Troubleshooting a short circuit

[sghill99]

I have a Viewsonic VX2235WM-3 with a short somewhere on the circuit board that is attached to the LCD screen. I have isolated the shorted fuse, tried to replace it and it shorts out right away.

I've seen posts that suggest connecting a light bulb in place of the fuse. What type of light bulb? After the bulb is attached, I assume I power up the monitor, but what am I looking for? I'd assume the light bulb will go on, and something on the circuit board will start to overheat. Is this correct?

If I can't power up the unit due to it's layout, is there anyway I can connect a small battery up to the circuit board and start troubleshooting that way?

Really I think what I need is guidance in troubleshooting a short circuit I've attached a couple of pictures. I've searched the forum for postings on shorts, but none of them really address how to troubleshoot it.

Thanks in advance.

[PlainBill]

Originally Posted by sghill99

I have a Viewsonic VX2235WM-3 with a short somewhere on the circuit board that is attached to the LCD screen. I have isolated the shorted fuse, tried to replace it and it shorts out right away.

I've seen posts that suggest connecting a light bulb in place of the fuse. What type of light bulb? After the bulb is attached, I assume I power up the monitor, but what am I looking for? I'd assume the light bulb will go on, and something on the circuit board will start to overheat. Is this correct?

If I can't power up the unit due to it's layout, is there anyway I can connect a small battery up to the circuit board and start troubleshooting that way?

Really I think what I need is guidance in troubleshooting a short circuit I've attached a couple of pictures. I've searched the forum for postings on shorts, but none of them really address how to troubleshoot it.

Thanks in advance.

Just a minor point to start off; fuses don't short, they blow, or go open. A fuse which reads as a short is a good fuse.

The point of using a light bulb is that it allows the circuit to operate without interrupting the power. Choose one whose current rating is about equal to the current rating of the fuse that it temporarily replaces. I wouldn't suggest using a battery, but I've used a variable power supply many times. The problem is they are expensive if you are only going to use it once.

Troubleshooting shorts (circuit overloads) is a complex process. Some are what I call 'hard' shorts - a shorted diode, a blob of solder between power and ground. Other are dynamic - two transistors both turning on at the same time. Those tend to be much harder to identify.

The second picture shows two voltage regulators, U1 and U2. Check the resistance between the three pins (the tab is connected to the center pin). Check the resistance from the output side of the fuse to ground. Check any large diodes for shorts.

PlainBill

[sghill99]

Will do, regarding your suggestions.

When you say check the resistance between the three pins, is that in circuit with no power? Probably a dumb question, but my lack of experience necessitates it. Also, when checking the output side of the fuse to ground, what part of the screen should I use as the ground? And checking diodes for shorts, would that be diodes on the circuit board that the fuse is on, or all of the circuit boards that are inside the monitor (power board and video or logic board)?

I've heard that if you replace the fuse and remove the suspected shorted component, then the fuse wont blow (although the monitor still wont work). Is this true? Would this method help in identifying the shorted component?

[PlainBill]

Originally Posted by sghill99

Will do, regarding your suggestions.

When you say check the resistance between the three pins, is that in circuit with no power? Probably a dumb question, but my lack of experience necessitates it. Also, when checking the output side of the fuse to ground, what part of the screen should I use as the ground? And checking diodes for shorts, would that be diodes on the circuit board that the fuse is on, or all of the circuit boards that are inside the monitor (power board and video or logic board)?

I've heard that if you replace the fuse and remove the suspected shorted component, then the fuse wont blow (although the monitor still wont work). Is this true? Would this method help in identifying the shorted component?

Do the resistance tests in circuit, no power. NEVER check resistance with power on!

You'll have to determine where the ground plane is on this panel. My approach is to look at the connector. If you a group of pins connected together, they are either power or ground. Power goes to the fuse (often you can see the link) so ground would be the other group. Also, the - side of electrolytic caps is a clue.

Just check the diodes on this circuit board.

Yes, removing a component you suspect and the fuse no longer blows is one technique.

PlainBill

[sghill99]

Here are the resistance measurements for the two voltage regulators. I wasn't sure what probe should be where, so I indicated where I put the black probe and where I put the red probe for each measurement. Does these measurements tell you anything?

Ground = Pin 1
Output = Pin 2
Input = Pin 3

ME 1117 3.3V Voltage Regulator
1-2 .888 kohm Black Probe on 1, Red Probe on 2
1-3 1.120 kohm Black Probe on 1, Red Probe on 3
2-3 2.009 kohm Black Probe on 2, Red Probe on 3

ME 1117 1.8V Voltage Regulator
1-2 465.7 ohm Black Probe on 1, Red Probe on 2
1-3 .889 kohm Black Probe on 1, Red Probe on 3
2-3 1.370 kohm Black Probe on 2, Red Probe on 3

[PlainBill]

Originally Posted by sghill99

Here are the resistance measurements for the two voltage regulators. I wasn't sure what probe should be where, so I indicated where I put the black probe and where I put the red probe for each measurement. Does these measurements tell you anything?

Ground = Pin 1
Output = Pin 2
Input = Pin 3

ME 1117 3.3V Voltage Regulator
1-2 .888 kohm Black Probe on 1, Red Probe on 2
1-3 1.120 kohm Black Probe on 1, Red Probe on 3
2-3 2.009 kohm Black Probe on 2, Red Probe on 3

ME 1117 1.8V Voltage Regulator
1-2 465.7 ohm Black Probe on 1, Red Probe on 2
1-3 .889 kohm Black Probe on 1, Red Probe on 3
2-3 1.370 kohm Black Probe on 2, Red Probe on 3

Well, you can do the math. The 3.3 V supply shows roughly 1K; that's less than 5 ma of current. The 1.8 volt supply shows about 500 ohms; again less than 5 ma drain. There is no hard short on either of those lines.

Of course, this doesn't take into consideration possible dynamic loads, non-linear loads, etc.

PlainBill

[sghill99]

I think I have identified this diode as shorted.

I need help in identifying it...

Anybody know what type it is?

Where I can find one?

Thanks in advance.
Scott