About The Burnt Protection Resistor In Electronic Circuit
Ever wonder why after the replacement of a resistor in circuit board and when power “On” the resistor burnt again? It was because the reisistor is a protection resistor. If you repair the vertical section of the CRT Monitor you would observe that along the supply line (VCC line) to the vertical driver/output IC there will be a low ohm (typically 0.47 to few ohms) resistor in series to the supply input of the ic. So if the vertical output IC developed a short circuit, the protection resistor will burn first thus just by replacing only the resistor would not solve the problem. You have to replace the vertical output ic as well.
This scenario is same as when you see a protection resistor burnt along the B+ supply line to FLYBACK TRANSFORMER. It could have been the HOT, flyback primary winding, internal capacitor had short circuit causing the protection resistor to open circuit or burnt! Always look out for surrounding components if you found a protection resistor burnt.
Now what if the resistor is not a protection resistor and it burnt? Resistors usually don’t burnt by itself, it was the other circuit that had caused the resistor to burnt. For example, there were many cases of resistors burnt surrounding the flyback transformer circuit. If you carefully trace back the circuit, you could know that it comes from the abl (automatic blanking limiter) pin of flyback transformer. It could be a shorted internal capacitor or a shorted internal high voltage diode that has made the resistors to burnt.
I hope from this post you will get some ideas as where to check when you come across a burnt resistor in the future!