Understanding The HA2500 Horizontal Output Load Test

Transcription

1 Understanding The HA2500 Horizonta Output Load Test Horizonta output stages are part of every CRT video dispay incuding cosed circuit monitors, computer monitors, video games, medica monitors, TVs. HDTVs, IDTVs, and the ike. The horizonta output stage produces CRT high votage and/or horizonta yoke current to defect the CRT s eectron beam side to side. Fu AC vots cannot be appied without component damage B+ suppy is dead or bad Horiz. output dead or suspect H.O.T. heats or fais Dispay dead or won t start B+ suppy squeas, burns-up components or bows fuses X-ray shutdown symptom Estimating repair costs The Load Tests detect probems in high votage, defection, or combination horizonta output stages and enabes troubeshooting. This Tech Tip expains when to use the Load Test and how it works. It expains how to propery set up and perform the Load Tests. Tech Tip #220 expains how to interpret the LOAD TESTS Readouts. If horiz. output stage function is norma or abnorma If short or high current oad exists on B+ power suppy If horiz. output stage puse timing is norma or abnorma If horiz. output stage is efficient or inefficient (high osses) Why Use The Horizonta Output Load Test Many probems invoving the horizonta stages and B+ power suppy are difficut to isoate with AC power appied to the chassis. Many defects in the horizonta output stage, B+ power suppy or HV/defection reguators reduce the B+ power suppy votage to the horizonta output stage. Since the stages interact so cosey, the defect is not easiy isoated. The horizonta output stage may oad the B+ suppy, pacing high current stress on horizonta output, high votage/defection reguator and B+ power suppy components. Damage to repacement horizonta output transistors and B+ suppy components often resuts when the probem is not isoated before appying AC power. Fig. 1: The Load Tests of the Sencore HA2500 Universa Horizonta Anayzer test horizonta output stages with no AC power appied to the chassis.

2 Fig. 2: Bock diagram of the Load Test and connections to a horizonta output stage. Especiay difficut to detect is a shorted fyback turn or a defect in a fyback derived secondary circuit. These probems cause an abnorma increase in current in the horizonta output stage oading the B+ power suppy. This type of short or oading cannot be detected with resistance tests in the horizonta output stage. The X-Ray or high votage shutdown circuits immediatey defeat the operation of the horizonta output stage by interrupting horizonta drive or B+ votage. Since conditions are momentary, conventiona votage measurements cannot be made to isoate the probem to the B+ power suppy. HV/defection reguator, horizonta output or shutdown circuits. In the past. many of these difficut symptoms coud be isoated using a variabe AC power suppy to reduce the AC votage to the chassis. Reducing the AC votage owered the B+ votage to the horizonta output stage permitting conventiona measurements. However, the B+ votage output from a switching mode power suppy cannot be reduced when the AC votage is owered. Therefore. a variabe AC power suppy is ineffective in troubeshooting horizonta, Bt suppy. startup, and shutdown symptoms in chassis with a switch mode power suppy. The HA2500's Horizonta Output Load Test provides a functiona test to quicky confirm if the horizonta output stage is free of severe defects or if probems exist. It quicky determines if the horizonta output stage is the cause of probem symptoms. The Load Test is usefu in determining potentia repair costs or to troubeshoot defects in the horizonta output stage. It protects repacement parts by verifying when horizonta output stage probems have been corrected and it IS safe to appy AC votage. Understanding the Load Test The Load Test simuates the operation of the horizonta output stage at its norma operating frequency and 1/10 its norma B+ votage. The test energizes the horizonta output stage producing aternating currents in the fyback and/or yoke just as if the chassis was operating. Measurements taken during the simuation are dispayed to indicate the conditions of the chassis horizonta output stage. To simuate the actua operation of a horizonta output stage without power to the chassis requires three things: 1. A B+ or DC votage appied to the chassis horizonta output transformer. 2. A horizonta output transistor to provide a switched current path to energize the output transformer or coi. 3. A drive signa to this horizonta output transistor at the proper frequency and duty cyce to switch it on and off propery. The HA2500 satisfies these three requirements. For a better understanding, refer to the Load Test bock diagram and horizonta output stage in Fig. 2. To functionay test the chassis horizonta output stage with no AC power to the chassis requires that the B+ votage to the horizonta output stage be suppied. A variabe DC power suppy 0-18 vots provides the votage for the Load Tests. The DC power suppy is adjustabe with the front pane LOAD TEST B+ VOLTS contro. The positive votage is output to the orange cip of the LOAD & RINGER TESTS ead in respect to the back cip or ground. The power suppy current is imited to 250 ma to provide short circuit protection. A MOSFET transistor serves as the chassis horizonta output transistor providing the switching action to energize the chassis horizonta output stage. The HA2500's interna horizonta frequency generator provides the proper horizonta drive signa to the gate of the transistor. The drive frequency is adjusted with the front pane COARSE and FINE contros to match the frequency of the chassis horizonta output stage. Drive may aso be decoded from an externa sync signa appied to the HA2500's Ext. Sync Input Jack. During the Load Test, if the chassis horizonta output stage is operationa, aternating currents and induced votages or fyback puses are produced. Since the Load Test B+ votage is 1/10 of norma, the current and votages are approximatey 1/10 of norma providing a ow eve and safer test. However, the LC or resonant timing of the chassis horizonta output stage is the same no matter what the eve of appied votage. This means the fyback votage puse duration is neary the same during the Load Test as it is during chassis operation. The Load Test is active in the LOAD TESTS SETUP and LOAD TESTS positions of the LOAD & RINGER TESTS Switch. Measuring circuits indicate parameters during the Load Tests Setup and Load Tests functions.

3 Fig. 3: Setup for performing the horizonta output stage Load Tests. Connecting The Load & Ringer Test Lead Performing the LOAD TEST requires connecting the three coored cips of the suppied LOAD & RINGER TEST LEAD to the chassis horizonta output stage. Before connecting the LOAD & RINGER TEST LEAD cips, be sure to unpug or remove AC power to the chassis being tested. The HA2500 contains interna protectron circuitry, but to remove any chance of damage to the anayzer and chassis, aways unpug the chassis AC ine cord before connecting the test ead cips. CAUTION Ony perform the LOAD TEST with AC power to the chassis removed. Aso be carefu not to come into contact with circuit points whie performing the oad test or when connecting or disconnecting test cips. The LOAD TEST produces current and inductive votage puses in the chassis horizonta output stage. It IS a good idea to reduce the LOAD TESTS B+ votage to minimum when connecting or disconnecting test cips or before unsodering components in the horizonta output stage. CAUTION Do not come in contact with energized horizonta output circuits points during the LOAD TEST. Reduce the LOAD TEST B+ vots for added safety when connecting or disconnecting test ead cips. The cips on the LOAD/RINGER TEST LEAD are abeed and coor coded for easy identification as foows. The abes indicate where the cips are connected to the chassis horizonta output stage to perform the LOAD TEST. Connect the back test ead cip to horizonta ground commony avaiabe at the emitter or source ead of the horizonta output transistor. Connect the orange test ead cip to the B+ votage input to the horizonta output stage transformer primary or coi winding. Connect the yeow test ead cip to the coector or drain of the horizonta output transistor. The Load Tests can be performed with or without the chassis horizonta output transistor in the circuit. If the transistor is eaky or shorted, the Load Test Setup may dispay a LIMITING message or indicate a severe output stage probem. If you suspect a shorted horizonta output transistor, remove it from the chassis and repeat the Load Tests Setup. Connecting The Load & Ringer Test Cips Without A Schematic To connect the LOAD TEST LEAD cips without the aid of a schematic diagram, first ocate the horizonta output transistor or transistors. There may be one or two horizonta output stages. When two horizonta output stages are used, one produces high votage whie the other Lead produces horizonta defection. A singe horizonta output stage produces both high votage and defection. Horizonta output transistor(s) are ocated near the fyback transformer or yoke connector pug on the circuit board. Most horizonta output transistors are variations of a TO-3P case stye and are mounted to a arge meta heat sink. The circuit path of the coector or drain shoud ead to a pin of the fyback or yoke transformer or coi. If you are unsure if the transistor is a horizonta output type, use a semiconductor cross reference book to match the transistor number and identify a repacement transistor. Check the uses specified for the repacement transistor and the breakdown votage rating BVce. Most horizonta output bipoar transistors have a rating greater than 1000 vots (typicay 1500 V) and MOSFET transistors greater than 800 vots (typicay 1000 V). Once you have identified the horizonta output transistor(s), connect the yeow test ead cip to the center ead (coector or drain). Identify the ground for the horizonta output circuitry. Horizonta ground typicay connects to the emitter or source ead of the horizonta output transistor directy or through a parasitic inductor or sma vaue resistor. In some instances the emitter connects through severa windings of the driver transformer to ground. The ground circuit path on the board typicay has a arge trace size. It may aso be recognized as the trace on the circuit board that the negative ead of many poarized capacitors share. To identify the B+ votage input point to the horizonta output stage, trace the circuit path from the coector or drain of the horizonta output transistor to the output transformer or coi. This is one side of the primary winding of the output transformer or coi in the horizonta output stage. With an ohmmeter, identify a pin or pins that Orange B+ B+ input to horiz. output stage transformer or coi. Yeow C or D Coector or drain of horizonta output transformer. Back E or S Horizonta Ground or H.O.T. emitter or source ead.

4 To Setup For The LOAD TESTS (LOAD TESTS SETUP) 1. Remove AC power to the chassis. 2 Reduce the LOAD TEST B+ votage to MIN. Fig. 4: Use the Load Tests Setup to set the frequency and B+ for an accurate Load Test. 3. Connect the LOAD & RINGER TEST LEAD cips to the proper circuit points. have continuity to the coector pin of the transformer. Typicay, one or two pins may be identified on a fyback transformer. The B+ input pin may be Identified on the circuit board for you If not, the B+ input typicay has an eectroytic capacitor to ground with a coi or ow vaue resistor eading to the B+ power suppy or reguator. A second transformer pin that may have shown continuity to the primary winding ikey has a diode connected to it as it is commony used as a 200 to 300 vot power suppy to the CRT. If you are unsure of the B+ Input. you can use a tria and error method to identify the B+ input using the Load Test Setup Aternatey connect the B+ or orange test cips to pins of the transformer which may be the Bt input whie monitoring the Load Tests Setup readouts. The connection which produces Load Tests Setup and Load Tests resuts that are norma or cosest to norma is ikey the proper input Improper connections wi show unusua Load Tests Setup and Load Tests resuts. Understanding The Load Test Setup The Load Tests Setup guides you in setting the horizonta frequency and B+ vots for the Load Test and confirms proper chassis connections. An improper horizonta frequency and/or B+ votage can cause Load Tests resuts that are outside of the typica range for a particuar chassis. Perform the Load Tests Setup before anayzing the Load Tests ma, %, and µs readouts. If you change the horizonta test frequency or B+ vots whie performing the Load Tests, repeat the Load Test Setup. During the Load Tests Setup, three parameters are simutaneousy metered and dispayed in the center fuorescent dispay. They incude the foowing. 1. Load Test Horizonta Frequency (khz) - Variabe with the HORIZONTAL FREQUENCY COARSE and FINE contros. 2. Load Test B+ votage (DCV) - Variabe with the LOAD TEST Bt VOLTS contro. 3. Peak-to-Peak (PPV)- Ampitude of induced votages in the chassis horizonta output stage resuting from an active LOAD TEST at the frequency and B+ seected. These three parameters guide you to the proper setup for performing the Load Tests at the chassis 1/10 eve and achieving the most accurate test resuts. The frequency readout indicates the horizonta frequency for the Load Tests seected with the HORIZONTAL FREQUENCY COARSE and FINE Contros. When the COARSE contro is set to "EXT". the test frequency is determined by the frequency of the appied sync to the EXT. SYNC INPUT Jack. The DCV readout indicates the HA2500's Load Test B+ power suppy votage appied to the horizonta output stage. The votage is adjustabe with the LOAD TEST B+ VOLTS Contro. Adjust the LOAD TEST B+ VOLTS Contro unti the dispay indicates a DC votage that is approximatey 1/10 of the norma B+ votage to the horizonta output stage being tested. 4. Set the LOAD & RINGER TESTS Switch to LOAD TESTS SETUP. 5. Set the HORIZONTAL FREQUENCY COARSE and FINE Contros to the horizonta operating frequency For muti-frequency monitors. seect the highest horizonta operating frequency. 6. Increase the LOAD TEST B+ VOLTS contro to approximatey 1/10 of the norma B+ votage to the chassis horizonta output stage. 7. Read the VPP readout dispay. in the digita The LOAD TESTS SETUP VPP (Vots- Peak-to-Peak) readout Indicates the ampitude of the Induced votage puses produced by the horizonta output stage being energized by the Load Test. If propery connected, a functiona horizonta output stage produces induced votage puses reative in ampitude to the seected test frequency and appied B+ votage. In horizonta output stages using bipoar transistors. VPP readings typicay range from 80 to 120 VPP. In horizonta output stages using MOSFET transistors, VPP readings during the Load Tests Setup range from 50 to 80 PPV. To achieve the most accurate Load Tests of muti-frequency CRT video dispays, set the horizonta frequency to match the highest horizonta frequency the dispay can produce. Whie a precise frequency is not important, avoid being more than 10 khz beow the monitor s highest frequency. Low test frequencies increase the ma readings often above the typica norma range because mode switched components are removed from the horizonta output stage. If you are unsure of the chassis norma B+ votage, Increase the LOAD TEST B+

5 Load Tests Frequency (khz) Dispay's Highest Horiz. Freq. Dispay's Highest Horiz. Freq. Dispay's Highest Horiz. Freq Dispay s Highest Horiz. Freq. VDC Setup Readouts VPP 1/10 norma B VPP (Highest Dispay Freq.) (Bipoar type) Can't increase DCV 0.0 VPP LLow VPP Most Likey Causes Proper Load Test Setup Load Test ma ~ 250 ma Current Limiting Severe B+ Load. 1/10 norma B+ 0.0 VPP Improper connections Open inductor/fyback circuit path. 1/10 norma B+ 1/10 norma B+ Chart 1: Likey causes of Load Test Setup Readouts. VOLTS Contro whie monitoring the VPP readout. Increase the votage unti the VPP readout is near 110 VPP (bipoar transistor) or 80 VPP (MOSFET transistor). This estabishes a satisfactory 1/10 eve for Load Testing most horizonta output stages. Occasionay probems in the horizonta output stage, improper circuit connections, or an improper test frequency or B+ votage do not permit a norma Load Test Setup. If the Load Test B+ votage cannot be increased beyond a certain DCV readout, it is ikey the power suppy is current imiting to 250 ma. This condition is indicated by a Limiting readout in the digita dispay and a Load Tests ma reading near 250 ma. Common causes incude an error in the B+ connection and a DC short or severe AC oad in the horizonta output stage. A 0.0 VPP readout during setup with a norma VDC appied indicates no inductive votage puses are being produced by the horizonta output stage. This may be caused by an improper connection or open in the horizonta output stage. Low VPP readouts may be caused by ower than norma B+ vots, a higher than norma horizonta test frequency, or an output stage oading or timing defect. Higher than norma VPP readouts indicate higher than norma B+ vots, a ower than norma horizonta test frequency, or a timing probem in the horizonta output stage. Severe horizonta output stage probems may prevent norma Load Test Setup. This does not prohibit you from using the Load Low VPP High VPP Load Test B+ too ow. Load Test Freq. too high. Load Test B+ too high. Load Test Freq. too ow. Tests. Set the Load Tests Setup as cose to norma VPP as you can and perform the Load Tests. Use the Load Test readouts to hep determine the nature of the horizonta output stage defect or setup probem. (See Chart 1) Performing The Load Tests In the LOAD TESTS position of the LOAD & RINGER TESTS Switch, the HA2500 simutaneousy meters and dispays three automatic measurements. The measurements accuratey refect the operation of the horizonta output stage being tested and indicate if the horizonta output stage is norma or contains a defect. The three LOAD TEST measurements are summarized in Chart 2. The "ma" readout Indicates the current being drawn by the horizonta output stage from the Load Test s B+ power suppy. This current refects the current Load Test Dispay Readout B+ Current miiamps (ma) drawn by the horizonta output stage from the chassis Bt power suppy. In other words, this is the power suppy oad current to the horizonta output stage. Since the Load Test B+ suppy is approximatey 1/10 of chassis norma Bt votage, the "ma" readout is approximatey 1/10 of the chassis norma. Readings typicay range from 10 to 70 ma in norma operating horizonta output stages and rise significanty when severe oading probems exist in the horizonta output stage. The Efficiency "%" readout measures what percentage of the input energy or current to the output stage is returned back to the B+ power suppy at the end of the horizonta cyce. Horizonta output stages are primariy tuned LC circuits with energy aternating between the magnetic fieds of the fyback and/or yoke and capacitance of the output stage. At the end of the horizonta cyce, the magnetic fieds coapse returning stored energy back to the power suppy. Defects add power osses greaty reducing the efficiency of the horizonta output stage and % of energy returned to the power suppy. Efficiency % readouts typicay range from 55 to 90% in norma horizonta output stages and decrease dramaticay in probem horizonta output stages. The "µs" readout is an automatic measurement of the puse duration or time of the inductive votage produced in the horizonta output stage being tested. The Load Test measures the time of the puse from the start of its rising edge to the end of its faing edge. The puse duration or time is determined by the Description of Test The current suppied by the Load Test B+ power suppy to the horizonta output stage under test. Efficiency percent (%) The % of the current input to the horizonta output stage at the beginning of the horizonta cyce that is returned to the power suppy at the end of the cyce. Puse Time microseconds (µs) The duration of the induced votage puse produced by the horizonta output stage Chart 2: Description of the three measurements performed during the LOAD TEST.

6 Fig. 5: The Load Tests indicate if the horizonta output stage contains a defect. inductance or capacitance of the fyback or coi primary, retrace timing capacitor(s), yoke, and yoke series components. The "µs" readout provides an indication of the LC timing of the horizonta output stage. The timing infuences the fyback puse ampitude which determines the amount of yoke defectron and CRT high votage. To perform the LOAD TEST: 1. Perform the LOAD TESTS SETUP. 2. Set the LOAD & RINGER TESTS switch to LOAD TESTS 3. Read the "ma", "µs", and "Eff." readout of the digita dispay. 4. Compare readout resuts to typica ranges isted in the Pu Chart. Form #6621 Printed In U.S A.