How to find short on PCB

[Scout]

I have a Gateway fpd2485w monitor Im trying to repair. It was broken when it was given to me(no power at all) so I replaced the electrolyte capacitors ,well the ones that were even the slightest bit bad. Checked the rest and the tested good. I replaced the fuse which was blown.

As soon as I hooked it up the fuse blew again.So I thought maybe a solder bridge or flux so I so I took magnifying glass went over the board and cleaned it as best as I could.Checked the capacitors to make sure I didnt put one in wrong and also checked to make sure the were good and tested the resistors on the board. Two resistors gave me no reading but I think they were just out of range of my multi-meter Plugged it in and again blew the fuse.

I Googled how to find a short on PCB & circuit,trouble shooting PCB and found a few articles and videos but nothing that helped me too much the one that seemed most interesting involved using 12v battery to power the board and look for shorts but I wasnt clear on what exactly I was supposed to do.

Anyway Im looking for some advice on how to track down that short/s.

Heres the PCB front and back If it will give ya an idea of what Im working with.Its cleaned up a little better than when pic was taken. Oh and the solder defect on the board were noth cause by me. Had to have been from manufacturer or some other cause but they seem to be pretty oxidized like they've been there a while.

THX in advance

 

[davenn]

is it the 120/240V mains fuse thats blowing or a lower voltage DC fuse ?

if its the mains fuse then you need to firstly check the bridge rectifier, its probably shorted that will cause an instant fuse pop

the rectangular part arrowed it should have 4 pins in a row on the solder side


OK the solder side ......

I have traced out the 2 mains AC lines from the socket to the bridge rectifier
you can see that they go to the 2 inner pins

there is a fuse shown (solder side) at the bottom of the image right by the ipower input socket. Is this the fuse that's blowing?

Dave

 

[Scout]

Yes its the t5.0a 125v fuse you referenced that is blowing. I tested the bridge rectifier and it seemed fine but Ill go ahead and try replacing it.

The one thing I couldnt test were the ceramic capacitors. They all visually looked fine but I couldnt test them with my cheapometer. Im gonna start by replacing the Bridge rectifier and 2 resistors that were suspect and Ill report back.

Is there anyway to test the board safely without have to blow a fuse? I have 4 left and I cant find them around here so have to get off the net. I dont mind if I waste 3 of them if I can get it repaired by the 4th.

I tested the board the second time without connecting other lines. The one to the inverter board and the one to the board with the power switch,rca vga,hdmi,usb... jacks. I wanted to isolate the short and not damage those board so left unattached. Is this ok do I have to do anything to complete the circuit like put a jumper in the connectors or any thing?
Thanks

 

[markm6164]

If you have a really good ohm meter you can trace down the short. The closer you get to the short the lower the resistance value will be. This is a safer way as you don't power up anything.

 

[(*steve*)]

Markm's suggestion is a good one if the problem is a dead short, however it may be something that doesn't look like a short.

One option that is sometimes employed is to wire a bulb in series with your power supply. If you've got 110V mains, then a 100W light bulb will effectively limit the short circuit current to 1A. The brightness of the bulb also tells you how much power the device it trying to draw. Beware that switchmode power supplies can act weird with a series bulb. You shouldn't run into too many problems as long as you limit the load to a couple of watts.

I would be suspicious of the output rectifiers and the chopper transistor. If either of these read a dead short across them then you have problems (but the measurements can be heavily influenced by the surrounding circuit.

 

[khankll]

a bit off of the toic .. but i personally use a reasonably good led torch under the pcb.. that gives me an excellant x-ray of pcb.. id u use a bit powerfull torch u can see the tiniest cracks or shorts..

 

[iqbaal khaan]

all of you have done a great job but still there is a way which is called "isolate and check". Start isolating the circuit components one by one starting from bridge rectifier to chopper's out put diodes.
1st you have to make a series board in which a 200 watts bulb is used in series of the monitor's supply. now you will not loose any fuse.
I have noticed 2 diodes on the other way to the bridge, If you can isolate and check them too.
For this purpose just remove the component you want to isolate and check if the short circuit is removed or not.
try it

 

[p1taylor]

torch idea is good

 

[Scout]

Bridge rectifier question

I just bought 5 bridge rectifiers and none of them appear to be working.

I Installed one on PCB because I was blowing fuses. I plugged PCB in and didnt blow a fuse so thought good prob solved. but then notice I still was not getting power out so tested rectifier and was getting fail test. Nothing going through. on either pair of prongs.

So thought maybe I got a bad one and one by one tested other 4. useing ohm meter Testing each pair of prongs switching leads And none of them appear to be working.No reading in either direction.. Is it likely I got 5 bad ones? or am I testing incorrectly. or something else?

Just wanna get some insight before I call them

 

[mechtronics]

what function on the meter are u using?

 

[Scout]

Ohm meter
I started at 200 thought maybe I just wasnt using enough turned it up to 2k then 20k and then 200k on one of the unused rectifiers was getting nothing

 

[duke37]

If the rectifier has gone short and blow a fuse, then it should show on the meter as a short not open circuit.

If you have a diode and an analogue meter, use the 20k range and see what happens in each direction of the test leads.

With a bridge rectifier, measure between an AC input and the + output, it should act like the diode test. If this is OK, then repeat with the other AC input..
Repeat last tests using the - output.

If you are using a digital meter, then it will probably measure the voltage drop in the forward direction.

I think you may have wired the bridge wrong. Show us the circuit.

 

[Scout]

Well i didnt really do any wiring for bridge its just a four prong through hole bridge rectifier. KBU8J to be exact and I tried to test the one I installed and other 4 and am getting no reading out of any of them.

Will post pic of it in few min

And to clarify a bit. PCB was blowing fuses and I was told it was likely bridge rectifier so I ordered it (well I had to buy 5) it came and I installed plugged monitor in and didnt blow a fuse was like great it fixed prob hooked up rest of harnesses plugged it in and still no power so took board ot to check rectifier and...

oh and fuse was still fine.

And here is PCB https://www.electronicspoint.com/find-short-pcb-t249275.html
Davenn put arrow to rectifier in second post of thread

 

[KrisBlueNZ]

A digital multimeter on ohms range may not work for testing diodes, which have forward voltages in the range 0.6V to 1V. Does the meter have a diode test range? Otherwise, make a continuity tester with a 9V battery, an LED, and a series resistor of around 1K. Each diode in the bridge should conduct in one direction and not in the other. You should test it out-of-circuit.

 

[(*steve*)]

Best to keep things in one thread

Not sure how safe your meter is, but if it was appropriately rated and you knew what you were doing, I would recommend reading the AC voltage across the ~ and ~ leads, and the DC voltage across + and -.

NOTE. If you do this and do it wrong, we won't hear from you again -- ever. To do this safely, you MUST only use one hand (keep the other one in your pocket) AND select the correct mode on the meter AND attach the leads BEFORE turning on the power. Then turn off the power before removing the leads with one hand.

I'm not kidding that doing this wrong can KILL YOU.

To do this you will almost certainly have to have probes with alligator clips (or similar) and you may not be able to attach the probes directly to the bridge rectifier leads (an option is to solder a short length of insulated wire to them.

If you manage to short either the mains or the rectified mains, you will hopefully only blow a fuse (and possibly destroy the bridge rectifier).

I don't use colour, bolding and underlining much, but if flashing was an option I would have used it.

 

[mechtronics]

ya was gonna say ur better off with a meter that has a diode test on it rather than using the resistance range. depending on what meter that you have of course. this function puts a 0.6 - 0.7 voltage onto the diode that u r testing and you should get a reading. as what was said previously use a diode , LED and a resitor then test it using continuity.

 

[JMW]

What size fuse is clearing and what is the voltage. Get a power supply with adjustable voltage and current. Plug your board in, and substitute your adjustable supply for the cleared fuse. Set the voltage for the proper level and the current for 10 to 20% of the fuse rating. Power up the circuit and the test supply simultaneously, the shorted component(s) will be the ones that heat up. You can go by touching, smoke or freeze spray (depending on the sensitivity or ESD characteristics of the board). Continue changing parts until the "test" supply rises to set voltage.
Used this technique on PCB's that were in the vicinity of lightning strikes, worked everytime and saves a bunch in cleared fuse money.

 

[Rleo6965]

Usually with that kind of problem. First I have to isolate which part of the board causing to blowing of fuse. I removed the power transistor or chopper and the inverter IC or board. Replaced the fuse with half the current capacity of the original fuse.
Then place a voltmeter on +V of bridge rectifier or the + of main filter capacitor using alligator clip as *steve* suggested for safety.
Apply power on pcb. If fuse did not blow and voltmeter read voltage almost same w/ filter capacitor working voltage. Then a possible short on chopper transistor or inverter board.

Warning. don't touch or solder chopper transistor or inverter board w/o discharging +DC voltage on main filter capacitor. Either your electrocuted and damaged the chopper transistor or the Inverter board. Discharged capacitor using the AC plug of your soldering iron. Soldering iron will act as bleeder resistor and therefore will not cause spark.

Let'us know the result.