Hannspree HF257 w/bad backlight

[jetadm123]

Re: Hannspree HF257 w/bad backlight

I have one of these I found on the road dead. It had a burned CCFL wire and a burned CCFL connector, where the wire crossed the connector, bad assembly. I cut off the burned stuff and soldered on new wire and connectors with heat shrink insulation.
I found a jumper on the inverter labeled 7.5ma, 8ma. I moved the jumper to 8ma before I tested. I wondered if the jumper was there for CCFL aging compensation. I forgot about it.
I then tested, back light worked fine and stayed lit. I put the monitor back together.
Now, if I turn brightness below 65 or so, the back light goes out in a few seconds. The safety circuit shuts down the inverter if it detect over or under current. I assume the jumper sets the range of current for the safety circuit. But it may affect the PWM. Does your board have this jumper? My board has all TAICON caps, did yours?

The power supply on your monitor is probably used for several different models. The 8mA setting would be used on a larger screen as those CCFL's require more current. If you moved the the jumper back to 7.5mA can you increase the brightness without causing a shutdown?

[sparkythefiredog]

Re: Hannspree HF257 w/bad backlight

I have one of these I found on the road dead. It had a burned CCFL wire and a burned CCFL connector, where the wire crossed the connector, bad assembly. I cut off the burned stuff and soldered on new wire and connectors with heat shrink insulation.
I found a jumper on the inverter labeled 7.5ma, 8ma. I moved the jumper to 8ma before I tested. I wondered if the jumper was there for CCFL aging compensation. I forgot about it.
I then tested, back light worked fine and stayed lit. I put the monitor back together.
Now, if I turn brightness below 65 or so, the back light goes out in a few seconds. The safety circuit shuts down the inverter if it detect over or under current. I assume the jumper sets the range of current for the safety circuit. But it may affect the PWM. Does your board have this jumper? My board has all TAICON caps, did yours?

[rasterdan]

Re: Hannspree HF257 w/bad backlight

Replacing the caps (even though they looked ok) solved the 12v problem. I still had the 2 second to black.
I pulled the inverter transformers and they measured both the same and very close to what the generic replacement said their values were so I am assuming they are ok. I can unplug the back light and the power supply still shuts down.

I believe it is a bad surface mounted Current Mode PWM controller. It seems to have the same voltage and wave form coming out of every pin except ground. The DVM also has every pin about the same whether power on or off, startup what ever. I may or may not continue with this piece o junk. Most of the blogs have wound up junking this board and you can't find one online. It now has some good parts for other repairs.

All the diodes and transistors etc seem to be ok, the 5 and 12v is working perfectly. I have wasted too much time on this beast.
I have a Samsung repair that should be an easy cap replacement (blown cans). That will solve my third monitor need for now. Before long, one of the neighbors will give me another dead one to dink around with. I have seen other power supplies used on this monitor so I may try a different supply when one comes along.
Thanks for the advice.

[retiredcaps]

Re: Hannspree HF257 w/bad backlight

I have a Hannspree HF257 25" display that goes black after a couple seconds.

"2 seconds to black" can be

a) bad caps
b) bad inverter transformer
c) bad ccfl and/or wiring

Since you already replaced all the caps, see

http://lcdparts.net/TransformerDetai...ProductID=2796

for b).

All the above are explained in more detail in posts #13 and #14 at

https://www.badcaps.net/forum/showthread.php?t=10419

[rasterdan]

I have a Hannspree HF257 25" display that goes black after a couple seconds. After reading many posts, I replaced all of the caps on the board even though the cans did not looked popped on top. I also replaced the 12 v voltage regulator for good measure since the 12 v would shut down after a couple seconds. One of my friends said that was because it was probably in over current and it is one of those non-breakable regulators with a safety built in.

So, all the caps and regulator replaced, still have the same problem. I can see with a flashlight that the display is working, just no backlight. I suppose that leaves the inverters. I have been looking for the inverters on many sites and while you can buy just about every one on earth, you can't buy this one.

After searching, it looks like this may be the same inverter on the TDK XAD819AR inverter but I can't seem to find that one either.

I got this monitor for free and the problem doesn't seem like it should be so difficult to fix. I also reheated several solder joints that looked like they could have had ring fractures, I even pulled the inverters and reinstalled in the opposite position to see if that made any difference and still no help.

I would also be interested in figuring out what is pulling down the 12v.

One last bit of information, before i replaced the caps, i replaced the 12V regulator and the monitor worked for a few minutes before shutting down. Is it possible that the caps have damaged the regulator and now that I have new caps, the regulator needs to be replaced again? I know some circuits once they have tripped a few times the safety portion is damaged and will just keep shutting down.

Thanks for any help on this one. It seems to have a very common failure mode with this model but I have not seen any posts that have actually fixed the monitor except one where they jerry-rigged another model's power supply into the monitor. I think that is a bit extreme.