Repairing a PVR power supply
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From: Philip Pemberton on 11 Jun 2010 13:02 On Fri, 11 Jun 2010 11:06:13 -0500, Jim Yanik wrote: > did you check the ESR of the electrolytics,particularly those around > your IC? All checked good, but were replaced anyway (they were off-brand Chinese things that I've seen fail in the past). Thus far I've replaced: TEA1522 Optocoupler Shunt regulator One shorted diode in the 5V loop. Looked good on the meter in-circuit, took it out of circuit and my DMM and the Atlas DCA reported it as dead- shorted. I've checked the resistors around the shunt regulator and the TEA, they're fine. All the other stuff around the TEA looks good, but still noting... "Beam me up, Scotty, there's no life out here." I'm getting sorely tempted to disconnect the secondary diodes for all but the 5V rail, then reconnect them one by one... -- Phil. usenet10(a)philpem.me.uk http://www.philpem.me.uk/ If mail bounces, replace "10" with the last two digits of the current year
From: Philip Pemberton on 11 Jun 2010 14:23 OK, I disconnected all the rails and it still wouldn't come up. Replaced the TEA1522 and it came up OK with only the 5V rail active. Fair enough, reconnected each rail in turn, turns out the SR560 Schottky on the 12V rail had shorted *again*. I'm now onto my last TEA1522 and getting more will be "entertaining" to say the least (and probably involve buying £10 worth of them). What gets me is that the shorted diode did NOT cause the PSU to go into shutdown: it blew the TEA instead... The shorted-turn trigger trips when the voltage at SOURCE is 0.75V. Thus, the 1R resistor that's in there now will cut the power when the primary pulls 0.75A, and the chip is rated to a drain current of 1A. This seems a bit close to me -- so I'm thinking of adding about 10R to this (two 0.7W 22R resistors in parallel to share the load). Good idea, bad idea? I'd try the Lightbulb Trick, but I'm willing to bet the contents of my wallet that it won't offer much protection against a secondary side short. -- Phil. usenet10(a)philpem.me.uk http://www.philpem.me.uk/ If mail bounces, replace "10" with the last two digits of the current year
From: JW on 14 Jun 2010 05:42
On Fri, 11 Jun 2010 11:06:13 -0500 Jim Yanik <jyanik(a)abuse.gov> wrote in Message id: <Xns9D947B503CED1jyaniklocalnetcom(a)216.168.3.44>: >Philip Pemberton <usenet10(a)philpem.me.uk> wrote in >news:4c12566c$0$17407$c3e8da3(a)news.astraweb.com: > >> On Thu, 10 Jun 2010 11:52:17 +0000, Philip Pemberton wrote: >> >>>> then replace >>>> the TEA1522P before wasting time with the opto isolator, shunt >>>> regulator, or other unlikely candidate. >> >> Swapped the 1522. PSU still dead, no output or startup. >> >> Suspects are now the opto, the shunt regulator, and the resistors which >> measured high. >> > >did you check the ESR of the electrolytics,particularly those around your >IC? I've seen where high-ESR caps kept switchers from starting. >They loaded the IC supply enough that it would not start. Huh? From what I've seen, high ESR caps will cause the switcher to shut down, but from over voltage sensing rather than over loading the outputs. Shorted caps will do what you say, but an ESR meter is not much good in those cases. A good low range ohm meter for tracking shorts is best in a situation like that. Better still if it has 4 wire inputs. |