Sharp LC-37x20e power on but only for few seconds

[clamel]

Hello, I'm a newby. I read a lot of post on the forum to search a case similar to mine, but nothing found to solve my problem.

I have a Sharp LC-37x20e. I think I have a problem with power supply unit rdenca230wjqz (DPS-240CP) by delta electronics.

If I leave tv unplugged from power for 1 day, then I plug it into the AC, the tv starts regularly (green led) and images, but after 1 minutes it shutdowns. No lamps, no messages, no standy-led after shutdown.

If I unplug power cord for a while, then reconnect, the tv starts (I see green led), but now it shutdowns after few seconds.

I downloaded service manual for this model and I tried to understand something.

So I tried so umount power board from TV and make some tests. I cannot see any bad capacitor, so I tried to perform something other.

In my tests, first I plug the power cord. I use my digital multimeter to read voltages on connectors.

On PD connector (see image in detail) I can read (using as ground the holes for screws at corners)

pin1 - NC -> 0V
pin2 - NC -> 0V
pin3 - NC -> 0V
pin4 - PS_ON -> 0V
pin5 - AC_DET -> 3.3V
pin6 - PNL_POW -> 0V
pin7 - SGND -> 0V
pin8 - BU5V -> 5V
pin9 - SGND -> 0V
pin10 - SGND -> 0V
pin11 - UR15V -> 0V
pin12 - UR15V -> 0V

I also read 0V on other pin connector at example INV60, UR12V etc. I understood it is normal because the power board is in standby mode. So I tried to connect 3.3V to PS_ON (it should be power on the board).

After that, I can see voltages UR15V, INV60, UR12V etc, but only for few seconds. Then they go to zero V. I think I have a power board problem, because I got same problem when it is attached to tv, and when power board is standalone.

I also tried to replace some components: D7121, D7918, C7150, C7151, C7152..

I think the my problem is due to a component malfunction (it goes only for few seconds) but I'm unable to understand where could be this component.. May someone give me some help please?

thank you very much.

Claudio

[Redrebel10]

Re: Sharp LC-37x20e power on but only for few seconds

I'm also new but I can suggest starting by testing all the voltages with everything hooked up normal, with you removing the board you would have to add jumpers to trick the psb to turn everything on, testing the pins in standby then testing after powering on. You might also find which one is causing the problem by testing each pin as it shuts down also. Good luck there are a bunch of very intelligent people on this forum.

[dskall]

Re: Sharp LC-37x20e power on but only for few seconds

When you force power supply on what do you read across the terminals of the big capacitor in dcv

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

I'm also new but I can suggest starting by testing all the voltages with everything hooked up normal, with you removing the board you would have to add jumpers to trick the psb to turn everything on, testing the pins in standby then testing after powering on. You might also find which one is causing the problem by testing each pin as it shuts down also. Good luck there are a bunch of very intelligent people on this forum.

thank you very much for you suggestion

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

When you force power supply on what do you read across the terminals of the big capacitor in dcv

Hello,
thanks for your reply.

When the power supply is only connected to AC I read 305V at big capacitor.

When I give 3.3v to PS_ON, in the meanwhile I read all correct voltages on connectors, I read 397V across terminals of big capacitor.

When voltages on connectors disappear I read 305V at big capacitor.

[dskall]

Re: Sharp LC-37x20e power on but only for few seconds

As a first step I would try freezing the four optocouplers while 3.3v to ps-on
And also ic7601

[jetadm123]

Re: Sharp LC-37x20e power on but only for few seconds

Possibly bad solder joint or bad startup cap. Try heating up the board with a hairdryer to see if it helps.

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

As a first step I would try freezing the four optocouplers while 3.3v to ps-on
And also ic7601

Hi, thanks for your reply.

Your suggestion give me the hint about thermal trouble. So I connect power board to TV, and to the AC. As usual the tv came up, but only for few time. After shutdown I disconnect AC power, then I tested with my finger searching for HOT components.
The octocouplers and ic7601 seems to be fine.


But I found IC7904 very very hot. So I tried to freeze it, then reconnect AC power.. the TV came up, but always for few time. So I tried to freeze IC7904 without disconnect AC power (only a bit freeze) and TV starts again. After that IC7904 as burned.

Tomorrow I'll replace that component, it is TNY264PN, I hope to found it.

I also hope that component burnt do not damaged other components. I can see anything strange in the board. I'll let you know..

thank you

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

Possibly bad solder joint or bad startup cap. Try heating up the board with a hairdryer to see if it helps.

hi thanks for your reply. Maybe I found the problem. Tomorrow I'll replace the suspect component.

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

Hello.. no good news.. I replaced IC7904 but now I not have 5v nor 3.3v.. when ic7904 burnt something other was damaged

[dskall]

Re: Sharp LC-37x20e power on but only for few seconds

Check soldering. Check across pins 5 and 8 for short

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

Check soldering. Check across pins 5 and 8 for short

I checked.. pin 5 and 8 are not in short

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

Hello after replacing R7920 and R7918 I get again 3.3v on oin ac_det but only 1v at bu5v.. someone has any idea?

[budm]

Re: Sharp LC-37x20e power on but only for few seconds

Hello after replacing R7920 and R7918 I get again 3.3v on oin ac_det but only 1v at bu5v.. someone has any idea?

That has to be measurement error since the AC_DET (it is the protection circuit that tells the main board if the incoming AC feeding the TV is within the operating range or not) circuit runs from the 5VSTBY (BU5V) power supply, so for you to get 3.3V on AC_DET pin that means the BU5V must be putting out more than 1V!

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

That has to be measurement error since the AC_DET (it is the protection circuit that tells the main board if the incoming AC feeding the TV is within the operating range or not) circuit runs from the 5VSTBY (BU5V) power supply, so for you to get 3.3V on AC_DET pin that means the BU5V must be putting out more than 1V!

I will try to recheck my measurement (I remember I measured many times, but I'll check again)... making other checks I found the octocoupler IC7906 (PC123Y82FZ0F) has different behaviour than other similar octocouplers (IC7900, IC7601, IC7903, IC7902). I tested in this way (I don't know if it is a valid test):

I put multimeter in continuity mode to read ohms.
I put red tip on pin 3. black tip on pin 4. Then with a 1.5v battery I put negative on pin 2 and positive on pin 1. So I read a resistance value on mlutimeter. If I disconnect positive from pin 1 I read zero from multimeter. And every time I connect positive on pin 1 I instantly read values from multimeter.

This is true for all octocouplers except for IC7906. The difference is that of this I read ohm different from zero when battery is not connected. So I think this component is broken.. I'm rigth ?

thanks

[budm]

Re: Sharp LC-37x20e power on but only for few seconds

"That IC601 is probably has blown Gate drive outputs when MOSFET's went out. I would not take a chance on a new pair of MOSFET's without replacing the IC at the same time." Are you sure it is not 'OL' on your Ohm display since your meter probes cannot have 0 Ohms resistance. What Ohm scale are you using? 200, 2K, 2M Ohms scale. The higher you go the less resolution you get.
Short your meter probes together, that will show the lowest resistance you meter can display.

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

That has to be measurement error since the AC_DET (it is the protection circuit that tells the main board if the incoming AC feeding the TV is within the operating range or not) circuit runs from the 5VSTBY (BU5V) power supply, so for you to get 3.3V on AC_DET pin that means the BU5V must be putting out more than 1V!

You are right I'm sorry.. I have to correct my previous findings. I have

AC_DET = 0.37v
BU5V = 1v

When I made measures it was so late

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

"That IC601 is probably has blown Gate drive outputs when MOSFET's went out. I would not take a chance on a new pair of MOSFET's without replacing the IC at the same time." Are you sure it is not 'OL' on your Ohm display since your meter probes cannot have 0 Ohms resistance. What Ohm scale are you using? 200, 2K, 2M Ohms scale. The higher you go the less resolution you get.
Short your meter probes together, that will show the lowest resistance you meter can display.

Yes it is OL.. sorry but yesterday I was very tired.. Howevery I desoldered IC7601 and when not in circuit the component measurements seems ok..

I'm very sorry for my bad measures

[budm]

Re: Sharp LC-37x20e power on but only for few seconds

Please disregard the ""That IC601 is probably has blown Gate drive outputs when MOSFET's went out. I would not take a chance on a new pair of MOSFET's without replacing the IC at the same time." comments, that is for the another thread anther TV, not your TV, sorry about that.
So right now we need to find out why your 5V Standby power supply is not working properly.

[clamel]

Re: Sharp LC-37x20e power on but only for few seconds

Yes.. before ic7601 burnt, 5v standby was ok..