Philips 170B LCD - OK for a few minutes then image starts going on and off

[Agent24]

Here's a real interesting one... a Philips 170B7CB/75 that I was told (hmm..) had fallen off a desk.

It works OK from cold, but after a little while (1-2) minutes the screen goes blank, then a few seconds later, the image comes back. Then the image goes on and off at random intervals of a few seconds.

PSU capacitors are Hermei and Elite, BUT they test perfectly fine for ESR\Capacitance out of circuit.
Logic board capacitors are UCC KY which I did not test.


Some detailed observations:

1) The backlights shut off but this seems because the inverter gets a shutdown command from logic board (backlight enable signal goes from 3v to 0v)
2) The power LED sometimes stays on, sometimes goes off.
3) Sometimes when the backlights come back on, the image is gone and it displays a white screen instead.
4) After some time it seems the CPU gets confused. Buttons lock themselves, OSD changes location, image settings become corrupted. A reset (in the OSD menu) fixes this.
5) Powering one of the CCFLs from a separate inverter, it can be seen that during the backlight off time, the image disappears too, and the LCD panel switches sometimes between full black and seemingly totally powered off.

I thought one of the logic board regulators could be failing. But I can monitor the voltage on both (3.3v and 1.8v) regulators and they both stay stable as a rock the whole time. (4th meter showing 1.8v rail not in photos)

What I did notice is that while the screen is blank, the 5v rail dips slightly from 4.99v to 4.73v. The 12v rail which is about 12.3v during operation, rises to 14.2v during the fault condition.
(In soft-off mode the voltages are about halfway between those, 4.89v and 13.2v)


Anyone heard of anythng like this before? It's very strange.

[Agent24]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

Just thought of something else to try.

Disconnected the LVDS connector from the panel and got (as expected) a white screen. This also starts flickering on and off after a short time. So I guess that rules out the panel and T-CON as being faulty, at least.

[Agent24]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

It's not the audio amplifier, or the button board either. Same problem with those disconnected as well.

Service manual mentions checking the MCU crystal for a no image problem, and I figure if it had taken a tumble the crystal might have ended up cracked or such, so I had a look at that but the waveform stays unchanged throughout the fault so it doesn't seem to be an issue.

I'm thinking it's something heat related since from cold it works for longer before going insane. Maybe time to get some freezer spray...


Since I'm down to PSU or Logic board definitely, I will probably try an external PSU+inverter and see if that makes any difference. That should narrow it down to which board is bad.

Also I notice the logic board and schematic have an RS232 and a SPI 'Debug' connector (unpopulated). Might see if anything interesting comes out of either of those.

[selldoor]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

Good pictures of front and backs of the boards may help??
I have had similar problems with different monitors one was fixed by replacing the BIG cap
so perhaps you could monitor that.
Another was on one with a separate inverter board- I had "repaired " it some months earlier
replacing the two caps - worked fine for a quick test so put it aside. My son borrowed it and a few days later came back with "it keeps switching off and on"- sure enough it did.
Went right through the power supply testing and ended up one of my solder joints on one of the inverter caps was crapping out pulling the whole thing down.
So I would try reflowing all the inverter section.

Also as it has been dropped I would reflow the BIG transformer as its weight and the momentum could have cracked the solder?

[Agent24]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

I already checked the primary filter capacitor, ESR and capacitance was OK too.
Solder joints on the SMPS transformer look good.

I found that when unplugging the VGA signal cable to force the monitor to momentarily power off the panel\lights, the PSU voltages changed in the same way as during the fault. I think it's probably the load changing (panel and lights going off) that causes the voltages to change, not the other way around. Which makes me suspect the logic board.

Anyway, I'm currently running the power supply by itself with some test lamps for fun. I doubt this will prove much though.

[Agent24]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

Well, the PSU seemed fine running by itself with test lamps (as I expected, though no load on 5v rail, so not conclusive) so then I tried running the monitor from the 12v and 5v rails of an ATX PSU, and a backlight from my external inverter.

I still got the problem with that set up, so I think I can say for sure it's the logic board at fault here.

Also I managed to make it go crazy faster by putting my desk lamp over the board and heating it up, so I really do think it's a heat issue too.

If I can pinpoint the part.. it should be fixable. But not if it's the MCU. Even though I could get one, it's too expensive for such an old monitor and I don't have the firmware image anyway. But for anyone who wants the Novatek Easywriter ISP programming software, it's here. Good luck with figuring out the ISP board wiring!

[selldoor]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

Do you mean unplug the vga signal cable that comes from the PC? I didnt think that would cause the lights to go out? How about trying a different vga cable. Could it be anything to do with the 5v pin on the vga (pin9)

[Agent24]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

Do you mean unplug the vga signal cable that comes from the PC? I didnt think that would cause the lights to go out? How about trying a different vga cable. Could it be anything to do with the 5v pin on the vga (pin9)

Most monitors turn the backlights and panel off temporarily when the video cable is plugged in or unplugged, before turning them back on to display the image or the "No signal" message.

I did this just to see if the 5v rail voltage drop was being caused by the PSU itself or the panel switching off. At first I thought the voltage was dropping and causing a system reset, but I think it's just how the PSU responds to the drop in current draw.

A different cable and another computer were one of the first things I tried. No difference.

I don't think Pin 9 would be related. As far as I know, that just powers the EDID EEPROM so the computer can read the monitor information, even when it's switched off.

[selldoor]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

Most monitors turn the backlights and panel off temporarily when the video cable is plugged in or unplugged, before turning them back on to display the image or the "No signal" message.

Guess I have never noticed though I usually have the monitor off when plugging in/out

Thought you would have tried another cable and yes I thought the 5v was only for the edid and only on some monitors but think I did see somewhere where the
5v was somehow causing a problem , but just clutching at straws

[Agent24]

Re: Philips 170B LCD - OK for a few minutes then image starts going on and off

I'm sure now the MCU is bad.

Alternate heating with a small hot air nozzle and cooling of the IC makes the symptoms appear and disappear.

Since I have no firmware to load into a new chip, it looks like I lost the fight this time.
But I did come up with a clever idea - instead of freezer spray, I had the idea of putting a chunk of ice from the freezer in a zip-loc bag and using that instead. Works great, for large parts like a 128-Pin QFP...
Not very accurate for anything else though.