Lenovo ThinkVision L191 19" goes blank after <1 sec

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Thanks for that. Here's a higher quality pic of power/inverter board.

Please see attached image - (2000 px wide)

[el3ctroded]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Some other things to check that are real quick tests:
Check for shorts on all pins of Q203, Q206, Q207
Do diode test on D201

J921: I think this is it's designator, hard to tell. But it looks like it isn't well-connected on the bottom of the pcb because of maybe being made too short. It's a long wire jumper next to Q203 and Q207. Make sure it's making good connection.

Better pic of the top of the psu plz.

[PlainBill]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Pin 10 of IC201 controls the power to the Royer oscillator. What is it's voltage after the backlights turn off. Also, it appears the feedback from the current sensing caps goes to pin 3 of IC 201. Please verify the top of R232 goes to pin 3 of IC201.

PlainBill

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Some other things to check that are real quick tests:
Check for shorts on all pins of Q203, Q206, Q207

No shorts on these.

Do diode test on D201

I'm not too sure what to expect for the diode - the meter's diode setting says 2k, and the two readings I got were:
0.266
0.729

J921: I think this is it's designator, hard to tell. But it looks like it isn't well-connected on the bottom of the pcb because of maybe being made too short. It's a long wire jumper next to Q203 and Q207. Make sure it's making good connection.

I'm not sure of this one. I can see the J921 label on the rear of the board, but not sure which connection to check. On the front of the board, there looks to be a space for a connector, but with nothing there (just the white printed line).

Better pic of the top of the psu plz.

See attached.

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Pin 10 of IC201 controls the power to the Royer oscillator. What is it's voltage after the backlights turn off.

I'm getting 0.09. Note I opted to put the other probe on one of the four nodes to the left of the screw-hole by the RoHS logo. Is that suitable?

Please verify the top of R232 goes to pin 3 of IC201.

Yes it does.

[el3ctroded]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

J921 wasn't used, so don't worry about that. In the old pic I couldn't tell it wasn't there.

D201: Your reading depends upon the type of diode used. This was probablly a schottky, so around 0.2 to 0.3 is good. However you shouldn't get reading both directions, but sometimes circuitry makes readings not as expected. Since you got readings in both directions, you will have to de-solder one side of the diode and re-do the test.

[PlainBill]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

I'm getting 0.09. Note I opted to put the other probe on one of the four nodes to the left of the screw-hole by the RoHS logo. Is that suitable?

Yes it does.

That was fine. I'm going to give you the Cliff's Notes version of what is going on with this circuit.

The return lines from the 4 CCFLs go through resistor - diode networks that charge C223. The voltage on C223 is dropped by diode D210 , further reduced by R231 and R232, then goes to IC201, pin 3. The threshold for this is IC201, pin 4. If pin 3 is at a lower voltage than pin4, the controller shuts down the output.

The easiest way to troubleshoot would be by forcing the CCFLS on, but I REALLY hate disabling the feedback loop, so we will proceed in another direction.

1. Measure the resistance across R225, R226, R227, and R228. The should be identical.

2. Set your DMM on the diode test scale and measure the voltage drop on D202, D204, D206, and D208. The should be the same.

3. Also measure the voltage drop on D203, D205, D207, and D209. Again, all four should be the same.

4. What is the voltage drop on D210?

PlainBill

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Hi again, interesting info. And you're spot on.

1. R225, R226, R227, and R228 are 0.98, 0.97, 0.98 and 0.97 on the 20kΩ scale.

2.
D202: 0.780 / 0.866
D204: 0.780 / 0.860
D206: 0.780 / 0.863
D208: 0.786 / 0.867

3.
D203: 0.870 / 1.789
D205: 0.863 / 1.781
D207: 0.862 / 1.783
D209: 0.865 / 1.785

4.
D210: 0.900 / 1. (this is how the meter displays it... though "1." is also displayed when the probes aren't on anything with the diode setting - I'm fairly confident they were contacting ok)

[PlainBill]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Hi again, interesting info. And you're spot on.

1. R225, R226, R227, and R228 are 0.98, 0.97, 0.98 and 0.97 on the 20kΩ scale.

2.
D202: 0.780 / 0.866
D204: 0.780 / 0.860
D206: 0.780 / 0.863
D208: 0.786 / 0.867

3.
D203: 0.870 / 1.789
D205: 0.863 / 1.781
D207: 0.862 / 1.783
D209: 0.865 / 1.785

4.
D210: 0.900 / 1. (this is how the meter displays it... though "1." is also displayed when the probes aren't on anything with the diode setting - I'm fairly confident they were contacting ok)

The resistors match, but I hadn't really expected a problem there. D208 has a very slightly higher voltage drop; still it's within 6 millivolts, not likely to be the problem. Those all look fine.

On the other hand, the D210 reading does NOT look right. Most DMMs apply 2 Volts through a resistor across the leads on the diode test scale and display the actual voltage. As far as I know, a small signal diode should NEVER have a .9 volt forward drop with such low current. If you have a high speed switching diode such as a 1N4148, try soldering it across D210. Otherwise, try a resistor in the range of 1K to 10K if you have one available. If neither of those are available, try shorting across the diode.

If you use a resistor or a jumper, only try it for a few seconds - just long enough to verify the backlights are not shutting down. I don't want to risk damaging anything.

PlainBill

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

el3ctroded - I've not yet de-soldered one side of D201, I'm not the most confident at desoldering and resoldering, but will if it's necessary...?

In the mean time, I've done PlainBill's test and shorted across D210 (didn't have any resistors handy) and this did allow the monitor to remain on. I left it for about 5 sec - definitely longer than it used to stay on. Does that mean D210 is the problem, or just an indicator of another problem?

Thanks. -JK

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Here's D210 close up.



If this needs replaced, any idea on the type/rating etc.?

[el3ctroded]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Don't worry with D201, obviously not a problem

D210 should be replaced with any of these: 1n4148 or 1n914 or 1n4448

[PlainBill]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

The real test would be to unsolder it and perform the diode check out of circuit. As I said earlier, the forward voltage drop seems to be too high. Perhaps someone else has more experience with 'reading' these diodes.

Radio Shack # 276-1122 is a package of 1N914 / 1N4148 diodes. I would suggest picking up one,, and performing three tests.

1. Check the forward voltage drop of one of the new diodes with your DMM. I would expect a reading of about .7 volts.

2. Check the voltage drop of the original diode. If it reads open in both directions it is definitely bad.

3. Replace D210 with one of the new diodes (yes, you will have to be slightly creative installing it). Then test the monitor. I'd watch it carefully for the first ten minutes of so for any sign of problems, then take it in steps before reassembling it.

PlainBill

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Excellent. I'll do these tests - it'll need to be tomorrow - and check local suppliers for a replacement.

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Well, unfortunately, the diode crumbled as I removed it. Either I was too heavy-handed, or it already had physical flaws - I don't know.

I'll get a replacement (i.e. one of these from maplin.co.uk tomorrow (Mon) as I work near one of their stores.

[PlainBill]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Well, unfortunately, the diode crumbled as I removed it. Either I was too heavy-handed, or it already had physical flaws - I don't know.

I'll get a replacement (i.e. one of these from maplin.co.uk tomorrow (Mon) as I work near one of their stores.

Odds are it had already cracked. That's why the inverter wasn't working.

PlainBill

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

OK, not sure what's going on. I got a 1N4148 from Maplin and soldered it in temporarily in place of D210 (making sure the black band was at the same end as the old D210), but got the same 2 sec to black behaviour.

I measured the diode in place and was getting the open reading in one direction ("1.") and 0.8ish (didn't record it) in the other direction.

Having taken it out the circuit again, I measured the diode and got the same, "1." and 0.899. I'm now kicking myself for not measuring the readings before connecting it and switching on. Could I have blown that diode?

[el3ctroded]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

1. is what you meter reads when not connected right? Diodes should read nothing in reverse polarity, which is what you got.

The diode seems to be reading high though. Did you only buy 1?

[jonathanku]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Yeah - 1. is the diode reading when not connected. And yes, I only bought 1.

I have noticed though, that this is the same reading as I had with the old diode. But if they both blew when connected to the monitor, that might explain it - assuming there's a different root cause. I'll get another couple tomorrow and test them both before connecting them to the circuit.

[el3ctroded]

Re: Lenovo ThinkVision L191 19" goes blank after &lt;1 sec

Lets make sure none of the resistors in that circuit have failed (usually will go open circuit). Measure resistance of R232, R231, R230, R229.