I posted a message about this a few weeks ago, regarding replacement of the blown lamp. After installing the new lamp, the set still won't start up; the "replace lamp" error light flashes instead. Checking the power input to the H.V. power supply (or lamp ballast I suppose), I measured a peak of over
300 VDC initially but it quickly drops to (and stays at) about 160. The 450 volt electrolytic cap connected across the power input suggests that the operating voltage should be much higher than 160. So what now? Is this a ballast problem or something on the main power supply? I can't afford to gamble any more money on this thing. The model and chassis numbers BTW are E44W46LCD and MB-042B respectively.
On Wed, 6 Aug 2008 14:16:16 -0700, "Chris F." put finger to keyboard and composed:
FWIW, I have a service manual for an LG MB-042C chassis.
The DC supply voltage for the ballast is 370 min, 390 typ, and 410 max. This is derived from directly across the main 220uF/450V bulk capacitor (C820). It is not regulated.
On Thu, 7 Aug 2008 11:32:33 -0700, "Chris F." put finger to keyboard and composed:
I don't know. If the ballast is drawing too much current, then I suppose it's possible for the DC supply to sag significantly. BTW, when I wrote that the 400V supply is not regulated, I should have said that this supply is derived from the rectified mains and passes through an active PFC circuit.
Maybe you could insert a 1 ohm 10W series resistor into the DC feed to your ballast and measure the voltage drop across it. That would tell you the current draw.
- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
On Sat, 9 Aug 2008 14:36:14 -0700, "Chris F." put finger to keyboard and composed:
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:
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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
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Please remove one \'i\' from my address when replying by email.
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.
On Sat, 9 Aug 2008 19:42:00 -0700, "Chris F." put finger to keyboard and composed:
Would an overload on the secondary side cause IC810 to shut off its PFC and voltage doubling function? Maybe you should try unloading some of the secondaries, eg the sound circuit.
Otherwise you may want to check what IC830 and IC851 are doing. I would expect that, because the secondary outputs are low, this should cause the optocoupler's diode to be driven hard, which should in turn mean that pin 4 should be close to ground. This should at least prove whether the voltage feedback is working. I would then check R837. This resistor determines the current limit of your circuit. IC810 could be reducing the drive to Q830 in response to an overload on the secondaries which it would sense via R837.
- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
On Sat, 9 Aug 2008 19:42:00 -0700, "Chris F." put finger to keyboard and composed:
Doh, I just realised that your set is behaving as if it is in standby mode. Notice the "POWER ON" signal at pin 1 and the ST-5.5V rail at pin 2 of PB41 which goes to the Tuner Board. I suspect that if you disconnect PB41 and short pin 1 & 2, the PSU will power up.
I would take care when doing this because disconnecting PB41 also removes power to the fan.
- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
In my case, it does appear that IC810 also performs some kind of voltage-doubling, via T801. There are no other components to do the job, in fact the circuit appears to perfectly match the schematic you provided me. I've since tried injecting 400V from an external power supply (to C820), surprisingly it doesn't make any difference at all. While the ballast now gets the correct 400VDC, the output voltages in the low voltage supply are still down by more than 50%. What's more, the "replace lamp" light is still blinking. I'm more confused than ever now. I'm only using that basic schematic you sent me first - I haven't been able to download the rest because eserviceinfo is not responding today. Maybe I need a break from this anyway. I guess a tech should never assume a blown lamp is just a blown lamp, when working on junk like this.....
You're right, and I should have realized that a long time ago. Shorting those pins did power the PSU and all voltages are now normal, but the lamp still does not come on. I guess the next step is to test the ballast, but for that I need the pinout, and eserviceinfo is still down.......
On Sun, 10 Aug 2008 12:51:46 -0700, "Chris F." put finger to keyboard and composed:
I was thrown by the fact that all the voltages were at 40%. You would normally expect that all rails would be at 0V apart from a single standby supply.
If the ballast supply is now at 400V, then this would imply that the PFC and voltage doubling function of IC810 is turned off in standby mode and turned on in run mode. This begs the question, how does IC810 know that the set is in standby mode? Perhaps this is the function of pin 5?
The uP on the Tuner Board appears to control and sense the lamp via connector P10. BTW, when you shorted pins 1&2 of PB41, I hope you removed the connector, otherwise one of the uP pins would have been subjected to 16V.
I have uploaded tuner PCB circuit 2-of-2 to my web space:
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It seems to me that if you can't get the set to come out of standby, then the M37151 uP (IC1) may be faulty, in which case your lamp problem may be a secondary issue, if it is in fact a problem at all.
- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
Franc, you're not going to believe what was wrong the whole time.... I hadn't yet replaced the cover to the lamp compartment, and last night I realized there was a switch under that cover to sense when it was open. Replaced the cover and the set works fine! Can't believe I was so dense as to not realize that right from the start, instead of going on this wild goose chase. I should have realized that the toughest problems often have the simplest solutions. I still thank you for your help though, it's good to know there are folks out there to help me when I really do need it.
I might not be out of the woods just yet.... although the set is working, the remote isn't responding and I can't get a universal remote to work either. The service manual does not include the control panel board so I have no way to trace the remote signal. I'm not sure what to do now, I'm worried that this may be something I caused while I had the set apart.......
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