Question about BenQ FP567s 15" LCD Monitor
Many people have noted two primary failures with this monitor. i have a couple time-saving troubleshooting tips, which actually apply to mostly all electronic devices. first, of course, bulging electrolytics [capacitors] should be considered suspect. i personally usually replace all the filter [power supply] caps as a matter of practice, since they are the 'hardest worked' component. anyway, a good way to test then [if you dont't have a capacitor tester capable of testing them at working voltage] is to check the voltage across the cap terminals, with your meter set to the A.C. volt scale. it should read close to zero, with only perhaps a few millivolts max. this test is only valid for the filter caps, many smaller caps are bypass or coupling caps, and you will get a reading. secondly, the transistors- many references have been made to shorted transistors in the inverter circuit. it has been stated that the transistors must be removed to test them. in almost all cases, they can be tested in circuit- first, if you're lucky enough to have an old analog vo. [such as the simpson 360, or tripplett 630] you can usually get an accurate test using the rx10 or rx100 scale. it is always possible to get an inaccurate reading depending on how it is configured in the circuit. - but a very good, accurate test is to measure the voltage between the emitter and base [while the unit is powered on, of course] you should get aproxomitately
.7 [seven tenths] of a volt if the transistor is on and conducting. this test applies to almost all electronics. the only variatin in this rule is if the transistor is a germanium type [you won't see them unless the device is from the late 1950's to mid 1960's] in which case the voltage reading will be more like 1.2 volts. also, in some cases, darlinton [multiple transistors in one case] occasionally behave differently, but you should still get a reading close to .7. on most inverter boards, the transistors are a TO-22 case. with the outer leads being the emitter and base, and the center lead the collector. in many devices, you may actually see the center [collector] lead cut, appearing to have no connection. this is because they are using the actual mounting tab, which connects to the collector, to make the connection. you should see the B+ [power supply voltage,] present at the collector to ground. i hope the readers find thid info helpful, and remember, these test procedures work for pretty much all electronics. - and one other thing- when testing capacitors, if you have an oscilloscope, you can take the reading across the cap terminals, and you should see a virtually flat line- if the cap is bad, you'll see a sawtooth pattern. one word of caution- if you use a scope, use a three-prong adapter on the ac plug of the scope, and defeat [don't connect] the earth ground of the scope. this is because many of the power supplies used have the common [ground] above earth ground potential, and you can actually damage it when you connect the ground lead of the scope-i've had it happen- especially in a few of the older sony tv's!
Posted on Jan 26, 2009
Tips for a great answer:
Basic LCD monitor and TV troubleshooting guide:
Learn about bad caps: http://en.wikipedia.org/wiki/Capacitor_plague
Capacitors kit: http://lcdalternatives.auctivacommerce.com/ he can make you a set of caps for you.
Or www.digikey.com just make sure to use caps with low ESR, 105c, high ripple current, long life rating such as PANASONIC FM or FC series
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