Do power supplies in shut down come on briefly?

In the process of trying to determin if my power supply is defective, or the video board on an LCD TV. I checked the voltage output pins and found the 5 volts stand by voltage was working fine, but nevertheless the set does not turn on.

Some one suggested I put my oscilloscope probe on the switched outputs while pushing the power button to see if the other outputs attempt to come on before going in to shut down.

Is this a valid test? Can I really expect to see the other output pins briefly come up if something on the video board is causing my power supply to go in shut down?

Testing the switched outputs revealed there was not even a spike, so my hunch is I have a bad power supply.

I'm wanting to determin if my power supply voltages are correct on my > dead LCD TV. On my power supply board the output voltages are printed, > but since the set doesn't turn on, the only voltage I see present is 5 > volts on the stand power supply pin. > I realize the power supply isn't completely turned on until the > microprocessor on my video board sends it the correct turn on pulse, > but isn't there more I can do aside from assuming the video board is > likely defective. I checked ESR on all capacitors, looked for shorted > diodes and transistors, burnt resistors, etc. What is a high failure > item on the video board which might cause this? The set doesn't come > on for even a second. > This is a Westinghouse model SK32H240S > Thanks in advance.

On many, although not all, LCD TV sets that I've had the 'pleasure' to work on (!) the standby supply produces +5v and +3v3. Normally, when you have one that won't come out of standby (do you actually even get a standby light showing?) the first thing to do is to locate the standby switching line to see if that is toggling. If it is, you then need to look at each individual output rail from the power supply to see if they start to try to come up. Often, they will, but fall over again so quickly that you don't see any activity at all in terms of the set coming to life. The rails are normally something like +3v3 switched, +5v switched, +12v, +24v high current (for the backlight inverter) and +33v.

If the power supply is trying to start, then the next thing to do is to unplug the backlight inverter board. It is common for these to fail, and if they impose a heavy load on the 24v rail, the power supply will shut back down before it ever really gets going. Another common failure for a heavily loaded rail, is a failed audio IC.

In general, LCD TV power supplies are *very* heavily self protecting, and they will either shut back down very quickly, or not even come up at all (as far as you can see) if they detect the slightest thing wrong with any of their rails - thats over-voltage, under-voltage, or excess load.

It can be very tricky to get to the bottom of what's causing an apparently dead LCD TV. I have also had system control micros fail, and also corrupted EEPROMs causing startup problems.

Arfa

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Dave
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Let's put it this way... Your hand won't jerk back from a hot stove until it comes in contact with the stove.

Reply to
William Sommerwerck

On Sun, 29 Jun 2008 11:29:51 -0700 (PDT), Dave put finger to keyboard and composed:

If a microprocessor has detected a fault that has caused it to shut down the supply, then you may have to cycle power to the unit in order for the uP to forget and try again.

- Franc Zabkar

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Franc Zabkar

Arfa

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Arfa Daily

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I hate to sound stupid, but How do I find the switched line from the control micro to see if it toggles? Didn't I do this when I checked all of the power supply pins going to my video board? I do have a glowing LED on the front of the set which turns on the minute I plug the set in, but it is continually on, and does not blink. I also tested every pin on my power supply board and found them to be dead, except for the 5 volt stand by voltage. I did notice when measuring the five volt line on the power switch it dropped to 0 volts when I pushed the switch. Am I misunderstanding anything your saying?

Reply to
Dave

OK. The fact that you are getting a standby light shows that the 5v you are seeing at the power supply, is definitely an output from it, not an input from elsewhere, that is not moving. Seeing the 5v at the set's standby pushbutton, again verifies that the 5v is reaching the main board, and is also getting to the system control micro (to which the standby button will almost certainly be connected). In response to that line being dropped from

5v to zero, by pushing the standby button, the system control micro should issue a "power on" signal to the power supply. This is the usual arrangement. It is highly unlikely that the standby pushbutton, is connected directly to the power supply.

So, assuming that the standby switching line's pin is not marked at the power supply, and that you do not have a set of schematics (or even a board interconnect diagram will do) then the only way that you can proceed, is to go looking for the standby switching line at the power supply, by looking at every external connector pin that doesn't have any legend to tell you what it does, with a 'scope set to DC coupling. As you move to each pin, you need to push the standby button, and wait a couple of seconds to see if the DC level on the pin you are checking, moves, or even if a pulse is produced. Wait ten seconds before moving on to the next pin to test, to allow any 'problem' that the system control micro may have detected, to reset to the 'try again' condition.

If you find a pin that's toggling, then the chances are that it is not your mainboard that's primarily at fault (as such) and that the problem lies either with the power supply, or one of the loads that it's feeding. If there is no pin that's toggling, then the system control is not issuing a "power on" signal, and trouble shooting the power supply, is not the right path.

Unfortunately, these things are not straightforward like in the 'good ol' days', and can be very difficult to fault-find on, without a lot of experience, and spare boards to try in them. Let us know if you manage to find the standby switching line, and we might be able to proceed a bit further, from there. Do you know what chassis is fitted by any chance ? It's not a Vestel is it ? Are there any numbers / letters at the corner of the PSU ?

Arfa

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Arfa Daily

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Thanks Arfa, There are a bunch of numbers on the power supply board, starting with a VLT70039.50 LOGAH REV5 UA-4141-1-LF I just recieved the service manual from Andrews on this set and its a joke. The only page that shows a schematic is so blury its useless.

I did find what I believe to be the return line from the micro going to my power supply board, It says "PWR ON" so I'm going to assume this is the right one, but I get no voltage what so ever when I press my power on button. Thanks again. Any further suggestions would be appreciated. Dave

Reply to
Dave

Thanks Arfa, There are a bunch of numbers on the power supply board, starting with a VLT70039.50 LOGAH REV5 UA-4141-1-LF I just recieved the service manual from Andrews on this set and its a joke. The only page that shows a schematic is so blury its useless.

I did find what I believe to be the return line from the micro going to my power supply board, It says "PWR ON" so I'm going to assume this is the right one, but I get no voltage what so ever when I press my power on button. Thanks again. Any further suggestions would be appreciated. Dave

OK Dave. That line is definitely the power switching line. If it has no voltage on it at all (check this with a 'scope set to DC, in case it just pulses too quickly to see on a meter) then there's a good chance that the micro is not telling the power supply to turn on. If at all possible from the manual, you want to trace this signal back at least to where it comes off the mainboard, and preferably all the way back to a pin on the micro, just to prove that there are no simple connector isues. Also, now that you have the manual, try to see if you can verify that standby supplies are 5v only on that set i.e. that you don't have say a 3v3 standby rail that's missing. Also, try to verify that there is not a 3v3 regulator on the mainboard that derives its input from the 5v standby rail, and that's not working. After that, is where it starts to get hairy ...

If the "power on" line is low, never changes - even immediately after a fully-unplugged-from-the wall-outlet repower - and the power supply is in standby, then the chances are that the line should shift from low (0v) to high (5v) in order to wake the power supply up. You can thus now verify the power supply by disconnecting the line (usually you can easily pop the pin from the connector shell to isolate any wire) and then connecting a low value resistor - start with 100 ohms - between the 5v standby supply, and the actual power supply pin where you have disconnected the wire from. If this brings the power supply to life, then the chances are that you have a bad mainboard, and you are unlikely to be able to fix that. You might try looking on e-Bay for replacement boards. These often pop up this side of the pond, at very reasonable prices, as sets often get broken up for spares as a result of insurance write offs from LCD panel damage.

One word of caution, I am assuming that you have enough knowledge of switch mode power supplies to realise that they are potentially *very* dangerous at their front ends. The main filter cap on one that's not running, can remain charged to a very high voltage, long after you disconnect it from line power. Also, most of these LCD PSUs have a resonant PFC stage at the front end, which will run, producing full front end rectified DC across the main filter cap, with as little as 40v ac input applied to the supply.

Good luck. Let us know if you make any progress.

Arfa

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Arfa Daily

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Even if I can't fix it I appreciate all your help in expanding my understanding of LCD power supplies. I will let you know if I have any luck. Dave

Reply to
Dave

Even if I can't fix it I appreciate all your help in expanding my understanding of LCD power supplies. I will let you know if I have any luck. Dave

No problem. There's something 'new' to challenge us every day ... d;~}

Arfa

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Arfa Daily

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