Hitachi CML155XW blank screen

[thebigdino]

Hi all,

I hope you could help me out. I got a few years old Hitachi CML155XWV LCD monitor 6 months ago. It was working fine until one day it stopped working. Whenever I plug in the 12Vdc 4A power supply (which I have plugged into another monitor and works) the green monitor power status indicator and the backlight flashes for about 1/2 a second and then everything blanks out (I shined a torch into the LCD screen. There was nothing showing).

I managed to find a repair manual for this model online and followed the instruction and replaced a smd fuse F002, I0012 p channel enhancement mode mosfet and D020 5.6v zener diode in the power regulator section. I went through the troubleshooting flow charts again and all the voltages were shown as correct (i.e. they were all within +- 0.1v of the suggested readings, although 5.15V for the regulated supply VCC for the video decoder chip).


It still wasn't working, so I checked all the capacitors for bulging etc. No bulging were apparent anywhere. I checked all the fuses on the boards and they were all fine. I unplugged the inverter board, no LED or LCD display. This is the point I've gotten stuck!

I have attached pictures of all the PCBs, so if you wouldn't mind please help me out! Thanks!

[alexanna]

Re: Hitachi CML155XW blank screen

Try the monitor without the inverter plugged in,dose the front power switch and PLED work normally?

[thebigdino]

Re: Hitachi CML155XW blank screen

Yep unplugged inverter board. same symptoms - Pled flashed green once, nothing from the LCD when torch is shone on it.

[alexanna]

Re: Hitachi CML155XW blank screen

If you would, post a link to the schematics.
If all of the fuses and voltages going to the signal board are correct usually the next thing to check would be the LDO volt regulators, it looks like there is a bunch of them. Look for an IC with 1117 stamped on it with a 1.8-2.5 or 3.3 to indicate the expected output.

[thebigdino]

Re: Hitachi CML155XW blank screen

Yep - I did that already. The outputs of the LDO voltage regulators on the left hand side of the video decoder chip were all either 3.27V or 2.5V stable - I did that as part of the troubleshooting flow chart.

http://dl.dropbox.com/u/15011404/hitachicml155xw.pdf is the link to the CML155XWV service manual. The schematic and the troubleshooting flow chart is within...

(sorry i couldn't attach it - the uploader kept failing!)

Thanks a lot so far btw.

[PlainBill]

Re: Hitachi CML155XW blank screen

The pictures are excellent,and I have a couple of ideas and two points I would like clarified. This is an older design with at least two, possibly three separate processors, at least one of which produces a significant amount of heat.

First the points that aren't clear.

1. Were the parts you replaced defective, (as in a reading was wrong), or were they suggestions in the troubleshooting chart? Did they result in any change in behavior?

2. When you apply power, the power led flashes. Does it respond to pressing the power button?

As far as the suggestions, it's early and I haven't looked over the schematic yet, so these suggestions will only be of a general nature.

The socketed IC upper left handles the control functions. It might be worthwhile removing it from the socket and reinserting to clean the contact areas.

When pushing the front panel buttons, do the voltages on the lines from the front panel change?

I had the 'repair job from hell' recently. The cause was the 'power brick', specifically the startup cap. The strange thing was the power brick would work perfectly at lower loads.

The graphics processor (I011) is in a BGA package. These are particularly prone to bad solder joints.

While it is premature to order one, have you checked for availability of signal cards?

PlainBill

[thebigdino]

Re: Hitachi CML155XW blank screen

Yes you're right - I think it's a model from 2002 so it's quite old. I will answer your first 2 questions first:

1. yes - I worked through the troubleshooting chart and initially found the smd 1608 5A fuse to the 12V to 5V converter had an open circuit so I replaced it. I also replaced the mosfet and the zener diode just in case. I didn't do a reading on the mosfet but I think I remember the zener diode breakdown voltage reading looked a bit dodgy so replaced it just in case the incident that caused the fuse to blow blew them as well.

The symptoms before and after the component changes was the same - i.e. green pled blinks once and everything goes blank. The reading at the zener diode is 4.97V (connected to the output of the 12/5 - DC/DC converter)

2. Heh - yes sorry forgot to mention that part. No, pressing the power button does not make anything respond. I have probed it - the switch works, has a good connection to the uController holder, and the line in for the switch drops to 0v when the switch is pressed.

Point to note though - when the power is plugged in all three major ICs (the ucontroller, the video decoder and the graphics processor chips are all hot after a minute or two. I probed the temperature to be around 37 degrees. I don't know if this indicates anything?

I think I might do the following - reflow the BGA using a temp controlled heat gun, take out and clean the ucontroller, and try find another power brick again! I said the power brick worked but I did plug it into a newer monitor which might have been a smaller load. who knows!

And no - I haven't managed to find a replacement signal board yet..I haven't really done this whole repairing lcd monitors thing before so I don't know where to look..:p Do you have any suggestions?

thanks a lot!

[thebigdino]

Re: Hitachi CML155XW blank screen

nope - no luck! cleaned the contacts on the uController and changed to an ATX power supply - still doesn't work!

[thebigdino]

Re: Hitachi CML155XW blank screen

reflowed the graphics processor...still no luck!

I did notice that when plugged in, all the I/O pins on the controller (top left) were high...does that mean the IC has been wiped?

any ideas?