Panasonic Viera TX-26LMD70 Powers off after a minute....

[reessi]

Panasonic Viera TX-26LMD70 Powers off after a minute....

Hi everyone,

When plugging in it powers on for around a minute or 2 depending how it feels, everything is working during that time, picture looks good. Then powers off with green led continuously flashing fast.

I have replaced the 3 filter caps on the PSU and the 2 filter caps on each Inverter board to no avail.

I was thinking could be tired backlight CFL's? But unplugging the signal cable to the Inverter and the backlight will stay on fine (I had to jump the MOSFET that turns on power to the inverter on the PSU as the set will not turn it on with the signal cable removed from inverter.

Any thoughts? I have schematic for the PSU, I think this is working since all it does is output 24V.

When plugged in the 24v for the main boards is always on, the inverter power turns back off when the set turns off but I jumpered the MOSfet so is always on to inverter, makes no difference.

On the PSu to next small power board along the top (see pic), there is 3 wires apart from 24v:

GND,
24V,

24v Det - 2.96v when on leads to missing components on PSU (missing from factory).

Sub_on - 3.22v when on then pulses on/off when set switchs off but strangly the PSU keeps the 24v on unless I remove the whole connector.

Inv_On - 3.22v is on when set is on and off after it goes off.

I have schematic for the PSU board, I have schematic for another chassis that contains the MAIN 2 boards called DG board but not service manual for whole TV and not include second Power board along the top.

The inverter has following data lines:

Err out on - I dont understand it - 3.15v when on, is generated from Main board, dis-connecting and backlight does not come on, does it mean error???

ext_pwm - 3.13v, what is this?

on/off - 3.24v - I assume from mainboard to turn on inverter but disconnecting whole cable and inverter is on all the time!

VBR-a - 3.14v - is it variable brightness?

Many thanks.

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

Actually the green flashing is not continuous its like 8 or 10 very fast blinks too fast to count and then a brief pause. Leaving it on and occasionally it will try to fire up again and backlight briefly come on. Needs unplugging for a few minutes to work again presumably when the caps have dis-charged.

[CapLeaker]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

to me it sounds like you have a problem with the inverter board. You can try to turn down the backlights as far as they go and see if your TV stays on longer.

Also the problem could be with one of the transformers that go bad, bad solder joints or bad HV cap (blue) on the inverter board.

You can try to compare the output winding of each transformer with your DMM in Ohm setting. They have to be within 3%.

A bad transformer will run hotter than good one. Watch for discolouring of the PCB underneath the transformers.

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

Thanks, I will do some more testing. I was trying to find a way for the tv to stay on without the backlight to see if it is a problem with backlight but also to dupe the tv since the backlight works fine on its own without being connected to the main board if I remove all the signal cables, it will stay on all day!

So does not that infer the inverter is working correctly? Or will it stay on but still report that the bulbs are out of spec?

Which signal is going back to the mainboard saying all is ok?

Cheers

[CapLeaker]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

sounds like you are losing the BLon from the main board. PSon and BLon are generated on the main board.

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

Hello again, after a year I decided to look at this again now we all have infinite time available..

After some more testing I have found that if I disconnect the power to the left inverter (Main one connected to mainboard) then the set boots up all be it without a backlight but no SOS 1 Blink REd any more.

I checked the ohm readings across the transformers as a previous suggested, both boards read 0.4 ohms on the input and output windings. I do not know if this is correct.

If I disconnect one tube from one board then it will stay on for a few seconds and vice versa with left side. If they are both connected they just flash for a second.

I can get them to stay as mentioned earlier in the thread but not with the TV booting.

I cannot understand the signals. Anyone suggest if I can dupe the tv to staying on? It will only boot without power to the inverter so no backlight or I can have backlight by dis-connecting the data wires to the TV but then the TV errors out.

I have tried alll combinations of the data lines. I thought maybe I can do something with the ERR out line, but what signal does it expect?

Cheers.

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

also i used a torch and turned brightness down to minimum but it did not help. is that affecting the backlight though or just the image sent to the lcd?

I read online the backlight cannot be controlled even in service menu as these tvs alter brightness to simulate better contrast ratio!

if its the bulbs are tired can there be a mod on the inverter board to not check this, how is it checking? by measuring current?

[dick_barton]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

.

[dick_barton]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

Err out on - I dont understand it - 3.15v when on, is generated from Main board, dis-connecting and backlight does not come on, does it mean error???

I would think that Error_Out is a signal to the mainboard signaling an error with the inverter which then switches off the inverters.
When you power up with everything connected and the backlights on, what is the voltage at Error_Out and also when it fails and the backlight go off?
Also what happens to the voltage at ON/Off when you initially switch on and the backlights come on and then when the backlights switch off

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

Hi, sorry for the delay.

I am totally baffled by the Error out signal. I have 2 schematics using these boards. On one it shows the direction of signals so error out from invertor to mainboard but when testing it does not make sense:

When connected the error out goes to 2.78v when the inverter is turned on and then to 0v when it turns off.

So I dis-connected just that wire and the invertor gives no output at all. But the mainboard is producing the 2.78v? But why, it clearly states on the schematic the signal should come from the invertor. Can it be that the invertor pulls the 2.78v low to ground to tell the mainboard there is a problem? But if so then leaving it dis-connected would have solved that?? Even when dis-connected the tv turns off with invertor SOS. How can it know there is a problem if nothing comes from the invertor?



Cheers.

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

The on/off goes to 3.15v when turned on! When disconnected there is a voltage coming from the invertor which again is confusing to me. When dis-connected there is 1.8v on the invertor side! If you short that to ground the invertor turns on so seams like it turns on if pulled high or low from 1.8v... It only has to be done momentarily and then it stays on or goes off, just changes state.

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

So I can choose to have tv boot but no invertor or invertor on but tv not boot. The tv boots if 24v is removed from invertor but data cable is left connected.

[reessi]

Re: Panasonic Viera TX-26LMD70 Powers off after a minute....

A new problem has surfaced just yesterday, sometimes when I plug in tv it flashes the green led to start booting then can hear tiny pulse noise from power supply and it turns off and then starts booting again! Not sure if related or new? It behaves as before once booting commences. It does not do it everytime and never did it last year when I was testing many times.