Hannspree HF257 - White Screen

[dranobob]

I have a 25" Hannspree Monitor that all of sudden went white (full backlight no image). Troubleshooting (steps listed below) led me to think I have a bad smd capacitor. I'm curious how to determine what the replacement capacitor is since there are no external markings? Picture of the top half:


Troubleshooting steps:
- Swapped logic board from identical working monitor (Good monitor still worked)
- Swapped power board from same monitor (Good monitor still worked)
- Returned parts to bad monitor
- With unit powered on, measured voltage going in/out of fuse on T-con board (5.03V)
- With unit powered on, measured voltage at VDD+5V pin on T-con board (measured 5.03V)
- With unit powered on, measured voltage at VDD+1.8V pin on T-con board (measured 1.80V)
(I couldn't find any other pins that were clearly power and didn't want to risk random pin tests)
- Noticed the back of the t-con board under the inductor was very hot on the defective monitor, but not on the working monitor.
- Started examining t-con board around inductor
- Noticed capacitor labeled C208 was slightly discolored
- Pushed on solder joints with meter probe and with little pressure the smd split in half horizontally. Just to make sure i wasn't too aggressive I tried several other SMD caps, and none budged under the same pressure.
Here's a picture of the area mentioned, this is after the top half of the smd popped off.



Of course, please feel free to pick at any holes in my troubleshooting techniques and any comments/advice is welcome.

[jetadm123]

Re: Hannspree HF257 - White Screen

c205-c211 are all probably mounted in parallel, which generally means they are all the same value. Ideally, you'd remove one of the caps, say, c205 and measure it's value with a capacitance meter. This will give you the value of c208. A 25 volt rating should work fine.

[bammbammfran]

Re: Hannspree HF257 - White Screen

if you remove the residual capacitor parts and take it out of the equation, you can turn it on and if it was an issue, it should give you a picture since the other caps are in what appears to be a parallel setup.
As far as replacing it, you would need a cap tester to check one of the other ones out of circuit to see what size you need to replace it.

[budm]

Re: Hannspree HF257 - White Screen

You should measure the DC voltage on the working board to see what the applied voltage is, they are used as filter caps for the switching supply (L200, D200, and some switching IC and MOSFET which drive that inductor L200).

[dranobob]

Re: Hannspree HF257 - White Screen

thanks everyone for the replies! I only had a cheap meter last night, so I'll need to go dig my nicer meter capable of testing capacitance out of some moving boxes. I'll update the post once I can find it

[Pauldesign]

Re: Hannspree HF257 - White Screen

I reckon you measure about 2 or 3 of the other parallel caps (de-solder one side of each) to accurately determine the identical faulty cap value, since measuring just one cap might not give you the right value due to tolerance / ageing.

Judging from the cap size, the faulty cap value will not be greater than 10uF. In the meantime, you may quickly connect a 10uF (check the polarity) cap as replacement and see if it helps.

Regards