As Skepticist says, use extreme caution when doing this though. Be very aware of unplugging equipment before inserting or removing the fuse-globe, if you go this way.
Yes I understood that. In his earlier post he described an extension lead he made up with the lamp in series with the active lead.
We used this method in a place I worked at many years ago for checking finished products on the factory floor. The lamps were built in to the tester. It was also a good way to get rough measurements such as millivolt drops, current draw etc. from the device under test. Sometimes a product would fry. That was mostly the older test equipment. When I started there, the move was to replace most of that equipment with PLC controlled solid state testers that had a high degree of measurement accuracy.
Frankly I never really though about using a lamp at home. I'll have to make one up.
Last edited by loopyloo; 11-07-17 at 10:20 AM.
As Skepticist says, use extreme caution when doing this though. Be very aware of unplugging equipment before inserting or removing the fuse-globe, if you go this way.
loopyloo (11-07-17)
Yep I'm very aware.
Anyway, looks like I'll have to put this TV away for a week while I'm waiting for parts.
I'll come back then....
If your series light bulb is a BC style, at switch off you can remove it and place it across the main capacitor, to discharge it.
loopyloo (26-07-17)
These are the blown parts I found :
4 FETs
2 fuses
1 HV cap
1 zener diode
Could be lucky that the damage is limited to the primary side.
Has the 8 pin IC been proven healthy? (& the opto isolators as well)
loopyloo (26-07-17)
I'd be applying an external source of Vcc (about 15V) to look for signs of life and lack of short-cct. Also check the internal mosfet and integral protection diode for health prior to actually applying mains power and risking new power mosfets. That first power-up would be a nervous time for me and a definite exercise for use of the series lamp to limit the disaster (if one were to occur that is
Agreed. I have a variable voltage, variable current bench power supply for this kind of thing and would use it in similar situations for low voltage testing.
If all appears well, THEN connect the mains via a current-limiting setup using a series lamp....otherwise one runs the risk of vaporised pcb tracks, newly-blown recently-replaced components AND in losing six out of ones seven lives.....all in the space of an instant.
loopyloo (16-07-17)
Yes that's the idea but check the mosfet/diode with a multimeter first. If the chip is ok you should be able to detect some activity when you apply Vcc within the operating range as stated in the datasheet (chip has brown-out protection so would shut down when Vcc is below that range).
That particular SMPS ship appears to be intended for a backup type supply as it's a simple 'flyback' low power design. I'm curious as to how the drive for pair(s) of mosfets (push/pull) is derived from it.
Last edited by Skepticist; 16-07-17 at 03:22 PM.
loopyloo (26-07-17)
Parts arrived today.
After checking a few things, I made up a 75W lamp(didn't have 100W) and powered it up.
No problem there so I plugged it directly to the mains. No bangs or flashes of blinding light so that was good.
Red LED came on then went off flashed twice and that's it. So far so good but more to do.
I've got 3.5v STB but no 12v or 24v, and no drive on.
Need to force power on and drive on to test but forgotten how to...Are they active high or low ? (3.5v or gnd) ... and is necessary to use a resistor ? say 1k ?
P.S. Looks like it's "power on" to stb and "inverter on" through 100ohm resistor to stb.
Last edited by loopyloo; 18-07-17 at 04:48 PM.
Didn't work.
Now gotta find out where the 12 and 24volts went to.
The 'INV_CTL' input has a pullup resistor so it's active low, there's a series resistor so simply ground it.
That signal has to get back to the primary side via an opto-isolator so I'd be tracing back from the connector to find which one it is and where the photo-transistor is connected to on the primary side. It should be easy to bypass the opto's output (with the mains disconnected) in order to force the supply to power up if the inverter circuit is intact.
What's unclear from the PCB pics is where is the controller for those main supplies (12V & 24V)?
I'm guessing it's on the copper side of the board close to the FETs as I can make out the shadow of an SMD layout there
Last edited by Skepticist; 19-07-17 at 12:52 AM.
loopyloo (19-07-17)
I saw that pullup but I also saw INV_CTL as active high. Seemed a bit odd to me so I guess it's just the way it's written, bit of a trap, INV underscore CTL.
I'm testing this by connecting the resistors/wires into the main board plug with it disconnected. That 100ohm resistor's on the main board isn't it ? Being LG they don't give you power supply circuits.
Last edited by loopyloo; 19-07-17 at 10:49 AM.
I've been all over the board randomly and can't seem to find anything else that's faulty.
Any idea how these transformers are rigged up ? All of the secondary side reads the same, short...
If the rectifiers on the secondary side are shorted out it looks like time to retire it. The thing is, the secondary side is a very low impedance circuit so can appear to be shorted out unless the rectifiers are desoldered for testing. What stands out for me is that 16 pin SMD controller chip which could easily have copped the HV spike that destroyed the FETs - you need to trace out the schematic of that circuit and use the datasheet for the controller to diagnose what's going there. Shouldn't have any trouble getting a new controller chip if required but that isn't necessarily going to be the end of the exercise.
loopyloo (26-07-17)
loopyloo (26-07-17),Skepticist (19-07-17),tristen (19-07-17)
Bookmarks