Judgment by the failure phenomenon, failure must be comprehensive protection circuit in the case with control. In order to determine the fault source is in the switch power supply circuit, or at the output or protection circuit, first pull the plugin X1, break operation output circuit dc power supply, with D11 (3) feet and cold end external false load a 100 w bulb. Found that external light bulb glow red short boot, and the testing norms of voltage of 140 v on both ends of the light bulb, hereby exclude bad comprehensive protection circuit switching power supply and the factors, and concluded that line in the output and the load circuit fault components. After shutdown with a model 500 multimeter R x 10 Ω blocked the road test line output tube Q552 collector of forward and reverse resistance are 0 Ω, remove the Q552 detection, the beam electrode has short circuit breakdown, Q552 breakdown caused by the damage factors: (1) Q552 has its bad performance; (2) the line output transformer T501 local short circuit or internal field output circuit fault or visual circuitry to working anomaly picture tube beam current is too large or tube touch very damage such as the line output Q552 collector current is too large and the broken down; (3) signal resulted from abnormal owe incentives or overexcitation causing pipe broken down due to increased power consumption; (4) + B output over-current protection monitoring circuit components to poor protection function failure, etc. So before replacing Q552, over-current protection monitoring device was detected in the road first Q553, D551, R557, found that virtual welding D551 anode pin open, all the other elements are normal. After repair welding D551, down dummy load, and then good plug-in X1 (D11), and with a 2 sc4927 manager substitution Q552 after boot, edge detection Q552 c voltage, edge observation screen grating quality, found that just boot Q552 c voltage is normal, and normal sound, but no image is only a light grating and full screen flyback bright line, after about twenty seconds, + B output over-current protection circuit control and automatic shutdown. Hereby confirm the cause of the problem is the tube beam current is too large. Should check mainly depending on the board (L) and its related circuit components. Unplug the tube when the power socket, the beam current is zero, boot detection visual put board plug-in L3 1 foot dc voltage is 90 v, obviously lower than the normal 220 v, on the basis of judgment depending on the power supply circuit formed bad. The machine visual put power 220 v is formed by line output transformer T501 (4) the foot line flyback pulse D559, C504 rectifier filter. Respectively when the power is removed suspicious components quantitatively test, found that filter capacitor C504 serious leakage variable values. Good, after replace the capacitor inserted tube, it is testing the L plate on visual put the drive tube Q360 ~ Q362 emitter voltage of 158 v, 159 v, 162 v, all within the normal range. Screenshot has been observed at this time, light, sound are back to normal, troubleshooting.
 
summary Because + B output over-current protection monitoring signal transmission elements D551 pin solder open, disable + B load over-current protection function, causing the output pipe line Q552 serious leakage circuit formed by the visual work power line (equivalent to the output load increase) and produced flow phenomenon, the work time is slightly longer Q552 broken down due to power consumption is too large, then forced the switch power supply IC801 internal control protection over-current protection circuit. If a + B output over-current protection function is normal, you won't get pipe because of over-current damage phenomenon, unless it is Q552 their poor performance caused by the damage of nature. Through this example troubleshooting also can see that: in the repair of damage of the key components of failure, never neglect the protection circuit monitoring components inspection, otherwise easy to cause the key components of the new replacement damage again.