CONTENTS
1
Anal An alyz yzin ing g The The HV Ci Circ rcui uits ts Wi With th Th The e VA VA62 62A A
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Anal An alyz yzin ing g The The Ho Hori rizo zont ntal al Out utpu putt Pul Pulse se Wi With th The Waveform Analyzer
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The Th e TV TVA92 92s s Ho Hori rizo zont ntal al Ou Outp tput ut Tes ests ts
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Unde Un ders rsta tand ndin ing g the the LC LC10 103 3s s InIn-C Cir ircu cuit it Capacitor Test
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Unde Un ders rsta tand ndin ing g Hor Horiz izon onta tall Out Outpu putt Sta Stag ges of Computer Monitors
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Analyzing The Horizontal Output Pulse With The Waveform Analyzer Of all the TV waveforms you have to analyze, the horizontal output transistor collector pulse is the most important because this output pulse is used to perform many other functions than to just sweep the CRT beam horizontally. It can easily be said that the horizontal output transistor is the heart of a TV. Let's take a careful look at nine of the key functions this all-important horizontal output waveform is responsible for, how to fully analyze it, and some possible problems. The horizontal output stage is practically responsible for the complete and efficient operation of the entire TV. The waveform at the collector of the horizontal output
transistor is the most important waveform you should check on every TV before you begin changing parts, and after every TV is repaired .
sensitive position and hook up to the horizontal output transistor collector, there is no need to panic as no damage will result.
Looking At The Horizontal Output Pulse With In Three Simple Steps
1) Connect the TV AC line to an isolation transformer, such as the Sencore PR57 POWERlTE ® . The isolation transformer protects you and your equipment from electrical shock and damage by isolating the HOT chassis from you and your Waveform Analyzer .
Sencore's Waveform Analyzers use a highly accurate low capacity probe network that lets you safely measure the horizontal output transistor and other pulse waveforms to 2,000 volts (DC + peak AC). Even if you should happen to accidentally leave your Waveform Analyzer input attenuator in its most
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Fig. 1: The horizontal output pulse is responsible for the efficient operation of the TV.
2) Hook up the probe ground to the TV chassis ground, then connect the probe to the collector of the horizontal output transistor.
3) Adjust the vertical volts per division control to the 200 volts position. Switch your time base control all the way down to the video preset position. Push in the horizontal preset button and you will see waveforms that include two lines of video information.
Fig. 3: Two common faulty waveforms which cause problems with the operation of a TV.
Fig. 2: A typical horizontal output pulse.
lines in the video picture. Look for a shorted secondary power diode, shorted IHVT diodes or shorted windings. The trace time should be clean of any noise before performing the measurements to follow .
What To Look For In This Waveform
Measuring The Waveform Parameters (Automatically)
Before you do any measurements, take a second or two to look over the waveform itself. It should look just like the horizontal waveform in Figure 2. The waveform should be symmetrical in shape during pulse retrace time. If it is not, look for a change in the value of the horizontal output transistor stage timing capacitors, or an excessive load on a B+ supply. The waveform should be symmetrical before you proceed with testing .
You'll need to make four measurements (along with the waveshape covered earlier) of the horizontal output waveform to be sure that it is operating safely or when you are troubleshooting the horizontal section of the TV:
The trace or scanning “on time'' of the transistor should also be relatively clean. Any excessive ringing is a clear indicator of deflection system problems such as a cracked integrated high voltage transformer (IHVT) core or open IHVT windings. The trace must be clean before you analyze the pulse any further. If it is not, look for other noise pulses riding along during the trace time. They could be causing faint noises or drive
FIRST: Push the DCV button for fully autoranged DV voltage measurements. SECOND: Push the VPP button for automatic peak-to-peak measurements. THIRD: Push the FREQ button for automatic and noise free frequency measurements. These three measurements are the first ones to be made. They tell you the condition of the regulated B+ supply, and that the TV is not in the shut-down mode. The next measurement is used to help prevent future component failures.
How Important Is The Duration Measurement Of The Horizontal Output Transistor Waveform? Of all the horizontal output transistor waveform parameters, the“duty-cycle” measurement tells you the most. Because of the many jobs that this critical circuit performs, TV manufacturers carefully specify the horizontal output transistor “duty-cycle” or time duration in exact microseconds as follows: Retrace time: 11.5 - 16 microseconds Trace time: 47.5 - 52 microseconds They make these specifications for a very good reason. If the time duration (dutycycle) is too short during retrace, speed and excessive voltage will be developed; therefore, excessive power will be dissipated. This generates heat which will cause TV parts damage in time. IMPORTANT: Always r efer to the manufacturers' schematic or literature for the particular chassis timing. The Waveform Analyzer is especially equipped to measure portions of a waveform with the DELTATIME feature.
To make this important measurement: 1. Align the pulse by using the VOLTS/DIVISION and the CAL. knobs so the top of the retrace pulse is on the 100% graticule marking. 2. Select the dual channel mode by pushing the A & B button. 3. Switch t he CHANNEL B INPUT COUPLING switch to ground, and align the trace with the VERTICAL POSITION control so it lies on the 10% graticule marking. 4. Press the DELTATIME button, 5. Adjust the DELTA BEGIN knob so the left-side of the intensified trace aligns with the left-side intersection of the CHANNEL A and CHANNEL B traces (Figure 4).
What If The Horizontal Output Was Only 5 Microseconds Off? Suppose you measure 9 microseconds instead of 14 microseconds for the retrace pulse. On a TV with this type of problem, the peak-to-peak value could be good, the DC reading could be close, and the waveform would look close enough. Even the frequency could be right on 15,734 Hz. This TV will work for a while. Shifting the retrace duty cycle 5 microseconds does not look like much, or even sound like much. But, to the horizontal output system, it sees a 35.7% reduction in retrace time meaning that retrace is faster and this generates higher voltage that means the horizontal output transistor is “on” just a little longer at full scan conduction. Increased conduction time means increased heat.
Increased scan time means increased scan derived power supply levels. The power supply capacitors have a longer time to charge and reach higher voltages. All the circuits are now stressed and must work at this higher voltage.
Isolate Start-up and Shut-Down Problems With The Horizontal Output Pulse The CRT can be used to watch for an instantaneous start-up pulse. Simply connect the Waveform Analyzer and preset the CRT controls as described earlier. Then, watch the CRT as you apply power to the TV's circuitry. If you see a pulse appear then disappear your start-up circuitry is operating correctly and the set is in the shut-down mode. If this happens, you have to service the chassis in a “powered down” condition., at either half the normal B+ level, supplied separtately, or reduce the ACinput power to half power (60 VAC) and monitor the collector of the horizontal output transistor with your scope. NOTE: If the chassis uses a switch mode power supply (SMPS) as the B + source, you need to determine if the SMPS is defective, or if the porblem is on the horizontal output stage. Refer to Tech Tip #205 "Identify SMPS Problems" for information on how to do this.
Fig. 4: The time duration measurement of the retrace puse should be made from the 10% to 10% levels.
For More Informat ion, Call T oll Free 1 -800 -SENCORE (1-800-736-2673)
6. Adjust the DELTA END knob to align the right-side of the intensified trace with the right-side intersection of the two traces. 3200 Sencore Drive, Sioux Falls, SD 57107
7. Read the digit al display directly in microseconds to see that you are within 11.5 to 16 microseconds .
Fig. 5: The digital display shows the timing.
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Form 4968 Printed In U.S.A.
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#118
The TVA92’s Horizontal Output Tests This Tech Tip presents a logical, step-bystep procedure for using the TVA92’s Horizontal Output Tests. While these tests can be used individually, this test sequence gives you the best opportunity to safely identify different types of horizontal circuit defects. If you need additional information on how the horizontal output stage operates or would like an indepth explanation of each test, please refer to the following Tech Tips: #207 “Understanding the TV Horizontal Output Stage” #208 “Making Horizontal Output Dynamic Measurements” #209 “Understanding the TVA92’s Horizontal Output Load Test” #210 “Understanding the TVA92’s Horizontal Output Device Sub & Drive.”
Recommended TVA92 Horizontal Output Tests Procedure The following outline presents an overview of the recommended procedure
for using the TVA92’s Horizontal Output Tests. I. HORIZ OUTPUT LOAD TESTS – AC power removed from the chassis (TV Off) A. Check current draw - mA B. Check output pulse time - uS II. HORIZ OUTPUT DYNAMIC TESTS – AC power applied to the chassis (TV On) A. H.O.T. removed, HORIZ DEVICE SUB & DRIVE off 1.Check unloaded B+ power supply - DCV 2.Check for horizontal drive to the H.O.T. - INPUT DRIVE B. H.O.T. removed, HORIZ DEVICE SUB & DRIVE on 1.Check current draw for excessive load - DEVICE SUB CURRENT 2.Check loaded B+ power supply - DCV 3.Check flyback pulse amplitude PULSE PPV 4.Check flyback pulse time PULSE TIME uS C. H.O.T. installed, HORIZ DEVICE SUB & DRIVE off 1.Monitor B+ power supply DCV 2.Monitor flyback pulse amplitude - PULSE PPV 3.Monitor flyback pulse time PULSE TIME uS
Performing the TVA92 Horizontal Output Tests The following procedure is a detailed look at the above outline. It describes how to do the tests, what readings to
expect, and what to do when a bad reading is indicated. This procedure covers the most common results and defects. I. Hori zontal Output Load Tests The first TVA92 Horizontal Output Tests to perform are the Load Tests. These two tests give an indication of any major defects in the horizontal stage. During the Load Tests the TVA92 supplies a B+ voltage at approximately 10% of normal to the horizontal output stage CAUTION
The HORIZ OUTPUT LOAD TESTS produce flyback voltages at the collector of the chassis horizontal output transistor and the fl yback secondaries. Do not come in contact with energized circuit points during the Load Tests. A. Current – mA 1. Remove power from the TV. These tests should never be performed with AC power applied to the chassis. The load tests can be performed with the H.O.T. in or out-of-circuit. If you find the H.O.T. is shorted, remove it and proceed with the Load Tests. 2. Connect the RINGER/LOAD TEST leads as follows: black lead to the H.O.T.’s emitter or equivalent connection if the H.O.T. is removed, yellow lead to the H.O.T.’s collector or equivalent connection if the H.O.T. is removed, and orange to the B+ connection on the flyback. 3. Set the HORIZ OUTPUT TEST selector to HORIZ OUTPUT LOAD TEST mA and note the current reading on
Fig. 1: Controls used to perform the HORIZ OUTPUT TESTS.
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TVA92 HORIZ OUTPUT TESTS RINGER TESTS YOKE & FLYBACK •
DYNAMIC TESTS DCV
•
•
SWITCHING • XFORMER HORIZ OUTPUT LOAD TEST
PULSE PPV PULSE
• •
•
uS •
•
mA T V O FF
T V O N
INPUT DRIVE
DEVICE SUB CURRENT
HORIZ OUTPUT DEVICE SUB & DRIVE (CURRENT LEVEL)
0
SUB ON
OVERLOAD
OFF
1.5A MAX
RINGER/LOAD TESTS
DYNAMIC TESTS
(Orange)
OFF: TVS HORIZ OUTPUT ACTIVE ON: TVS HORIZ OUTPUT SUBISTITUTED
1500V MAX
! SSIS
FLOATING GROUND 1000V ISO
g. 2: Connecti ons to perform the HORIZ OUTPUT LOAD TESTS.
the LCD. The acceptable range is 5-80mA. 4. A reading of greater than 80mA indicates excessive current. To isolate the defect, disconnect the yellow clip lead from the collector and note the current reading. a) DC leakage is indicated if the current stays above 15mA. The defect is caused by a DC leakage path on the primary side of the flyback, such as a damper diode, retrace capacitor or leaky component in the B+ line. b) AC leakage is indicated if the current falls below 15mA. A defective IHVT, horizontal yoke, or secondary loading can cause a short of this type.
B.Timing – uS 1. Set the HORIZ OUTPUT TESTS selector to HORIZ OUTPUT LOAD TEST uS and note the timing reading on the LCD. 2. The acceptable range is 11.3-15.9uS with a stable reading. 3. A reading that is stable but outside this range indicates a defective timing component. a) A reading that is greater than 15.9uS is likely caused by an open yoke or yoke series capacitor. b) A reading that is less than 11.3uS is likely caused by defective retrace timing capacitors, IHVT, or excessive loading on the secondaries.
ote: A reading close to or above 250mA ndicates a direct DC short to ground, sually caused by a shorted H.O.T. or amper diode.
Note: A few chassis manufactured by NAP may normally return timing readings slightly less than 11.3uS.
5. A reading of less than 5mA or dashes indicates an invalid connection or open in the circuit. Be sure the mA reading is within the acceptable range before proceeding to the Dynamic Test.
4. A reading that is fluctuating indicates that the flyback pulse waveshape contains ringing or multiple pulses. This is likely due to a defective flyback, excessive loading on the secondaries, or leakage.
Note: A few chassis may return a reading that fluctuates from normal to 0.1-0.5uS. This is due to the different impedance in the leads and circuit, not a defect in the set. 5. A reading of dashes (“- - -”) indicates improper lead hook-up or an open in the circuit. Be sure the pulse time is within the acceptable range before proceeding. II.Horizontal Output Dynamic Tests The three sections of the TVA92’s Horizontal Output Dynamic Tests provide a quick and easy B+ voltage measurement, input drive test, horizontal output waveform analysis, and horizontal output device substitution. A. Dynamic B+ and Input Drive Measurements The first Dynamic Test enables you to check the regulated B+ supplied to the horizontal output stage and the input drive to the base of the H.O.T. 1. Remove the H.O.T. from the chassis and connect the Dynamic Test Leads
TEST:
NORMAL RANGE
BAD RANGE
mA
5-80 mA
<5 mA or >80 mA
µS
11.3 – 15.9 µS
<11.3 µs or >15.9 µS
Tabl e 1: HORIZ OUTPUT LOAD TESTS Good/Bad ranges.
HORIZ LOAD TEST READOUTS mA µS
MOST LIKELY CAUSES
----
----
• Improper Connections • Open Flyback • Open Output Stage Circuit Paths
BAD
----
• Severe B+ Supply Short Or Leakage Path • < 5 mA = Open Flyback Or Circuit Path
GOOD ----
• Open Flyback • Improper “Collector” Connection • Open Ringer/Load Fuse
GOOD GOOD
• No Severe Loading Or Timing Defects
BAD
• Severe B+ Leakage And/Or Flyback Secondary Short Or Leakage Path • Flyback Transformer
GOOD
GOOD BAD
• Defective Output Timing Components • Flyback Transformer • Severe Flyback Secondary Short Or Leakage Path
BAD
• • • •
BAD
Severe B+ Leakage Flyback Secondary Short Or Leakage Path Flyback Transformer Defective Output Timing Components
NOTE: Fluctuating µS readout values indicate abnormal flyback pulse ringing or timing.
Table 2: Possible HORIZ OUTPUT LOAD TEST readings and l ikely causes.
as follows: red to the collector connection, blue to the base connection, and black to the emitter connection or circuit ground. Note: A few chassis do not connect the H.O.T. emitter to ground. In this case, connect the black clip-lead to circuit ground, not the emitter connection. 2. Set the HORIZ OUTPUT TESTS selector to DCV, apply AC power to the chassis, and note the voltage reading on the LCD. This reading should closely match the schematic’s value for regulated B+. If the reading does not stabilize to this value, turn the HORIZ OUTPUT DEVICE SUB &
DRIVE on just enough so the SUB ON LED lights. This will provide feedback to the power supply if necessary. The DCV reading should be near the schematic’s value. If it is not, the power supply is malfunctioning and should be repaired before continuing. Turn the DEVICE SUB & DRIVE off before continuing. 3. Set the HORIZ OUTPUT TEST selector to INPUT DRIVE, apply power to the chassis, and note the reading on the LCD. The LCD should read “ ON.” Some chassis’ horizontal circuits run off of a scan derived supply. With these types of sets you need to turn the TV off and back on again watching the LCD to see if it will momentarily
flash “ON.” If it reads “ON”, or will momentarily when the set is powering up, a drive signal is present. If it reads “OFF” there is a defect previous to the base of the H.O.T. This defect does not necessarily need to be repaired before continuing. B. Dynamic H.O.T. Sub & Drive These next steps allow you to substitute for the H.O.T. and operate the TV at full voltage without risking an expensive replacement H.O.T. 1. Set the HORIZ OUTPUT TESTS selector to DEVICE SUB CURRENT. 2. Turn the HORIZ OUTPUT DEVICE SUB & DRIVE slightly on until the SUB ON LED lights and watch the current reading on the LCD. If the current exceeds 500mA, turn the HORIZ DEVICE SUB & DRIVE off. There is likely a defect in the circuit that needs to be repaired before continuing. If the current stays below 500mA turn the knob quickly to the 12 or 1 o’clock position (higher for larger sets). Adjust the HORIZ OUTPUT DEVICE SUB & DRIVE control to get normal horizontal deflection without “foldover” in the center of the CRT display. The current reading may now be over 1A depending on the size of the set. 3. With the HORIZ OUTPUT TESTS selector, check the DCV to see that the power supply is regulating and the PULSE PPV and PULSE TIME uS to measure the amplitude and width of the horizontal output pulse to be sure that the horizontal output stage is operating properly. Repair any problems before continuing. C. Dynamic Hori zontal Output Parameter Measurements This final step monitors the horizontal circuit’s operation at full voltage so you can be sure that it is working properly with the H.O.T. installed. 1. Install a good H.O.T. and reconnect the Dynamic Tests leads as described above. Note: Be sure the HORIZ OUTPUT DEVICE SUB & DRIVE knob is in the OFF positi on. 2. Apply AC power to the chassis. 3. With the HORIZ OUTPUT TESTS selector, check DCV, PULSE PPV, and PULSE TIME uS for correct values.
TVA92 HORIZ OUTPUT TESTS RINGER TESTS
130
RINGS mAuS KVPP
DYNAMIC TESTS DCV
YOKE & FLYBACK •
HORIZ OUTPUT TESTS
SWITCHING XFORMER
•
•
PULSE TIME uS
•
•
HORIZ uS OUTPUT • LOAD TEST mA
VERTICAL YOKE DRIVE LEVEL
PULSE PPV •
•
•
T V O F F
T V O N
INPUT DRIVE
DEVICE SUB CURRENT
HORIZ OUTPUT DEVICE SUB & DRIVE (CURRENT LEVEL)
0
0
SUB ON
-
+
OVERLOAD
OFF
1.5A MAX
RINGER/LOAD TESTS
DYNAMIC TESTS
OFF: TVS HORIZ OUTPUT ACTIVE ON: TVS HORIZ OUTPUT SUBISTITUTED
VERTICAL YOKE DRIVE OUTPUT 50V 1A
1500V MAX
! DISCONNECT YOKE FROM CHASSIS
FLOATING GROUND 1000V ISO
Fig. 3: Connecti ons to perform the HORIZ OUTPUT DYNAMIC TESTS.
SYMPTOM
PROBABLE CAUSES
B+ = 0 Volts
• Open Fuses • Bad B+ Supply • Shorted B+ Path
Low B+ Volts
• B+ Power Supply Regulation • Low AC Voltage
High B+ Volts
• B+ Power Supply Regulation • Open Loads On B+ Supply
Pulse PPV = 0 V
• No B+ • No Input Drive • Open HOT • Open Flyback Primary
Low Pulse PPV
• Leaky Retrace Capacitor or HOT • Flyback Loading • Reduced Value Of Yoke Capacitor • Bad Yoke • Low B+ • Insufficient Input Drive
High Pulse PPV
• Retrace Capacitors • Flyback Shorted Turn • High B+ (regulator)
Pulse Time = 0 µS
• No B+ • No Input Drive • Open HOT • Open Flyback Primary
Pulse Time < 11.3 µS
• Flyback Loading • Flyback Shorted Turn • Retrace Capacitors
Pulse Time >15.9 µS
• Yoke • Yoke Series Capacitor
Multiple Pulse Times
• Flyback Loading • Flyback Shorted Turn • Leaky HOT Damper Diode, Yoke, Retrace Capacitors, Yoke Or Yoke Capacitor
Input Drive “ON”
• Drive present to base of HOT
Input Drive “OFF”
• No Drive To Base Of HOT
HOT = Horizontal Output Transistor
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For More Information, Call Toll Free 1-800-SENCORE (736-2673)
S ig. 4: Possible HORIZ OUTPUT DYNAMIC TEST indications and their possible causes.
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#231
3200 Sencore Drive, Sioux Falls, SD 57107 1-605-339-0100 • www.sencore.com Form #6905 Printed In U.S.A.
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Understanding The LC103’s In-Circuit Capacitor Test Capacitors continue to be found in electronic circuits in record breaking numbers. In fact, the number of capacitors used in the manufacturing of electronic circuits continues to rise each year. In 1997, U.S. factories sold over 50 billion capacitors. As these capacitors age or are stressed by circuit voltages and heat many will fail causing improper circuit operation. Finding a bad capacitor and replacing it to restore normal circuit operation is challenging. First, you must identify which capacitor is suspect. Second, the capacitor must be unsoldered, removed, tested and reinstalled if good or replaced. These steps can be time consuming and you also risk damage to the circuit board traces or the capacitor. In fact, many manufacturers suggest replacement of surface mount capacitors when unsoldered and removed. Time and money is wasted if the removed capacitor is good or a replacement doesn’t fix the problem.
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The Sencore LC103 “ReZolver” provides a patent pending test of capacitors while still soldered in-circuit. The in-circuit capacitor analyzing test determines if the capacitor is good, bad, or if it should be removed for further tests. This Tech Tip covers how to test capacitors in-circuit COMPONENT TESTS IN-CIRCUIT
CAPACITOR
OUT-OF-CIRCUIT CAPACITOR
CAPACITOR
VALUE
ESR
GOOD/BAD
DIELECTRIC
CAPACITOR
ABSORPTION
LEAKAGE
INDUCTOR
INDUCTOR
VALUE
RINGER
INDUCTOR GOOD/BAD
Fig. 1: The Sencore LC103 “ ReZolver” provides a patent pending in-circuit capacitor test to reduce servicing time and expense.
with the LC103’s In-Circuit Capacitor Good/Bad test and explains how to interpret the test results.
looking at the test instrument could easily cause you to slip off the capacitor resulting in improper measurements, frustration and potential circuit damage.
In-Circuit Capacitor Testing Challenges
Sencore has overcome these mechanical difficulties with the innovative Adjustable In-Circuit Test Probe. The Adjustable In-Circuit Test Probe (AP291) joins two probe tips and provides an adjustable spacing wheel. The probe mechanically adjusts providing the versatility to fit the lead spacing of capacitors ranging from surface mount to large electrolytics.
Obtaining meaningful and reliable test results when analyzing a capacitor in-circuit has many complications. First you must make an electrical connection to each of the capacitor’s test leads and maintain a stable connection while performing the tests. Second, you must perform analyzing tests that determine with a high reliability if the capacitor is good or if it may have a defect and should be removed for further testing. You also need the flexibility to test a wide range of capacitors found in today’s circuits to be comprehensive. Finally, you need a simplified solution to interpreting the in-circuit capacitor test readouts to avoid confusion. The first challenge is simply making connection to an in-circuit capacitor. Electrical connections to each of the capacitors leads while soldered in-circuit is complicated by a wide range of capacitor types, values, sizes and mechanical lead basings. Since most capacitors do not expose enough lead length for clip lead connections when soldered in-circuit, connections must be made on the solder side of the circuit board. Surface mount capacitors are already mounted directly to the solder side. Connection to the soldered side of the circuit board requires 2 sharp probe tips. Connecting to each of the capacitor legs requires a hand for each probe, leaving no hands to operate a test instrument. Even if you were able to hold each probe with one hand, reaching or
The angled tips provide ease in probing surface mount electrolytic capacitors. A push button switch conveniently located on the test probe enables the LC103’s in-circuit capacitor test to avoid probe slippage. For most applications, the probe can be adjusted and connected to the in-circuit capacitor with one hand. In addition, the LC103 beeps when the first complete measurement is complete and the readings are momentarily frozen on the LC103 display after the test button is released to be sure you have sufficient time to view the in-circuit test result.
S
Fig. 2: A push button switch conveniently located on the test probe enables the LC103’s in-circuit capacitor test.
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While capacitors fail in several ways a combination of two common measurements, capacitor value and equivalent series resistance (ESR), can determine if a capacitor is likely good or suspect in-circuit with a high level of reliability. Aluminum Electrolytic capacitors and tantalum capacitors commonly fail from increased ESR prior to changing value and increasing in leakage. Other capacitor types commonly change value. Testing both value and ESR provides the most comprehensive and accurate in-circuit test results. An ESR tester alone would mistakenly report a shorted capacitor or circuit short as good. Likewise, a capacitor value test alone would miss capacitors with excessive ESR. Accurate in-circuit capacitor testing can be hindered by the presence of components in parallel with the capacitor. Capacitance, resistance, inductance and semiconductor junctions in parallel with the capacitor may influence the accuracy and reliability of in-circuit capacitor tests. At times the parallel components may have little effect on the accuracy of the tests but at other times the parallel components cause significant changes to the test results. It is important to know when the parallel components are effecting the in-circuit capacitor measurements.
The LC103’s In-Circuit Capacitor test performs several sophisticated tests to determine if parallel components are present which may be effecting the accuracy of the in-circuit capacitance value and ESR measurements. The tests include a test to determine how much current is needed to hold a capacitor charge. Current exceeding the original charging current by 20% indicates parallel resistance that can impact the capacitor value test. A second test uses a selection of test frequencies and analyzes the Xc of the circuit. A capacitance value is determined and compared to a capacitance value determined with an RC time constant value measurement. Large differences in the capacitance values indicate parallel components which would impact the in-circuit measurement accuracy.
rating. A calculator would be needed to determine if the measured capacitance value is within a normal tolerance. The LC103 provides Good/Bad test analysis with every in-circuit capacitor test to help determine if the capacitor value and ESR is within a normal range. ESR evaluations are based upon maximum allowable limits established by component manufacturers and the Electronic Industries Association (EIA). Capacitor measured values are automatically compared to maximum and minimum values calculated from the entered value and tolerance of the capacitor being tested.
LC103 In-Circuit Capacitor Testing The LC103 offers two alternatives for Good/Bad testing a capacitor with the InCircuit Capacitor Good/Bad Test function. You may perform a basic Good/Bad test of the capacitor or a complete EIA Good/Bad test. Both testing alternatives perform the same analyzing tests but use different references for Good/Bad interpretation. The display readouts vary slightly depending upon the test alternative.
Fig. 3: The LC103 analyzes the capacitor for parallel components that would alter the accuracy of the in-circuit test results.
To perform a basic Good/Bad check apply power to the LC103 and attach and zero the test probe. Connect the Adjustable In-Circuit test probe to the capacitor legs and push & hold the front panel CAPACITOR GOOD/BAD push-button switch or the small push-button switch on the In-Circuit Test Probe. The test results are shown in the COMPONENT TEST RESULTS display.
The LC103’s In-Circuit Capacitor test function measures the capacitance value It can be difficult to determine if a and ESR of an in-circuit capacitor. These capacitor ESR readout is normal or not measurements are simultaneously as capacitor ESR values vary among displayed in the COMPONENT TEST different capacitor types and also vary RESULTS display. Capacitance value with the capacitor’s value and voltage measurements range from 0.002 uF to 20,000 uF. Capacitor ESR measurements are In-Circuit Capacitor Test Component Parameters Good/Bad Judgement Factors displayed for capacitors ranging Basic Good/Bad Check None Measured capacity and 50V in value from 0.02 Tantalum ESR Chart if >1 µF uF to 20,000 uF. Ceramic (10Ω) if <1uF Measurement voltages are below PN EIA Good/Bad Test Capacitor type, value, Based on the EIA chart for entered capacitor type forward bias volttolerance, rated voltage Measured capacity versus entered value/tolerance ages so the tests are unaffected by semconductor Chart 1: The LC103 performs a basic Good/Bad test of the capacitor or a complete EIA Good/Bad test. Both testing junctions. alternatives perform the same in-circuit analyzing tests but use different references for Good/Bad interpretation.
The display readouts shown during the basic Good/Bad check include the capacitance value, capacitor ESR and a “GOOD??” or “BAD??” or “SUGGEST REMOVAL” display readout. ESR is not displayed for capacitor values below 0.02 uF. The good or bad evaluation is based upon the ESR measurement and the measured capacitance value. For measured capacitance values over 1 uF, the measured ESR is compared to the maximum ESR values for a similar value tantalum capacitor as determined by the EIA. For measured capacitor values less than 1 uF, a 10 ohm good/bad reference is used. ESR values of 10 ohms or more are considered “BAD??” while less than 10 ohm are considered “GOOD??.” Question marks accompany both the good or bad readouts during a basic Good/Bad check because the LC103 can not compare the measured capacitance value to the rated value of the capacitor being tested. When you see the question marks, remember to check the LC103’s capacitance measurement to the capacitor’s rated capacitance value to determine if it is within a normal tolerance. Note: Double Layer Lytics and High R Double Layer capacitor values are beyond the range and testing capability of the In-Circuit Capacitor Good/Bad test. The In-Circuit Capacitor Good/Bad test should not be used on these capacitor types.
IMPORTANT
Do not hold-in the CAPACITOR GOOD/BAD switch or Adjustable Test Probe push-button switch while connecting the Test Probe to an in-circuit capacitor. The LC103 circuitry may be damaged because capacitor discharge protection is lost. A complete EIA Good/Bad test evaluates both the measured in-circuit capacitance value and ESR. The display readouts shown during the EIA Good/Bad test includes the capacitance value, capacitor ESR and a “GOOD” or “BAD” or “SUGGEST REMOVAL” indicator. An ESR measurement readout is not displayed for capacitor values below 0.02 uF. The good or bad evaluation is based upon both the measured capacitance value and measured ESR. The measured capacitance value is compared to the entered value and tolerance. The measured ESR is compared to the maximum ESR determined by the EIA for the entered capacitor type. If the measured capacitance value is out-of-tolerance and/or the ESR exceeds the maximum determined by the EIA, a “BAD” readout is indicated. If the capacitance value is within the rated tolerance and the ESR is below a maximum EIA level, a “GOOD” readout is indicated.
To Perform an In-Circuit Capacitor Basic Good/Bad Check: 1. Apply Power to the LC103. 2. Connect the In-Circuit Ajustable Test Probe to the LC103’s TEST LEAD jack. 3. Perform the Lead Zero Adjustment. 4. Connect the probe tips to the capacitor leads. 5. Push & hold the In-Circuit CAPACITOR GOOD/BAD push-button or test probe push-button. 6. Read the COMPONENT TEST RESULTS display
Fig. 4: The Adjustable In-Circuit Test Probe provides reliable in-circuit connections and push-button test ease.
To Perform an In-Circuit Capacitor - EIA Good/Bad Test: 1. Apply Power to the LC103. 2. Connect the In-Circuit Adjustable Test Probe to the LC103’s TEST LEAD jack. 3. Perform the Lead Zero Adjustment. 4. a.Enter the capacitor - Component Type Example: Push the “ALUMINUM LYTIC” push-button. b. Enter the capacitor value. Example: Push the 2, 2, 0, uF, push-buttons. c. Enter the capacitor value tolerance. Example: Push the 2, 0, +%, -%, push-buttons. d.Enter the capacitor’s rated voltage. Example: Push the 5, 0, V, push-buttons. 5. Connect the probe tips to the capacitor leads. 6. Push & hold the In-Circuit CAPACITOR GOOD/BAD push-button or the test probe’s push-button. 7. Read the COMPONENT TEST RESULTS display
Understanding the “ SUGGEST REMOVAL” In-Circuit Capacitor Good/Bad Test Readout A “SUGGEST REMOVAL” message is sometimes displayed during either the in-circuit capacitor basic Good/Bad test or EIA Good/Bad test. This message indicates that the LC103’s tests have identified components in parallel with the capacitor being measured and that the parallel components are influencing the accuracy of the capacitor value and/or ESR measurements. For an accurate evaluation of the capacitor’s value and/or ESR the capacitor must be unsoldered from the circuit and tested with the LC103’s out-of-circuit capacitor tests. Most “ SUGGEST REMOVAL” messages are accompanied by capacitor value and ESR test readouts. These readouts may not be accurate because of parallel components but are often helpful in
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determining if the capacitor likely has a problem. Occasionally, the readings may help you avoid removal and testing time. For example, a capacitor value readout that is much higher than the rated value of the capacitor is likey caused by a capacitor in parallel with the one being tested. If the schematic shows a capacitor in parallel with the one being tested that results in a total capacitance near the displayed value, the value of the capacitor being tested is likely fine. At other times, you may know from previous experience what to expect for capacitance and ESR readouts with the in-circuit capacitor tests across a particular capacitor.
Fig. 5: The “ SUGGEST REMOVAL” readout indicates there are components in parallel with the capacitor being measured that will influence the test results.
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F o r M o re I n f o r m a t i o n , Call Toll Free 1 -800-SENCORE (1-800-736-2673)
S 3200 Sencore Drive, Sioux Falls, SD 57107 www.sencore.com
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Form #6939 Printed In U.S.A.
#236
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UNDERSTANDING HORIZONTAL OUTPUT STAGES OF COMPUTER MONITORS Today's computer, medical, security, design and industrial video display monitors operate at a host of different horizontal resolutions or scanning frequencies. Many change modes to display video at several scan frequencies while others adapt to display a range of horizontal scan frequencies or resolutions. All CRT based video displays have horizontal stages including a horizontal output stage. Some use two horizontal output stages, one to produce high voltage and another to produce horizontal yoke current. The frequency of the horizontal output stage(s) must match the video's horizontal sync or scan frequency. Despite the wide range of operating frequencies and uses, several basic horizontal output circuit configurations are common.
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All horizontal output stages operate in a similar manner. A power supply voltage (B+ voltage) is applied to the stage, typically to one side of the transformer or coil. (See Fig. 1.) The power supply provides the current to energize the transformer or coil. When energized, alternating sawtooth currents in the transformer primary or coil winding are produced. Induced voltages from the transformer or coil develop high voltage and/or yoke current. An H.O.T. (horizontal output transistor) provides a path for current to energize the transformer primary or coil winding. The transistor's emitter-to-collector current path is switched fully on or off by an input drive signal. Horizontal output stage transistors can be a conventional bipolar type or newer N-channel enhancement MOSFET.
This Tech Tip examines the common horizontal output stage configurations found in multi-frequency CRT video displays. It provides details on how these stages operate and what to expect for normal voltages and waveforms to improve your troubleshooting.
A retrace or timing capacitor (Ct) is used to tune or time the horizontal output stage. With the H.O.T. switched open, the timing capacitor forms an LC tuned circuit with the transformer or coil winding. The timing capacitor slows the rate of the transformer or coil's collapsing magnetic field controlling the level of induced voltage.
The Basics Of A Horizontal Output Stage
A damper diode parallels the horizontal output transistor. The diode is biased on with induced voltage from the flyback or coil during a critical time in the output stage cycle. The damper diode's conduction path prevents reverse breakdown current in the horizontal output transistor
All horizontal output stages have 4 key components and require two essential inputs. These are listed below.
Fo u r K e y Co m po n e n t s: 1.Horizontal Output transistor 2.Transformer Primary or Coil 3.Retrace Timing Capacitor 4.Damper Diode
Tw o Es s e n t ia l In p u t s: 1. B+ Power Supply Voltage 2. Input Signal Drive
resulting in transistor heating and failure. The damper diode permits energy stored in the transformer or coil at the end of the output stage cycle to be returned to the B+ supply. The damper diode is a fast switching high voltage, high current diode.
High Voltage or Deflection Only Horizontal Output Stages Multi-frequency video display monitors may have one or two horizontal output stages. A display with one horizontal output stage combines the yoke and flyback transformer into a circuit which produces both high voltage and horizontal yoke deflection current simultaneously. When a video display contains two horizontal output stages, one horizontal output stage is responsible for producing high voltage while a second output stage produces horizontal yoke current. A multi-frequency video display is more likely to have two horizontal output stages if the CRT size is greater than 15 inches. Separate HV and deflection horizontal output stages are rarely found in monitors with CRT sizes less than 15 inches or in televisions because of the added cost. Larger CRT display monitors requiring higher yoke current and larger current changes to accommodate multi-frequency operation are more likely to have separate high voltage and deflection output stages. Design of a horizontal output stage to satisfy the yoke current requirements and maintain reliable operations is simplified when separated from the high voltage generating horizontal output stage. Separating the output stages allows each to operate with much less current (power)
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BIPOLAR
Flyback Transformer
MOSFET B+ Volts
Flyback Transformer
+V
B+ Volts
H.O.T.
+V
D1 Damper Diode
H.O.T. D1 Damper
CT Retrace/Timing Capacitor
Driver Amp.
CT Retrace/Timing Capacitor
Optional Parallel H.O.T.
Fig: 1: Typical bipolar and MOSFET high voltage only horizontal output stages and their driver stage.
requirements than if combined. A typical high voltage only horizontal output stage consist of a flyback transformer, timing or retrace capacitor, damper diode and horizontal output transistor(s) as shown in Fig. 1. Either a bipolar transistor, MOSFET transistor or paralleled MOSFET transistors may be used. Because of the reduced current in a high voltage only horizontal output stage compared to a combination stage, a MOSFET horizontal output transistor can be used reliably. In cases where the current is still substantial, matched paralleled MOSFET transistors may be used to divide the H.O.T. conduction current for increased reliability. Considering the reduced costs of MOSFETs and drive components, the cost of a mosfet compared to a bipolar horizontal output transistor is slightly better. Two important differences exist when using a MOSFET horizontal output transistor compared to a bipolar type. First, the flyback voltage pulses induced into the output stage must be reduced because MOSFETs have a lower breakdown voltage rating than bipolar transistors. Secondly, the input drive signal and driver circuits must be different to match the differences in the MOSFET's transistor's operating characteristics. For these reasons bipolar and MOSFET output transistors cannot be interchanged. Today’s MOSFET horizontal output transistors typically have a maximum voltage rating from drain to source of either 800 volts or 1000 volts. In comparison, bipolar tran-
sistors have a maximum voltage rating collector-to-emitter of 1500 volts. Typically, flyback pulses in MOSFET output stages are at least 100 volts under their maximum rated voltage or less than 900 volts peak-to-peak. In comparison, flyback pulses in a bipolar transistor output commonly exceed 1000 VPP. Lesser induced voltages are compensated for with a different flyback transformer turns ratio to produce the needed high voltage. A MOSFET transistor is a voltage operated device while a bipolar transistor is a current operated transistor. Switching a bipolar transistor on requires that the drive produce base current of several hundred milliamps. The base drive current switches the transistor fully on enabling it to conduct collector currents of several amps. A MOSFET output transistor turns fully on when positive voltages greater than 4 volts are applied to the gate. The input signal typically ranges from near 0 volts (H.O.T. off) to between 5 and 15 volts (H.O.T. on). When switched on the MOSFET transistor reduces its drain-to-source resistance path to less than 2 ohms permitting peak currents to build in the flyback primary winding. Because MOSFET and bipolar output transistors have different input drive requirements, the horizontal driver stages for each are considerably different. Driver stages for bipolar output transistors use an amplifier and current stepup transformer to produce the needed drive current to the bipolar transistor's low impedance base. Driver stages for MOSFET output transistors use an amplifier to provide a changing voltage to the MOSFET'S high impedance gate.
High Voltage Only Horizontal Output Stage Operation Operation of the high voltage only horizontal output stage is fundamental to all horizontal output stages. When the H.O.T. is driven on by the drive signal, B+ current increases through the H.O.T. collector energizing the transformer or coil winding. The current is opposed by the inductance increasing at a near constant rate reaching a peak of several amps. The magnetic field builds in the transformer or coil's core during this time inputting the power required to produce high voltage. The B+ supply voltage, coil inductance, H.O.T. conduction time, beta and base current drive all effect the level of energizing current buildup. When the H.O.T. is switched open, the timing capacitor (Ct) is effectively placed into the circuit forming an LC resonant circuit. Immediately after the H.O.T. is switched off, the magnetic field of the transformer or coil begins to collapse. The collapsing magnetic field causes current to flow through the low impedance of the B+ supply capacitor charging Ct. This is the beginning of the retrace time and corresponds with horizontal sync. As the timing capacitor charges, a rising voltage is produced at the collector or drain of the output transistor. The voltage reaches its peak as the magnetic field is fully collapsed. The timing capacitor performs a critical function in slowing down the rate of the collapsing magnetic field. If the capacitor value decreases or is opened, the field collapses much more rapidly producing a much higher induced voltage, several
thousand volts or more. The induced voltage would produce excessive high voltage and/or deflection and quickly damage the horizontal output transistor. Because of this key role in controlling the flyback or kickback voltage, the timing capacitor is often called a "safety capacitor".
comprise the yoke's current path. A typical deflection horizontal output stage is shown in Fig. 2. Because of the yoke's high current requirements, bipolar output transistors are used. A high frequency coil or transformer replaces the flyback transformer.
After the magnetic field has completely collapsed, Ct begins to discharge, causing current flow back into the transformer or coil in the opposite direction. A magnetic field builds, but with the opposite magnetic polarity. This action completes the second part of retrace and corresponds to the falling portion of the voltage waveform or pulse at the collector or drain of the H.O.T.
The horizontal yoke and its series components parallel the timing or retrace capacitor of the horizontal output stage. Included in the yoke's current path is always a linearity coil and an "S-shaping" capacitor. These components shape the rise and fall of the alternating current in the yoke to produce a linear and uniform deflection on the CRT. The series components can be arranged in any order. Other components that may be found in the horizontal yoke's current path are a pincushion transformer and efficiency or width control transformer or coil (not shown in Fig. 2).
When Ct has completely discharged the magnetic field of the transformer or coil begins to collapse. The collapsing field induces a voltage with a polarity that forward biases the damper diode. The damper diode conducts producing an inductive circuit similar to when the H.O.T. was conducting. The damper diode allows the magnetic energy of the transformer or coil winding to decay at a controlled inductive rate returning energy (current) back to the B+ supply capacitor. As the magnetic field is nearly fully collapsed the horizontal output transistor is turned on and the cycle repeats.
Deflection Only Horizontal Output Stage A deflection only horizontal output stage is responsible for producing yoke deflection current. This horizontal output stage contains the basic components of a horizontal output stage plus components which B+ Volts
Coil or Transformer
H.O.T. D1
CT
Linearity Coil Horiz. Yoke S-Capacitor
Fig. 2: Basic deflection only horizontal output stage.
Operation of a deflection only horizontal output stage is the same as described for the high voltage only horizontal output stage, but with the additional path for yoke current. To produce yoke current that is in sync with the video's horizontal retrace time, a common horizontal drive signal originating from the locked horizontal oscillator feeds the separate HV and deflection output stages. The common horizontal drive synchronizes the high voltage and deflection output stages to produce flyback pulses at nearly the same time. Current for the horizontal yoke is derived from the output stage's retrace or timing capacitor. When the H.O.T. is turned on, the bottom side of the S-shaping capacitor connects to the top of the linearity coil. Because the S-shaping capacitor is fully charged from the previous cycle, it begins to discharge through the horizontal output transistor. The resulting current flow produces an expanding magnetic field in the linearity and yoke coils. The polarity of the increasing current deflects the CRT's electron beam from the center to the right. At the same time, B+ power supply current flows through the H.O.T. to energize the transformer or coil winding. When the horizontal output transistor is switched open, the retrace timing capacitor effectively is placed in parallel with the yoke and its series components increasing the resonant frequency or rate of current
change in the yoke. The yoke's magnetic field rapidly collapses producing current which charges the retrace timing capacitor and S-shaping capacitor. Because of the difference in capacitor values, most of the energy is returned to Ct. Corresponding with this time, is the collapsing magnetic field of the B+ transformer or coil which replenishes or fully charges the retrace capacitor. You may recall this is the rising edge of the inductive "kickback" voltage pulse at the collector of the output transistor. Now, fully charged, Ct becomes the current source for the yoke for the remainder of the cycle. This time corresponds to the 1st part of retrace when the CRT's electron beam is quickly returned to the center of the display. During the 2nd part of retrace, Ct and the S-shaping capacitor produce discharging current through the yoke in the opposite direction. The current rises to a peak building the magnetic field in the yoke and quickly moving the electron beam from the center of the CRT to the left. Also, during this time, Ct is energizing the B+ supply transformer or coil. When the retrace capacitor and the S-shaping capacitor are fully discharged, the yoke's magnetic field begins to collapse. This corresponds with the collapsing magnetic field of the transformer or coil energized by the B+ supply. The induced voltage forward biased the damper diode into conduction. The circuits timing now agrees with the timing during the right trace time when the H.O.T. was conducting. The yoke's collapsing magnetic field returns energy to the circuit charging the S-shaping capacitor. Yoke current moves the CRT's electron beam slowly from the right to the center of the CRT. When the yokes magnetic field is collapsed, the damper diode stops conducting. This corresponds with the beginning of the H.O.T.'s conduction and the cycle repeats.
Indirect Flyback Driven – HV Only Horizontal Output Stages In most high voltage horizontal output stages the flyback primary winding is in the direct current path of the horizontal output transistor. However, there are several nonconventional horizontal output stage configurations in which the flyback primary is not in the H.O.T.'s conduction path.
For example, Fig. 3 shows a high voltage only horizontal output stage in which the flyback transformers primary current is provided by the retrace capacitor Ct. This configuration is nearly the same as the deflection only horizontal output stage of figure 2. The only difference is that the yoke is replaced by the flyback transformer primary winding. Recall that when the H.O.T. is switched on by gate drive, B+ supply current flows to energize the coil and produce an expanding magnetic field. When the H.O.T. is switched off, the coil's magnetic field induces voltage and charging current to Ct. Ct then becomes the supply or current source of flyback current. Current alternates in the tuned circuit including the flyback transformer primary in the same manner as described earlier for the deflection only horizontal output stage. B+ Volts
Coil
Flyback Transformer
CT
15mH
Cs
Fig. 3: Indirect current drive to the flyback transformer.
Combination High Voltage and Deflection Horizontal Output Stages A combination high voltage and deflection horizontal output stage produces high voltage and yoke current simultaneously. There are 3 common combination horizontal output stage configurations found in multi-frequency display monitors. They include: 1. Single Damper type 2. Split or Dual Damper type 3. Emitter Driven type
Single Damper Combination Horizontal Output Stage A common combination horizontal output stageis the single damper horizontal
output stage. Thesingle damper combination horizontal output stage is used in the majority of television receivers. The single damper output stage produces high voltage and deflection with the fewest parts and component costs. It offers good performance and reliability for single frequency operation. The single damper output stage is also popular in computer display monitors that operate over only a few frequency modes or a limited operating frequency range. The single damper diode output stage can be recognized by the fact it has only one damper diode. (See Fig. 2). The diode is placed from the H.O.T. collector to emitter. The damper diode may be a discrete component or integrated into the bipolar horizontal output transistor. In a single damper diode horizontal output stage, the flyback transformer primary winding connects from the collector of the output transistor to the B+ power supply. The yoke and series components connect between the H.O.T. collector and emitter or ground. The timing capacitor provides energy to produce yoke current much like the deflection only horizontal output stage. Operation of the single damper horizontal output stage is identical to the deflection only horizontal output stage explained earlier. The only exception is that the coil between the collector and B+ supply is replaced with the primary winding of a flyback transformer. Conduction of the H.O.T. energizes the primary of the flyback transformer. When the H.O.T. is switched off the collapsing magnetic field of the flyback transformer charges Ct. The charge in Ct produces current in the yoke and flyback transformer primary. The damper diode shunts the H.O.T. and is biased into conduction by the induced voltage from the flyback and yoke to return magnetic energy to the power supply. While the single damper diode combination output stage reliably produces high voltage and deflection current with the fewest components and costs, it is limited in multi-frequency applications. This is because any changes to the output stage to increase high voltage would also increase yoke current and
vice versa. With extreme horizontal frequency changes it becomes difficult to change the operating parameters to both establish normal high voltage and proper yoke deflection current simultaneously. For a complete explanation of a single damper diode combination horizontal output stage request Sencore Tech Tip #207.
Split Damper Combination Horizontal Output Stage A popular combination horizontal output stage found in multi-frequency video displays uses two damper diodes. The damper diodes are placed in series from the collector to the emitter of the H.O.T. (See Figure 4) This horizontal output stage also uses two timing capacitors placed in series between the H.O.T.'s collector and emitter. A connection in the middle of the damper diodes and timing capacitors splits the output. The split damper diode and timing capacitor configuration provides a means to control the level of yoke deflection current while not impacting the flyback current and resulting high voltage. It furthermore provides a method of achieving pincushion correction and other dynamic modifications of the horizontal yoke current. An understanding of the operation of a split damper horizontal output stage can be gained by analyzing the currents and resulting voltages during 4 times during the horizontal cycle, starting with the conduction time of the H.O.T. (See Fig. 5). When the H.O.T. is switched on by base drive, current increases through the flyback primary Flyback Transformer
B+ Volts
B+ Filter Capacitor
D1
C T1
Linearity Coil
S-Shaping Yoke D2
C T2
Fig. 4: A split damper combination horizontal output stage.
Multiplier
B+ Volts
Multiplier
CT1 D1
Linearity Coil
C T1
B+ Cap.
Charge ++++ 80%
Linearity Coil
Yoke
Discharge CT2
A
B+ Cap.
++++ B+ Cap.
CT1
D1 Charge
Yoke
++ ++ Shaping Capacitor
D2
B+ Volts
B+ Volts
B+ Volts
++ ++ ++++
Charge
+ +
CT2 ++++ Charge 20% CT2
C T2
D2
+ +
C
B
D
Fig. 5: Operation of a Split Damper Combination Horizontal output stage cycle.
creating an expanding flyback magnetic field. Current is also supplied from the Sshaping capacitor charged from the previous cycle. Current flows from the S-shaping capacitor through the bottom damper (D2), H.O.T. linearity coil and yoke. Yoke current deflects the CRT's electron beams from center to the right of the picture. When the H.O.T. is switched off, the magnetic fields in the flyback and yoke collapse. The flyback's collapsing magnetic field produces induced voltage and charging current to timing capacitors CT1 and CT2. Values of Ct1 and Ct 2 are chosen so approximately 80-90% of the charge is delivered to CT1 and 10-20% to CT2. The yoke’s induced voltage produces charging current to CT1 and the S-shaping capacitor. The difference in capacitor values returns the greatest charge to CT1. This portion of the cycle is the 1st part of retrace which quickly returns the CRT beam from the right to the center of the picture. With the flyback and yoke magnetic fields fully collapsed, capacitors CT1 and CT2 begin to discharge. Capacitor CT1, now fully charged, supplies discharge current along with the lesser charged S-shaping capacitor to the horizontal yoke. The yoke current moves the CRT's electron beam from the center to the left completing retrace. Capacitors CT1 and CT2 produce current through the flyback primary, but in the opposite direction, producing an expanding magnetic field. When the timing capacitors are discharged, the flyback’s magnetic field collapses biasing on the damper diodes. Diodes D1 and D2 conduct providing a current path for the magnetic energy of
the flyback to recharge the power supply filter capacitor. The collapsing magnetic field of the yoke charges the S-shaping capacitor with current flowing through D1. Yoke current moves the CRT electron beams from the left slowly to the center. When the yoke magnetic field is fully collapsed, the S-shaping capacitor nears full charge. The H.O.T. is then switched on to repeat the horizontal cycle.
Emitter Driven Combination Horizontal Output Stage Most horizontal output stages in multifrequency display monitors are single damper or split damper type, but occasionally a modification to these will be
encountered. One such change is found in several computer display monitors from Gateway and several other manufacturers. The modification changes the manner in which the H.O.T. is switched on and off by the input drive signal. (See Fig. 6) All other operations of the output stage are the same as the single damper combination output stage. The emitter driven horizontal output stage places a switching MOSFET at the emitter lead of the bipolar horizontal output transistor to ground. The horizontal drive signal is applied to the gate of the MOSFET turning its source-to-drain conduction path on or off. Effectively the emitter lead of the H.O.T. is connected to ground when the MOSFET is switched on. The supply B+ Volts
Multiplier
Flyback Transformer Base Drive Transformer
Linearity Coil
+12V
1KΩ
H.O.T. CT1
Yoke
Bias Resistor Pincushion Transformer
Damper C T2
S-Shaping Capacitor
Horiz Drive
MOSFET Emitter Drive Transistor
Fig. 6: Emitter driven combination horizontal output stage.
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#236 Emitter Output HV
Deflection Only B+ Volts
B+ Volts
S-Shaping Capacitor Linearity Coil Yoke
Multipler
DCV
Pulses
Coil or Flyback Transformer
Flyback Transformer
Fig. 7: Typical Emitter Output high voltage or deflection only horizontal output stages.
voltage to the base of the H.O.T. permits a low level of base current enabling the H.O.T. to start conducting flyback primary current. As the current slowly increases in the flyback primary, the base drive transformer induces voltage to the base lead. The voltage draws additional base current enabling the H.O.T. to produce additional collector current. In this manner, the inductive base drive dynamically increases the base current to produce the increasing collector current. The base drive transformer also improves the switching speed to turn the H.O.T. off. This H.O.T. switching method provides more efficient H.O.T. operation reducing its heat and improving stage reliability.
Emitter Output Horizontal Output Stage (HV or Deflection Only) Most HV or deflection only horizontal output stages include the flyback or energizing coil between the H.O.T.'s collector and the B+ power supply voltage. This produces a voltage or flyback pulse output at the collector or drain of the H.O.T. A rare deviation of this typical horizontal output stage results when the flyback or coil is placed at the emitter lead of the H.O.T. (See Fig. 7) The emitter outputs current to the flyback or coil, thus an emitter output horizontal output stage. This emitter output configuration can be used as a high voltage or deflection only horizontal output stage or combination horizontal output stage.
The flyback primary or coil winding leads to ground typically through a voltage regulation stage (not shown) which regulates the ground side of the current path. A damper diode and timing capacitor parallel the H.O.T. The yoke and its series components draw energy from Ct to produce deflection. Operation of an emitter output type horizontal output stage is nearly the same as those with the flyback or coil in the collector lead. However, because the emitter is not grounded and contains the inductive component to ground, DC voltages and flyback induced voltages are developed at the emitter in reference to circuit ground. For this reason, the stage's output is seemingly at the emitter of the H.O.T. To better understand the emitter output, consider the horizontal cycle. During the conduction time of the horizontal output transistor, the emitter is effectively connected to the B+ supply voltage at the collector. Current flows through the flyback or coil winding to energize the stage. When the transistor is switched opened, the coil's magnetic field collapses producing current to charge Ct. The voltage at the emitter decreases with the induced voltage in the flyback or coil winding as Ct charges. When the magnetic field is fully collapsed, Ct discharges through the flyback or coil winding. This causes the voltage at the emitter to increase or become less negative. The charging and discharging action of Ct results in a negative going
induced voltage pulse at the emitter of the H.O.T. in respect to circuit ground. During damper diode time, the collapsing magnetic field returns energy to the circuit producing current flow though the damper diode. The current charges the B+ power supply capacitor. During damper diode time, the B+ supply voltage is switched to the emitter. This configuration produces no waveform at the collector of the H.O.T. as it is connected to the B+ supply voltage. A DCvoltage measurement at the collector reads the B+ supply voltage. Because of the configuration and switching action of the stage, a DC voltage and waveform at the emitter reflect the normal operation of the stage. Negative going flyback pulses of several hundred volts peak-to-peak are typical. The DC voltage at the emitter reflects the B+ supply voltage to the output stage. This is determined by a regulation stage typically along the ground current path on the input side of the flyback or coil.
For More Information, Call Toll Free 1-800-SENCORE (736-2673)
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#236
Form #6955 Printed In U.S.A.
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