Dell Monitor has "No Power"!!

[Tayninh55]

Hello again! I picked up a "freebie" Dell Monitor that the person said "would not power up"! I brought it home and hooked it up to see what it would do. IT WORKED PERFECTLY! So, I gave it to my grandson to use and it continued to work for almost a year,,,,,,,,,,THEN IT STOPPED POWERING UP again. I brought it back home and, sure enough, no power when I pushed the "on" button. No light "anywhere that I can see". Thought maybe "bad caps", so I tore it apart and everything "looks" to be ok,,,,,,but I know it isn't. None of the caps are bulging or look bad to the eye. I was just reading on here about "a bad diode", but since it was "supposedly" acting the same way before I got it, and then worked for me, I know the diode didn't fix itself. Thought about a loose connection,,but couldn't find any. I've fixed a couple of tv's before, WITH YOUR HELP, so, what do you think?
It is a 19" Dell Monitor LCD, Model E196FPb. I searched, but didn't find this model on any of the other threads.

[philliesfan30m]

Re: Dell Monitor has "No Power"!!

what voltage do you have across the big cap? be careful it is high voltage and make sure you have the black on neg. and red on pos. and don't touch anything else when checking. but before that is the fuse ok?

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Phil, The Big cap (120uf, 450V) shows 173 volts, the fuse shows continuity.

[Rtech]

Re: Dell Monitor has "No Power"!!

CN701 will carry voltages to the Logic Board,and,probably,on the bottom of the board it will tell you what voltages should be on which pins,these need to be tested.Other common Faults looking at the Board,would probably be PF801(fuse),Q808 and 809,Q815 and 816,as these,and there assocoated caps and output Inverter transformers are typical Royer circuitry,as used in many Benq Boards.

[Davi.p]

Re: Dell Monitor has "No Power"!!

Rtech you don't think that with a fault on inverter circuit he must see the status led lit?
To the OP: test in diode mode the main mosfet Q601 on power board, the "red red silver gold" big resistor near it and the 2 double diodes on the heatsink. Watch better with a magnifying glass all the primary side solder joints.

[viziks]

Re: Dell Monitor has "No Power"!!

This is a BenQ board with C5707 transistors in Inverter. Check Voltages on the 2 diodes mounted on heatsink at bottom left of picture.There should be 5V and 12V on them.
If voltages are present the problem is either a defective main board or defective key switches. Since you say this problem resolved itself initially and worked for a year there are distinct chances of the six micro switches being leaky and causing this problem.Remove connection fom mainboard and check switches individually. If any switch shows leakage replace all of them.

[R_J]

Re: Dell Monitor has "No Power"!!

Check the solder connections on all 4 inverter transformers and c5707 transistors, use a magnifier, if there are poor connections check the c5707 transistors for being shorted.

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Won't be able to check further until after weekend. Thanks!

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Viziks, Not sure how to check the diodes on the heat sink. I put the meter's black lead on the center lead of the diode, then the meter's red lead on each of the outside leads of the diode. The first diode (closest to the edge of the board) had a reading of 5.03V, and the other diode had a reading of 18.8V. That's not the exact voltage you quoted, so maybe I'm not checking it right? I'll check back on Monday.

[Davi.p]

Re: Dell Monitor has "No Power"!!

I don't understand why you get that voltage, anyway the correct way is to put the black pb on a screw hole or on to the heatsink, the other on the central pin, tester in vdc. Anyway 18 and 5 v seems corect to me, i remember one of those Benq board with 35v capacitors on the inverter supply so 12v can be few.

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Davi.p,,,,,,,,I know you won't believe this, but I got the same readings checking the diodes they way you told me to do,,,,and the way I had done it originally. I did have more time this time and discovered something. I'm not sure if it is the source of my problem. I went back to "recheck" the voltages again, and this time I had no reading at all at first. Then, as I moved the board some, I would get a reading one time, but none the next. I started moving things around, and discovered that one of the wires going from the main voltage (120V) going from the "inside" of the power plug to the circuit board felt like it was weak where it was soldered on. It had a rubber coating on it but it felt like it was broke. I took a knife and removed the coating to see that "it was partially broken,,,,and only held on by a few strands. I cut it off and re-soldered it and now I get a reading every time. I know this was probably the reason for the "intermittent" readings. Now I guess I'll have to re-assemble it in order see if it fixed the whole problem??

[Tayninh55]

Re: Dell Monitor has "No Power"!!

On second thought, I decided to wait a while before putting it back together to see if you guys have any other ideas on components to check. The light on the on/off button is still not glowing, I pushed it four or five times and it doesn't show. Also, while I had my meter on the diodes, I had my wife push the on/off button and it didn't make any difference on the voltage, it just stayed the same while working the on/off button. Does this sound right? Thanks again for any help!

[viziks]

Re: Dell Monitor has "No Power"!!

Did you check the switches ?
The voltages are correct,No problem with power section.

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Are you talking about the on/off, and other buttons on the front of the monitor? These are connected to the main board by a long flat "ribbon wire" to the circuit board? I would just be checking these for continuity, right?

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Is this the "switch" you are talking about? There are only four buttons on this switch but there are "seven" contacts on the end of the ribbon. I suppose one of the contacts is the ground, but by looking at the ribbon, and both sides of the board, I can't tell which one it is. I tried just touching one contact on the end of the ribbon cable with the black lead, then touching each of the others with the red lead, but got no reading. Is this the right way to do it? or is there another way? Can you tell which contact should be the ground. I am attaching a couple more pictures so you can understand what I am talking about.

[bammbammfran]

Re: Dell Monitor has "No Power"!!

If you could remove the small green power board from the front trim and post a picture of the other side of that board, that would help

[bammbammfran]

Re: Dell Monitor has "No Power"!!

check power board to main switch connector for power
CN701 connector pins
9 10
7 8
5 6
3 4
1 2

looks like 9 and 10 should be 3.3volts dc
7 and 8 should be 5vdc
5 and 3 ground (6 and 4 may also be grounds)
1 and 2 are the on off and dim pins

if that is ok, check the power switch
on the power switch 7 contacts of ribbon, it looks like 3 are grounds (center one and the second one in from each end)
you should probably have around 3 volts on all 4 other pins
if you have 3 volts
go to the power button switch
two pins on the switch are ground and 2 should have that 3 volts
if you have 3 volts at the power switch (while the meter is reading- push the switch to see if the voltage drops to 0) If it does not drop-- the switch is bad
If this is the case run a wire from a ground ( Metal of the monitor anywhere should do) and touch the 3 volt pin on the power button switch for a second. this should turn it on

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Hi,,,,,I'm still showing 4 volts, and 18.3 volts on the diodes on the heat sinks BUT I CAN NOT get any voltage to show at the CN701 connector pins, where the two boards go together. I can push the on/off button and nothing shows, had meter set on 20 volt range. Sorry about the delay getting back with you, but I had to take time out to work on my car!

[budm]

Re: Dell Monitor has "No Power"!!

What DC voltages do you get at all 3 pins of U6 on the main board?

[Tayninh55]

Re: Dell Monitor has "No Power"!!

Budm,,,,,,,,I don't know if I'm doing something wrong or not, but I don't read any voltage on any of the three pins of U6 either.