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Thread: LG TV wipeout

  1. #221
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    Dog gone perfect.
    Both gates. Q101 on top.


    Last edited by loopyloo; 16-08-17 at 11:53 AM.



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    The plot thickens....
    That's perfect switching so the chip passes the test
    Link the external 12V supply to the Q101 drain as well so the resonant circuit gets switched - if the external supply can hold its voltage up with the extra load that is

    If it can pass that test go for the mains test using the onboard Vcc supply

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  • #223
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    Quote Originally Posted by Skepticist View Post
    The plot thickens....
    That's perfect switching so the chip passes the test
    Link the external 12V supply to the Q101 drain as well so the resonant circuit gets switched - if the external supply can hold its voltage up with the extra load that is

    If it can pass that test go for the mains test using the onboard Vcc supply
    With the supply linked to Q101 drain : supply holding, 2volts on RFmin and the waveform is the same with the exception that Q101 gate now peaks around 24v. Q102 still peaking 12v.
    Lookin' good....go for gold ?
    Last edited by loopyloo; 16-08-17 at 12:12 PM.

  • #224
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    Powered it from it's own supply and the result :: back to square one.
    RFmin dropped straight back to 85mV.
    At least we now know switching is working.
    That test lamp's not limiting too much current is it ?

    Think I'll pull the fuse and lift D510 and R502 and drop the external supply in there, also short pins 3&4 of IC503 to see what Q501 does.
    Last edited by loopyloo; 16-08-17 at 01:45 PM.

  • #225
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    The series lamp's resistance is very low when the filament is cold so if it's not more than a very dull glow it shouldn't be strangling the supply voltage
    What does the lamp do at switch on? with no load connected it should just blink once or twice as the caps charge up then settle back to a very dull glow at the most

    Take care not to feed 340V back into your external supply or there'll be yet another repair job

    What happened to the onboard Vcc when RFmin voltage dropped?
    Last edited by Skepticist; 16-08-17 at 02:52 PM.

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    Quote Originally Posted by Skepticist View Post
    The series lamp's resistance is very low when the filament is cold so if it's not more than a very dull glow it shouldn't be strangling the supply voltage
    What does the lamp do at switch on? with no load connected it should just blink once or twice as the caps charge up then settle back to a very dull glow at the most

    Take care not to feed 340V back into your external supply or there'll be yet another repair job

    What happened to the onboard Vcc when RFmin voltage dropped?
    The lamp gives a brief flash then stops glowing entirely. It's a 75 watter so prob looking at about say 400mW cold and 600mW hot..
    Yeah I won't be feeding 2 different supplies at once
    I didn't look at the onboard supply at the time, I'm heading to the shed again right now. Ill check that supply then swap to the external to check Q501 out.
    Last edited by loopyloo; 16-08-17 at 05:51 PM.

  • #227
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    In that last test with the onboard supply, Vcc was at 9.6v.

    I remove the fuse and lifted R502 and D510 at their node and connected the external supply at "the node", shorted IC503's output side and of course replaced the33k res too. 12v supply produced 9.6v Vcc. I increased the supply slowly up to 17v yet Vcc maintained 9.6v. No matter what supply voltage Vcc would not go above 9.6v.

    Here's the bad bit. I then lifted R117 so effectively only Q501 and its parts were in circuit. As I increased the supply voltage between 10-17v, Q501's emitter followed it. So whatever the supply voltage was at, pretty much the same was on the other side of that transistor.

    So IC101's circuit work fine alone and Q501's circuit works fine by itself, put them together and satan appears.
    It's like Vcc is self regulating but at the wrong setting.
    This is better than a good murder mystery. Crikey !
    Last edited by loopyloo; 16-08-17 at 07:44 PM.

  • #228
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    Maybe the devil's in the diode IE ZD502. The emitter of Q501 should be roughly equal to the voltage on the base - 1V and ZD502 is to ensure Vcc max doesn't exceed approx 14V so it's a bit of a worry that you were able to see 17V on Q501's emitter - something odd there indeed. Check the voltages on Q501 E, B to ground with 17V on the collector with and without a short across IC503 pins 3&4. I think the reason the emitter voltage follows with no load attached is due to C509 charging up and no load for it to discharge into. So the problem looks like the Zener breaking down at a lower voltage than specified, could also be Q501 but the steady 9.6V makes that unlikely.

    More traps here than you could shake Indiana Jones at
    Last edited by Skepticist; 16-08-17 at 09:50 PM.

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    Wasn't ZD502 one of the initial casualties?
    I suppose it is a 1n4744 (15V 1W)?
    if it was in fact a 1n4741 (11V) it would explain quite a lot

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    I think ZD502 is the one I replaced with 1watt 1N4744 I got from Jaycar. I'll have to check on that. Schematic shows 15v zener but looking in the parts list it's a MTZJ15B 500mW with a min/max of 13.89v 14.62v. I guess that's close enough
    Also C504, C510 and C509 were replaced although these were from another board not new. All 4 of those parts were in the destruction zone.

    I'll gets some measurements off that zener.
    Last edited by loopyloo; 17-08-17 at 12:48 AM.

  • #231
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    If you put some load on the Vcc circuit (while it's totally isolated from IC101 and the IC601), a 1k resistor across C509 would do
    With D510 & R502 lifted, IC503 pins 3 &4 shorted, apply 17V from the external supply to the collector of Q501
    Q501 E reading 9.6V, C 17V so the base voltage (cathode of ZD502) should reveal exactly what's going on there
    IE if B is something like 10V then the zener (or possibly C516) is the cause, if B is approx 15V then it's Q501
    So try with C516 out of circuit if the voltage is pointing to the zener.

    EDIT: There's always ZD102 (15V) which could be temporarily swapped into ZD502's position to prove the case of a zener problem if the base voltage points that way
    Last edited by Skepticist; 17-08-17 at 01:11 AM.

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    Quote Originally Posted by Skepticist View Post
    If you put some load on the Vcc circuit (while it's totally isolated from IC101 and the IC601), a 1k resistor across C509 would do
    With D510 & R502 lifted, IC503 pins 3 &4 shorted, apply 17V from the external supply to the collector of Q501
    Q501 E reading 9.6V, C 17V so the base voltage (cathode of ZD502) should reveal exactly what's going on there
    IE if B is something like 10V then the zener (or possibly C516) is the cause, if B is approx 15V then it's Q501
    So try with C516 out of circuit if the voltage is pointing to the zener.

    EDIT: There's always ZD102 (15V) which could be temporarily swapped into ZD502's position to prove the case of a zener problem if the base voltage points that way
    Without load resistor - C 17v, B 15v, E 15v
    With load resistor - C 17v, B 15v, E 1.4v

    Don't have too many spare transistors around the place, but I've got some BC337 if suitable for testing.
    Last edited by loopyloo; 17-08-17 at 02:04 PM.

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    Very strange readings there - have you tested the junctions in the transistor with an ohmmeter?

    Any general purpose NPN transistor will do as a replacement

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  • #234
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    Quote Originally Posted by Skepticist View Post
    Very strange readings there - have you tested the junctions in the transistor with an ohmmeter?

    Any general purpose NPN transistor will do as a replacement
    Yeah I used the diode setting, got strange readings for that too but in circuit around 600mv C-B and 2100 both ways B-E I think it was.
    I'll pull that transistor and check it then replace with a BC337. Se what happens.
    Last edited by loopyloo; 17-08-17 at 04:05 PM.

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    That emitter-base measurement looks way wrong to me, collector base is ok.

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  • #236
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    The original transistor seems to be open one way.
    With the BC337 and load resistor C17v, B15v, E14.4v
    Looking better.
    Might remove the load and drop in IC101 now.

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    Was that with the temp 1k load resistor? (15mA load)

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    Quote Originally Posted by Skepticist View Post
    Was that with the temp 1k load resistor? (15mA load)
    Yes it was..

    So I removed that load and lowered the supply to 12v then reconnected IC101 and guess what ????........2v RFmin woohoo !
    Nearly time to try it on it's own power.

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    On a winner now I believe
    A definite fault found and fixed

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    Also put the oscilloscope on those FET gates and got 2 beautiful waveforms.

    Powered it up on mains.
    Left R621 unconnected at this time but everything else was in.
    2v showed up on RFmin......and.........12 and 24 volts are back !!!
    I flipped the board over to watch for the micon led and sure enough after a while it started flashing the 5 times code, hope that's not another problem.

    Next stop, back into the TV and connect the main board and backlights. The big test.
    Last edited by loopyloo; 17-08-17 at 08:33 PM.

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