Monitor problems

[rigeback]

I got his monitor thats gone blank "white" even on boot up. I opened the housing and checked all conections and all seems okay. The video card has
7 x 47 ouf caps that might be the cause?
Any sugestions would be much appreciated.

[seanc]

Re: Monitor problems

Check the board on the LCD panel for a fuse which might have blown. Please also supply pictures of the power supply.

[rigeback]

Re: Monitor problems

Thanks for the quick reply, I've got this pic of the power supply that I should have posted.
This fuse you mentioned is it located under the plastic liner on the back of the LCD panel, it must be real small?
Thanks.

[rigeback]

Re: Monitor problems

I've opened the lcd panel back but I can't see anything that resembles a fuse, or am I looking in the wrong place? Please advise if I have to split the panel.
Many thanks.

[newbie1]

Re: Monitor problems

Apologies if i have resized your image. But i believe the fuse is marked 'F1' just off the LVDS connector. I have marked it (red circle) on the resized image attached. Hope it helps.

[rigeback]

Re: Monitor problems

No worries, I have checked the fuse as you indicated for continuity and there is none! How should I go about changing this fuse if this is the correct method testing?
Your help is greatly appreciated,
Thanks.

[PlainBill]

Re: Monitor problems

No worries, I have checked the fuse as you indicated for continuity and there is none! How should I go about changing this fuse if this is the correct method testing?
Your help is greatly appreciated,
Thanks.

Remove the old fuse using a temperature controlled hot air rework station and solder in a replacement. If you don't have a hot air rework station, a soldering iron and Chip-Quik will remove it easily. Clean up the pads with solder braid and solder in the new fuse.

Note: the fuse might have failed spontaneously, but that is not certain. While you are at it, check the resistance across the ceramic caps (C25 and C27-C30) in the area.

PlainBill

[newbie1]

Re: Monitor problems

Out of curiosity, how do you choose the type (F,FF,FS,T), current & voltage ratings to replace it, if there is no schematic diagram or markings (either on the SMD fuse or PCB)?..

[PlainBill]

Re: Monitor problems

Out of curiosity, how do you choose the type (F,FF,FS,T), current & voltage ratings to replace it, if there is no schematic diagram or markings (either on the SMD fuse or PCB)?..

If that were true, it would be difficult. However there is a marking on this fuse; it appears to be a 'K', which corresponds to 1.5A. The voltage rating is immaterial - the supply voltage is usually 5 volts; the lowest voltage SMD fuse I've seen is 32 volts.

PlainBill

[rigeback]

Re: Monitor problems

I have digging high and low to find a replacement for this fuse, old mother boards and even dismantled an old lap top.
Is it not possible to just hot wire this fuse because I don't think I'm gonna find one around here?

[PlainBill]

Re: Monitor problems

I have digging high and low to find a replacement for this fuse, old mother boards and even dismantled an old lap top.
Is it not possible to just hot wire this fuse because I don't think I'm gonna find one around here?

My first reaction was to wish I could grab a baseball bat, reach through my monitor, and give you a good swat. Here's why.

About 30 years ago I worked for a chemical research and manufacturing firm. My job was keeping the electronic equipment running. One day 'Jim' turned on an older piece of equipment and it blew a 1 Amp 125 Volt fuse. He decided to pop in a 2 Amp 125 Volt fuse. That blew too, so he figured a 10 Amp, 32 Volt fuse would last. This time there was a flash and smoke from the rear of the equipment.

At that point he decided to call the expert - me. I discovered that Jim had been twiddling with various screws and had managed to short the power leads to the heater. When the 10 Amp fuse blew it took out the power control module, specifically a couple of SCRs and the pulse transformer which drove the gate of the scrs. Simple repair, huh?

Well, this piece of equipment was over 10 years old, and it was an experimental version. The company that built it was no longer in existence. The chance of finding another unit to cannibalize was nil. All we had was the destroyed module and a hand-drawn schematic. The part numbers of the SCRs and the transformer were still visible. The SCRs were no longer manufactured, and what references I could find were inconsistent as to their specs. The transformer was still available, but at a $50 minimum order surcharge. (At the time, $50 would buy a very nice dinner for 2 at a fancy restaurant). I was eventually able to locate replacement SCRs, but of a different case style, which required modifying the heat sinks.

It was several months before the repair was complete. The parts and labor cost was nearly half of what the equipment had cost originally, and was a significant fraction of the cost of buying a new one. All because some thumb fingered fool had decided to bypass a fuse.

NOW, if you have determined why the fuse blew in the first case, or you don't really care if the monitor is destroyed, go ahead. But have a video camera going se you can post the carnage on You-tube.

PlainBill

[momaka]

Re: Monitor problems

I agree with PlainBill - do not replace the fuse with a wire! More often than not, those fuses do not blow for no reason. Likely a shorted ceramic cap.
As PlainBill pointed out, check resistance across C25 and C27-C30. Since C27-C30 are all connected in parallel, you can also just measure resistance between Vaa and ground (Vaa is the test point to the right of C27) instead of measuring resistance for C27-C30.
Let us know what results you get.

[rigeback]

Re: Monitor problems

Okay guys I've tested C-27 - C-30 and I get a consistant reading of 1020 between Vaa.
C-25 on the other hand gives a fluctuating reading 725 - 730. I must admit the batteries on my tester need changing so the readings might be a bit off!
Please advise what readings I should be getting if these seem off the mark?

[PlainBill]

Re: Monitor problems

Okay guys I've tested C-27 - C-30 and I get a consistant reading of 1020 between Vaa.
C-25 on the other hand gives a fluctuating reading 725 - 730. I must admit the batteries on my tester need changing so the readings might be a bit off!
Please advise what readings I should be getting if these seem off the mark?

Those readings indicate the caps are not shorted. A reading of less than 10 ohms would indicate a short. Readings in the range of 100 ohms would be worrysome.

PlainBill

[momaka]

Re: Monitor problems

Yup, those readings are quite reasonable.
Next, check resistance between test point VGH and ground and also between VGL and ground.

Then, check diodes D1, D2, D6, and the big diode under the inductor (D3D4???). To do that, set your multimeter either to diode check or to 2000 Ohm scale (if it's manual ranging). Put black (-) probe on cathode (the side of the diode marked with a white stripe) and the red probe (+) on anode (side NOT marked with a white stripe). Note the reading on your multimeter. Now reverse the probes and note the reading again. Let us know what numbers you get.

Lastly, measure resistance across capacitors C21, C22, C24, C26.

[rigeback]

Re: Monitor problems

Guys, got some more numbers!
VGL = 32.8
VGHP1 = 830
VGH1 = 1940 (Intermitant, does not hold)
D-1 = 815 /1170
D-2 = 1350 / 1350
D-3&4 = 43.2 / 107.5
D-5 = 995 /208
D-6 = 1680 / 0 (Intermitant, does not hold)
C-21 = 1325
C-22 = 1290
C-23 = 813
C-25 = 725
C-26 = 342
The big sucker with (100 marking NR D-3&4) gives no reading (0)?
The items listed (Intermitant, numbers do not hold) gives a quick reading then nothing, might be my tester?
I will be going away for a week and might buy some equipment like new tester and cold soldering gun!

[momaka]

Re: Monitor problems

VGH1 = 1940 (Intermitant, does not hold)
D-6 = 1680 / 0 (Intermitant, does not hold)

It's okay if the measurement doesn't hold to a single value. As long as you get a resistance above 500 Ohms that keeps climbing, you don't really have to worry about it.
For D6, try scratching the leads with your multimeter's test probes (but do not apply much pressure) and then retake the measurements. Sometimes, there's leftover flux on the joints from the factory, and this may cause those intermittent readings. VGH1 is a test point, so it shouldn't have any on it, though.

The big sucker with (100 marking NR D-3&4) gives no reading (0)?

Is this the round inductor (L1/L2) marked with "100" on top you're referring to? If so, you should get a very low resistance across it, most likely under 5 Ohms.

VGL = 32.8
...
D-3&4 = 43.2 / 107.5

Okay, these measurements are a bit worrisome.
VGL is the negative supply rail for the TFT. On the monitors I've worked with, this rail had a much higher resistance, mainly because it's usually a low-current rail. Can't say for sure if yours is bad, since C26 checked out good. Check resistance (2000 Ohm scale) on C43 and C46 (bottom left, 3rd picture, post #4) - perhaps VGL may be connected to them. Let me know what they measure.

As for D3D4, the forward reading seems way too low as well. Put your red (+) probe on the anode of D3D4 (side not marked with a white stripe) or on the lower pad of the inductor, and measure resistance to ground. Let me know what reading you get.

The readings on all of the other components look good.

The items listed (Intermitant, numbers do not hold) gives a quick reading then nothing, might be my tester?

If you get a quick reading that then disappears and you get a 1 on the left side of the screen, then the resistance is higher than 2000 Ohms (assuming you were using the 2000 Ohm scale). This is actually the proper behaviour for manual-ranging multimeters. With that said, your multimeter is more than likely working fine.

[rigeback]

Re: Monitor problems

Hi guys back again with new batteries and a new multimeter!
I have remeasured some of the diodes and there seems to some variations
D-1 = 750 / 830 Suspect gives reading only auto tester
D-2 = 0.925
D-3&4 = 37.0 / 146.7 - resistance to ground = 38.8
D-5 = 1.094 -1.001 / .475 - .876
D-6 = .874 one way only
C-26 = / 0.825
C-27/28/29/30 = 1019
C-46 = 1.916 / 0.739
C-48-43? = 0.349 / 0.572 (C-43 looks likr 48?)

[momaka]

Re: Monitor problems

D-3&4 = 37.0 / 146.7 - resistance to ground = 38.8

Again, those readings in bold seem very low.

If I am interpreting this correctly, the 38.8 Ohm is on the 5v rail (the one connected to the fuse F1). So, check resistance across C1 (or C2, or C32). Also measure resistance between top (single) pin of U1 and ground. Post back what results you get.

I'm beginning to think either Q2 or one of the capacitors (C1, C2, and C32) on the 5v rails may be shorted. But let's see from your readings.

[rigeback]

Re: Monitor problems

I have reverted to using the automatic resistance tester as it seems more accurate with these low readings.
C-1 = 147 / 39.7
C-2 = 146.3 / 39.5
C-32 = 146.2 / 39.1
U-1 = 42.3 (single) pin of U1 and ground)
Q-2 = 40.1 (single) pin of U1 and ground)