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Old 07-26-2010, 12:03 PM   #1
retiredcaps
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Default CMV CT-934D won't power on - secondary voltages disappear

CMV CT-934D won't power on - no power LED

SUMMARY
======
This is a CMV CT-934D made in 2004 LCD. It won't power on. The power LED does not turn green.

THINGS TESTED
==========

1) Measured steady 164V DC across main filter capacitor.
2) With the power board ONLY (i.e. logic board is disconnected), the secondary voltages 19V and 5V DC are present at connector location CN100. The 19V and 5V DC voltages are steady and do no fluctuate.
3) Fuse FP1 on logic board reads 0.3 ohms.
4) All connectors from power board connector CN100 to logic board location CN2 read 0.3 ohms. That is, the connectors are fine.

PROBLEM
======

1) Press the power button and the power LED does not come on.
2) When the logic board is connected to the power board, the voltages found at CN100 disappear. The 5V DC disappears immediately. The 19V DC drops slowly from 19V to 0V. I can see the voltage dropping on the multimeter. It drops about 1V every 5 or 10 seconds. The voltages at CN2 on the logic board show the exact same behaviour as CN100 (drop and disappear).

QUESTION
=======

What could cause the secondary voltages to drop/disappear when the logic board is connected to the power board? When the logic card is disconnected, the secondary voltages are solid and steady on the power board.
Attached Images
File Type: jpg logic-back.jpg (255.6 KB, 1772 views)
File Type: jpg logic-front.jpg (523.6 KB, 3282 views)
File Type: jpg overall.jpg (426.2 KB, 1591 views)
File Type: jpg powerboard-back.jpg (384.9 KB, 2898 views)
File Type: jpg powerboard-front.jpg (285.1 KB, 6922 views)
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Old 07-26-2010, 01:12 PM   #2
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by retiredcaps
CMV CT-934D won't power on - no power LED

SUMMARY
======
This is a CMV CT-934D made in 2004 LCD. It won't power on. The power LED does not turn green.

THINGS TESTED
==========

1) Measured steady 164V DC across main filter capacitor.
2) With the power board ONLY (i.e. logic board is disconnected), the secondary voltages 19V and 5V DC are present at connector location CN100. The 19V and 5V DC voltages are steady and do no fluctuate.
3) Fuse FP1 on logic board reads 0.3 ohms.
4) All connectors from power board connector CN100 to logic board location CN2 read 0.3 ohms. That is, the connectors are fine.

PROBLEM
======

1) Press the power button and the power LED does not come on.
2) When the logic board is connected to the power board, the voltages found at CN100 disappear. The 5V DC disappears immediately. The 19V DC drops slowly from 19V to 0V. I can see the voltage dropping on the multimeter. It drops about 1V every 5 or 10 seconds. The voltages at CN2 on the logic board show the exact same behaviour as CN100 (drop and disappear).

QUESTION
=======

What could cause the secondary voltages to drop/disappear when the logic board is connected to the power board? When the logic card is disconnected, the secondary voltages are solid and steady on the power board.
Probably a short on the 5V rail on the logic card. Most monitor power supplies use a transformer with three secondary windings. One secondary (I often refer to it as a tertiary winding) powers the SMPS controller. Another is rectified to provide 5 volts. The last winding is rectified to provide the inverter voltage - anywhere from 12 to 20 volts, depending on design. On an SMPS design with current limiting on the primary an excessive load on a secondary will cause the SMPS to shut down. Because there is very little load on the 19V supply, it's voltage drops slowly.

Now you have the fun of locating the short on the logic card. I would suggest investigating the four voltage regulators, U8, U10, U12, U13. If it turns out I am wrong (it happens) and there is no short, investigate the small capacitor in the SMPS primary.

Edit: Interesting, I just noticed there are TWO optoisolators on the power supply. How interesting. What, pray tell is U101 on the power supply?

PlainBill
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Last edited by PlainBill; 07-26-2010 at 01:18 PM..
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Old 07-26-2010, 02:08 PM   #3
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by PlainBill
Now you have the fun of locating the short on the logic card. I would suggest investigating the four voltage regulators, U8, U10, U12, U13. If it turns out I am wrong (it happens) and there is no short, investigate the small capacitor in the SMPS primary.

Edit: Interesting, I just noticed there are TWO optoisolators on the power supply. How interesting. What, pray tell is U101 on the power supply?

PlainBill
I measured the ohms on U8, U10, U12, and U13. U10 with red on pin 1 and black on pin 2 measured 146 ohm (I measured this 4 times). All other voltage regulators readings were well over 800 ohms.

Pin 3 of U8 and U10 are tied to the 5V rail. I figured this out by using the continuity beep test by probing around.

On the CN110 connector (read vertical down), the 2nd pin is GND, 3rd is 5V and last pin is 19V.

The small cap at CE107 was the first thing I replaced when I didn't get secondary voltages (before I discovered it was stable without the logic card). The original cap was a Capxon 22uF 50V.

I replaced it with a Samxon GK 22uF 25V. When I first replaced it, I didn't realize my mistake of substituting in a lower voltage cap. It wasn't until I went to write this response and checked the original cap that I had made this mistake.

Anyways, I don't have a 22uF 50V spare. And I had the same problem of disappearing voltages before with the original Capxon cap. If I had to, I could try and see if one of my working boards has a 22uF 50V that I could borrow to figure this one out.

U101 is LM358 PBGP. Datasheet is at

http://www.datasheetcatalog.com/data.../5/LM358.shtml

PS. I also changed out 2 caps that were rated 10V (all others are rated 25V), just to rule out bad caps on the 5V rail. I used Rubycon MCZ and Nichicon HN(M). Same problem with disappearing voltages.

Last edited by retiredcaps; 07-26-2010 at 02:12 PM..
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Old 07-26-2010, 04:26 PM   #4
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by retiredcaps
I measured the ohms on U8, U10, U12, and U13. U10 with red on pin 1 and black on pin 2 measured 146 ohm (I measured this 4 times). All other voltage regulators readings were well over 800 ohms.

Pin 3 of U8 and U10 are tied to the 5V rail. I figured this out by using the continuity beep test by probing around.

On the CN110 connector (read vertical down), the 2nd pin is GND, 3rd is 5V and last pin is 19V.

The small cap at CE107 was the first thing I replaced when I didn't get secondary voltages (before I discovered it was stable without the logic card). The original cap was a Capxon 22uF 50V.

I replaced it with a Samxon GK 22uF 25V. When I first replaced it, I didn't realize my mistake of substituting in a lower voltage cap. It wasn't until I went to write this response and checked the original cap that I had made this mistake.

Anyways, I don't have a 22uF 50V spare. And I had the same problem of disappearing voltages before with the original Capxon cap. If I had to, I could try and see if one of my working boards has a 22uF 50V that I could borrow to figure this one out.

U101 is LM358 PBGP. Datasheet is at

http://www.datasheetcatalog.com/data.../5/LM358.shtml

PS. I also changed out 2 caps that were rated 10V (all others are rated 25V), just to rule out bad caps on the 5V rail. I used Rubycon MCZ and Nichicon HN(M). Same problem with disappearing voltages.
The lower voltage cap shouldn't be a problem. Many manufactures don't make 22F caps in the lower voltages.

On this one, I would suggest tracing the outputs of the two optoisolators. One should be used for feedback for voltage regulation. I just came across an application note that used a second optoisolator to shut down the SMPS controller.

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Old 07-27-2010, 02:26 AM   #5
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by retiredcaps
I measured the ohms on U8, U10, U12, and U13. U10 with red on pin 1 and black on pin 2 measured 146 ohm (I measured this 4 times). All other voltage regulators readings were well over 800 ohms.

Pin 3 of U8 and U10 are tied to the 5V rail. I figured this out by using the continuity beep test by probing around.
More important is to identify the voltage at their pins - looking for one (or more) with an output dragged down to ground or similar.
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Old 07-27-2010, 02:56 PM   #6
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by PlainBill
On this one, I would suggest tracing the outputs of the two optoisolators. One should be used for feedback for voltage regulation. I just came across an application note that used a second optoisolator to shut down the SMPS controller.
Can you send a link for that application note? I'm having a tough time finding the datasheet for PC617?

U100 and U103 (optoisolators) is part

B T2
PC617
Sharp

Pins 1 and 2 are on the secondary side. Pins 3 and 4 are on the live power side.

I tested both with the diode functionality on pins 1 and 2 and got 1.04 in one direction and 0L in the other. Pins 3 and 4 give 0L in both directions.

I measured ohms between pin 1 and 2 and they are not shorted. Same for pin 3 and 4.

I'll do the trace.

All the diodes on the back in that section test okay.

ZD102 measures0.72 and 0L with the diode test.
D102 measures 0.59 and 0L with the diode test.
ZD101 measures 0.72 and 0L in the diode test.
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Old 07-27-2010, 02:59 PM   #7
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by pedro
More important is to identify the voltage at their pins - looking for one (or more) with an output dragged down to ground or similar.
I'm not sure I completely understand what you are asking, but the input and output pins of all the voltage regulators on the logic card measured 0V I if remember correctly. This would make sense since the 5V disappears right away.
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Old 07-27-2010, 04:28 PM   #8
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by retiredcaps
Can you send a link for that application note? I'm having a tough time finding the datasheet for PC617?

U100 and U103 (optoisolators) is part

B T2
PC617
Sharp

Pins 1 and 2 are on the secondary side. Pins 3 and 4 are on the live power side.

I tested both with the diode functionality on pins 1 and 2 and got 1.04 in one direction and 0L in the other. Pins 3 and 4 give 0L in both directions.

I measured ohms between pin 1 and 2 and they are not shorted. Same for pin 3 and 4.

I'll do the trace.

All the diodes on the back in that section test okay.

ZD102 measures0.72 and 0L with the diode test.
D102 measures 0.59 and 0L with the diode test.
ZD101 measures 0.72 and 0L in the diode test.
The PC617 is a jellybean part. Equivqalent part numbers for the optoisolator are LTV817BM and PS2561L1-1-VW. It's an LED facing a phototransistor with an insulating material (glass?) separating them. As the output voltage increases, the current through the LED increases. This causes the phototransistor to turn on, increasing the voltage on the feedback input of the SMPS controller.

I'll try to figure out what power supply schematic I was looking at with the dual optoisolators.

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Old 07-27-2010, 04:47 PM   #9
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Remember, this application note is for what may be a different SMPS controller (you didn't mention (and I didn't ask)) what that part number is. However, it describes something similar to what this monitor has. Look at the circuit on page 10, particulary the part in blue.

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Old 07-27-2010, 07:31 PM   #10
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by retiredcaps
I'm not sure I completely understand what you are asking, but the input and output pins of all the voltage regulators on the logic card measured 0V I if remember correctly. This would make sense since the 5V disappears right away.
Ah, I was hoping that the death of the 5V was "fast but not instantaneous", so that by monitoring the various reg outputs you might see a transient reading on some (but none on the culprit with the shorted output).
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Old 07-27-2010, 09:16 PM   #11
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by PlainBill
Remember, this application note is for what may be a different SMPS controller (you didn't mention (and I didn't ask)) what that part number is. However, it describes something similar to what this monitor has. Look at the circuit on page 10, particulary the part in blue.

PlainBill
Thanks.

The concept is what I'm trying to understand first. I'll study it and see if I can take it to the next level.
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Old 07-27-2010, 09:18 PM   #12
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by pedro
Ah, I was hoping that the death of the 5V was "fast but not instantaneous", so that by monitoring the various reg outputs you might see a transient reading on some (but none on the culprit with the shorted output).
When I hit the switch on the power bar and get the probes in place, the 5V rail is already down in the 0.35V range. And I don't think my multimeter is that fast to capture a 5V -> 0.35V drop.
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Old 07-28-2010, 12:28 AM   #13
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by retiredcaps
When I hit the switch on the power bar and get the probes in place, the 5V rail is already down in the 0.35V range. And I don't think my multimeter is that fast to capture a 5V -> 0.35V drop.
If you can't get the probes pre-connected (i.e. solder a short stub onto a relevant track) what I would do - and it comes with the usual caveat(*) - is to put an external current-limited (but not foldback) 5V supply onto the video board, and "smoke out" the fault. By allowing it to pull a decent current the offending device can often be located. The monitor's PSU states 2.5A @ 5V, so I'd start with something less than 0.5A and see what gets warm to touch. You DO risk blowing a voltreg if its load is the short and it can't handle the current/power into the short, but it DOES help to locate the short. The only other route that works for me is to start lifting voltregs - either totally or one of the I/O legs - and see when the short disappears.

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Old 07-28-2010, 12:52 PM   #14
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by retiredcaps
When I hit the switch on the power bar and get the probes in place, the 5V rail is already down in the 0.35V range. And I don't think my multimeter is that fast to capture a 5V -> 0.35V drop.
Retiredcaps,

Did you check these CrapXon caps on the p/s for ESR and capacitance? They don't look domed but you say this monitor was made in 2006.

If the logic board regulators are OK (i.e., not shorted), then go ahead and try with a current limit p/s and feed 5V to the logic board.
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Old 07-28-2010, 04:08 PM   #15
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Go ahead and replace the caps anyway - i've had a similar issue on LG monitor (power supply would shut down when the display board was plugged in, but running the CCFLs alone worked fine) and all there was to it was the caps. If it still doesn't work, at least you ruled them out.
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Old 07-28-2010, 10:51 PM   #16
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by lucky13
Retiredcaps,

Did you check these CrapXon caps on the p/s for ESR and capacitance? They don't look domed but you say this monitor was made in 2006.

If the logic board regulators are OK (i.e., not shorted), then go ahead and try with a current limit p/s and feed 5V to the logic board.
I don't have an ESR tester. I did replace all caps that were on the 5V rail, but not on the 19V rail. The capxon are all 25V rated which I assume is on the 19V rail.

I may try and left the legs off the regulators to see if I can narrow down the short.

I tested the voltage regulators in an earlier post and none are shorted using a resistance test. If you have another multimeter test to suggest on the voltage regulators, let me know.
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Old 07-28-2010, 10:55 PM   #17
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by Th3_uN1Qu3
Go ahead and replace the caps anyway - i've had a similar issue on LG monitor (power supply would shut down when the display board was plugged in, but running the CCFLs alone worked fine) and all there was to it was the caps. If it still doesn't work, at least you ruled them out.
Thank your for sharing. I'll keep it in mind.

One reason I'm hesitant to change the caps right away is I'm trying to learn more about the sense circuitry. I'm in no rush or need to fix this monitor quickly.

It could very well be the caps causing the problem, but I want to see "proof" through detective work and inference rather than just a "blind" cap job.
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Old 07-29-2010, 03:54 AM   #18
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Well, if you don't have an oscilloscope to actually see what's happening there isn't much to learn by just looking at the board. When i changed the caps in that monitor the result was visible on the scope - a dramatic reduction in ripple on the 5v rail.

I've worked without a scope for quite a while, but at one point i decided that i can't work like that anymore (that was a few months ago, when i decided to get into SMPS stuff seriously), and spent $120 on an old Russian scope. It's built like a tank (also about as heavy as one but oh well...), works up to 100-something MHz, and the seller also gave me the full service manual and schematics for the thing. I also bought some 50-ohm coax and a couple nice BNC connectors, and 3 hours later i had my own 1x and 10x probes without spending another $25. The body of the 1x probe is a mechanical pencil, for the 10x i had to use a marker as the voltage divider wouldn't fit in anything smaller. :p

Last edited by Th3_uN1Qu3; 07-29-2010 at 04:01 AM..
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Old 08-27-2010, 06:15 PM   #19
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Quote:
Originally Posted by Th3_uN1Qu3 View Post
Go ahead and replace the caps anyway
So today I replaced the caps one at a time and then retested for disappearing secondary voltages. I hate recapping all and if it works not knowing which one was faulty.

By repeating the above process, I eventually recapped all the caps on the power/inverter board and I still get disappearing voltages.

All caps I used are new Rubycon low ESR caps (ZL or MCZ).

I'm not giving up though.
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Old 08-28-2010, 07:02 AM   #20
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Default Re: CMV CT-934D won't power on - secondary voltages disappear

Although 'logic' says the fault is on the 'Logic' PCB(pardon the pun..).I assume that if you had another Logic PCB,you would have tried that to confirm that is the case ????.
A situation that can occur with any Power Supply,is that although a Meter can measure the Voltage outputs ie NO Load,,,when a Load is applied the Voltage disappears,as the PSU cannot supply the Current requirement,even though there is NO shorts in/on the Load itself.
I have done a few measurements this morning,and perhaps the following may be of interest:-
Set up the PSU/Inverter ONLY..ie NO Logic PCB,Plug in the CCFL's,and on the connector which normally goes to the Logic PCB,apply Approx 3volts DC(from an external battery perhaps),between the ON/OFF pin and Ground.This will light the Lamps.HOWEVER I had lying around a little PC Fan,a Standard 12v,250Ma one,and a little experimenting showed that applying 12v to it, it actually took 62Ma,BUT applying
5V to it the current increased to approx 160Ma,So with the Set up as above,if you have a Fan,or similar, put it on the same connector between the 5v pin and Ground.The question is DO the CCFL's light ???and the Fan spin ???
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