Viewsonic vp171b - Bad Caps?

[22turbo]

Hello everyone,
I have a vp171b 17 in LCD monitor that has a problem with the backlight, it randomly turns off. I know it is just the backlight because if I shine a flashlight on it, the image is still there. I took the monitor apart and found a bunch of 100uF caps by taicon and 2 25uF by hermei. None of them or bulged or leaking in any way though. My question is, do you think that the problem could be caused by a bad cap that is shorting out when warm or after it has been in use for a while, or do you think it's something else. On my capacitance tester, it showed most of the caps at around 83uF, so I wonder if that is too much tolerance.

Also, if the caps are the problem would you choose panasonic FC or nichicon HE. I have both brands available to me. Thanks

Dan

[kc8adu]

Re: Viewsonic vp171b - Bad Caps?

always post pics.
btw bad caps can look fine on your tester and fail esr test.

[PCBONEZ]

Re: Viewsonic vp171b - Bad Caps?

I took the monitor apart and found a bunch of 100uF caps by taicon and 2 25uF by hermei. None of them or bulged or leaking in any way though.

Caps can fail with no visual indication.
As kc8adu said they can have bad ESR but the uF is just fine.
-
No telling if your's are bad but both of those brands occasionally cause problems. Taicon is partially owned by Nichicon but that doesn't make them Nichicon Caps.

My question is, do you think that the problem could be caused by a bad cap that is shorting out when warm or after it has been in use for a while, or do you think it's something else.

Yes it 'could be' but there are other possible problems and not all have to do with caps. Sometimes even just a bad solder joint will change the circuit when things warm up.
Caps are indeed a very common problem.....
Good *chance* that's all it needs to be fixed.

On my capacitance tester, it showed most of the caps at around 83uF, so I wonder if that is too much tolerance.

Most low ESR caps are +/-20% for uF when new.
uF also tends to go down with age and that is (or should have been) taken into account by the designer.

Would you choose panasonic FC or nichicon HE.

There is more to it than that.
The old caps may be anything from GP to Ultra Low ESR.

Along with uF and volts there are ESR and Ripple ratings to consider.
Way off ESR can unbalance the circuit just as uF. (Depends on what circuit is.)
Too low a ripple and the caps may overheat and die young.
-
Ideally you want to get as close to the original as you can and to know that you need to look up the old caps specs if possible. (It's not always.) And to do that you need to know the series (models) of Hermei and Taicon these are. - If there are no data tables to be found then you guess what grade they are based on where they are used in the circuit. (Why kc8adu asked for a pic.) If you have doubts then fail up to a better grade than you may use otherwise.
-
That said, FC and HE are fine for most caps in an inverter.
But also, I just did an inverter for a VA721 that required (to match the originals) a few caps (not all) to be rated closer to Panny FM.
FM has much better ripple and ESR than either HE or FC.

http://www.hermei.com.tw/catalog_e.htm
[Have your internet condom on. Google is currently reporting this as a bad site for some reason.]
http://www.google.com/interstitial?u...products_e.htm


http://www.taicon.com.tw/product/company-index.htm

.

[22turbo]

Re: Viewsonic vp171b - Bad Caps?

I don't have a pic yet, but the series of caps are as follows.

HERMEI LZ series (On the Inverter Board)

2 - 180uF @25v
(Purple with silver writing)

TAICON
17 - 100uF @16v "C0501(M)" Written on side along with (VT)
5 - 470uF @16v "C0513(M)" Written on side along with (VT)
(Both black with white writing)

1 - 1000uF @10v "C0511(M)" Written on side along with (HF)
(Green with white writing)

Hopefully I can get a pic soon
Dan

[22turbo]

Re: Viewsonic vp171b - Bad Caps?

I actually found this picture of the corner of the board on Google. The board to the left of it with the two purple caps (HERMEI) is the inverter. I have no idea why a larger cap should be installed. Any ideas?

[PCBONEZ]

Re: Viewsonic vp171b - Bad Caps?

I'm making a huge assumption that you buy from Digikey so I'll stick to what series they carry.


The HERMEI LZ - 180uF @25v -- Ripple 1140 and ESR .030
That's a high grade actually.
A Panny FC is much worse at 555 and .117.
HE and FM don't come in 25v 180uF and HE would be too low anyway.
Safest match I see is a Panny FM 50v 180uF -- Ripple 1430 and ESR .033
[That is if the physical size works.]


TAICON
The "C"-whatever is the date code. I don't know what the "C" means (plant?, shift?) but the numbers after the C are the 2 digit year and week of the year.
The following (M) is the standard notation for +/- 20% on the uF.
- VT or HF would be the series.

VT is a general purpose cap.
HF is Low ESR.



100uF @16v VT -- Ripple is 135
470uF @16v VT -- Ripple is 395
These are GP and don't even list ESR. - FC/HE would be fine for these.

1000uF @10v HF
in 8x20mm can -- 1250 / .041
in 10x16mm can -- 1430 / .038
That's better than both FC and HE.
Almost exact match to Chemicon KZE so KZE would be fine.
FM is better than KZE so if you have a thing for Panny they'd work.

.

[22turbo]

Re: Viewsonic vp171b - Bad Caps?

Wow! Thanks a lot for the info PCBONEZ, DigiKey is fine by me, so I'll buy from there. I'm an electrical engineering student, so I know about capacitors, but not much about esr and such. This project is right up my alley, so it will be fun to work on. I'll replace the caps and reflow any solder joints that look questionable (although the board looks pretty good) and report back with the results. If that doesn't work I will diagnose further. In that case I may try and see if viewsonic made a service manual for the monitor. Thanks again!

Dan

[22turbo]

Re: Viewsonic vp171b - Bad Caps?

Thanks again for all the help above guys! I finished replacing the capacitors a couple of days ago, and while it seemed to fix the problem for a little while, it seems to have come back with a vengeance. When I turn the monitor on, it works fine for a couple of minutes and then the backlight shuts off (Heat Related?). If I power cycle the monitor, the backlight will turn on for about 2 seconds and then shut off. If I do this enough times (for about 15 minutes), it will start to stay on, and will work perfectly for hours. I replaced all the caps according to the instructions above, and if someone wants a pic, I guess I can take the monitor back apart and get some now. If anyone has any idea what the next area I should troubleshoot is, let me know. I'm fairly sure it's not the CCFL's, as they dont have many hours on them.

Thanks Again,
-Dan-

[PCBONEZ]

Re: Viewsonic vp171b - Bad Caps?

Hmm,
Maybe a bad solder joint somewhere? Not necessarily one you did either.
They happen at the factory often enough.
Possibly even at the end of a tube (lamp).

.

[linuxguru]

Re: Viewsonic vp171b - Bad Caps?

Hmm - it could be a cap problem still, could also be an unrelated temperature/moisture-related problem, from the symptoms. Visually inspect all joints, and also look for other caps in the backlight and PSU circuits. Hermei is crap, Taicon is medium-quality.

[kc8adu]

Re: Viewsonic vp171b - Bad Caps?

the glue under the 2 toroids eats the traces.
pull them,clean off the glue,and repair the eaten traces.
one is a thuhole that i repair by passing edm wire through and soldering.
bet that fixes it.

[22turbo]

Re: Viewsonic vp171b - Bad Caps?

the glue under the 2 toroids eats the traces.
pull them,clean off the glue,and repair the eaten traces.
one is a thuhole that i repair by passing edm wire through and soldering.
bet that fixes it.

I'll take a look at that soon! Thanks

-Dan-

[volto]

Re: Viewsonic vp171b - Bad Caps?

Ive got this same monitor, and it stays on just fine, but after its been on for a long time, certain shades of red make it glitch out, and i get a flickering horizontal row of green pixels on the same line where the pixel of red is. If I cycle the monitor off and back on, the problem is gone for a couple more hours, this is what makes me think it might be a capacitor/heat issue. Anyone got an idea? Don't mean to thread hijack but i figure same monitor, same thread.