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**Bobdee** · 12-02-2009, 10:01 AM

Re: Coby Flat Panel TV Bad Inverter

Get the Datasheet to the 8 pin IC on the back of the board, probably a mosfet for transformer, check for shorts between G D S
common failure on Inverters, you may just get lucky and find one

**Krankshaft** · 12-02-2009, 10:31 AM

Re: Coby Flat Panel TV Bad Inverter

The bulb test proves nothing you need to plug the CCFLs into a known good inverter to test them. If the caps aren't bad then it sounds like an inverter shutdown from a bad CCFL.

**Bobdee** · 12-02-2009, 10:51 AM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by Krankshaft

The bulb test proves nothing you need to plug the CCFLs into a known good inverter to test them. If the caps aren't bad then it sounds like an inverter shutdown from a bad CCFL.

Of course now I see, (I think) because it has one mosfet, then If that failed its not possible for the CCFLs to come at all

**fixit** · 12-02-2009, 02:32 PM

Re: Coby Flat Panel TV Bad Inverter

I tested the bulbs individually, not at the same time. I think it would be rare that both bulbs would fail at the same time, unless the inverter induced it. Which still brings me back to the inverter.

I do not have a good good inverter to test with, but could see about buying a new bulb to test with if you think it is a high prob. that the bulb is the issue.

**PlainBill** · 12-02-2009, 03:05 PM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by fixit

I tested the bulbs individually, not at the same time. I think it would be rare that both bulbs would fail at the same time, unless the inverter induced it. Which still brings me back to the inverter.

I do not have a good good inverter to test with, but could see about buying a new bulb to test with if you think it is a high prob. that the bulb is the issue.

You have an unusual situation here. It's not impossible that it's a bad CCFL, but it's not likely in my opinion. I think the best way to troubleshoot this is by determining why the inverter is shutting down. And the best way to do that is by checking the inverter controller - the 16 pin IC on the bottom of the board. What are the numbers on that IC?

Also, a better picture of the bottom of the board would help - one taken from directly above.

PlainBill

**Krankshaft** · 12-02-2009, 03:13 PM

Re: Coby Flat Panel TV Bad Inverter

Both of the transformers are driven by one inverter PWM IC if anything is wrong on either of the outputs it ceases operation completely by killing the drive waveform to the switching transistor on the primary of the switching transformer.

To test the CCFLs either use a replacement CCFL or purchase a CCFL backlight kit that's used for computer cases. Just be sure to fuse the 12V input to the backlight inverter because these things can blow in a puff of smoke if they encounter a bad CCFL.

This is of course as long as bad caps and bad solder joints have been ruled out first.

The backlight working for a second rules out the driver transistor since it blowing would in most cases blow a fuse.

In an even rarer instance it could be a shorted inverter transformer but luckily that doesn't happen too often.

**fixit** · 12-02-2009, 08:22 PM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by Bobdee

Get the Datasheet to the 8 pin IC on the back of the board, probably a mosfet for transformer, check for shorts between G D S
common failure on Inverters, you may just get lucky and find one

Thanks for the feedback. IC part number is STM6920A. I was able to find a datasheet . I checked for shorts on G D S and found none. It still could be bad.

Would you recommend replacing it anyway and if so any recommendations on where I can purchase one?

Thanks for the help

**fixit** · 12-02-2009, 08:26 PM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by PlainBill

You have an unusual situation here. It's not impossible that it's a bad CCFL, but it's not likely in my opinion. I think the best way to troubleshoot this is by determining why the inverter is shutting down. And the best way to do that is by checking the inverter controller - the 16 pin IC on the bottom of the board. What are the numbers on that IC?

Also, a better picture of the bottom of the board would help - one taken from directly above.

PlainBill

Thanks, I have added a close up picture of the ICs on bottom of the board. The 16 pin IC is BIT3713.

Any ideas to troubleshoot are greatly appreciated.
Thanks

Attached Files

**PlainBill** · 12-03-2009, 06:15 AM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by fixit

Thanks, I have added a close up picture of the ICs on bottom of the board. The 16 pin IC is BIT3713.

Any ideas to troubleshoot are greatly appreciated.
Thanks

BIT3713 is now known as a BIT3193. I'll have to see if I can find an application note on it. We're going to have to see if we can figure out WHY it is shutting down. This could take a while. Suddenly the demands on my time are increasing. And I really need a higher resolution picture of the entire board. The resolution of the latest picture is excellent - better than I need, actually, but I must be able to trace the circuits from both CCFL connectors back to the control IC (BIT3713 AKA BIT3193).

PlainBill

**Bobdee** · 12-03-2009, 07:24 AM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by fixit

Thanks for the feedback. IC part number is STM6920A. I was able to find a datasheet . I checked for shorts on G D S and found none. It still could be bad.

Would you recommend replacing it anyway and if so any recommendations on where I can purchase one?

Thanks for the help

There are two mosfets in the one package, if you have checked both for shorts and found none then I would suspect that to be good, they can be a common problem on inverters
KS and PB will sort it out for you no doubt

**fixit** · 12-03-2009, 04:25 PM

Re: Coby Flat Panel TV Bad Inverter

Bobdee - thanks for the help and information

PlainBill - thanks for comments. Per your request, I have added a bottom view of the inverter, please let me know if this is not good enough quality for your needs.

As I noted in my original post, the caps look OK, no bulges. But if you think I need to eliminate variables and should replace them, I can. The list is as follows:

6 - Capxon KF 1000 uf 25V
1 - Capxon RK 100 uf 16V
1 - Capxon KM 10 uf 50V

Any help to xref these to readily available quality caps would be appreciated.

Do I need to worry about replacing any of the pf caps?

Thanks

Attached Files

**fixit** · 12-03-2009, 06:02 PM

Re: Coby Flat Panel TV Bad Inverter

Just an update. I was able to find a mod kit at Frys for 8 bucks. I tested each LCD screen bulb individually by connecting it to the mod kit inverter and the bulbs worked OK and did not go blank as they do when connected to the TV inverter.

I have at least eliminated the TV bulbs being the issue, which is what I thought but wanted to confirm.

**PlainBill** · 12-03-2009, 09:39 PM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by fixit

Just an update. I was able to find a mod kit at Frys for 8 bucks. I tested each LCD screen bulb individually by connecting it to the mod kit inverter and the bulbs worked OK and did not go blank as they do when connected to the TV inverter.

I have at least eliminated the TV bulbs being the issue, which is what I thought but wanted to confirm.

I've marked up the picture you attached. The two points I've marked 'R' in red are the return lines from each CCFL. I've also circled the pin on the connector in brown. The one on the left should go to the solder pad circled in bright red. I think a jumper conducts the signal to the point circled in dark red. Please confirm that. I'm not sure where the signal from the pin on the right comes out from under the goop.

Pin 15 (outlined in blue) on the controller is the current sense pin. I THINK it goes over to the diode (also circled in blue). Please confirm that.

A very brief summary of the operation of the controller: When it is commanded ON it drives the transformer and CCFLs at full power. After a few seconds it begins monitoring the voltage on pin 15. If it is below 1.3 volts it turns off the CCFLs. Check the voltage on pin 15 when the backlights turn on; also check the voltage at each return line (brown circles).

PlaBill

Attached Files

**fixit** · 12-08-2009, 03:34 PM

Re: Coby Flat Panel TV Bad Inverter

PlainBill, here is my feedback for you.

The one on the left should go to the solder pad circled in bright red. I think a jumper conducts the signal to the point circled in dark red. Please confirm that.
Answer - Yes that is correct

I'm not sure where the signal from the pin on the right comes out from under the goop.

Answer – I removed the goop and traced it in yellow, see attached photo

Pin 15 (outlined in blue) on the controller is the current sense pin. I THINK it goes over to the diode (also circled in blue). Please confirm that.

Answer – pin 15 goes to a different diode than you indicated, I traced it in blue, see photo

A very brief summary of the operation of the controller: When it is commanded ON it drives the transformer and CCFLs at full power. After a few seconds it begins monitoring the voltage on pin 15. If it is below 1.3 volts it turns off the CCFLs. Check the voltage on pin 15 when the backlights turn on; also check the voltage at each return line (brown circles).

Answer – pin 15 voltage was 0.03v when I turned the TV on and went to 0.04v when the backlights turned on for a couple seconds and then went blank. The return line voltage put the meter in to over voltage when the backlights came on and then dropped to nothing when the backlight turned off a couple seconds later.

I do not know what this all means, but if you do, I really would appreciate the help. Thanks

Attached Files

**PlainBill** · 12-08-2009, 06:06 PM

Re: Coby Flat Panel TV Bad Inverter

Originally posted by fixit

PlainBill, here is my feedback for you.

The one on the left should go to the solder pad circled in bright red. I think a jumper conducts the signal to the point circled in dark red. Please confirm that.
Answer - Yes that is correct

I'm not sure where the signal from the pin on the right comes out from under the goop.

Answer – I removed the goop and traced it in yellow, see attached photo

Pin 15 (outlined in blue) on the controller is the current sense pin. I THINK it goes over to the diode (also circled in blue). Please confirm that.

Answer – pin 15 goes to a different diode than you indicated, I traced it in blue, see photo

A very brief summary of the operation of the controller: When it is commanded ON it drives the transformer and CCFLs at full power. After a few seconds it begins monitoring the voltage on pin 15. If it is below 1.3 volts it turns off the CCFLs. Check the voltage on pin 15 when the backlights turn on; also check the voltage at each return line (brown circles).

Answer – pin 15 voltage was 0.03v when I turned the TV on and went to 0.04v when the backlights turned on for a couple seconds and then went blank. The return line voltage put the meter in to over voltage when the backlights came on and then dropped to nothing when the backlight turned off a couple seconds later.

I do not know what this all means, but if you do, I really would appreciate the help. Thanks

BINGO!!! We have a winner!!! You may have to carefully trace the circuit to identify the defective component. I'll try to identify a schematic that illustrates what is happening, but here's a text description.

The inverter puts out 700 - 1000 VAC to drive the CCFLs. Rather than ground the return line from the CCFL, it goes to a dual diode. When the output voltage is negative, the diode shunts it to ground. When it is positive, the diode conducts and charges a small capacitor through a resistor network. Pin 15 of the inverter controller monitors the voltage on that capacitor. If the voltage is greater than 1.3 volts it 'knows' the CCFLS are both operating properly. If it is less than 1.3, it turns off the inverter. The fact that the voltage at pin 15 was so low while the voltage at the return line was so high indicates an open component.

I've identified the two dual diodes by circling them in red. I suspect you will find one side of one of the diodes shorted. Try to trace the circuit back to pin 15 of the inverter controller.

PlainBill

Attached Files

**PlainBill** · 12-09-2009, 07:42 AM

Re: Coby Flat Panel TV Bad Inverter

Look at this service manual for an Acer AL-1916V. If you go to the very last page and look at the lower right corner you will see the circuit I am referring to. The Colby uses dual diodes, the Acer uses 1N4148 diodes. Trace the circuit back to the controller. Of course, the Acer uses a different controller, but the principle is the same.

Good hunting!!!

PlainBill

**fixit** · 12-09-2009, 08:59 AM

Re: Coby Flat Panel TV Bad Inverter

Thanks PlainBill for the service manual. I will dig into this and get back to you.

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Coby Flat Panel TV Bad Inverter

Coby Flat Panel TV Bad Inverter

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