Hi all,

Have a touch screen from a milking robot not working.
Problem seems to be the driver for the LCD backlight, it looks like a boost converter, would I be right to assume the component marked 2A is a surface mount fuse?
It's not conducting anyway, the monitor could be up to 20 years old.
I can't see any burnt components on the board, popping a new fuse in got to be worth a try?
Cheers Geoff

 

hi Geoff,
Welcome to AAC.
Is the top of that blue electrolytic capacitor bulging,? , if yes, that is the component I would try changing.
E

 

Thanks Eric,

I'll have a look when I'm back, been on here a while just don't get out much!
I've got two with the same symptoms (different times) I'll pull the other apart and see what that looks like

Cheers Geoff

 

Brighter pictures please.

 

Have seen a few fuses die from thermal cycling over the years, I would put it on a bench power supply, connect the lamp, set the current limit to a couple of amps and bypass the fuse. If it starts and runs at a good amount under 2A then you may be good to go, just replace the fuse. Definitely worth checking that electro as Eric stated though, that's pretty old for an electro.

 

Measure the resistance across the electro as that would be the power supply. If it is low, it could be a crook electro or the switching transistor.
I have found often the electros dry out so they do not abzorb the spikes from the switching and these spikes can pop the transistors.
Can you remove the electro and measure it?
Then, when it has been fixed, replace the fuse with a 2A polyswitch as they are resetable.

 

Here's some better pictures!
A quick prod about with the multimeter, 2A has 12v on its output so can rule that out.

The electrolytic I discharged, with my meter on 200k resistance gradually rises to open circuit which is very similar to a good one on my desk!

Guess the two diodes on the high voltage side are for rectification?

What sort of voltage should I expect?

Might be better to replace with a available unit if I can find the specs?

Cheers Geoff

 

It's described as an inverter and available second hand (from America!), looking at the pink wire would it likely be ac output through those wires?
The thinner white wires meet at the socket and is marked return on the board.

 

Are you sure the inverter is the problem? It may be the flouro tubes themselves. This inverter is the type that generates a few hundred volts AC to drive tubes.
Have you managed to measure the high voltage out to see if it is working or not? Be careful, it can bite!

 

Yes I've had the plastic safety cover off and measured but there's no output, I'm not licking it just incase

Might be worth scoping it I suppose?
I'll strip the monitor further tomorrow, might it be possible to put a led strip in? Replace the flouro tubes

 

I've stripped the other broken one down now, same symptoms, bit more prodding with multi meter,
No ac output
Coil winding seem ok but nothing going into coil?
Most likely the drive transistors?
I'm not sure, I've found what I think to be a replacement for £22 so have ordered one to try.

Just for information it's a lg lb121s02 screen with cold cathode fluorescent tubes.

It does look like could slide a led strip in there, what voltage do they generally require? Do they need a driver? Have 24v and 12v available easily.

Cheers Geoff

 

Part turned up on Saturday so I now have one working monitor!
I'm going to fault find the other inverter now.

Could be the tubes on the old faulty one, it did light up briefly when power turned off? (With the new inverter)

Thanks everyone