Zenith DLP Set Not Working After Lamp Replacement
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From: Franc Zabkar on 7 Aug 2008 23:41 On Thu, 7 Aug 2008 19:57:30 -0700, "Chris F." <zappyman(a)hotmail.com> put finger to keyboard and composed: > What I did so far was just disconnect the lamp ballast. The voltage across >C820 is still only 160V, and I also determined that all voltages derived >from the switching supply (16, 32, etc) are proportionally low - the 16V >supply is only measuring 6V, for example. > I don't know too much about PFC circuits but I assume something has to >take place to get the correct 400V across that cap, something which clearly >isn't happening at this point. I also have no experience in this area, but I wonder if a leaky D811 could be causing C820 to be discharged by Q801 during PFC cycles ??? > If you could Email me the schematic of that power supply I'd really >appreciate it. I've got almost $150 in this thing now and giving up just >isn't an option right now. This is the PSU section: http://www.users.on.net/~fzabkar/MB-042C/ The whole manual is about 24MB zipped. I have uploaded it to eserviceinfo. To get all 13 parts, replace 370nn with 37046-37052 and 37054-37059 in either of the following URLs: http://www.eserviceinfo.com/download.php?fileid=370nn http://www.eserviceinfo.com/downloadsm/370nn/LG_RT44SZ80LB.html - Franc Zabkar -- Please remove one 'i' from my address when replying by email.
From: Franc Zabkar on 8 Aug 2008 02:13 On Fri, 08 Aug 2008 13:41:55 +1000, Franc Zabkar <fzabkar(a)iinternode.on.net> put finger to keyboard and composed: >On Thu, 7 Aug 2008 19:57:30 -0700, "Chris F." <zappyman(a)hotmail.com> >put finger to keyboard and composed: > >> What I did so far was just disconnect the lamp ballast. The voltage across >>C820 is still only 160V, and I also determined that all voltages derived >>from the switching supply (16, 32, etc) are proportionally low - the 16V >>supply is only measuring 6V, for example. >> I don't know too much about PFC circuits but I assume something has to >>take place to get the correct 400V across that cap, something which clearly >>isn't happening at this point. > >I also have no experience in this area, but I wonder if a leaky D811 >could be causing C820 to be discharged by Q801 during PFC cycles ??? I'm assuming you have checked that C820 has not gone open or low in value ... - Franc Zabkar -- Please remove one 'i' from my address when replying by email.
From: Chris F. on 9 Aug 2008 17:36 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. "Franc Zabkar" <fzabkar(a)iinternode.on.net> wrote in message news:6ton9491b0gg82pi3i4nne2agke56h3d2h(a)4ax.com... > On Fri, 08 Aug 2008 13:41:55 +1000, Franc Zabkar > <fzabkar(a)iinternode.on.net> put finger to keyboard and composed: > >>On Thu, 7 Aug 2008 19:57:30 -0700, "Chris F." <zappyman(a)hotmail.com> >>put finger to keyboard and composed: >> >>> What I did so far was just disconnect the lamp ballast. The voltage >>> across >>>C820 is still only 160V, and I also determined that all voltages derived >>>from the switching supply (16, 32, etc) are proportionally low - the 16V >>>supply is only measuring 6V, for example. >>> I don't know too much about PFC circuits but I assume something has to >>>take place to get the correct 400V across that cap, something which >>>clearly >>>isn't happening at this point. >> >>I also have no experience in this area, but I wonder if a leaky D811 >>could be causing C820 to be discharged by Q801 during PFC cycles ??? > > I'm assuming you have checked that C820 has not gone open or low in > value ... > > - Franc Zabkar > -- > Please remove one 'i' from my address when replying by email.
From: Franc Zabkar on 9 Aug 2008 17:20 On Sat, 9 Aug 2008 14:36:14 -0700, "Chris F." <zappyman(a)hotmail.com> put finger to keyboard and composed: >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 -- Please remove one 'i' from my address when replying by email.
From: Chris F. on 9 Aug 2008 22:42
> 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??? 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..... |