AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

[Agent24]

I have here an AOC LM726 / TFT1780PSA+

When the picture comes up it seems fine (although this only happens for less than a second)

Symptoms:

At plug in, with no signal input I get:

Green LED for a few seconds
Changes to Orange LED
Screen/Backlight flashes/Speakers click
LED goes off
Repeat.


With Signal:

Green light comes on
Picture flash on screen and light goes off
Repeat


My first thought was that it's a PSU problem but since I have never tried to repair an LCD monitor I'm unsure if this behavior points to a fault in the backlight/inverter or not.

I opened it up and found 2 bloated Hermei on the PSU/inverter board. 470 uF and 1000 uF. There are actually 2x each of these caps but only 1 of each was bloated.

I replaced both 470 uF as that was all I had, but I noticed no change in the fault.

I have attached some photos

Can I unplug the backlights and power on to see if they are a problem? (Blue and pink wires on left side) Or will this cause damage to the inverter circuit?

[PlainBill]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

I have here an AOC LM726 / TFT1780PSA+

When the picture comes up it seems fine (although this only happens for less than a second)

Symptoms:

At plug in, with no signal input I get:

Green LED for a few seconds
Changes to Orange LED
Screen/Backlight flashes/Speakers click
LED goes off
Repeat.


With Signal:

Green light comes on
Picture flash on screen and light goes off
Repeat


My first thought was that it's a PSU problem but since I have never tried to repair an LCD monitor I'm unsure if this behavior points to a fault in the backlight/inverter or not.

I opened it up and found 2 bloated Hermei on the PSU/inverter board. 470 uF and 1000 uF. There are actually 2x each of these caps but only 1 of each was bloated.

I replaced both 470 uF as that was all I had, but I noticed no change in the fault.

I have attached some photos

Can I unplug the backlights and power on to see if they are a problem? (Blue and pink wires on left side) Or will this cause damage to the inverter circuit?

It's not recommended, and is unnecessary. Hermei is a known bad brand of cap. The 470 uF are probably for the 12V (inverter) supply, the 1000 for the logic board.

Remember, a bulging cap is always bad; one that isn't bulging isn't necessarily good. I STRONGLY suggest you replace all of the Hermei caps (except the large 450 volt cap) with new Panasonic FCs. I recommend buying from Digi-Key, you'll get fresh product and quick shipping. The whole set of caps should cost you about $10.00, including shipping.

PlainBill

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Why is it (unplugging the CCFL) unnecessary?

After reading some other posts, I would have thought that a faulty lamp drawing excess current could be tripping protection circuitry? (or is this not possible?)

If unplugged, I would assume the monitor may start but without backlights


I know Hermei are rubbish and this board also uses Elite and maybe some other junk too (didn't look too closely)


Do you really think even just 2 bad caps could cause this kind of problem? (I think it's the way they're positioned - right above the heatsink for the rectifiers, with the monitor standing up heat would rise right up onto them)

[Bobdee]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Why is it (unplugging the CCFL) unnecessary?

After reading some other posts, I would have thought that a faulty lamp drawing excess current could be tripping protection circuitry? (or is this not possible?)

If unplugged, I would assume the monitor may start but without backlights


I know Hermei are rubbish and this board also uses Elite and maybe some other junk too (didn't look too closely)


Do you really think even just 2 bad caps could cause this kind of problem? (I think it's the way they're positioned - right above the heatsink for the rectifiers, with the monitor standing up heat would rise right up onto them)

yes it will shut down if it draws excess current, but the reverse applies as well
It will shut down if too little is drawn
disconnect 1 or 4 tubes it shuts down
take Bills advice those caps are your problem, they are bad and no doubt the problem

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

I'm compiling an order for various capacitors to send off to RS. Since they have a minimum order of 5 of any type of capacitors, I'd rather do a whole lot at once. Plus then I only have to pay for shipping once.

I'll post back when I've done that and see what happens...

[EGuevarae]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

I'm compiling an order for various capacitors to send off to RS. Since they have a minimum order of 5 of any type of capacitors, I'd rather do a whole lot at once. Plus then I only have to pay for shipping once.

I'll post back when I've done that and see what happens...

RS?
Radioshack?

Man, go to DigiKey instead.

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Not Radioshack, RS Components. http://newzealand.rs-online.com/web/

Also known as Radio Spares, Radionics and Allied Electronics (in USA)

Pretty much the only place to get decent brand (Panasonic/Rubycon etc) capacitors here in New Zealand.

Jaycar might still do Samxon but per capacitor the price is higher, so if I do a bulk order (because of the 5 piece minimum qty) from RS it's cheaper

[EGuevarae]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Not Radioshack, RS Components. http://newzealand.rs-online.com/web/

Also known as Radio Spares, Radionics and Allied Electronics (in USA)

Pretty much the only place to get decent brand (Panasonic/Rubycon etc) capacitors here in New Zealand.

Jaycar might still do Samxon but per capacitor the price is higher, so if I do a bulk order (because of the 5 piece minimum qty) from RS it's cheaper

Ohh. Sorry, but I would have hated me until the end of the time if it was RadioShack and I didn't warn you... it was in the better of the intentions.

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

No worries, I appreciate your concern. I know I wouldn't want me to buy from RadioShack either

My fault really I guess, though, since I know most people here say RS when they mean RadioShack

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

I decided to do some more tests... (note: this is with the bad capacitors still in there)

I tried all the backlights with my case mod inverter and they all seemed to be OK

I tried disconnecting the audio board but this didn't change anything

Then I tried disconnecting the signal cable that goes to the LCD panel and noticed this:

Power on -> Light goes green
Then light goes orange
Then screen comes on bright white (but for longer than before, maybe 4 seconds)
Then turns off BUT orange light STAYS ON and there is no repeat cycling of power etc.

Plug that connector back in and then the same problem as before.

What can this mean?

[EGuevarae]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

I decided to do some more tests... (note: this is with the bad capacitors still in there)

I tried all the backlights with my case mod inverter and they all seemed to be OK

I tried disconnecting the audio board but this didn't change anything

Then I tried disconnecting the signal cable that goes to the LCD panel and noticed this:

Power on -> Light goes green
Then light goes orange
Then screen comes on bright white (but for longer than before, maybe 4 seconds)
Then turns off BUT orange light STAYS ON and there is no repeat cycling of power etc.

Plug that connector back in and then the same problem as before.

What can this mean?

With caps like the ones installed, it can't be told. That is the first step to rule out a PSU failure. You just reduced the load the PSU is under (you removed the +5v load that goes from Logic board - which in turn draws from PSU- to the pane itself). Good thing is that the lamps are good (or so it seems).

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

I know it's been a long time... 4 months! but I bring an update: eventually got around to doing this today...

So, I replaced everything on the PSU/Inverter board (except the main filter capacitor) with a mixture of Panasonic and Rubycon

Fired it up and it worked first time! Still going even several hours later too...


Attached are some photos for your viewing pleasure

(Pity the design sucks though, with the rectifier heatsink directly underneath the capacitors )

But then the lamps will probably die before the new capacitors do

[Wizard]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Always replace all if you cannot check with ESR meter especially on generic monitors (even it is HP, acer etc) due to monitor made by some one else.

Cheers, Wizard

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Do you think I should replace that 400volt capacitor too?

It IS a Hermei but I thought these ones usually didn't need it - that and the fact they cost $15 from RS with a minimum qty of 5 put me right off...

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

I have another question, do you think it's OK to replace the Hermei 150uF 25v LZ with this:

http://newzealand.rs-online.com/web/...duct&R=3150704

The thing is the Hermei apparently can take 700mA ripple while the Panasonic only 555mA...

[Bobdee]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Do you think I should replace that 400volt capacitor too?

It IS a Hermei but I thought these ones usually didn't need it - that and the fact they cost $15 from RS with a minimum qty of 5 put me right off...

I have never replaced a big filter cap ever, it has always been accepted and
recomended on this forum to leave out the big cap as they very rarely fail, only recently in the the last 6-8 wks have one or two members recomended replacing it, At the cost you got to pay is it worth it, you will know what to fix if it Ever failed (no doubt now I have started something)

[PCBONEZ]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

That's right. - Those caps hardly ever go bad.
When they do there's usually a power surge involved or some other AC power problem.

Sometimes I replace them anyway just to have done every cap.
Sometimes I replace them to get a 105c rated cap in there if the unit looks like it might have heat/cooling issues.
Most of the time I just leave them alone.
.

[Agent24]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

What do you think about replacing the hermei 700mA with panasonic 555mA though?

I should probably get some better ones shouldn't I?

[PCBONEZ]

Re: AOC LM726 Power cycling but picture seems OK (when it's displayed) + 2 bulging Hermei

Yes. Get better caps.