Smps Faults Tips

Here are some tips for smps faults – Smps blows fuse – if fuse blows in smps , there is short circuit in the ps circuit . first understand the circuit of smps . What is the method of switching , parts used , and make test points . Main filter is centre point for checking smps . in this fault use resistance method for locating shorted part . Measure resistance across the two terminals of main filter , capacitor action must come . Instead of capacitor action it shows low resistance across mf than there is shorted part I the supply line , disconnect all parts one by one connected with the supply line , by disconnecting which part removes short , check that part or line for short , replace . Probable faulty parts may be – shorted – bridge rectifier or diodes making rectifier , mf , any protection part if used , smps out put transistor or switching regulator ic (STR) ,sm transformer , short on output of smps can also cause fuse blow , transient protection capacitors placed across rectifier or diodes .

Smps dead no out put from smps – There might be many faults for this symptom . If fuse do not blows , and there is no output than follow the following tips – Check voltage on mf. If no voltage on mf , than ac to dc circuit is to be checked . check ac voltage on mr , if no voltage than check wire, switch, fuseor FR(r- before rectifier) open (search for reason for blow of fuse & FR ) , mr open . If voltage available at mf than ac to dc part of circuit is ok . now consider type of smps circuit . If str is used check voltage on collector or drain terminal of str , if voltage is not available than check parts coming in the path of supply from mf to str , check for open line , lr , smps transformer . If voltage on collector terminal is available than measure voltage on base or gate terminal if voltage is not coming than check parts giving supply to base pin of str , if voltage is coming than check for oscillator failure , parts giving feed back for oscillation , or parts used for protection , short in out put of smps , or circuit loading smps . If transistor circuit is used in output and driver and IC in oscillator circuit and output is not coming , than first check driver transistor base or gate voltage if voltage is not normal than check oscillator circuit by measuring voltage of ic and comparing it with normal voltage to access fault , supply to ic , oscillator faulty , protection on , time constant parts faulty . if voltage on driver base is normal than check voltage on base or gate of output transistor , if voltage is not normal than check circuit between base of drive to base of out put by measuring voltage decide fault by comparing fault time voltage with normal voltage . Driver transistor and it’s supply, secondary parts , driver transformer faulty , if voltage on output transistor is normal than check output transistor voltage . If collector or drain voltage is missing than check parts coming in the supply path, lr (limit resistance) , fuse , eht primary , if voltage on collector voltage is coming than check output transistor and replace it . Always check for protection circuit if smps starts for a fraction of a second.Donot force circuit to start , search for fault only. Never disable protection circuit without considering it’s effect on circuit. Always discharge MF before attempting any repair work . If chasis is LIVE than always use isolation transformer or be carefull to take precautins for live chasis circuit, otherwise you will get SHOCKS or any kind of damage can take place.

If you are not sure, always start circuit using VARIAC or series lamp or use any 100 to 150 volt old transformer. To locate faulty line you may apply 12v on the line from any urg ps used for tape recorders .this will reduce damage risk to minimum You get full time to search fault, without damage.

Smps out put is low – if smps givel low voltage output than the fault is mostly in the error amplifier, control, softstart circuits . Output loading may also affect the output voltage some time. Measure voltages and compare them with normal voltage given the circuit diagram, probable parts may be faulty zenar diode in the error amp, faulty control circuit parts,transistor, IC , opto coupler faulty, filter on error voltage line faulty.

Smps output is high – if smps output is high first shut down set , check fault either in switch off position or use variac or low voltage transformer . Disconnect tv’s other sections by disconnecting base of h-output transistor. Never keep on in this fault it may damage other parts also. Check for – error amp circuit, zener diode, optocouoler, filters on error amp line, transistor, IC, resistance.

Humming – if there is humming in the ps then picture and raster will shake or produce hum bars in raster or picture. Check power filters on all power lines, check for leakage in diodes of mr . if parts are ok. Than check for weak earth connections , open shielding , signal components close to ac parts or have common ground .