Hi

[Crafty42]

390k is not a meter range that you use often, and on some meters the range is supplied by a different battery. Does the ohm meter read zero when you touch the probes together?

The meter I'm using is HDM350, it has setting for resistance from 200 to 20M, and yes, it basically reads 0 when touching them. Less than 1 anyway on the 200 setting. 0 on the rest.

The meter is this one: Digital Multimeter Trouble (Pictures)

 

[Crafty42]

With the set off and unplugged, and its switch turned on, measure the following on the X1 ohms scale: See if you can follow this in the schematic.

Hot (narrow blade) of the AC plug to the cathode (the end with the bar) of D605.
Neutral (wide blade) of the AC plug to the cathode of D606.

No reading on the 200 scale. Not sure what the X1 ohm scale is.

 

[mneary]

No reading on either? X1 ohms is the old scale that is 200 on digital meters.

OK, (we conclude that) both sides of the AC line are disconnected from their respective diodes. So go down the line and measure across each component (F601 and L601) and confirm that their resistance is less than 10 ohms. Inspect the board and make sure their connections are good. Use the meter to confirm that there are no breaks in the traces (measure them from the solder point on one end to the solder on the other end.

If all seems good (everything less than 10 ohms) then check the power cord. Measure from the narrow blade to F601. Measure from the wide blade to the place that it enters the board.

 

[Crafty42]

No reading on either? X1 ohms is the old scale that is 200 on digital meters.

OK, (we conclude that) both sides of the AC line are disconnected from their respective diodes. So go down the line and measure across each component (F601 and L601) and confirm that their resistance is less than 10 ohms. Inspect the board and make sure their connections are good. Use the meter to confirm that there are no breaks in the traces (measure them from the solder point on one end to the solder on the other end.

If all seems good (everything less than 10 ohms) then check the power cord. Measure from the narrow blade to F601. Measure from the wide blade to the place that it enters the board.

I'm not sure I followed what you were saying, but I was getting 0.1 readings across F601 and L601 on the solder side of the board. I don't see any breaks, but not saying my solder job on the line filter is good.

About to call it a night, but I'm looking forward to playing around some more with this.

Thanks for your help!

 

[sheldonstv]

thank you mneary for the corrections my english is a bit trying at times but most of the time i get it right.....what you need to do is now set your meter to its lowest resistance range and check from the plug end of your mains lead to the fuse F601,then to D605 cathode going through 1 half of your line filter .....you should have continuity to this point resistance wise if not its either a track break or faulty line filter(unlikely) or cable break .....do the same test from the plug end of your cable on the other pin to D606 cathode you should have continuity through the other half of your line filter from your plug end to D606 cathode....
up to this point you are dealing with raw ac mains so you could also take voltage measurements (with your meter set to read ac voltage) place 1 probe on either end of the fuse holder F601 and the other probe to the neutral side of C601,L601- you should have full ac mains at this point-then transfer your leads to D605 cathode and D606 cathode (1 lead to each....)and at this point you should still have full ac mains present.....tell me what ac voltage you have up to this point??

 

[Crafty42]

thank you mneary for the corrections my english is a bit trying at times but most of the time i get it right.....what you need to do is now set your meter to its lowest resistance range and check from the plug end of your mains lead to the fuse F601,then to D605 cathode going through 1 half of your line filter .....you should have continuity to this point resistance wise if not its either a track break or faulty line filter(unlikely) or cable break .....do the same test from the plug end of your cable on the other pin to D606 cathode you should have continuity through the other half of your line filter from your plug end to D606 cathode....
up to this point you are dealing with raw ac mains so you could also take voltage measurements (with your meter set to read ac voltage) place 1 probe on either end of the fuse holder F601 and the other probe to the neutral side of C601,L601- you should have full ac mains at this point-then transfer your leads to D605 cathode and D606 cathode (1 lead to each....)and at this point you should still have full ac mains present.....tell me what ac voltage you have up to this point??

No need to apologize, I was able to follow what you posted earlier, though sometimes the technical words get me. This may be getting more complicated than I thought. Anyone skype? I could put the camera onto the board and we can walk thru it... other option is pictures or something, but this is all in learning for me and some fun in fixing something, if anyone was interested. I'm EST, and leaving tomorrow around noon, but tonight and the morning might be good. Then there's always some other time as well.

As for the above. I tried checking the continuity, but I'm not sure when you talk about "plug end" I was getting 120 VAC to the fuse if I contacted the hot wire and the fuse, but nothing when contacting the ground to the fuse. I'm starting to wonder about that line filter... I'm not getting any voltage to it at all now.

I tried quite a few things above and got continuity at some places, but i'm not positive i'm putting the probes onto the right areas. I'm also trying to do this all on the solder (under) side of the board. Anyway, if anyone is willing to help out, it is much appreciated, but I don't want to take up too much time from anyone, only if you're willing.

 

[unclejed613]

don't worry about an isolation transformer just yet, you've got a bit of work to do before you need one. test the diodes D605, 606 607, and 608. if your meter has a diode check function, you will read about 0.7V (usually between 0.5 and 0.8) across one with the black meter lead on the striped end, and open circuit (it may start at a low value and begin climbing) with the leads reversed. if you have any that read low both ways, it's shorted. next you want to test the switching transistor Q601, from the middle pin to the right one. with the red meter lead on the center pin, it should read open circuit, with them reversed should be about 0.7V (same as the diodes). those are the major components which will blow the fuse

 

[unclejed613]

looks like somebody tried to spam this thread...

lesson... don't name a thread "Hi.."

 

[fordo]

If you read about 120 VAC on the 2 inputleads of L601and only about 1 volt or so on the 2 outputleads of L601 then it kind of looks like L601 is probably open (burnt windings). If this is what you read, try this: with the tv unplugged, compare resistance readings of the 2 coils of L601 without unsoldering anything. Without knowing anything about that particular filter, my guess is that both coils should read fairly small values, probably less than 10 ohms and both coils should read about the same (2 readings). The readings should also be the same when you reverse the polarity of your meter leads (2 more readings). If all 4 readings aren't pretty close together, (within an ohm or two) I'd suspect the filter. If it appears to be the filter you could then pull it and take the readings again to make sure.

On the assumption you think L601 may be defective:

There's two possibilities I can think of: L601 first shorted (maybe from the possible lightning strike) then opened up as the windings fried. If that's all that happened replacing L601 should do it. However, if anything else is "cooked" downstream and that's what messed up L601, you have to find that first or you'll probably waste L601 again.

See what you can find out.

FYI, if you haven't found out yet: DVM = digital voltmeter (also called a multimeter)

 

[unclejed613]

from the OP's description, something downstream cooked L601, most likely the bridge rectifier or switching FET. L601 is not critical to the power supply working, so it could in the worst case, be bypassed. there are hundreds of other devices that a useable replacement could be pulled from once the power supply is repaired. it's possible one of the traces going to or from L601 burned open when the "foil fuse" was used. as the OP found out, "foil fuses" are a big no-no, as they cause cascaded damage from what might have begun as a single shorted component. i think i need to add foil fuses to my "I void warranties" blog.

 

[Crafty42]

I haven't had time to mess with it any more. Maybe some tonight, depending on what time I go to bed.

Fordo, The line filter was fried before, I replaced it, but I'm not sure I did a very good job of it.

Unclejed, I don't know if it has a diode check function, unless that's the one icon on there that I don't know what it is, I'll have to check and read up on it.

Thanks again everyone...I'm learning.

 

[unclejed613]

diode check function icon looks like the arrow and line symbol that shows up in the address bar of your web browser when you're on this web page.....

 

[Crafty42]

Yep, that'd be the icon on there.

 

[Crafty42]

don't worry about an isolation transformer just yet, you've got a bit of work to do before you need one. test the diodes D605, 606 607, and 608. if your meter has a diode check function, you will read about 0.7V (usually between 0.5 and 0.8) across one with the black meter lead on the striped end, and open circuit (it may start at a low value and begin climbing) with the leads reversed. if you have any that read low both ways, it's shorted. next you want to test the switching transistor Q601, from the middle pin to the right one. with the red meter lead on the center pin, it should read open circuit, with them reversed should be about 0.7V (same as the diodes). those are the major components which will blow the fuse

Looks like D607 and 608 are shorted. They read open both ways, while 605 and 605 read near 0.5V. I'm reading open circuit across Q601 no matter what leads are where. By open circuit, I better check here, by open circuit, is that a reading of 0.000 or 1? I took it to mean 0.000.

 

[mneary]

Short circuit is 0 ohms. Open circuit is usually just a single flashing '1'.

 

[unclejed613]

yes shorted is 0.000 or close to it. open is a flashing 1, or flashing OL or flashing 1999 or anything like that. so you have 2 diodes shorted, caused by applying power through the bypassed fuse (not to mention all of the smoke and fire that ensued on the line side of the bridge rectifier). since the AC was applied long enough to toast the line filter choke, chances are that all of the driver circuitry for the FET got a big dose of line voltage AC as well, and is probably also toasted. i'm surprised you didn't smoke the large filter cap (the aluminum can-like thing), as those definitely don't like AC.

 

[Crafty42]

crap, well, I was hoping it was just an item or two, but if there's more that could be fried, I just might go an buy a new main board for $50. Thanks to everyone for all the help!

 

[HiTech]

Mom says she think a few lightning strikes caused the failure, and although hoooked up to a surge protector, she believes enough strikes to the house caused it to go. Maybe if we'd replaced the fuse correctly the first time, we'd be good to go, but now I'm at a lose as to what it could be. If anyone has ideas, feel free to let me know.

You've chanced on a fix right there without realizing it! If your family is certain that voltage surges may have killed the set, esp. with a surge protector in place check with your homeowners insurance plan. It may cover repair or replacement. Even the Power company can assist by providing records of electrical activity on specific dates if you request it from them. Have the surge protector checked to see if it is bad as well, further proof of a nasty surge occuring. Remember that most surge protectors only survive once or twice despite led indicators and such that show they are still operational. Their spike clamping and other protection features are compromised after they've taken a strike. Only the very best makes and expensive surge protectors can handle repeated surges... and up to a certain point until they are compromised as well.

 

[unclejed613]

the best surge protectors use gas tubes, but the ceap ones use MOV devices. once an MOV has taken a couple of hard hits, they can begin leaking current. gas tubes can take many hard hits before their characteristics are compromised (usually higher breakdown voltage from electrode erosion). some of the early MOV surge supressors clamped at 130V, and so would get leaky after a few months use, and eventually the MOVs would burn up. the more reliable ones used 150V MOVs and would last years. gas tubes have clamping voltages from about 150V and up, and can handle larger surge currents without permanent damage.

 

[Crafty42]

Ok, well, got the new main board in, and it fires up, but the screen is just white with some faint vertical lines, but I'm getting sound. Any thoughts? If I can still fix it cheaply, I don't mind that I sunk $60 into it, but if it's going to be much more, I may scrap the idea.