Download Smps repair sometimes can be easy and sometimes are quite

Smps repair sometimes can be easy and sometimes are quite difficult to
solve the problem. The power supply fault could be an open start
up resistor only or could be even more than ten components burnt due to
heavy lightning strike. In this article, I’m going to explain to you the power
supply problems that caused by defective components at secondary side.
Sometimes no power, power blink and low output power symptoms are
due to problems in the main circuit board. It is not necessary the fault must
be in the primary power section and shorted secondary output diodes.
The fault can be further down the secondary output lines which are in the
main board. If you had measured all the components in the power supply
section and could not find the defective component, then try troubleshoot
components beyond the power supply section. A shorted small
ceramic capacitor in the main board can cause the whole power supply to
malfunction.
A Switch Mode Power Supply
All smps has outputs at the secondary section. Some designed have four
outputs, some have five and so on. Each of the output is connected to an
individual circuit. For instance, a 6.3 volt output supply is send to the
picture tube so that it could light up and heat the cathode gun. A 5 volt
supply will usually entered into microprocessor and EEPROM IC as vcc
supply, 60-100 volt to B+ pin of flyback transformers and so forth. If one of
the components or circuit developed a short circuit, the voltage (vcc
supply) line will drawn a lot of current and thus the power will shut down,
fluctuate, low output power or even no power at all. You must know how to
isolate or repairs the problem otherwise you will be wasting your time
troubleshooting good components in the power supply section.
Set your analog meter to times 1 ohm range and check all the secondary
output (after diodes) between of ground and all the output voltage line. Put
your red probe to the cold ground and the black probe to each of the
outputs of secondary diodes. It should measured only one reading and
sometimes the pointer will kick up a little bit. This is a normal reading for a
good output voltage line. This happened due to the main board resistance
and some undischarged voltage from the secondary output filter capacitor.
If you are checking the negative output then you have to reverse the
meter’s probe. If any output voltage lines showed two same reading (low
ohms readout), then suspect that there is problem in that output line.
Carefully follow the suspected output voltage line and gently remove
components one by one at a time to isolate the short circuit. From my
experienced it is usually caused by a shorted IC, a transistor, a diode or
even a resin type ceramic capacitor. If you repair Monitor smps, if one of
the output voltage lines shows two readings, please do not think that the
lines is having a problem. The real caused as to why do you get two
readings are due to that you are checking the filament (heater) circuit. The
line goes to the picture tube’s filament and direct to cold ground, that’s why
you have two readings. Please don’t set your analog meter to times 10 Kilo
ohm otherwise all the reading that you measured will show two readings
which is not accurate.
The Back Portion Of A Switch Mode Power Supply Section
True case study- A Samtron Monitor 4Bn sent for repair with the complaint of no
power. After troubleshooting the smps section for sometime, I’ve concluded that
the power supply is working and suspect something shorted after the secondary
output diodes and this made me to trace beyond the power supply unit. By using
the explanation above, I managed to traced and found a shorted small resin
ceramic capacitor located at the 80 volt line in the Monitor color board. The
capacitor shorted to ground and made the power supply to stop operating.
The value of the shorted capacitor was .22 microfarad 100 volt. Replacing
only the small capacitor brought the power supply back to life again. In
smps repair, we as an electronic repairer must know how to distinguish as
which section is giving problem. Spending and concentrating your whole
time to check on a working section is just a waste of time, you should only
concentrate on section that have problems so that you could locate the
fault and make the equipment work again.
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