Thoughts on sony bravia KLV-S40A10 - power issues - advice

matthewy · Joined: 24 Dec 2008 Posts: 14

I've done afew repairs in the past to random stuff like motherboards, sound systems and lcd monitors but not really a expericenced repair man and generally the repairs are done by inspection and spotting the obvious bad parts such as bad caps and shorted transistors.

I have picked up a sony bravia ( KLV-S40A10 40" ) lcd for a low cost from a friend in hopes of repairing it however, nothing really stands out. he suspects power surge caused damage and I have hence thought it would be a basic repair (but appears not as basic as I had first thought).

The symptoms are that it doesnt power up, the relay in the power supply section is constantly clicking (and the frequency of the clicks range from maybe 3 times a sec to 10 or so times a sec) as soon as power is connected.
The red Led flashs at a very quick rate too (not error code, the rate is prob the same as relay switching rate).
The 5v standby voltage is jumping up and down but using a dc voltage setting, it hovers around 2v. I suspect that the standby voltage is what is driving the relay....

Obviously I'm quite keen to give this a go and try fixing it. Has anyone had similar issues with similar lcd tv's and can guide me to areas to check?

Also, with tv's like this, should the standby voltage always to avaliable and at say the 5 volts or is it also controlled by other circuit boards which may be bad and causing the voltage to jump up and down?

Schematics are avaliable online which have been what i've been digging into.
http://vanejanev.googlepages.com/SONY_lcd_tv_bravia_KLV-S26A10.pdf
http://hdandk.googlepages.com/KLV-S32A10Phan1.pdf
http://hdandk.googlepages.com/KLV-S32A10Phan2.pdf

Any advice and help appreciated.

Matt

jts1957 · Joined: 24 Nov 2008 Posts: 2476 Location: Far, Far Away

5 volt stand by should be there "all time set is plugged in."
Is it there (at 5 volts) UNTIL set is "turned on," or does it start clicking, flashing, etc. as soon as plugged in (without having to hit the "POWER" button)?

matthewy · Joined: 24 Dec 2008 Posts: 14

As soon as the ac power is connected the tv clicks and the standby measures ~2V (jumos from 1.8 to 2.4V). I think the standby voltage is being turned on / off Sad

matthewy · Joined: 24 Dec 2008 Posts: 14

Part of what i'm trying to figure out is if the voltage is bad which is causing everything else to fail or if something else is failing which is causing the standby voltage to cut off through a protection circuit. Not sure what teh best way to check this is. Ususaly with lcd monitors etc, I would start swapping parts to issolate the issue but I only have this 1 lcd tv Sad

jts1957 · Joined: 24 Nov 2008 Posts: 2476 Location: Far, Far Away

If the standby source can be isolated from the rest of the set, this will let you know if the "voltage is bad."
I suspect a heavier than normal load (shorted component?) being reflected back each time set tries to "start."

vtech · Joined: 08 May 2006 Posts: 1264 Location: USA

The "bad voltage" could easily be loaded down because of some type of short(just like it was mentioned)

As an example, lets say there is something wrong with the inverter which will instantly put a massive load on the supply & shut down. I know it is like the "chicken or the egg" scenario.... Just about the only way to tell is to try to isolate any and all loads as much as possible. Sometimes it is easier said than done though.

One trick that can be handy is the use of external power supply(preferably with an amp meter to monitor current); you can always substitute a known good source provided your reference ground is common.

In this case, you can always apply a 3~4.5 volt external to the standby line and check the current draw. (in case of a short, you'd instantly see a pegged meter)
I have used this on many different circuits to determine the viability of a source. Of course you MUST know your ground reference & always stay about half the volt below the supposed value.

matthewy · Joined: 24 Dec 2008 Posts: 14

Thanks Vtech and Jts, good adice.

I'll first try measuring the amps comming out of the power board (theres a connection for standby going to one of the main boards so can disconnect that pin and go from there.

Vtech, when you say feed external power to it, do you mean plug in the mains (as usual) and then add another adaptor on top feeding say 4.5V straight to the standby line / ground?

Thanks heaps for your help.

Matt

matthewy · Joined: 24 Dec 2008 Posts: 14

K, christmas day and I had a few hours to play around again. So measuring standby current by disconnecting one of the pins and using the digital MM, approx 50ma (seems low to me).

I have disconnected the power to the inverters and the dimming controls and seeing the exact same result so feeling somewhat confident it is not to do with the inverters.

I have also tried disconnecting the standmy power completely and although nothing works (not even clicking sounds) I can see that the unloaded ps is a stable 10 volts.

I'll keep playing with it but again, any advice welcomed.

matthewy · Joined: 24 Dec 2008 Posts: 14

Threw 4.5 volts into the standby line and the relay stays on this time! Smile

The led is indicating 7 blinks which translate to "7 times Internal temperature rise (mainly on the panel side)". dont know if this is a lie or not.... but quite exciting

vtech · Joined: 08 May 2006 Posts: 1264 Location: USA