Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

[keb]

I have repaired many of these units by replacing the caps (except the large cap on the hot side of the board), however, this one has a problem that is beyond my expertise.

When turning on the monitor, the power button turns blue, the screen turns on for a split second, then turns off. The power button blinks and the screen flickers yet again. This cycle continues until the unit loses power.

The power board is a Yuyang Telecom YP1704G. Stock with Samxon capacitors.

I am in a unique position as I cannot upload pictures because cameras are prohibited where I work (I work in a prison). I could link to another website where a picture of a similar board can be found, but I'm not sure that would help in this instance.

Although I have a limited electronics background, I suspect the problem is a low power issue as a result of a defective transistor or diode, but I don't want to simply go on a hunting expedition taking things apart unnecessarily.

Any help will be appreciated.
A visual inspection of the board reveals no clues as to whether there are any additional defective components.

[bammbammfran]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

That sure is a blast from the past
I just dug one up from my goodies

first thing to check is to see if any of the red caps are bulging at the top or if the bottom is showing any signs of leaking
also check the large cap that is laying down is bulging at the top or bottom

if no bulging
use a dc tester and check what the 5 volts at the wiring harness is doing with the board all by itself
if it is around 5, check it again with it connected to the vga board
if 5 is low or turns on a drops, the small cap by the largest cap should be changed first (this small cap can fail without showing physical symptoms)

[keb]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

I'll try checking the voltages in the morning. I've already replaced the caps on the cold side, but because I have never seen the hot side caps go bad in all the monitors I have fixed, I didn't change them. I'm not sure I have one of those in stock. I'll have to dig around and see if I can find one.

Incidentally, none of the caps were bulging, but because I know the Samxon capacitors regularly fail, I always change them when a monitor fails. It has always fixed the issue, but not this time.

I'll reply back with the test results as soon as I can.

Thanks for the help.

[Agent24]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

I recently fixed an AOC LCD monitor that was power cycling due to shorted transistors in the inverter circuit.

The good thing about the inverters is you have two identical circuits side by side. Test all the semiconductors in them with diode test function on your meter, and compare between each circuit. If the results are quite different, or you get shorts, you may have found the problem.

It could also be a faulty PWM Controller in the PSU or faulty DC supply capacitor\diode for the PWM Controller. This would be the small capacitor right next to the mains filter one.

[selldoor]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

It could also be the inverter transformers - I had an LG monitor that kept cycling the 2 seconds to black and it was the transformer I dont know why it
kept cycling though.
I have tried to find a pinout but failed. No power to the board, set meter on ohms 2000 and check the resistance on the 2 transformer legs that go to the lamp sockets.

[keb]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

Sorry for the delay. Yesterday was hectic.

Anyway, both +5 leads are at +12 and the +12 is 0 with the power board not connected. When I connect the board, both +5 leads cycle between +5 and +12. The +12 lead is still 0.

I have already changed the small cap (47uf 50v) and it still does the same thing.

[selldoor]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

Anyway, both +5 leads are at +12

Are you sure about that - if its just a typo go into edit and change it then
I will change this

[bammbammfran]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

check your 12 volts rectifier diode for shorts
it is the one of the two on the one heatsink and it is the farthest from the power plug that goes to the main board.
check center pin to outer pins
mbr10150ct


PS: the 12 volts does not make it to the 12volt connector
you need to check your 12 volts at the J202 jumper that is next to the plug

[bammbammfran]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

you can also check for 12v at the 2 jumpers at J303 and J304
if there is no short at the 12v rectifier pins and the 12 at these jumpers is messed up, disconnect both of these jumpers at the side of the group of capacitors and try testing again. these jumpers feed the inverter side of the board and if once removed, your 5 volts is back to closer to normal, then you may have a shorted 4502C transistor on the inverter side.

[keb]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

ok, I am officially angry ... it has been a while since I have used my meter and when I switched it over to ohms, I noticed that it wouldn't zero ... great, dead battery ... or so I thought ... when I went to replace it, what do you suppose I found? it seems that someone (I have a sneaking suspicion who!) apparently used it on a live circuit and completely toasted the board .. and while I am certain the +5 is indeed +12 (I checked other known voltages to be sure) I am dead in the water until I can either fix this meter or bring one from home ... and getting outside tools into the institution is a real pain and can take days to get approval, if not weeks!

[bammbammfran]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

NO POINTY OBJECTS !!!

See if the warden has one !!

[keb]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

J303/J304 are ground .. voltage between J303/J304 and +5 is +12 when disconnected and cycled +5 to +12 when connected.

While I was checking, I tested a few other locations. These are the results:

GND to J305/J306: +30 / +32 to +22
GND to COM (D201): +30 / +32 to +22
GND to COM (D202): +12 / +12 to +5

I did notice an interesting phenomenon, on D202, when the voltage switches between +12 and +5, there is a steady discharge to about +3.5 before it jumps back up to +12. The complete cycle takes about half a second.

[keb]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

Additional info:

With the jumpers removed the voltage at +5 stabilized at +12 and did not cycle. The voltage on D201 and D202 common also stabilized at +32 and +12 respectively.

[bammbammfran]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

oop's
I meant J305 and J306 are the 12 volt pins to the inverter

the other ones are ground and may or may not completely isolate a short on the inverter side
so remove the other two
Sorry

[keb]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

no problem, with all 4 jumpers removed, voltage on +5 is +12 and D201/D202 common voltages remain unchanged at +32/+12

[bammbammfran]

Re: Troubleshooting Gateway FPD1965 - Blinking LCD and Blinking power button

did you recheck your tester using a battery ?
and
What are you using as a ground pin for testing ?
I would use the large aluminum heatsink if you are doing testing from the top
and if testing from the bottom use the heatsink tabs or the negative side of one of the capacitors on that corner of the board