Issue with Westinghouse 19" LCD Monitor (large pics)

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Question: Regardless of if a cap is in the circuit........should there be a low resistance of several kilo-ohm between gate and drain?

Maybe. Notice there are a number of resistors in the circuit. Now if the resistance is there OUT of circuit, that's a problem.

I'm hoping you can find a problem with a component, because the alternatives are to pull the transformer and try powering up the inverter to see if anything blows, or to map the interconnections between 30+ points so it is possible to create a schematic.

PlainBill

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Question: Regardless of if a cap is in the circuit........should there be a low resistance of several kilo-ohm between gate and drain?

[dondrusco]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

I still haven't tried to generate a schematic, but I notice there is a capacitor in that circuit. Could it be that it is holding enough charge to turn the FET on?

PlainBill

Hi,

I found only paid sites with schematics. This power board is the same, as in Acer AL1916W (probably you know about it)

http://www.s100-manuals.com/search.asp?q=AL1916W

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

What cap did you mean? I looked at the circuit but i did not see a cap connected to that transistor.

Your questions seems very plausible but i just dont know the answer......wish my electronics was better.

Look at the attached picture. The controller IC (U200), everything within the blue outline, and the two power FETs (solder points circled in black) are the driver circuit for the inverter transformer. Pin 8 of U200 drives the gate of the lower transistor directly. The gate of the other transistor is driven by Q208, driving through R210. The drive to the transformer is coupled back through C234. Without a schematic (or maybe even with one ), I'm having trouble figuring out just what is happening in this circuit.

PlainBill

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

What cap did you mean? I looked at the circuit but i did not see a cap connected to that transistor.

Your questions seems very plausible but i just dont know the answer......wish my electronics was better.

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

I finally finished testing the transistors:

Testing of transistors 1, 2, 3, and 4 (see enclosed picture):

Note:
Transistor 1: Is a K72 N-channel Mosfet. Equivalent digikey part number: 568-1369-1-ND
Transistor 2: Is the W2T PNP Bipolar. Equivalent digikey part number: 568-1744-1-ND
Transistor 3: Is a K72 N-channel Mosfet. Equivalent digikey part number: 568-1369-1-ND
Transistor 4: Is the W2X NPN Bipolar. Equivalent digikey part number: 568-1743-1-ND


Note: For transistor 1 and transistor 3 (K72 devices). From data sheet there is a diode from source to drain. This will also be measured.

Note: I made the ohm measurements with both polarities....ie…swapped leads of DMM.


Testing of transistor1: K72 device
Ohm-Measurement Red/Black Black/Red
Gate-to-drain: 9.3Meg-ohm 0.9Meg-Ohm*
Gate-to-source: 3.1Meg-ohm 3.1Meg-Ohm
Drain-to-source: -1.3Meg-ohm 1.9Meg-Ohm

Source-to-drain-diode: 0.579Volts

*: I had a very interesting result when I measured the Gate-to-drain resistance. If I measured the diode turn on voltage from source to drain and THEN measured the resistance from Gate-to-Drain, with black lead on gate and red lead on drain, the resistance measured was in the kilo-ohm range. If I wait ~ 4 to 5min and then measure the resistance I get the 0.9Meg-ohm value. But it's the “kilo-ohm” measurement that I get when ever I first measure the diode that concerns me…………what I don't know is it the device or something else in the circuit causing this. I DID NOT see this in the other K72 transistor.



Testing of transistor3: K72 device
Ohm-Measurement Red/Black Black/Red
Gate-to-drain: 1.5Meg-ohm 8.6Meg-Ohm
Gate-to-source: -1.6Meg-ohm 2.4Meg-Ohm
Drain-to-source: 4.9Meg-ohm 4.6Meg-Ohm

Source-to-drain-diode: 0.693Volts










Testing of transistor4: W2X device NPN bipolar

Ohm-Measurement Red/Black Black/Red
Base-to-collector: 4.69kilo-ohm 4.69kilo-Ohm
Base-to-emitter: 4.5Meg-ohm 4.5Meg-Ohm
Collector-to-emitter: 4.5Meg-ohm 4.5Meg-Ohm

Diode turn on voltages:
Base-to-collector: 0.611Volts
Base-to-emitter: 0.638Volts





Testing of transistor4: W2T device PNP bipolar

Ohm-Measurement Red/Black Black/Red
Base-to-collector: 5.86Meg-ohm 4.18Meg-Ohm
Base-to-emitter: 4.5Meg-ohm 4.5Meg-Ohm
Collector-to-emitter: 13Meg-ohm 1.6Meg-Ohm

Diode turn on voltages:
Collector-to-base: 0.697Volts
Emitter-to-base: 0.95Volts





If there is something else someone wants me to measure let me know. Also, can someone comment on the unusual testing of transistor 1.

I still haven't tried to generate a schematic, but I notice there is a capacitor in that circuit. Could it be that it is holding enough charge to turn the FET on?

PlainBill

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

I finally finished testing the transistors:

Testing of transistors 1, 2, 3, and 4 (see enclosed picture):

Note:
Transistor 1: Is a K72 N-channel Mosfet. Equivalent digikey part number: 568-1369-1-ND
Transistor 2: Is the W2T PNP Bipolar. Equivalent digikey part number: 568-1744-1-ND
Transistor 3: Is a K72 N-channel Mosfet. Equivalent digikey part number: 568-1369-1-ND
Transistor 4: Is the W2X NPN Bipolar. Equivalent digikey part number: 568-1743-1-ND


Note: For transistor 1 and transistor 3 (K72 devices). From data sheet there is a diode from source to drain. This will also be measured.

Note: I made the ohm measurements with both polarities....ie…swapped leads of DMM.


Testing of transistor1: K72 device
Ohm-Measurement Red/Black Black/Red
Gate-to-drain: 9.3Meg-ohm 0.9Meg-Ohm*
Gate-to-source: 3.1Meg-ohm 3.1Meg-Ohm
Drain-to-source: -1.3Meg-ohm 1.9Meg-Ohm

Source-to-drain-diode: 0.579Volts

*: I had a very interesting result when I measured the Gate-to-drain resistance. If I measured the diode turn on voltage from source to drain and THEN measured the resistance from Gate-to-Drain, with black lead on gate and red lead on drain, the resistance measured was in the kilo-ohm range. If I wait ~ 4 to 5min and then measure the resistance I get the 0.9Meg-ohm value. But it's the “kilo-ohm” measurement that I get when ever I first measure the diode that concerns me…………what I don't know is it the device or something else in the circuit causing this. I DID NOT see this in the other K72 transistor.



Testing of transistor3: K72 device
Ohm-Measurement Red/Black Black/Red
Gate-to-drain: 1.5Meg-ohm 8.6Meg-Ohm
Gate-to-source: -1.6Meg-ohm 2.4Meg-Ohm
Drain-to-source: 4.9Meg-ohm 4.6Meg-Ohm

Source-to-drain-diode: 0.693Volts










Testing of transistor4: W2X device NPN bipolar

Ohm-Measurement Red/Black Black/Red
Base-to-collector: 4.69kilo-ohm 4.69kilo-Ohm
Base-to-emitter: 4.5Meg-ohm 4.5Meg-Ohm
Collector-to-emitter: 4.5Meg-ohm 4.5Meg-Ohm

Diode turn on voltages:
Base-to-collector: 0.611Volts
Base-to-emitter: 0.638Volts





Testing of transistor4: W2T device PNP bipolar

Ohm-Measurement Red/Black Black/Red
Base-to-collector: 5.86Meg-ohm 4.18Meg-Ohm
Base-to-emitter: 4.5Meg-ohm 4.5Meg-Ohm
Collector-to-emitter: 13Meg-ohm 1.6Meg-Ohm

Diode turn on voltages:
Collector-to-base: 0.697Volts
Emitter-to-base: 0.95Volts





If there is something else someone wants me to measure let me know. Also, can someone comment on the unusual testing of transistor 1.

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Sorry, i hope i did not cause you too much trouble. I new how to test the bipolars but not the fets.

Thanks. And again i hope i did not cause you too much trouble.

Understand the limitations of these tests. They will detect a shorted transistor - bipolar or FET. They should detect a bipolar transistor that is open internally. They will not detect an FET that is open internally.

As for the problem with the inverter, I am trying to figure out a way to locate the defective parts while damaging as few other parts as possible.

PlainBill

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Sorry, i hope i did not cause you too much trouble. I new how to test the bipolars but not the fets.

Thanks. And again i hope i did not cause you too much trouble.

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Any good idea on how to check Fets?

Also, assuming the devices are all good.........is there some other location that i should check on the board?

I really don't have the energy for this....

A simple, generic in circuit test for bipolar transistors:
Step 1: Use an ohmmeter set on the lowest resistance scale - on mine, 200 ohms full scale. Arbitrarily identify the transistor leads as e, b, c. Measure the resistance from e to b, e to c, and b to c. You should see an open circuit condition in most cases. A notable exception is the base - emitter connection on a horizontal output transistor that is transformer coupled to the driver transistor.

Step 2: Set the ohmmeter on the diode test scale. Place the black probe on the e lead, touch the red probe to b, then to c. Note the readings. Move the black probe to the b lead touch the red probe to the e lead, then the c lead. Note the readings. Move the black lead to the c lead and touch the red lead to the e lead, then the b lead. Again note the readings. You should see a diode action (a reading of about .7 volts) on only two of the six readings.

A simple, generic in circuit test for FETs:
Use an ohmmeter set on the lowest resistance scale - on mine, 200 ohms full scale. Arbitrarily identify the transistor leads as d, g, s. Measure the resistance from d to g, d to s, and g to s. You should see > 200 ohms in most cases.

PlainBill

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Any good idea on how to check Fets?

Also, assuming the devices are all good.........is there some other location that i should check on the board?

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Just wanted to make sure i did not confuse anyone. The part that was replaced, that was shorted, was replaced sometime ago. This did not fix monitor. Monitor still blows open fuse when inverters are soldered into board. When inverters are not soldered into board fuse does not blow.

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

1) Black circles: Lot of flux there.
I measured the resistance between each of the leads. Readings in the Meg-ohm range

2) Red circles: (see picture)
I have labeled each red circle with numbers 1,2 and 3,
Resistance readings:
1-2: 10ohms
2-3: 3Meg to 3.3Megohms depending on polarity of leads
1-3: 3Meg to 3.3Megohms depending on polarity of leads



3) Check transistors:
Note:
K72 is an N-channel fet. Equivalent digikey part number: 568-1369-1-ND
W2X is an NPN bipolar. Equivalent digikey part number: 568-1743-1-ND
W2T is an PNP-bipolar Equivalent digikey part number: 568-1744-1-ND

I have not checked all the transistors in this circle. I need to look up how to do this with a DMM. I will post again with results

Note: The W2x device originally was “bad” and I replaced it. The base and collector were shorted. How did I know this. From the circuit the base and collector are in parallel with a surface-mount resistor which has a value of ~ 4kohm. I new it was shorted when I measured between Base and collector and got a value much less than the 4kohm resistor value. Currently the W2x base collector resistance equals the surface mount resistor value.
I could not find the diode you asked for D212.


4) The yellow circles are not connected together and they are not connected to either the white or dark red.

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Hi,

Enclosed are the pics you asked for. If you need better pics please just let me know. I can try and do better.

Also, you will notice in one of the top pics that the mosfet inverters are "standing straight up"........this is because i used the new devices and i did not "bend" them down since i wanted to be able to remove them again easily.

thanks for your help with this!

I've marked up one of the pictures and have some points for you to check.

Black circles: There is a lot of flux there. Make sure there are no hidden shorts.
Bright red circles: Are they connected by a jumper?
Large blue circle: Check the transistors and D212 for shorts. Has anybody bothered to identify what these are? (bipolar, mosfet, etc?)
Large blue circle: Are the points in the yellow circle connected together? Are they connected to either the dark red or white circle?

That's it for tonight. I'm taking the grandkids to look at Christmas lights.

PlainBill

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Hi,

Enclosed are the pics you asked for. If you need better pics please just let me know. I can try and do better.

Also, you will notice in one of the top pics that the mosfet inverters are "standing straight up"........this is because i used the new devices and i did not "bend" them down since i wanted to be able to remove them again easily.

thanks for your help with this!

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Ok I measured as requested:

I am enclosing two photos. One shows the overall board from a backside view. The other tries to show where i measured per your request.

The first measurement:
A1 to A1: Measured 0.749K-ohms
A2 to A2: Measured 0.750K-ohms

Does this tell me something?

I'm not sure, does it? It tells me that the transformer secondary is not shorted. It doesn't tell me anything about the transformer primary. Unfortunately, it is unlikely that an ohmmeter will be able to detect shorted primary windings. That would take a transformer ringer.

Now I will need a good picture covering at least the area outlined in blue on the attached picture, and a picture covering the corresponding area on the top side of the board.

And does anyone know if a service manual or schematic has been found that covers this monitor?

PlainBill

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Ok I measured as requested:

I am enclosing two photos. One shows the overall board from a backside view. The other tries to show where i measured per your request.

The first measurement:
A1 to A1: Measured 0.749K-ohms
A2 to A2: Measured 0.750K-ohms

Does this tell me something?

[PlainBill]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Is this thread still active?

Basically with the inverter mosfets in the circuit and the ccfls disconnected i open up the fuse F200........anyone have anyideas? I dont think its the actual mosfet since i used a new one......but it might be i guess. can anyone help?

Yes, the problem is the circuit is drawing too much current. Now the task is to figure out WHY.

I see three possibilities: Improper drive signal to the mosfets; shorted transformer; excessive load on the transformer. While disconnecting the CCFLs is not a good idea, you HAVE eliminated them as a likely cause.

The first step would be to measure the resistance of the secondaries of the transformer. Compare the resistance between the terminals circled in red to the resistance between the terminals circled in blue. Report that. If they are the same, take pictures of the back and top of the board. They don't have to be quite as large as the attachment, but I want them to include the driver transistors and if possible the controller IC.

PlainBill

[Bobdee]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Is this thread still active?

Basically with the inverter mosfets in the circuit and the ccfls disconnected i open up the fuse F200........anyone have anyideas? I dont think its the actual mosfet since i used a new one......but it might be i guess. can anyone help?

Might be worth trying that again with only the top tubes connected and then only with the bottom pair connected, one side may not blow the fuse, that will tell you what side is shorting

[Welchs101]

Re: Issue with Westinghouse 19" LCD Monitor (large pics)

Is this thread still active?

Basically with the inverter mosfets in the circuit and the ccfls disconnected i open up the fuse F200........anyone have anyideas? I dont think its the actual mosfet since i used a new one......but it might be i guess. can anyone help?