Weird LCD TV fault?.

[Nigel Goodwin]

OK, has anyone any ideas about the failure mode behind this? - the two attached files are the top and bottom views of one of the PCB's from inside a Sharp TV LCD panel.

The opposite ends of the tubes have individual connectors up to the inverter board, but this end simply links pairs of tubes in series.

As you can see from the pictures, each socket has three mounting points, only ONE of which is actually used. So how come the burn-up happened at a joint that has no possible current going through it?.

This isn't the first one I've seen like this - and the burn-up is probably my fault, running it in pieces with the trip circuit disabled - but there's no real option, these specific sets commonly fail with 'tube errors', but these are usually VERY intermittent (this one's been running disassembled for over a week).

Incidently, I noticed it had failed by a slight burning smell. and on checking I noticed one tube had gone out - so even though the tubes are in series the other one still worked. When I removed the tubes, the faulty one actually snapped off clean at the end.

At least you can get the tubes and boards for these Sharp sets, good job because complete panels for Sharp cost 3-4 times the price of the set!.

 

[duffy]

Weird one. Those CCF tubes get a high ignition voltage spike to start them, maybe got arc-over to something else.

 

[Nigel Goodwin]

The only thing I could think of was arcing to the chassis? - but it doesn't happen at start-up, only while it's running.

As the tubes are in series I wondered if one suddenly stopped working that might put the full voltage on the midpoint (at least double normal voltage) and perhaps that's the reason?.

 

[duffy]

Doesn't take much to get an arc started, is there a possibility someone spilled a fizzy drink in the cabinet?

 

[Nigel Goodwin]

Doesn't take much to get an arc started, is there a possibility someone spilled a fizzy drink in the cabinet?

No, it's been running stripped down in the workshop for a week - and there's no signs of any ingress - it's also not the only set I've seen with the EXACT same fault.

I've also got another with the same fault (but not burnt-up yet)

Hence the reason for asking.

 

[Nigel Goodwin]

OK - further update.

I had the second set left running in pieces, as I did the previous one - but unfortunately I've been off work since last Saturday looking after my wife who has a chest infection, and was too ill to left alone. So the set has been running unattended every work day, as it powers up when the workshop power is turned on.

Needless to say. this has burnt-up as well but somewhat worse, as I wasn't there to keep an eye on it, and the board has burnt entirely in half!.

Again I can only presume it's due to one of the tubes going out, and the voltage at the mid-point going to high, and arcing to the chassis.

I've ordered a new PCB for it, they come from Germany, so should be Monday or Tuesday - presumably that will cure it - I've already got a pair of new tubes awaiting fitting.

BTW, the previous set is still running fine, with the back on and all trips reconnected.

 

[duffy]

Sounding like some pretty inexcusable engineering over at "Sharp".