Badcaps Forums

Badcaps Forums (
-   Troubleshooting Computer Displays (
-   -   Philips 234e problem (

dicky96 07-30-2019 10:23 AM

Philips 234e problem - flickering screen
Hi guys, I have this Philips 234e that I picked up at a car boot sale

There was no PSU with it but looking around for replacement power supplies I found this is a 19V monitor

I have it connected to my bench PSU, it powers up, the white LED on the front lights up, then the screen flickers/flashes (not evenly) for a couple of seconds, the white LEd goes out and it shuts back down - only to repeat the process.

Stupid question first - how do I actually open this thing up? There are only two screws on the back near the input sockets. I removed those. I assume it has some sort of hidden clips somewhere that hold it together.

There is a little oblong slot on the back with a padlock icon by it

Normally you can find tear down videos for this sort of thing, but I had no joy with this one. I'm hoping someone else has worked on these.


R_J 07-30-2019 01:26 PM

Re: Philips 234e problem
The padlock icon is for securing the monitor with a kensington security cable, nothing to do with opening the case, I would look for a seam in the case and see if it will seperate.
attach some pictures of the monitor
This video might help?

dicky96 07-31-2019 06:25 AM

Re: Philips 234e problem
2 Attachment(s)
OK thanks
I'd already found that video but it misses the first part of the dismantling process but anyway I've got it opened

The main circuit board (which I guess also has a power supply integrated on it) is making a funny buzzing noise when the monitor tries to power up

sort of bzz-bzz-bzz-bzzz.......................bzz-bzz-bzz-bzzz.............

I disconnected the ribbon to the backlight but that made no difference

When i disconnect the LVDS cable the problem goes away and the monitor draws a 80mA milliamps. If I then reconnect the back lights it's drawing a steady 0.65A

So it is acting like there is a short on the Panel PCB. But I can't see an obvious short. I checked across all the larger capacitors, and from what look like buck regulator coils to ground. No shorts there.

I have what I assume is a Voltage regulator IC, SM4043 but I can't find a datasheet. I can see on my scope the buck regulator(s) are trying to start up when I get those buzzing noises, but can't.

Definitely a datasheet for SM4043 would help. Then I could try power the panel PCB from my bench PSU.


dicky96 07-31-2019 06:35 AM

Re: Philips 234e problem
duplicate post

R_J 07-31-2019 08:35 AM

Re: Philips 234e problem
1 Attachment(s)
From another service manual it has this description but no schematic:
R/TP, 48
The manufacturer seems to be Silicon Mitus. I found a datasheet for a SM4028 but it does not have a pinout, so It might be difficult to find one for the SM4043, it might be custom?

dicky96 07-31-2019 11:24 AM

Re: Philips 234e problem
Yes this is being rather elusive. Aliexpress have a number of vendors who have SM4043. None I have seen link to a datasheet

I think i will have to apply the 5V and 3.3V from bench PSU to the panel edge PCB and see if the the SM4043 runs. Then I can see what sort of current it draws if it does. As I said I can't find any shorts on the panel PCB. I also tested for shorts from all the test points on the panel PCB to ground.

Is it possible to have a screen panel that basically acts like a short circuit load when under power but shows nothing obvious on a multimeter?

Maybe the problem is with the regulator on the main board which converts the input 19V to 5V/3V for the Panel PCB, if I can determine the SM4034 works with a bench supply, I can look their instead.

R_J 07-31-2019 11:49 AM

Re: Philips 234e problem
You could try loading the 5v and 3v with resistors to see if can supply any current. that might be less dangerous. It could be you bench supply can't supply enough current when the monitor first trys to start up. Is the 19v dropping when the monitor trys to turn on?

dicky96 07-31-2019 11:57 AM

Re: Philips 234e problem
Yeah I also thought that, I was hoping I could measure current with bench psu supplying 3.3v+5v to panel pcb to try determine a suitable resistive load........

dicky96 07-31-2019 11:58 AM

Re: Philips 234e problem
But I guess if I load 5V / 3V to a few hundred milliamps that would be a start. I've never looked how many watts a panel actually consumes

No the 19V is not dropping that I can see. The constant current LED does not light when the monitor trips. It's a dual 0-30V 0-3A linear psu but to be sure tomorrow I'll put it into parallel mode :-)

It's this one but different brand name. I also have another 0-30V 0-5A that I can try.

dicky96 08-01-2019 06:02 AM

Re: Philips 234e problem
2 Attachment(s)
Hmm the results were quite interesting

I connected a 5R6 resistor to the 5V line with the panel PCB disconnected but only got a few extra mA flowing when I expected about 900mA into that load. When I checked I only had about 0.35V on the 5V line. It didn't 'chirp' though

I then put other resistors in series with the 5r6 and tested again:

5R6 = 0.35V
10R3 = 2.29V
15R6 = 3.20V
35R6 = 4.2V
105R6 = 4.78V
275R6 = 4.99V

So the 5V line can not supply much current. I didn't bother look at the 3V3 line

Looking at the main board, 5V originates directly from what I assume is a FET marked GA4X29, pins 5,6,7,8 (Q492). This had some horrible looking corrosion around it so I cleaned that off but it makes no difference.

Assuming GA4X29 is a FET then It is not clear where the gate of the FET (pin 4) goes but pins 1,2,3 (source? Drain?) seems to be connected to pin 5 of the nearby APW7089 step down converter (U705). and the largish inductor marked .220.

That does not seem to make a lot of sense looking at the typical application circuit on page 2 of the datasheet

It would also seem that the 5V line should be able to output more than a few mA but it is not obvious what is wrong with it.

Why did you say it could be dangerous to drive the 5V input on the panel edge PCB directly?

I can get a replacement working main board for this monitor for 30 euros. As I only paid a few euros for it and wanted to keep it for myself that wouldn't be a bad option, and it would still be a good purchase... but it would be nice to fix this one if reasonably possible.


dicky96 08-01-2019 07:47 AM

Re: Philips 234e problem
Actually come to think about it, Q492 is probably just acting as DC switch to feed the 5V rail to the panel edge PCB, controlled by the microprocessor, so it's not connected to U705 pin 5 directly but via the inductor.

I need to check that is switching on properly, but that would not explain the chirping sounds

R_J 08-01-2019 09:22 AM

Re: Philips 234e problem
1 Attachment(s)
Q492 is a AO4411 fet. I would think the +5volt line should supply a at least 1 amp and likely more like 2~3 amps
If there was corrosion on Q492, I suspect there is corrosion elsewhere and maybe a corroded trace or feedthru.

What does'nt make sense about the U705 circuit? Pin 5 (LX) is the output which connects to a coil and then to Q492 fet, Q492 is likely a switch that turns off part of the circuit during standby.
What about the input voltage to U705, does it com straight from the 19v input? Check the resistors around the ic, maybe they are corroded as well and changed value

R_J 08-01-2019 11:59 AM

Re: Philips 234e problem
It looks like there is still some corrosion on U402 & U403, Check that you have voltage on U705 enable pin
While load testing the 5 volt line, did the voltage drop on both sides of Q492?

dicky96 08-01-2019 12:40 PM

Re: Philips 234e problem

Originally Posted by R_J (Post 908042)

What does'nt make sense about the U705 circuit? Pin 5 (LX) is the output which connects to a coil and then to Q492 fet, Q492 is likely a switch that turns off part of the circuit during standby.

What didn't make sense is I thought source or drain of Q492 was directly connected to U705 Pin 5 LX..... until I was driving home from the workshop and it suddenly occurred to me that Q492 was probably a switch (like you also think) and was not connected directly to U705 but via the inductor instead. Then it did make sense :) Hence post +11

I'll check the other voltages like you suggest. Thanks for your continued help


R_J 08-01-2019 12:58 PM

Re: Philips 234e problem
I missed that in #11, A chirping sound usually means there is a short and the power supply is trying to start but shuts down for protection, then retrys.

dicky96 08-01-2019 03:49 PM

Re: Philips 234e problem

Originally Posted by R_J (Post 908068)
I missed that in #11, A chirping sound usually means there is a short and the power supply is trying to start but shuts down for protection, then retrys.

Which is why I am wondering if the panel itself can somehow act as a short when powered even though I can't see a short on the panel PCB with my meter.

That is something I don't have the experience yet to know - but may well learn from this repair :o:

R_J 08-01-2019 04:13 PM

Re: Philips 234e problem
You said if you disconnected the flat white cable between the t-con board and the main board and the buzzing noise stoped, Did the backlights come on? they should have. There are a lot of components on that t-con board so something could be bad, but there is a 2.5 amp fuse (F1) on that board, I would think if the fault was on that board the fuse would have blown. I suspect you have a problem with the +5 volts not being able to supply any current.

dicky96 08-02-2019 06:44 AM

Re: Philips 234e problem
Hi R_J
OK we can eliminate some of this.... things are not always as they seem lol :tongue:

The reason I couldn't get the 5V line to give any current ended up to be because one of my crocodile clip test leads was faulty acting as a high resistance and I just happened to have my meter probe on that end of the lead!

Measuring the 5V direct on the PCB at the LDVS connector and with good test leads, the PSU quite happily dumps about an amp into a 5.6 ohm power resistor which gets nicely warm just to confirm the fact.

So that was one red herring :whistle:

With the LVDS cable disconnected the backlight is working

With the LVDS connected when it chirps the backlight flashes on and off

Without the backlight connected it still chirps when LVDS is connected

So it is acting like there is a short on the panel edge PCB but I just cant find one on my meter (checking Caps and test points to ground - again!)

I can see that when I get a burst of the chirping sound, I get 3.3V on the panel edge PCB. This seems to be generated on the edge PCB because the 3.3V test point does not connect directly to the LVDS ribbon.

I also get 4.7V on the larger inductor L1 for a few seconds

As for the two smaller inductors near the SM4043 (L2,L3), L3 goes to about 1.5V and I can see the other one goes to 7V or more sometimes, but can go to 3.4V or less - it varies each time, maybe depending on the length of the burst

With my scope I can see a similar thing with each inductor getting a burst of high frequency drive. This suggests the SM4043 is at least trying to drive all the buck inductors.

I think i have two options now.

1. Supply 5V to the panel edge PCB from my bench supply and see what happens/gets warm - but you suggested that may not be safe though you didn't elaborate further.

2. Remove L1 L2 and L3 in turn to see if the 5V will stay present with either one removed, hopefully to get more clue as to where the problem is


dicky96 08-06-2019 04:24 AM

Re: Philips 234e problem
1 Attachment(s)
I took a guess that the supply generated by L2 has a problem, so I unsoldered L2.

I was right. With the LVDS cable attached I now get backlights, I have 5V on L1 and 1.8V on L3, and no more chirping.

L2 has 12V on one pad and 0V on the other. The pad with 12V connects to one end of R86 and pin 40 of SM4043. The 0V pad reads open circuit to ground on both ohm and diode test mode (positive meter lead to pad). It reads like a diode junction with negative lead to pad.

So I've narrowed down the problem but I still can't find the fault. A datasheet for SM4043 would be really useful at this point.

Any suggestions? I could apply some voltage to the load pad of L2 I guess...


R_J 08-06-2019 09:05 AM

Re: Philips 234e problem
Whats the panel make and model, sometimes you can find some information that way. Or contact the chip maker
That circuit seems to be the VGH boost converter so it likely boosts the voltage to around +15 ~ +20 volts
Did you check the caps around the +12v boost circuit, The panel is supplied with +5 volts so the +12v must be from a boost converter somewhere

All times are GMT -6. The time now is 05:40 AM.

Powered by vBulletin ®
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.