Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

[quattro alex]

I am in need of a little assistance please. This samsung would occasionally start up but sometimes would take 20 mins or so.

Looking at the power board revealed multiple capxon caps that were bulged and even the small caps had very high ESR. So I replaced all the caps with Panny FC/ FM's thinking this would sort it but unfortunately not. I did a bit more checking and found that the main ceramic resistors were open so I replaced these but still no joy, further checking revealed no secondary voltage at TM801s. I then decided to look at the board P/N BN96-03058A rev1.2 and found a working replacement same P/N but later revision 1.5. A few improvements to note straight away were extra ground wire from the primary cap, increased capacitance up from 1000Uf to 1500uf on the 24v side and an extra rectifier.

With the replacement board in I am now getting the 5v, 13v and 24v rails. Unfortunately I am not getting the backlight coming on. I then checked the caps on the inverter board and they were NKcon rubbish 100uf 35v that was out of spec. So I replaced these with Panny FM's and yesterday I was able to see the snowy screen and some menu options briefly with the aid of a torch for the first time but I can't even get that today. Now it may be that in trying to get into the service menu using side buttons (i don't have the remote) I have changed the source and can't get it back?

Here are the following voltages that I have so far:

CONNECTOR CNM 801 on power board

Power Off / On 4.35v
STD BY 6.01v
All GND'S 0.00v
All 12v's 12.06v
All 5.4V 5.39v
13v 12.83v

Connector CNM804

BL OFF/ON 5.18v
A/D 0.66v
P/L 4.91v
GND 0.00v
24v DET 5.12v

Connector CNM802
All 24v's 24.45v
All GND's 0.00v
B/L 5.18v
P/D 4.81v

Voltages taken from positive of Inverter caps
C121 17.4v
C120 17.4v
C119 0.00v
C118 0.00v
C117 0.00v
C116 0.00v
C115 0.48v
C114 0.48v

I have also noticed that the IC4001 on the main board is heating up to around 65c and the set powers down after around 20 mins.

Questions:

1. I don't hear any melody on start up but then again I am not sure if there should be one?

2. I had read somewhere that in swapping power boards you may need to enter service menu to adjust backlight settings, would this be true in this scenario?

3. I am trying to figure out if this is now a main board issue or an inverter board issue still, what can I do next to narrow it down?

As usual any advice is greatly appreciated as it's been a good 2 years since my son was born and a lot of the knowledge I gained here has been lost with 'baby brain'

[ReeceyBurger123]

Re: Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

Have you checked the little Smd fuse on the inverter ? These have a habit of randomly blowing.

[Kilroys]

Re: Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

Interruption of one of the transformers.
Measure the outer legs.
Should be about 1 to 1.5 Kohm,depending on which type it is

[quattro alex]

Re: Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

Have you checked the little Smd fuse on the inverter ? These have a habit of randomly blowing.

I've just taken the inverter board out of circuit and it appears all of the SMD fuses (marked with a '3') are open.

As a temporary measure for testing only could I use a 3.15a quick blow glass fuse?

[quattro alex]

Re: Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

Interruption of one of the transformers.
Measure the outer legs.
Should be about 1 to 1.5 Kohm,depending on which type it is

Just checked and they are all between 0.950 and 0.965 k ohm, so I am assuming that's ok?

[ReeceyBurger123]

Re: Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

Yes you could, before you do have you checked the mosfets that drive the transformers for S/C ?

[budm]

Re: Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

You probably have some shorted out Power MOSFETs on that inverter board.

[quattro alex]

Re: Samsung LE27S73BD - Power board replaced, Inverter caps replaced - No backlight

Yes you could, before you do have you checked the mosfets that drive the transformers for S/C ?

Good point, I am assuming you mean the FDS8958A mosfets? The only one which is significantly different is the bottom one 'U2'. If I put negative on pin 1 and check Pins 5-8 they are in the 5.5ohm range compared to 2-11 Meg Ohm for the others. Is there anything else ( I see R26 is a 1ohm SMD resistor) that would explain that apart from a short?