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Power supply repair tips

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Sometimes no power, power blink and low power symptoms are due to problems in the main board. It is not necessary the culprit must be in the power section (primary) and secondary output. It can be further from the power supply section which is in the main board. If you have checked all the components in the power supply section can could not locate the fault, try checking beyond the power supply section. In this article i will explain to you on how to troubleshoot faults beyond power supply circuit.

Every switch mode power supply have output at secondary side. Some design have three outputs, some have four, five and so forth. Each output are connected to different circuit. For example, 5 volt output mostly goes to the microprocessor and eeprom Ic, 12 volt to display or jungle ic, 100 volt to B+ circuit and etc. If one of the ic or circuit developed a heavy short circuit, the voltage (vcc) line will draw a lot of current and thus the power will blink, power becomes low or even totally no power at all. To isolate this problem is simple, just use your analog multimeter set to times 1 ohm and measure all the secondary output between ground and all the output voltage line. Place your red probe to the cold ground and the black probe to each of the output. It should show only one reading and sometimes the needle will move up a little bit. This is normal due to the board resistance and some undischarge voltage from the filter capacitor.

If you are measuring negative output then you have to reverse the probe. If any output lines register two same reading (low ohms reading) suspect that there is fault in that line. Trace from the suspected line and slowly remove components one by one to isolate the shorts. From experienced it is usually caused by a  shorted capacitor, transistor and ic. If you repair monitor power supply, if one of the output lines  shows two readings, please don't think that the lines have developed a short circuit. The reason you have two readings are due to you are measuring the heater circuit! The line goes to the crt filament and straight to cold ground, thus you have two readings!

True case example- A Samtron monitor 4Bn sent for repair with no power symptom, upon checking the primary and secondary side found no defective components so this lead me to check beyond the power supply circuit. Using the tips given above, i managed to traced a small resin ceramic capacitor shorted at the 80 volt line.
This line is supplying voltage to the video driver ic and the shorted capacitor value was 224 100v (.22microfarad).

Conclusion-Sometimes in electronic repair we as a technician or engineer have to think out of the box because electronic board consist of lines of circuit where one section spoilt will effect the other.