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Philips Plasma with faulty PS-424-PH PSU (no Vs, Ve, D5v)

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    Philips Plasma with faulty PS-424-PH PSU (no Vs, Ve, D5v)

    Good day fellow cappers, got a pretty complex project on my hands here, but I feel like I'm getting pretty close, so let's tinker this together: there's this LARGE Philips TV which has power supply issues. The power board is a Samsung ps-424-ph which has no Vs, Ve and most imporantly, no D5v and D3V3 outputs either. The biggest issue is that I can't find a schematic for it, only the repair guide which I've attached for your convenience.
    You plug the board in (on its own, without anything else attached), the two relays click, stay like that for a few seconds and then switch off again. The two large caps charge up to about 350v, but just from that initial pulse until the relays cut off the power going to the rectifier, at which point they slowly begin to discharge (I can even drain them manually with a lightbulb and it makes no difference).
    The LED on the daughter-board on the left side of the PSU returns an error code, namely a 13-blink code, indicating missing D5V and sure enough, I get nothing out of both the D5V output and the 3v output. Funny thing is that I also located another problem: two shorted MOSFETs (Q8022 and Q8020) that drive the large Ve-Vs transformer (notice I've already removed one of them) and of course a blown fuse (F8007). The blink code says nothing about that, though it could be because it's a shorter blink code that the 5v one and the board displays the longer ones first when there's more than one error. There's no doubt Ve-Vs are dead due to those busted FETs, but they're pretty expensive to replace and I don't want to waste money on them only to discover a truck-load of other dead components, so I first want to tackle the faulty D5V bus. I managed to track down the problem all the way to the primary side, now hopefully I'll be able to explain this well enough so we don't get lost, since this is one of those double-sided boards to make matters worse. To put it simply, IC8017 has no VCC. IC8017 is a 1207A (supposedly an NCP1207A by its full name, according to the datsheet). This IC drives Q8016, which in turn drives transformer T8002, which is fed into the secondary side - standard SMPS, if you think about it (I've taken a close-up of that general area). The thing is, VCC is supposed to come through C8069, but I get nothing on either side of that cap. Following the trace from the cap (which acts as a rectifier in this case), I can see it comes from an auxiliary winding of T8002. This is where it gets confusing and complicated as F, especially with no schematic: it seems to me this AUX winding (the bottom 2 pins of T8002) has no way to run without those two FETs, since I see another trace running to a capacitor in that general direction (C8067 visible at the very top of the last picture) which goes to the first pin of the transformer. Could that be why I'm not gettig squat out of that transformer, or am I wrong ? The repair guide doesn't talk about this, so I'm wondering how T8002 even works :| All other fuses are OK and I couldn't find any shorts in any of the major components....any tips or at least a schematic ? Feel free to ask for any further info I might've missed. Cheers.
    Attached Files
    Wattevah...

    #2
    Re: Philips Plasma with faulty PS-424-PH PSU (no Vs, Ve, D5v)

    The capacitor connected to pin 6 which data sheets says typically 10uF is charged to VCC via constant current supplied through the HV pin 8. I would check this capacitor and also any resistors connected to pin 8.
    Last edited by dick_barton; 11-15-2016, 06:59 AM.
    Willing to help but I'm no expert.

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      #3
      Re: Philips Plasma with faulty PS-424-PH PSU (no Vs, Ve, D5v)

      Originally posted by dick_barton View Post
      The capacitor connected to pin 6 which data sheets says typically 10uF is charged to VCC via constant current supplied through the HV pin 8. I would check this capacitor and also any resistors connected to pin 8.
      Mine's a 100uf cap. I checked the resistors connected to pin 8 and they all read ok, so the next step is to replace this cap. It's odd that I'm not getting anything AT ALL on this cap, so the next step is to replace it. If that doesn't change anything (which I doubt it will), then what's next ? Changing the IC itself I presume.
      Wattevah...

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        #4
        Re: Philips Plasma with faulty PS-424-PH PSU (no Vs, Ve, D5v)

        Just replaced the cap and nothing changed: no VCC on pin6. By comparison, the "standby IC" which controls the standby 5v, gets 12v on its 6th pin and it's nearly the same exact IC, so either the first IC's busted or the diode that's placed before pin 6 is faulty, though it reads the same values as the other one and I get absolutely nothing on any of the components around it...I read a volt somewhere on that large resistor R8127 but other than that, nada ! :|
        Wattevah...

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          #5
          Re: Philips Plasma with faulty PS-424-PH PSU (no Vs, Ve, D5v)

          I believe I found out why I'm not getting the VCC as expected: after a lot of measuring and analysis of the circuit, I was able to put together a simplified schematic which illustrates the issue. It turns out I was right about one thing: the small 5v/3v transformer is not getting any power because of the missing fuse which blew along with those two large FETs which I'm trying to replace. The AUX winding of the transformer (which is rectified and fed as VCC to the IC) obviously cannot run without the main winding, since it cannot produce power out of nothing, can it ? No fuse > no power to the main winding of the small transformer (T8002) > no power out of the AUX winding of T8002 > no VCC. I also drew the Vs transformer in there, just to give you an idea of what's going on. Makes so much more sense now...I was aware I removed the fuse since it was blown anyway, but I couldn't see the bloody traces running to it because of all those BS heatsinks taking up half the board :|
          Attached Files
          Wattevah...

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