Switching PSU Theory of Operation / Repair Tips

Lot of comments on mine: :-) boy that small one snowballed into big paper. :-P That helped! Also I have comments on capacitor bleed down after unplugged from outlet. Some didn't have bleed down resistor(s), required by safety in peecee PSUs I think but some did have bleed down resistor fail or simply didn't exist. How I know? I got bit by charged main filter capacitor in servicing modern JVC TV. Btw that one didn't have that bleed down resistor even JVC is decent brand! Now I test for that for any reason and flatten it off with 15k ohms resistor. :-P My story: One hand rests on certain area circuit board - BUZZ buzz buzz! AAAAGGGGGGH! In loud, shrill voice to make singers proud. DC felt like buzz like I get with raw AC from that outlets. That capacitor didn't discharge at all even after a week. That's very good quality capcitor, JVC! :-) This reminds me on this high voltages in both monitors and TVs. Give BIG due respect to high voltage in any monitors and tv, biggest ones is around 35,000 or so volts. Enough energy on HV to kick like mule and hurts like hell but not kill you. The danger is your hand or part of your body fly away and hit something else (injury or get cut.) High voltage is used to impart focused electrons emitted from electron guns to high speed against phosrphous (sp?) that glows when hit. Tube bell is made of painted graphite (that characteric dark grey) and glue binder on two parallel plates construction with glass as dielectric basically tiny capacitor to smooth HV pulses (more of sprikes from the flyback coil, inner surface has HV, outside surface on that tube is at ground potiential for HV circuit at few volts in respect to true frame ground especially in TVs because it does not have true ground design because of two prong plug while mesuring voltages on that circuit board, for that reason, I always use circuit board ground not that grounding wires across tube bell. In compact Macs, tech sheets warns DO not to use dead short to discharge tube! Properly done requires homemade tool or proper tool that has resistor to bleed out tube's stored HV gradually. Shorting out that tube with a noisy bang using piece of wire, this burns out the chips on that logic board, seen this happen before. Also this is preferred on everything else since the anode well on the inside conductive coating connection may burn open by high current then tube becomes junk. $$$. When off or unplugged still have HV charge present for few days to a week before that tube becomes discharged. Freshly charged tube has enough punch to jump a inch or two gap - LOUD SNAP. And, one more thing, after manual discharged, tubes always regains some charge after few minutes, don't be surprised by a tiny zap or see tiny spark. :-) Cheers, Wizard PS: Even good monitor or tv, sometimes HV leak out and bit you if you're careless. I saw this happen when tech rested that grounded screwdriver gently on glass near edge of suction cup - CRACK! Tech then looked surprised turned to me and I said oooh. PSS: Snapping, crackling, hissing noise is sign of HV leak or overvoltage fault this one must be serviced NOW because of HV had skyrocketed due to component failure somewhere and excessive X-ray emssions limiter or trip had failed also.