LG L1710B - bitten off more than I can chew

[pfrcom]

To start off with, its power LED just blinked continuously
This pic shows power board in its original condition

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

I first replaced all the tiny capacitors because their values seemed a bit high on my Dick Smith ESR meter, measured in situ
Hope dashed for a quick easy fix - power LED still blinked
I then replaced remaining capacitors, although only the four around IC BIT3105P measured high ESR in situ
Replacements purchased from local specialist electronics supplier's low ESR category, most of the bigger ones being Hitano
Power LED still blinks, although less frequently than original
Repeated cycle of staying on for about 7 seconds, then going off for about half a second
Accompanied by faint tick which seems to be coming from the area around the LED itself
Help appreciated

[budm]

Re: LG L1710B - bitten off more than I can chew

The ticking sound can mean the power supply is detecting fault so It goes into shurdown - restart cycle, or it has a hard time starting up. Did you also replace the start up caps?

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

Did you also replace the start up caps?

I guess those would be included in the tiny capacitors I replaced first, wouldn't they ?

Two of 4.7uf 50v, one of 10uf 50v & one of 47uf 50v

[budm]

Re: LG L1710B - bitten off more than I can chew

Well, I do not know which one you are referring to: "I first replaced all the tiny capacitors because their values seemed a bit high", if you did, then next thing will be to see if the loads are shorted or not by running the board without the main board connected, this power supply-inverter board is always running when you apply the power.
Check the DC voltage at the output of the secondary rectifier to see if it still pulsing, the shorted circuits can be in the imverter section due to shorted MOSFET's.
The symptom you have is the same as LGl1715S:
http://s807.photobucket.com/albums/y...G/LG%20L1715S/

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

Thanks budm

My expertise isn't electronics, so I'm learning (I hope) as I go, and appreciate your patience

Please don't misinterpret my post #4, it was a genuine question not a smart arse comment

Your photobucket album looks good - need some time to give it the attention it deserves

I didn't try powering the FSP board on its own, because I didn't know whether it needed to be switched on or not, like an ATX motherboard

And because the power switch is on the little controls board that connects to what I think of as the video board - is that what you call the main board ?

[budm]

Re: LG L1710B - bitten off more than I can chew

The power supply-inverter board is always on 24/7 the minute you plug it into the wall socket. The power switch just turn off the inverter circuits and the power to the T-CON board.

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

Check the DC voltage at the output of the secondary rectifier to see if it still pulsing

Will need some hand-holding to do this - is secondary rectifier mounted on the heatsink surrounding the four caps, in the bottom left corner of picture ?

[selldoor]

Re: LG L1710B - bitten off more than I can chew

I will let Budm answer that in the meantime perhaps measure the voltages on the connector between the power board and the main board.
Have everything connected up and switched off meter on 200vdc put black probe on a ground screw and check each pin on the connector with the red probe. As you have pulsing keep the test going to see if the pulsing shows
in the voltages. Then repeat with it switched on.
If you have a pin called pson or p on test this pin again but hold the probes on the pin with it off then switch it on with the probes in place ( may need help with that) and report wht happens.

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

meter on 200vdc

My DMM's a Fluke 77 (original) which I bought new in the 80s - why?, because similar was what IBM issued their Customer Engineers, who had the best

So it's autoranging

I measured on the pads beneath the connector which are labelled as shown in this pic

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

First though, I did a bit of rework because it looked like I had a solder bridge from negative of C661 to the track above it

This pic shows what it's like now, when I did the measurements, which are:

With board only, plugged to mains 240v
5v = steady 4.97
12v = steady 11.76
On/Off = steady .000

Then main board connected, complete assembly still plugged to mains 240v, but power button off and LED off:

5v = steady 4.92
12v = steady 12.40
On/Off = steady .059

After pressing assembly's power button, LED lights up then ticks off after about 7 seconds, for about half a second, repeats that cycle

5v = steady 4.86 until tick off, re-initializes briefly at 4.94, settles at 4.86

On/Off = steady .065 - best I can tell, maintains that through tick off

12v settles at 13.37 - after tick off re-initializes briefly at 12.14, then 13.3something, climbs by .01 until reaches 13.39, drops back to 13.37, where it stays until tick off

[budm]

Re: LG L1710B - bitten off more than I can chew

That ON/OFF voltage reading does not look right, I expect to see >3VDC for the inverter to turn on. By the way, when you are testing these voltages, do you have all the backlights lamps connected?
Please take the resistance readings of the MOSFETS.

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

By the way, when you are testing these voltages, do you have all the backlights lamps connected?

No, logistically too challenging

Please take the resistance readings of the MOSFETS.

Meter shows 00.2 ohm if probes are shorted together

Assuming the dot denotes #1, and others going away from it on the same side are #2, #3 & #4

xx or x in lieu of numeric digits denotes a changing value - values given are where they settled

Mosfets are marked 4600 AD3E19

U2
#1 1.3xx Mohm
#2 3.50 Mohm
#3 1.2xx Mohm
#4 1.3xx Mohm

U3
#1 1.4xx Mohm
#2 3.65 Mohm
#3 1.4xx Mohm
#4 1.49x Mohm

U4
#1 1.46x Mohm
#2 3.65 Mohm
#3 1.3xx Mohm
#4 1.4xx Mohm

U5
#1 259x Ohm
#2 2.89 Mohm
#3 00.2 Ohm
#4 35.x Ohm

[retiredcaps]

Re: LG L1710B - bitten off more than I can chew

U5
#1 259x Ohm
#2 2.89 Mohm
#3 00.2 Ohm
#4 35.x Ohm

http://pdf1.alldatasheet.com/datashe...MD/AO4600.html

U5 is shorted.

http://www.ebay.com/sch/i.html?_saca...Ao4600&_sop=15

$1.02 USD + free shipping for two.

PS. In my 2 seconds guide, I wrote a procedure on how to test a mosfet for a short. I suggest you take a look and retest the other mosfets to get all possible combinations with reference to the linked datasheet for AO4600.

[budm]

Re: LG L1710B - bitten off more than I can chew

I would change them in pair, might as well order at least 4.
When you finished installing the new IC, do not turn on the board without having the lamps connected, it will put out big high voltage (1500~2000V) arc at the lamp connectors when it try to start up.

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

Thanks for your help budm, retiredcaps and selldoor

Have just ordered two pairs of the AO4600, will post a sequel in due course

Considering working L1710Bs are on offer here for around $20, this one will be well over-capitalised

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

AO4600s arrived yesterday - very good service, especially considering their price, and the season

Here's U4 & U5 after originals removed

For the lifted pad I put a dab of Super Glue underneath, then very carefully flattened it back into place and left it to cure overnight

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

Here's U4 & U5 with the replacements

Sorry budm, mindful of your suggestion, but I couldn't face the prospect of replacing U2 & U3 whose measurements indicated they were probably functional - what with lifting U5's pad, and if it ain't broke ...

Reassemble it so I can plug in the backlights lamps, then the acid test, power it on ...

... steady green LED, not going Off & On, hmmm promising ...

... connect it to a computer, and hallelujah there's an image

Thanks again for your help budm, retiredcaps and selldoor - without it, definitely would not have been a successful outcome

[pfrcom]

Re: LG L1710B - bitten off more than I can chew

Some more questions please

This is the set of Capxons I replaced early on, each of which coincides with an AO4600

Is it fair to blame the Capxons for the demise of the AO4600 ?

With the wisdom of hindsight, I probably should have waited for better caps instead of using brandX (can't recall actual brand) from local specialist electronics supplier's low ESR category

Finally this question

I have some Rubycon YXF 47uf 63v in transit

Could I use those in lieu of the brandX 47uf 25v with which I replaced the original Capxon ?

[retiredcaps]

Re: LG L1710B - bitten off more than I can chew

but I couldn't face the prospect of replacing U2 & U3 whose measurements indicated they were probably functional - what with lifting U5's pad, and if it ain't broke ...

Now that everyting is working, it is a good time to ohm out U5 and see where the lifted pin has continuity to. That way, if the superglue fix doesn't work over time, you can then run a jumper wire from the U5 pin directly to the ohmed out component.