C820 is good, haven't checked D811 yet. From what I figured out so far,
IC810 has to serve two purposes; first, to drive the PFC circuit which
creates the 400VDC, and second, to drive/regulate the switching power supply
which supplies the low-voltage areas. The gate of Q830 has a strong 1V p-p
signal, but there's nothing at all on the gate of Q801, except of course for
the very short burst when the set is first plugged in. What I'm not sure of
is whether or not these two sections of the IC are dependant on each other;
if not, I could just remove the PFC section and insert 400VDC onto C820 from
an external power supply, just for testing purposes of course.
Is it possible that IC810 is defective? I can't seem to come to any other
conclusion right now.
I don't have any experience with PFC circuits, but the first of the
following two datasheets has an application circuit that describes a
two-chip design:
http://focus.ti.com/lit/ds/symlink/ucc28600.pdf
http://focus.ti.com/lit/ds/symlink/ucc28051.pdf
One chip does the active PFC stuff while the other is a quasi-resonant
PWM controller.
The SMA-E1017 is described as a "PFC&QRC COMBO". I haven't been able
to find its datasheet, but the "ST" in the parts list would suggest
that it is made by ST Microelectronics.
I would think that Q830 would be getting maximum gate drive because
IC810 is sensing a low output on the secondary, so that part of the
chip appears to be working. If Q801 is not being driven, then I would
think that the circuit would be behaving like an ordinary non-PFC
circuit, in which case it would make no difference if you open
circuited the drain.
Have you measured the voltage across C805? You say that you are seeing
160V across C820. My calculations suggest that, if C820 were OC, then
you should measure an average DC voltage of ...
2/pi * sqrt(2) * 240V = 216V
So either something doesn't make sense, or I'm misunderstanding how a
multimeter works.
BTW, are you posting from Canada, as your IP suggests, in which case
how are you expecting to get 400VDC from a 120VAC supply ... assuming
your circuit is similar to the one I have posted? I'm posting from
Australia where the mains is 240VAC, BTW. Does your PSU have a voltage
doubler section at or near the bridge rectifier (D801)??? Is there a
connection between the neutral conductor and the midpoint of two bulk
caps??? If there is no voltage doubler, can the E1017 IC boost the
supply from 160V to 400V via T801??? If so, then you may want to check
R810 and R811 for OC. BTW, the notes around T830 indicate that there
are North American and "domestic" versions of this SM transformer, so
this would suggest that the PSU circuit applies universally. In fact
the Australian manual states that the acceptable input voltage range
is 110VAC-240VAC (10%) at 50/60 Hz. So maybe IC810 performs PFC *and*
voltage doubling???
If it helps, some searching would suggest that your set may be a
rebadged LG 44SZ81 or RU44SZ80L. The manual I uploaded was for an
RT44SZ80LB.
- Franc Zabkar