Daewoo L700C 17” LCD – power supply problem?

[momaka]

A friend of mine gave me this monitor to fix for him. It's a 17” Daewoo L700C LCD with a CML-700C POWER power board.

The problem with it is that it doesn't always work if it gets unplugged from the wall.
When it works, it works fine and wakes up if a signal is fed to it and it can stay on for as long as the signal is present (the only odd thing I can find here is a high-pitched noise when the back light turns on). But when it doesn't work, there's a constant high pitch sound coming from the power supply board and the monitor does not respond to any signal or the power button on the front. Unplugging the monitor and plugging it in a few times can make it work again so I don't think it's an inverter issue. If the monitor has been sitting unplugged for over half hour, it always seems to power up fine the first time. Unplug it and plug it again and it might not work.
I measured the output voltages of the PSU board, and here are the results (the 3.3v, 5v, and 5vsb rails are generated by voltage regulators):

standby ------------ on ----------- not working (dead)

7.61v ------------ 7.61v ------------ 6.36v ------------ 3.3v (supply to 3.3v regulator)
5.52v ------------ 5.52v--------------2.91v ------------ 5v (supply to 5v and 5vsb regulators)
1.03v ------------ 3.29v ------------ 0v ---------------- 3.3v
1.29v ------------ 4.83v ------------ 1.90v ------------- 5v
23.8v ------------ 14.6v ------------ 19.74v ------------ 12v
4.56v ------------ 0.07v ------------ 0.70v ------------- on/off
5.04v ------------ 5.03v ------------ 0.92v ------------- 5vsb
4.86v ------------ 2.14v ------------ 0.88v ------------- BCON
0.03v ------------ 4.54v ------------ 0.05v ------------- BLON

Note that whenever the monitor is not working, the 5vsb is also not present. Also, 12v rail is showing 23.8v when monitor is in standby/off mode! Does this seem normal at all? As far as the PSU design, 12v rail is unregulated and there is no feedback from it to the PWM controller (PWM only gets feedback from the unregulated 5v/5vsb supply rail via optocoupler).

Visually, none of the capacitors look bad (I don't have an ESR meter to test them though). The PSU board does have some serious darkening due to heat near the regulators/rectifying diodes and there are some 85C rated caps in that area. So what do you think is the problem, bad caps perhaps?
By the way, here's a list of the capacitors on the PSU board:

2x Sam Young KMG, 16v, 1000uF, 10mm dia., 17mm h., 105C
3x Samxon GS, 16v, 470uF, 8mm dia., 85C
1x (unknown brand) RMU 25v, 1000uF, 10mm dia., 20mm h., 105C (logo looks like a tree stump)
1x Samxon GS, 25v, 100uF, 6mm dia., 85C
1x Samxon GS, 16v, 100uF, 6mm dia., 85C
1x Samxon GS, 50v, 33uF, 6mm dia., 85C
1x Samxon GS, 50v, 1uF, 5mm dia., 85C
1x Sam Young, 50v, 1uF, 5mm dia., 85C
1x Samxon LP, 400v, 68uF, 25mm dia., 21mm h.., 85C

There are also 2x Sam Young KMG, 35v, 220uF caps on the inverter board. Inverter board is made by PyeounaHwa Electcom, model P1704DA Rev.00. Resistance on HV side on both transformers measures okay (1000 Ohms, ~980 Ohms). No cracked transformer joints.

I didn't mention caps on the video board because they are all SMT and under 100uF (they appear to be Sam Young by the printed logo). There is also no 12v rail going to the video board so I don't think this is the problem either.

If anyone has any other ideas what could be wrong, please let me know. My guess is that caps are the problem here, but I would rather not do a full recap as some of the caps are located right under the heatsinks (though there's probably not much choice here).
Sorry for the long post, by he way.

[elbtax]

Re: Daewoo L700C 17” LCD – power supply problem?

I would definitely replace all the caps. Use 105deg, low esr caps. Order from digi-key or mopuser. Those 85 deg, general purpose caps should never be used in a psu. The big cap next to the Bridge rectifier should be o.k. Good luck.

[mcfischer]

Re: Daewoo L700C 17” LCD – power supply problem?

I recommend checking the ESR of the big cap (AC input). If lower than 2 or 3 ohms replace it or just replace it.
Mike

[mcfischer]

Re: Daewoo L700C 17” LCD – power supply problem?

Oops. My last post recommended replaciing the big cap if LESS that 2 or 3 ohms. That is backwards. Lower esr is good. I fixed a Samsung yesterday that had those symptoms (noise when high current but worked) ESR of the big cap was higher than 100 ohms. I use a currect monitor and saw an increase in current draw with the noise.
Mike

[momaka]

Re: Daewoo L700C 17” LCD – power supply problem?

Thanks for the replays everyone. Unfortunately, I don't have an ESR meter so I can't test the mains cap. I might just replace it, nonetheless.

And now an update on the monitor: I haven't ordered replacement caps yet (there's another monitor I'm going to check first before ordering stuff), but I did a quick test by changing only one cap on the 5v/5vsb supply rail and as expected, the monitor is now working perfectly and no longer makes loud noises when the backlight starts up. 12V rail is starting to look a lot more normal too. Most importantly, the 5vsb regulator no longer feels like it's on fire (it barely gets warm at all now). Pity, cause I made a custom heatsink for it. So that's definitely a cap issue here. I also took some readings of the voltages after this quick recap.

Soft off ------- Standby ------- On

4.79v --------- 4.79v --------- 4.79v --------- 3.3v (supply to 3.3v regulator)
6.34v --------- 6.34v---------- 6.34v --------- 5v (supply to 5v and 5vsb regulators)
1.03v --------- 1.02v --------- 3.31v --------- 3.3v
0v ------------- 0v ----------- 4.99v --------- 5v
16.18v ------- 15.83v -------- 13.53v -------- 12v
4.53v --------- 4.53v --------- 0.07v --------- on/off
5.02v --------- 5.03v --------- 5.03v --------- 5vsb
4.83v --------- 4.83v --------- 2.14v --------- BCON
0.03v --------- 0.03v --------- 4.56v --------- BLON

The capacitor I used as replacement was Nichicon HC, 10v, 470uF, 10x12.5 (D x H). According to the Nichicon datasheets, this cap is rated 0.03 Ohms ESR and 1230 mA ripple. Probably quite an upgrade over the original 16v, 470uF Samxon GS which is only an 85C general purpose cap. I'll admit that going with a lower voltage cap here was a bit of a gamble since I didn't know if the 5v/5vsb supply line would go over 10v. Turns out it doesn't, at least so far. Once I get the PSU fully recapped, I'll measure the voltages again and let you know if lower voltage caps can be used elsewhere too. I'm suspecting that the caps behind the 3.3v/5v regulators can be subbed with 10v or 6.3v parts as well. The only place where I'm not planning on changing the caps with lower voltage ones (and I don't recommend anyone else do it either) is the 3.3v supply and 12v rail because these don't have over-voltage and short circuit protection (or any other protections really). In case anyone is wondering what I'm talking about, here's how the caps are arranged on the PSU board:

3.3v rail:
1x – 16v, 1000uF [Sam Young KMG] after rectifying diode (this is the 3.3v supply to the KA78R33 voltage regulator)
1x – 16v, 470uF [Samxon GS] after KA78R33 voltage regulator

5v and 5vsb rails:
1x – 16v, 470uF [Samxon GS – replaced with Nichicon HC] after rectifying diode (this is the 5v supply to the two KA78R05 voltage regulators below)
1x – 16v, 1000uF [Sam Young KMG] after KA78R05 voltage regulator (5vsb rail)
1x – 16v, 470uF [Samxon GS] after KA78R05 voltage regulator (5v rail)

12v rail:
1x – 25v, 1000uF [unknown brand, RMU series] after rectifying diode
1x – 25v, 100uF [Samxon GS] after rectifying diode

[momaka]

Re: Daewoo L700C 17” LCD – power supply problem?

Okay, the update for this was due long ago, sorry about that (was away for a month).

I finally recapped and tested the monitor - it works great now and there are absolutely no noises coming from the PSU/inverter when it comes on. I did this without removing the heatsinks - just used a small screw driver to pry each cap, one side at a time (this was possible only because the heatsinks don't go all the way down to the PCB). To insert the new caps, I pre-cut their leads (in order for them to fit under the heatsinks), then used a small screwdriver again with a piece of folded tape at the end to hold the cap to the screwdriver (makes placement of the caps under the heatsniks very easy).
As for the voltages on the output rails, they are still pretty much the same to what I posted in the last update above. Only 12v rail now stays under 16v.
Here are also the caps I used and what they replaced:
2x Sam Young KMG, 16v, 1000uF ---> 2x United Chemicon KZE, 10v, 1000uF
3x Samxon GS, 16v, 470uF ---> 3x United Chemicon KZE, 10v, 1000uF
1x (unknown brand) RMU 25v, 1000uF ---> 1x United Chemicon KZE, 25v, 470uF
1x Samxon GS, 25v, 100uF ---> 1x United Chemicon KY, 25v, 820uF, 10mm dia.
1x Samxon GS, 50v, 33uF ---> 1x Panasonic FC, 50v, 33uF
1x Samxon LP, 400v, 68uF, 25 x 21mm ---> 1x Nichicon LS 400v, 68uF, 20 x 25mm.

As you can see, most of the specs of the replacement caps have nothing to do with the originals, so I have a couple of notes:
1) Since the 12v rail has only 2 caps (1000uF and 100uF) both connected in parallel, I decided to put the 820uF cap in place of the 100uF because the 1000uF one was under the heatsink and there wasn't enough height clearance. Spot for the 100uF cap was already silkscreened for a 8mm cap, so a 10mm fit no problem. As for the 1000uF cap, it was replaced with a 470uF one. Overall capacity for the 12v rail is now still the same more or less (1000uF + 100uF before recap and 820uF + 470uF after).
2) While I used lower voltage caps on the 3.3v and 5v/5vsb rails, I don't recommend doing so for the 12v rail. Even after the recap, it's still very close to 16v, so 16v caps might not be adequate.
3) There's no need to use low ESR caps after the regulators on the 3.3v/5v/5vsb rails, just ones rated for 105C (those heatsinks get very, very hot). I used KZE because there wasn't much of a price difference there.

The only problem I couldn't get rid of is the heat - this PSU board still runs very hot, even only after 10 minutes of use. I'm not sure how long the new caps will last there - the heatsinks are doing a good job of roasting them. There's some hot glue on one of the custom heatsinks I've made (see pictures) - it gets a bit soft after the monitor's been on for a while (if that's any indication of how hot it gets inside), and that heatsink runs cool compared to the others.
I was thinking about painting the metal back cover of the LCD with black spray (since I have it anyways). Perhaps it will absorb some of the heat this way? Or is this a bad idea?

Other than that, I think I'm done. Feel free to criticize or point out anything as this is only my second monitor recap.

[englandr753]

Re: Daewoo L700C 17” LCD – power supply problem?

I have the exact same model with what appears to be the same problem. Unfortunately I'm not that experienced in soldering but have done some in the past. I see that I have nothing to lose to try to do the repair. Can you circle the specific capacitors you replaced to give me a guide as to what to look for? I have a buddy of mine that I will try to get to help me out with this as I think he has done more soldering than me. I have a desoldering tool as I expect I would need to do this correctly.

Any tips? ...and how much did these capacitors cost you total to make this repair?

thanks!

[retiredcaps]

Re: Daewoo L700C 17” LCD – power supply problem?

Any tips? ...and how much did these capacitors cost you total to make this repair?

I like these soldering videos by Curious Inventor on Youtube. Start with this, but he has more on his channel.

http://www.youtube.com/watch?v=I_NU2ruzyc4

If you live in the USA, to recap everything in post #1, it will typically be less than $10 USD from digikey.com.

[momaka]

Re: Daewoo L700C 17” LCD – power supply problem?

Hi englandr753, welcome to the Badcaps forums!

I made a map of which cap goes where. I guess I should have done that earlier. Reading back on my posts, they do seem a bit hard to follow.

Anyways, for the numbered items below, refer to the attached picture in this post.

#1: This was the cap that failed in my monitor. Originally it was a 16v, 470uF Samxon GS. I replaced it with a 10v, 1000uF United Chemicon KZE. In my tests, the voltage on this cap did not go above 6.5v, so feel free to use either a 10v or a 16v cap here. If you want to be safe, though, use a 16v cap.

#2: Was a 25v, 1000uF cap originally. I couldn't find a low ESR cap with those specs that would be small enough to fit under the heatsink, so I used a 25v, 470uF United Chemicon KZE. Since this cap is connected in parallel with #3, I had the option to use a bigger cap there instead (which I did, see #3 below).

#3: Orignal was 25v, 100uF Samxon GS. Replaced with 25v, 820uF United Chemicon KY.

#4: Was a 16v, 1000uF cap originally. Like #1, I replaced it with a 10v, 1000uF KZE.

#5 and #6: Here you can use either 6.3v, 10v, or 16v caps as the highest voltage is 5v. 470uF to 1000uF would be fine.

#7: Same as #5 and #6, but use a 1000uF cap.

#8, #9, and #10 I didn't replace because I didn't have in stock. You may want to replace #8, though, as it's pretty close to that hot heatsink. It's a 50v, 1uF I believe.

#11: This cap was not installed on my PSU board. If your PSU has this cap, replace it with one of the same exact voltage and capacitance rating. If not present, then leave it as is.

Notes:
- The cap marked 50v, 33uF was originally a Samxon GS. I replaced it with a Panasonic FC with same exact specs. If your PSU has a capacitor of different specs in that spot, stick with the same exact specs as the original cap.
- You do not need to replace the big 400v cap. I just happened to find it at a reasonable price.
- For the replacement caps, get only low ESR caps rated for 105 degrees Celsius (it's optional for the 400v one, though). Below is a list of suitable replacements (most of them are avaliable on digikey.com and mouser.com):
Panasonic - FC, FK, and FM series
Nichicon - PW, PM, and HE series
United Chemicon - KY, KZE, LXY, LXZ series
Rubycon - ZL* series (for #5, #6, and #7, YXA would also work)
Samxon - RS* (and perhaps a few others)
Sanyo - WX
* - avaliable on Badcaps.net

Any tips?

See the beginning of post #6 above. It tells how I removed the caps under the heatsinks without actually removing the heatsinks.
Also, be careful not to bridge something when you solder on this PCB - for some reason solder sticks really easily to it, even spots where there are no traces.

...and how much did these capacitors cost you total to make this repair?

About $3.50 for all of the parts, but it came to ~$10 with the shipping. All parts were purchased from Mouser.com.

Let me know if you need more help with the monitor.

[englandr753]

Re: Daewoo L700C 17” LCD – power supply problem?

Very nice info. This makes it much easier for me to consider trying now. Just to compare the issue one more time before I go off and take on this project, my monitor in the beginning would sometimes work and sometimes not but over time it degraded and now doesn't seem to work at all. The display shows no signs of a signal and the power light flashes rapidly but is green as if a signal is present.

Does that sound like the same thing your monitor was doing?

Thanks again for the updated info....

[retiredcaps]

Re: Daewoo L700C 17” LCD – power supply problem?

Does that sound like the same thing your monitor was doing?

A clear focused picture of your boards would help. A bulging capacitor is easy to spot. See the following instructions on how to insert pictures ...

https://www.badcaps.net/forum/showthread.php?t=1868

[momaka]

Re: Daewoo L700C 17” LCD – power supply problem?

The display shows no signs of a signal and the power light flashes rapidly but is green as if a signal is present.

Does that sound like the same thing your monitor was doing?

Close, but not quite. Mine would appear either totally dead (no power light at all) or would work fine. Once unplugged from the wall, though, it may or may not work the next time it was plugged in.
Since yours sounds like it has a slightly different symptom, it might also be a good idea to post some pictures of your monitor's inverter and PSU board (both front and back), as retiredcaps suggested. Chances are it's still a bad caps issue, but you should also check a few components on the inverter board, just in case.
Now, if you're absolutely sure you have the same exact inverter board as mine, then you can use the procedures below to check your inverter (you will need a multimeter for this). There are two important components to check on the inverter: the fuse and the transformers. If they test good, then the inverter is likely okay.
Let's start with the fuse. With the multimeter set to resistance check and the power to the monitor disconnected (never check resistance on a live circuit, otherwise you can damage your multimeter), check the fuse circled in red in the image below. A good fuse will show a very low resistance - usually close to 3 ohms or lower, depending on how accurate your multimeter is. Let me know what readings you get.
The second thing we check is the high-voltage side on the transformers. Again, with the multimeter set to resistance and power from the monitor disconnected, put the two probes on the spots circled in blue and note the resistance. Now try the spots circled in yellow. What readings do you get?
If the readings for the inverter turn out to be good, then a recap is probably all that this monitor needs. Speaking of caps, you should check what your PSU board has, even if it appears to be the same as mine. If there are any caps that are different from mine, let me know. Best approach would be to list all of the caps and their specs (brand, series, voltage, capacitance, diameter).