Toshiba 42AV555D not starting up

[notallbad]

TV was always left on standby overnight, so I'm told. One morning went to switch it on and nothing.

Power on and front panel led flashes 3 times, no life after that.

Disconnected main av cables and power up to get:

P811A - Pin 2 - 4.76v

P810A - Pin 9 - 3v

How do I check the other voltages on the PSU? Which pins do I jumper?
When the AV board is connected the 3v drops out and the 4.76v slowly drops out.

Suspect its most likely to be the main av board.

Main cap steady at 320v whether AV board connected or not.
Thanks

[notallbad]

Re: Toshiba 42AV555D not starting up

With main AV Board disconnected and a Resistor across pin 2 5v and pin 8 power tv gives:

Connector P811A
Pin 13 18v audio - 18v
Pin 11 24v - 24v
Pins 4,5 and 6 12v - 12v

Connector P810A
Pin 7 AC-Detect - 4v

But still lacking the other voltages, perhaps the same needs to be done with P810A pins 5 P-LCD and Pin 8 P-Sig?

[notallbad]

Re: Toshiba 42AV555D not starting up

Not entirely sure the led blinking is a correct indication of the fault as the power is cutting out before the blinking completes.

[ammarben]

Re: Toshiba 42AV555D not starting up

perhaps a short circuit or bad cap in the main AV Board

[Andrew F. Ali]

Re: Toshiba 42AV555D not starting up

Replace the Main Board.

[notallbad]

Re: Toshiba 42AV555D not starting up

Thanks, I have replaced the only electrolytic cap thats on the av board which read around 700uf out of cct (1000uf rated), but still no luck. I believe its unusual for the smd caps to go bad.

I would replace the whole av board but £45 is not cheap if its not the problem and the fault has not yet been established.

I would still like to test the psu whilst not connected to the av b/d to eliminate the psu being faulty.

[Andrew F. Ali]

Re: Toshiba 42AV555D not starting up

The Power Board is working according to your posts #1 and #2. I would suggest a Main Board. Try e-bay US....when I buy US, I pay a little less than $7.00 to $1.00 USD...so I know how expensive it might be....I don't know the conversion factor Pounds to USD..

[notallbad]

Re: Toshiba 42AV555D not starting up

Ok, so I get steady 5v on power up standalone.

Hook up a 4.8v 0.75a bulb across 5v and gnd but it dies a sudden death, lights briefly and dims out.

Hook up a 1.5k resistor between 5v and power tv and get steady 12v,24v and 5v with 375v on main filter cap. Then also connect the 4.8v bulb between 5v and gnd and again nothing.

If I use the 4.8v bulb across 5v and power tv instead of the resistor I get steady 5v,12v and 24v but the bulb does not light up.

Sorry to persist with this but just want to be certain the psu is ok.

[ammarben]

Re: Toshiba 42AV555D not starting up

try to trace why main bord is not sending PSON
check all voltage regulators

[ljintexas]

Re: Toshiba 42AV555D not starting up

No ps-on signal from main board.

[notallbad]

Re: Toshiba 42AV555D not starting up

But what I still don't understand is that with a load (i.e 4.8v bulb) on the 5v supply from the psu whilst standalone (i.e av b/d disconnected) and the psu dies.

Surely its a bit premature to be tracing the ps_on signal as the tv is not even staying on standby. Correct me if I am wrong but isn't the ps_on initiated by the remote control or front control panel 'power on' buttons?

[budm]

Re: Toshiba 42AV555D not starting up

What is Volt and Wattage rating of the lamp? That 5V STBY is only rated at about 0.25A, You should check the cold resistance of the lamp.

[notallbad]

Re: Toshiba 42AV555D not starting up

That would explain it, the bulb is rated at 4.8v 0.75a (so works out to be 3.6w) and the cold resistance measures under 1 ohm (0.8 - 0.3 lead resistance = 0.5, not allowing for dmm accuracy tolerance).

So assuming now that the psu (and 5v rail) is ok it would seem plausible that there is a high current drain from the av b/d. Not the easiest board to trace but shall try my best to identify possible culprits.

[budm]

Re: Toshiba 42AV555D not starting up

Post 1 "Disconnected main av cables and power up to get:

P811A - Pin 2 - 4.76v" To me that 5V is too low, you only getting 4.76V without the main board connected.
Can we see the straight shot picture of the whole power supply both top and bottom side?

[notallbad]

Re: Toshiba 42AV555D not starting up

Re-visiting the psu I checked the diodes to find one shorted, D841 - PR1007L (LT8728).
Dmm on diode setting revealed a low reading both ways so unsoldered one leg to confirm its the diode, gave a reading of 26 both ways, then resistance read 29ohms both ways.

What can this be replaced with?

Checked the resistor R841 which appears ok.

[budm]

Re: Toshiba 42AV555D not starting up

[notallbad]

Re: Toshiba 42AV555D not starting up

Here are the psu photos as requested, hope they are ok, hopefully not needed now.

[budm]

Re: Toshiba 42AV555D not starting up

That Diode is the snubber diode for that STBY SMPS IC.

[notallbad]

Re: Toshiba 42AV555D not starting up

That sounds very promising then.

So just need to find a suitable replacement.

[notallbad]

Re: Toshiba 42AV555D not starting up

Would a 1N4007 be suitable?

1000v
1A
700v rms
DO-41 case style
Forward Volt drop 1.0v
Junction capacitance 15pf

Forward Volt drop slightly lower and cap higher though!