| LG 710E 17” CRT Monitor came in with the | | | | display but with B+ fet the power blink. In this case, one |
| complaint of power blink. A power blink problem can | | | | would always check the B+ fet first am I right? |
| be cause by either in primary or secondary section or | | | | However the B+ fet turned out to be good! |
| even in both sections. | | | | Now where is the culprit hiding? Troubleshooting |
| A defective power IC, power fet breakdown when | | | | sometimes can be very frustrated and most of the |
| under load, high esr in electrolytic capacitor, increased in | | | | time is always fun. One of my methods that I liked to |
| current sense resistor value, a shorted zener diode, a | | | | use is to touch for any unusually hot components on |
| leaky transistor, open circuit or resistor has changed | | | | board with my finger. (Please take note for all |
| value in primary side can cause power to blink. As for | | | | electronic repairers-switch off and unplug the power |
| the secondary side, a shorted secondary diode, | | | | cable and discharge the big filter capacitor in primary |
| flyback, HOT, horizontal yoke coil, damper diode, B+ | | | | side before you begin to touch any components). |
| fet, an open filter capacitor, shorted IC and etc can | | | | Usually after power off, I would touch on the flyback |
| cause power to blink as well. | | | | first then to other components surrounding the high |
| Normally what I do is to remove the B+ fet first and | | | | voltage section. While browsing the components with |
| connect a light bulb to distinguish whether is the | | | | my finger I stumbled upon a resin coated ceramic disc |
| primary or after the B+ circuit components that | | | | capacitor which is quite hot. It is located near the |
| caused power to blink. However in this case, I did not | | | | flyback transformer and in fact the capacitor was |
| put the light bulb and I directly removed the B+ fet and | | | | there is to act as a filter capacitor for the 400 vdc line |
| power on the monitor. For your information, this B+ fet | | | | output as shown from the photo. |
| is the N-channel mosfet and this B+ fet is part of the | | | | After the cap was removed and checked with an |
| boost circuit. | | | | analog meter, it showed a shorted reading even in the |
| After removing the B+ fet, not only the power | | | | X1 ohms range. A good capacitor should show a |
| didn’t blink, there was display as well and the anode | | | | charge and discharge activity in the X10 k ohms range. |
| voltage reads about 12 Kilo volt (a good monitor should | | | | A new capacitor replacement for this LG 710E 17” |
| have about 24 KV). The reason why you got 12 kilo | | | | CRT Monitor cured the fault. |
| volts is because the boost circuit is not functioning due | | | | The reason why the monitor power blinks is because |
| to the removal of the B+ fet. The display shown a | | | | the faulty capacitor become shorted when in full |
| small width and the horizontal size control is already | | | | operating voltage (boost circuit is functioning and |
| set to the maximum (you can’t get maximum width | | | | approximately 60 -70 vdc enter the B+ flyback pin). |
| because the anode voltage is only 12 KV). You can | | | | When the B+ fet was removed (boost circuit not |
| control or set everything in the On Screen Display | | | | working and only about 40 vdc enter the B+ pin of |
| (OSD) except you can’t bring horizontal size to | | | | flyback) the monitor is not working under full operating |
| maximum. | | | | voltage and the faulty capacitor can still tolerate (partial |
| With having a display, it is a good clue to me because I | | | | short) and became hot. I believe if the monitor is |
| knew that many circuits is working such as power | | | | switched “on” for a little longer, it would cause |
| section (so you don’t suspect primary side | | | | the capacitor to develop a direct short circuit and if this |
| anymore), flyback transformer, yoke coils, micro-p, | | | | happen, removing the B+ fet to test the monitor would |
| eeprom, horizontal and vertical circuit, color circuit and | | | | not make any different (the power would still blink). |
| even the Osd circuit. Without the B+ fet, there is | | | | |