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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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ticking smps
how can i fix a ticking smps on a mitac L1450PD
it uses a UC3842 controller CNX82A optocoupler and TL431A shunt. there are four lines on this monitor, 150v 87v 24v 6.3v all reading low. how do i find the line causing the problem? |
#2
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This is not something that can be done over an email. It is the skill and
experience of using the proper tools and knowledge to be able to trace through the operation of the supply, and to be able to check and or try out the various suspect components. Many times in these supplies the electrolytic caps give problems. You can use an ESR meter to test them. -- Jerry G. ===== "BigTom" wrote in message ... how can i fix a ticking smps on a mitac L1450PD it uses a UC3842 controller CNX82A optocoupler and TL431A shunt. there are four lines on this monitor, 150v 87v 24v 6.3v all reading low. how do i find the line causing the problem? |
#3
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ive checked esr on all the caps and theyre ok.
ive got a another working monitor to test voltages/resistances etc. ive read the sci.electronics.repair.of.smps. are there any other good web sites where i can learn more about smps. Jerry G. wrote: This is not something that can be done over an email. It is the skill and experience of using the proper tools and knowledge to be able to trace through the operation of the supply, and to be able to check and or try out the various suspect components. Many times in these supplies the electrolytic caps give problems. You can use an ESR meter to test them. |
#4
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BigTom wrote:
ive checked esr on all the caps and theyre ok. ive got a another working monitor to test voltages/resistances etc. ive read the sci.electronics.repair.of.smps. are there any other good web sites where i can learn more about smps. Jerry G. wrote: This is not something that can be done over an email. It is the skill and experience of using the proper tools and knowledge to be able to trace through the operation of the supply, and to be able to check and or try out the various suspect components. Many times in these supplies the electrolytic caps give problems. You can use an ESR meter to test them. Did you check for an over current condition causing the PS to shut down? |
#5
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i dont know the circuit well enough to test for it.
the output voltages on the smps are less than half of normal. and even the voltage on the diode side of the optocoupler is low. Ken wrote: BigTom wrote: ive checked esr on all the caps and theyre ok. ive got a another working monitor to test voltages/resistances etc. ive read the sci.electronics.repair.of.smps. are there any other good web sites where i can learn more about smps. Jerry G. wrote: This is not something that can be done over an email. It is the skill and experience of using the proper tools and knowledge to be able to trace through the operation of the supply, and to be able to check and or try out the various suspect components. Many times in these supplies the electrolytic caps give problems. You can use an ESR meter to test them. Did you check for an over current condition causing the PS to shut down? |
#6
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BigTom wrote in :
how can i fix a ticking smps on a mitac L1450PD it uses a UC3842 controller CNX82A optocoupler and TL431A shunt. there are four lines on this monitor, 150v 87v 24v 6.3v all reading low. how do i find the line causing the problem? ticking indicates the PS is trying to start,but tripping a protective current limit. I'd check for shorted diodes on the secondaries and use an ESR meter to check the electrolytics. -- Jim Yanik jyanik at kua.net |
#7
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BigTom wrote in :
i dont know the circuit well enough to test for it. the output voltages on the smps are less than half of normal. and even the voltage on the diode side of the optocoupler is low. Download the application notes for the UC3842,it's a common switcher control IC. That will give you an idea of how the PS works. -- Jim Yanik jyanik at kua.net |
#8
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"Jim Yanik" bravely wrote to "All" (03 Jan 05 17:07:22)
--- on the heady topic of " ticking smps" JY From: Jim Yanik . JY Xref: aeinews sci.electronics.repair:5701 JY BigTom wrote in : how can i fix a ticking smps on a mitac L1450PD it uses a UC3842 controller CNX82A optocoupler and TL431A shunt. there are four lines on this monitor, 150v 87v 24v 6.3v all reading low. how do i find the line causing the problem? JY ticking indicates the PS is trying to start,but tripping a protective JY current limit. JY I'd check for shorted diodes on the secondaries and use an ESR meter JY to check the electrolytics. Weak switching pass transistors can also result in ticking though the psu might even start intermittantly but the regulation will be poor. A*s*i*m*o*v .... If all else fails, hurl it across the room a few times! |
#9
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"BigTom" schreef in bericht ... Jerry G. wrote: This is not something that can be done over an email. It is the skill and experience of using the proper tools and knowledge to be able to trace through the operation of the supply, and to be able to check and or try out the various suspect components. Many times in these supplies the electrolytic caps give problems. You can use an ESR meter to test them. ive checked esr on all the caps and theyre ok. ive got a another working monitor to test voltages/resistances etc. ive read the sci.electronics.repair.of.smps. are there any other good web sites where i can learn more about smps. From sci you should know the SMPS is starting but switches off before it is really up and running. From the datasheet of the 3842 you should at least have a general idea of the workings of that chip, especially from the example circuit. Now you have two main causes that makes the regulator switch off: - Overcurrent. Somehow the voltage across the current sense resistor becomes too high. Maybe a short in the transformer but an overload or short on the secondary side is also possible. I ever had a current sense resistor that has got to high a value. - Power failure. These regulators start using a start circuit that provides just enough current just to start. When the circuit comes up it requires more current which usually is provided by the transformer. (Just look at the example on the datasheet .) This piece of power supply may fail itself but the failures mentioned above may also cause this problem. Failing transformers can be found by using a ringer. Secundairy overloads or shorts can often be found using an Ohmmeter. Pay special attentiion to the secondary rectifiers and their smoothing capacitors. Keep in mind that parts that look all right when measured this way nevertheless may fail under load. Also keep in mind that the cold filament has a low resistance. If this all does not give a clue you can try to find the failing secondary part by temporary removing the rectifier. (Which is not riskless but your last chanche.) petrus bitbyter |
#10
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"petrus bitbyter" wrote in
news "BigTom" schreef in bericht ... Jerry G. wrote: This is not something that can be done over an email. It is the skill and experience of using the proper tools and knowledge to be able to trace through the operation of the supply, and to be able to check and or try out the various suspect components. Many times in these supplies the electrolytic caps give problems. You can use an ESR meter to test them. ive checked esr on all the caps and theyre ok. ive got a another working monitor to test voltages/resistances etc. ive read the sci.electronics.repair.of.smps. are there any other good web sites where i can learn more about smps. From sci you should know the SMPS is starting but switches off before it is really up and running. From the datasheet of the 3842 you should at least have a general idea of the workings of that chip, especially from the example circuit. Now you have two main causes that makes the regulator switch off: - Overcurrent. Somehow the voltage across the current sense resistor becomes too high. Maybe a short in the transformer but an overload Bad filter caps on secondary,very common.ESR meters are good for these. or short on the secondary side is also possible. I ever had a current sense resistor that has got to high a value. - Power failure. These regulators start using a start circuit that provides just enough current just to start. When the circuit comes up it requires more current which usually is provided by the transformer. (Just look at the example on the datasheet .) This piece of power supply may fail itself but the failures mentioned above may also cause this problem. Usually the housekeeping supply's electrolytic cap,by my experience. Failing transformers can be found by using a ringer. A rare failure,IMO. Secundairy overloads or shorts can often be found using an Ohmmeter. Pay special attentiion to the secondary rectifiers and their smoothing capacitors. First place to look,IMO.(other than obviously burned parts.) Keep in mind that parts that look all right when measured this way nevertheless may fail under load. Also keep in mind that the cold filament has a low resistance. If this all does not give a clue you can try to find the failing secondary part by temporary removing the rectifier. (Which is not riskless but your last chanche.) petrus bitbyter -- Jim Yanik jyanik at kua.net |
#11
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****After many hours of studying the circuit and with the help of
everyone here, the ps problem has been SOLVED. It was R907 o/c, part of the current sense circuit of UC3842. Many thanks to everyone involved in this project. Jim Yanik wrote: BigTom wrote in : how can i fix a ticking smps on a mitac L1450PD it uses a UC3842 controller CNX82A optocoupler and TL431A shunt. there are four lines on this monitor, 150v 87v 24v 6.3v all reading low. how do i find the line causing the problem? ticking indicates the PS is trying to start,but tripping a protective current limit. I'd check for shorted diodes on the secondaries and use an ESR meter to check the electrolytics. |
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