15% of its initial value.This square wave is connected straight tothe clock inputs of shift registers IC2aand IC2b.

3. The LED bargraph display: IC2consists of a pair of identical four-bitserial-in/parallel-out shift registers, con-nected to form a single eight-bit unit,with each output driving one LED in the`bargraph' display via resistors R17 toR24. The serial data input of the firststage (pin 15) is permanently connectedto the positive supply, or logic 1.

One measurement

For the first 5us after the commence-ment of a new 2ms measuring pulse,both shift registers are reset to zeroes onall outputs, as described earlier. At thesame time the initial positive pulseapplied to the LOPT drives IC1b's out-

put, connected to both shift registers'clock inputs, to a low (logic 0) level -unless the test leads are shorted.

If the LOPT primary circuit is OK, itwill ring during the next several hundredmicroseconds. For each ring above about15% of its initial value, it will cause ahigh-going pulse to be applied to theshift register clock inputs, resulting inthe logic 1 on IC2 pin 15 being movedone shift register stage further along. Itdoesn't matter if the LOPT rings morethan eight times - all LEDs will stillremain illuminated.

So the overall result is that one LEDilluminates for each LOPT ring cycleabove 15% of the initial level, and thiscondition remains until the start of thenext 2ms measuring pulse. Phew!

Usage & limitations

In order to assess the usefulness of thisdesign, we gave several prototypeLOPT/FBT testers to technician friendsto evaluate for many months, then askedfor their comments and thoughts on howto put the tester to best use.

The first response is from Larry Sabo,an experienced monitor technician inOttawa, Canada who also suggested thefront panel layout:

One of the first things I do to check out

a monitor is connect the tester betweenthe HOT collector and ground. If no oronly a few LEDs light, I check the HOT,damper diodes and tuning caps forshorts using a DMM. If these are OK, Icheck for an open fusible resistor in thecircuit feeding B+ to the LOPT, and forshorts/leakage in diodes on the LOPTsecondaries. I also check the bypasscapacitor on the DC supply to the LOPTprimary for excessive ESR.

If these check OK, I ring the horizontalyoke with its connector unplugged. It willnormally ring seven times on its own. Ifthe yoke rings OK, I unsolder all but theLOPT primary winding and ground pins,and ring the primary. If the primary stillrings low with everything else discon-nected, the LOPT is probably defective.

Most LOPTs on their own will ring 8+times, but some ring only four or five,even when they are perfectly normal. Soit is prudent to confirm the diagnosis byringing an identical known-good LOPT,if at all possible.

Sometimes an LOPT is defective, butstill rings normally with the tester, e.g.due to leakage or arcing that only occursat full operating voltage. The problemwill sometimes be manifest by heavyloading of the B+ supply, spurious ring-ing and/or reduced voltages on the HOT

Text and illustrations courtesy of Electronics Australia Page 3

Fig.3: Use this PCB overlay and the facing photo as a guide in assembling thetester.

Resistors(All 5% 0.25W carbon) R1,2,3,14 1MR4 2.2MR5,16 47kR6,7,9,R17-24 1kR8 270 ohmsR10,15 4.7kR11 33kR12 150kR13 10k

CapacitorsC1 100uF 16/25VW

RB electrolyticC2,3,5,7 0.047uF MKTC4 0.01uF MKTC6 100pF disc ceramic

SemiconductorsD1,2,3 1N914 / 1N4148 silicon

diodeIC1 LM393 dual comparatorIC2 4015 / MC14015 / CD4015

dual 4-bit shift registerLED1,2,3 Rectangular red LEDLED4,5 Rectangular yellow LEDLED6,7,8 Rectangular green LEDQ1 BC328 / 2N5819 PNP

silicon transistor

MiscellaneousPCB, 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-tions.

Parts List

collector, or excessively high EHT result-ing in HV shut-down.

Because this tester uses impulses ofonly 650mV to minimize the forwardbiasing of semiconductors, such defectswill not be reflected in the ring count.Under these circumstances, I check formeasurable leakage resistance betweenthe EHT cap and the other LOPT pins. Itshould be unmeasurable, otherwise theLOPT is defective.

If I have gone through the above testsand have these symptoms and a normalring count on the tester, the diagnosiscan usually be confirmed only by substi-tuting a known-good identical LOPT, orby testing with a chopper similar to theone described in Sam Goldwasser'sElectronics Repair FAQ, located on theInternet at http://pacwest.net/byron13/sam/flytest.htm.

Something else I do when testing aLOPT is to supply it with a reduced B+to enable scoping the HOT and measur-ing EHT (in situations where the monitorgoes into HV shutdown). To reduce theB+, I use two light bulbs in series, oneend to B+ supply, centre-tap to LOPT B+connection, other end to ground. Onebulb is 60 watts, the other is 100, so I canreverse the end leads and increase ordecrease the B+ value used in testing.

At the outset, when I have power sup-ply cycling but have confirmed there areno shorts from HOT-C to ground, I sub-stitute a dummy load (60W bulb) for theLOPT where the B+ enters, to see if thepower supply works with the LOPT outof the equation.

Overall, the LOPT tester can identifyabout 80% of LOPT failures. When try-ing to solve a puzzle, if someone offersinformation that is right 80% of the time,it's a lot better than having to guess100% of the time, especially if the ante isthe 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, withthe usual precautions of ensuring thatthe under-test unit power is off and anycaps are discharged.

When testing an LOPT in circuit, itmight be necessary to disconnect some ofthe LOPT terminals, and/or yoke plugsthat could load it down and upset thereadings. The tester will often not detectbad HV diodes in integrated split-diodeLOPT units, nor shorts/arcing that isvoltage dependent - but then no otherpassive tester does either.

I have found it useful for checking TVdeflection yokes, both horizontal andvertical. A good yoke lights at least fiveand 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, eventhough the winding itself is fine.

The tester can be used for checkinghigh-Q transformers such as those usedin SMPS's. However, my experience hasshown that it will not provide more thana two or three LED indication for goodTV horizontal drive transformers. It canbe used for these, however - to indicateshorts (no LEDs lit). On the other handthe ESR Meter (Dick Smith catalog num-ber K-7204) can do much the same withthese low resistance transformers.

Wayne Scicluna services TVs inSydney, and is the technician who talkedme into developing the tester in the firstplace. Here are his hints:

If you've already checked for the moreobvious leaky and shorted semiconduc-tors and capacitors etc., and are still get-ting a low reading on the tester, there aresome other traps to avoid.

You need to get a good connection withthe test leads, because contact resistancecan cause a low reading. The sameapplies to defective solder joints in thehorizontal output stage, especially on theLOPT itself and HOT. In fact connectingthe tester with clip leads, flexing theboard and wiggling components is agood way to show up bad solder joints inthis area.

Body conductivity can also cause alower than normal reading if you'retouching the test leads and your skin isdamp. Low readings can also be causedby having the test leads reversed, i.e.,connecting 'HOT Collector' to chassis,and by faults in an external voltagetripler.

How to build it

Before soldering anything to the print-ed circuit board, hold it up to a brightlight and examine the copper side care-fully for fine track breaks and especiallywhiskers or bridges - particularly wheretracks pass close to component solderpads.

Referring to the board overlay in Fig.3,begin installing the components, startingwith the resistors and diodes and work-ing your way up to the tall ones includingthe four PCB pins for `GND', `HOT' and`+6V' terminal connections- but leaving

Text and illustrations courtesy of Electronics AustraliaPage 4

Value 4 Band (1%) 5 Band (1%)

270R Red-Vio-Brn-Brn Red-Vio-Blk-Blk-Brn1K Brn-Blk-Red-Brn Brn-Blk-Blk-Brn-Brn4.7K Yel-Vio-Red-Brn Yel-Vio-Blk-Brn-Brn10K Brn-Blk-Org-Brn Brn-Blk-Blk-Red-Brn33K Org-Org-Org-Brn Org-Org-Blk-Red-Brn47K Yel-Vio-Org-Brn Yel-Vio-Blk-Red-Brn150K Brn-Grn-Yel-Brn Brn-Grn-Blk-Org-Brn1M Brn-Blk-Grn-Brn Brn-Blk-Blk-Yel-Brn2.2M Red-Red-Grn-Brn Red-Red-Blk-Yel-Brn

Resistor Colour Codes

Printed Circuit Board to Spacers4 x Screw M3 x 6mm (zinc plated)

Front Panel to Spacers4 x Screw Countersunk M3 x 6mm (Blk)

Front Panel To Case4 x Screw Countersunk No4 x 6mm (Blk)

Screw Size and AllocationGuide

Value IEC Code EIA Code

100pF 100p 101K0.01uF 10n 103K0.047uF 47n 473K

Capacitor Codes

The assembled PCB, which supportsvirtually all of the circuitry.

the LEDs off the board for now. Takecare with the orientation of the polarisedcomponents, including the IC sockets.

With everything but the LEDs installedon the PCB, once again illuminate itfrom the top, and check for and correctany solder bridges or other problems.

Now turn your attention to the frontpanel, mounting the banana sockets andthe power switch in their respectiveholes.

Attach the tapped spacers to the cor-ners of the board using plain 3mmscrews, and solder long component leadoffcuts 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 theleads of all the LEDs through theirrespective holes in the board. Make surethe coloured LEDs are in their correctplaces, 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-downon a soft flat surface. Manoeuvre all ofthe LEDs into their cutouts in the frontpanel, and push each LED down slightlyto ensure its face is level with the front ofthe panel. In the unlikely event that aLED won't fit, use a small file or similarto remove the excess powder coatinginside the hole.

Now solder all the LEDs into place,then connect the test lead sockets and theclosest terminal of the power switch totheir respective wires from the board,and finally the red battery snap wire tothe 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 tothe terminals, and switch the unit on. Ifeverything's OK then the bottom red (`1')LED will illuminate, and shorting thetest leads will cause it to go off.

An effective way to test the unit is to

connect the test leads to the primarywinding of a known good LOPT out ofcircuit, which should bring all eightLEDs on. Then thread a loop of solderaround the ferrite core of the LOPT(simulating a single shorted turn), andthe LED count should drop to 1-3 as theloop 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 tothe printed circuit board. Note, as the component overlay shown is viewed fromthe copper side of the PCB, wiring terminations for the Power and HotCollector/GND should be made to the PCB pins on the component side of theboard.

ACN 000 908 716CNR Lane Cove & Waterloo Roads

North Ryde NSW 2113PH: (02) (lnt 612) 9937 3200 Fax: (02) 9888 3631

Text and illustrations courtesy of Electronics Australia Dick Smith Electronics © ZA8738-2

adhesive tape to stick the battery holderinto the bottom of the case, with the cellsaligned in a `north-south' direction foreasiest access. All that remains to be

done now is to screw the front panel intoplace and try out your tester on someLOPTs and their associated circuitry.

Finally, our sincere thanks to Larry

Sabo, Michael Caplan and WayneScicluna for their assistance in complet-ing this project. We couldn't have done itwithout you!

Case

Battery holder

PCB

Front panel

M3 x 6mm

Countersunk

screw

Double-sided

tape

Case pillar

No4 x 6mm

Countersunk

S/T screw

Banana

socket

M3 x 6mm

Pan head

screw

Spacer

Fig.5: Shows how the PCB with LEDs is mounted to the frontpanel using 19mm hex tapped spacers. The battery holder isfixed to the bottom of the case by two pieces of double-sizedtape.

Winding a Test CoilIn 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 toeither 1 or 2 LEDs giving a good indication that the unit is working correctly.

Parts Supplied1 x Balun core (R 5440)1 x Enamel copper wire (30B&S or 0.25mm dia x 2 metres)

Assembly Notes