Component identified I think?

[HandyMan]

Hi everyone,

After the brilliant and spot-on help I got before when I couldn't identify a component, I thought I'd return with another component I can't identify.

(EDIT) after looking on Farnell UK website, seems it may be a tantalum electrolytic, they also stock the IC that I mention further down... pity about minimum order fee (EDIT)

Bit of background first: bought myself a faulty laptop for my parents. They're getting on a bit but do enjoy getting on the Internet, especially eBay! So my wife offered to get them a laptop so they could go anywhere and enjoy their browsing. We bought a faulty one as it's possible to get a very good spec for far less than it would cost working/new and I planned to repair it as I've repaired laptops before (not enjoyable... thousands of surface mount components that make ants look big) with a good success rate.

This one has turned out to be a bit more difficult but I've finally found the trouble. It's a Medion MD9580-A laptop incidentally, I say this because I found a forum where 8 people had the same fault symptoms so maybe someone might benefit from what I've discovered.

The 19V input voltage was going straight to ground, causing the LED on the power supply, which should be solid green, to flash/blink/flicker rapidly as it struggled with the current load. Laptop was dead.

Cause:
CB76 short-circuit
U28 short-circuit

It seems that CB76 went short, sending voltage straight to ground, this in turn meant a great deal of current passing through U28 on its way to CB76 and hence to ground, causing U28 to become short-circuit between all 8 of its legs. As to why any of this happened, I don't know. Whether other damage has been done, I cannot say until I have found replacement components and tested it. With these parts removed, the short is removed, so I'm hoping that nothing else is damaged and that there isn't still an underlying cause for the failure.

I would recommend that anyone with this laptop (also known as an ASUS L8400C) check the CPU fan. A spot of glue holding the middle of the fan base to the copper heatsink breaks away and the fan rides up with the consequence of fan blades rubbing against top fan cover. This stops the fan cooling the CPU adequately, resulting in it getting so hot you may find the plastic casing above it warped from the heat. The failed components I've found are near the CPU, U28 is almost adjacent to the CPU on the top part of the board and CB76 is on the underneath of the board and is almost directly beneath the CPU... my fan had the trouble above with associated melting of the top case plastic, so maybe heat caused the failure (?). I've used a spot of superglue, not sure how it'll hold with the heat, though.

Anyway, here is where I need help, please. Although the IC only had 7811A written on it amongst other things, I managed to discover it was this: an IRF7811A power MOSFET. Which I'm pleased about, no need to pester anyone else to find out for me. Just need to find a supplier for it now (here's hoping... :roll: ). Trouble is, I'm not sure what CB76 is.

It is a black rectangle 7x4x1.5 mm (LxWxD) and has a white stripe at one end, this white-striped end has a + symbol next to the printed rectangle and associated stripe on the PCB, so I take it they are polarised capacitors. It has this written on it: 100 (and underneath) 1642 d.

I took an identical one from the PCB and measured it with the capacitance meter and it read 128uF. So the 100 marking might mean 100uF?

(EDIT) Thing is, Farnell stock a multitude of different voltages for 100uF, so how do I know what voltage it is from the numbers on the case which I have given? (EDIT)

Thanks,
James

 

[Optikon]

Hi everyone,

After the brilliant and spot-on help I got before when I couldn't identify a component, I thought I'd return with another component I can't identify.

(EDIT) after looking on Farnell UK website, seems it may be a tantalum electrolytic, they also stock the IC that I mention further down... pity about minimum order fee (EDIT)

Bit of background first: bought myself a faulty laptop for my parents. They're getting on a bit but do enjoy getting on the Internet, especially eBay! So my wife offered to get them a laptop so they could go anywhere and enjoy their browsing. We bought a faulty one as it's possible to get a very good spec for far less than it would cost working/new and I planned to repair it as I've repaired laptops before (not enjoyable... thousands of surface mount components that make ants look big) with a good success rate.

This one has turned out to be a bit more difficult but I've finally found the trouble. It's a Medion MD9580-A laptop incidentally, I say this because I found a forum where 8 people had the same fault symptoms so maybe someone might benefit from what I've discovered.

The 19V input voltage was going straight to ground, causing the LED on the power supply, which should be solid green, to flash/blink/flicker rapidly as it struggled with the current load. Laptop was dead.

Cause:
CB76 short-circuit
U28 short-circuit

It seems that CB76 went short, sending voltage straight to ground, this in turn meant a great deal of current passing through U28 on its way to CB76 and hence to ground, causing U28 to become short-circuit between all 8 of its legs. As to why any of this happened, I don't know. Whether other damage has been done, I cannot say until I have found replacement components and tested it. With these parts removed, the short is removed, so I'm hoping that nothing else is damaged and that there isn't still an underlying cause for the failure.

I would recommend that anyone with this laptop (also known as an ASUS L8400C) check the CPU fan. A spot of glue holding the middle of the fan base to the copper heatsink breaks away and the fan rides up with the consequence of fan blades rubbing against top fan cover. This stops the fan cooling the CPU adequately, resulting in it getting so hot you may find the plastic casing above it warped from the heat. The failed components I've found are near the CPU, U28 is almost adjacent to the CPU on the top part of the board and CB76 is on the underneath of the board and is almost directly beneath the CPU... my fan had the trouble above with associated melting of the top case plastic, so maybe heat caused the failure (?). I've used a spot of superglue, not sure how it'll hold with the heat, though.

Anyway, here is where I need help, please. Although the IC only had 7811A written on it amongst other things, I managed to discover it was this: an IRF7811A power MOSFET. Which I'm pleased about, no need to pester anyone else to find out for me. Just need to find a supplier for it now (here's hoping... :roll: ). Trouble is, I'm not sure what CB76 is.

It is a black rectangle 7x4x1.5 mm (LxWxD) and has a white stripe at one end, this white-striped end has a + symbol next to the printed rectangle and associated stripe on the PCB, so I take it they are polarised capacitors. It has this written on it: 100 (and underneath) 1642 d.

I took an identical one from the PCB and measured it with the capacitance meter and it read 128uF. So the 100 marking might mean 100uF?

(EDIT) Thing is, Farnell stock a multitude of different voltages for 100uF, so how do I know what voltage it is from the numbers on the case which I have given? (EDIT)

Thanks,
James

Sounds like a tantalum cap. Another piece of evidence is that they fail short circuit and do not perform well under heated conditions. When you replace it, if you cannot solve the heat problem, consider using a "fused" tantalum. These are only slightly more expensive and provide a series fuse in the part so that they will fail open and not cause further damage.

Taking a guess at the voltage reting I would say 16V since that is a standard rating and appears in the number. BUT, you can always put in the highest voltage rating you can find that will fit the footprint and you'll be ok because chances are good the rating wasnt higher otherwise you would have found it. Size of these things is directly proportional to voltage rating and for a given footprint, it can only support so high of a volt rating.

Look at typical dimensions for 25V and 16V if on average it looks like the 16V will fit but 25V barely wont, then 16V is a safe bet. Of course if you can "fit" the 25V - thats even better yet. Try it!

 

[HandyMan]

Hi, Thanks for your reply

I was wondering why it was short-circuit, do you reckon that it's most likely the heat that's caused the short-circuit failure then? I ask because I have no experience of the tantalum capacitors whatsoever! There are other identical caps near and nearer to the heat source (CPU) than this one, but they are fine, just the whole probability thing at work? Or have I misunderstood, I've re-read your post and see you didn't say they fail S/C because of heat. So they often fail S/C when they fail, but do you know what conditions cause them to fail?

I read on a datasheet for the IC 'IRF7811A' that it is a buck converter and ideal for CPU core DC to DC conversion. Considering its location, it seems that it may possibly be used for the CPU core voltage, and from what I could see of the printed circuit appearing to go from this IC to underneath the CPU, that may be right.

That's worried me a lot now, as I'm wondering if the CPU has been damaged. Did the CPU get hot from the fan rubbing and not cooling it enough, causing the CPU to cook its insides, shorting down to ground, that current rush somehow did in the capacitor (but would it do that really?) and the IC. Or, is it more likely that the capacitor failed first, the short to ground meant too much current which cooked the IC. More likely, but why did the capacitor fail. Hopefully CPU is alright. It's a Mobile Pentium III 1GHz CPU so I would think it has a high temperature tolerance and I also thought Intel CPUs had integral overheat protection to stop them cooking themselves.

So maybe the CPU didn't fail, causing the damaged components. But then I worry because that failed buck converter IC meant that 19V was able to go straight through to the core of the CPU (if that's really where it goes) but, if the capacitor went short first, would that 19V going to ground via the S/C capacitor damage the CPU?!

Seems like all I can do is replace the two components and switch on, hoping the CPU survived and wasn't the cause of the failure (i.e. failed itself first).

Fused tantalum caps sound ideal. I'll see if I can find them. Looks like I'll have to order the IC from Farnell which means minimum order of £20 exc. VAT - shame - but they may do fused caps.

Thanks for the info on voltages. That's good advice and will be taken.

James

 

[Styx]

I dissagree Tant caps can handly temperature, You can get then that operate at 125C with only a voltage derate. Electrolytic that cannot handle temperature

 

[Optikon]

I dissagree Tant caps can handly temperature, You can get then that operate at 125C with only a voltage derate. Electrolytic that cannot handle temperature

125 yes but no one really knows how hot it was with faulty fan. And even tantalum caps rated for that will have there useful life severly de-rated.

 

[Optikon]

Hi, Thanks for your reply

I was wondering why it was short-circuit, do you reckon that it's most likely the heat that's caused the short-circuit failure then? I ask because I have no experience of the tantalum capacitors whatsoever! There are other identical caps near and nearer to the heat source (CPU) than this one, but they are fine, just the whole probability thing at work? Or have I misunderstood, I've re-read your post and see you didn't say they fail S/C because of heat. So they often fail S/C when they fail, but do you know what conditions cause them to fail?

I read on a datasheet for the IC 'IRF7811A' that it is a buck converter and ideal for CPU core DC to DC conversion. Considering its location, it seems that it may possibly be used for the CPU core voltage, and from what I could see of the printed circuit appearing to go from this IC to underneath the CPU, that may be right.

That's worried me a lot now, as I'm wondering if the CPU has been damaged. Did the CPU get hot from the fan rubbing and not cooling it enough, causing the CPU to cook its insides, shorting down to ground, that current rush somehow did in the capacitor (but would it do that really?) and the IC. Or, is it more likely that the capacitor failed first, the short to ground meant too much current which cooked the IC. More likely, but why did the capacitor fail. Hopefully CPU is alright. It's a Mobile Pentium III 1GHz CPU so I would think it has a high temperature tolerance and I also thought Intel CPUs had integral overheat protection to stop them cooking themselves.

So maybe the CPU didn't fail, causing the damaged components. But then I worry because that failed buck converter IC meant that 19V was able to go straight through to the core of the CPU (if that's really where it goes) but, if the capacitor went short first, would that 19V going to ground via the S/C capacitor damage the CPU?!

Seems like all I can do is replace the two components and switch on, hoping the CPU survived and wasn't the cause of the failure (i.e. failed itself first).

Fused tantalum caps sound ideal. I'll see if I can find them. Looks like I'll have to order the IC from Farnell which means minimum order of £20 exc. VAT - shame - but they may do fused caps.

Thanks for the info on voltages. That's good advice and will be taken.

James

Well they do not necessarily fail _because_ of heat. I merely think that the elevated heat situation probably was stressing the parts limiting there life. That cap may have lasted many thousand more hours had the heat not been so high. When up against capacitors like ceramics, the tantalums (and most electrolytics for that matter) do not have nearly the life.

Other potential causes are reverse polarity and overvoltage stress. Something else in the power supply could have failed and caused a sever reverse polarity on the cap causing it to fail.

No guarantees of course but the uP is probably ok. There is much better heat sinking on the uP and all of the IO pins have overvoltage protection diodes. I doubt if it cooked the uP. But like I said, no guarantees.

Since the tantalum is apparently part of a power supply another potential cause is ripple current rating. If this capacitor experienced ripple currents beyond its ratings (tant are usually pretty good but not immune!) then over time it would heat up and accelerate failure time. But this would be a design issue and not necessarily something more believable like fan failure.

Without making sweeping generalizations about that particular system. The most unreliable components generally are:

#1) Fan. (mechanical in nature and high duty cycle)

#2) Mechanical relays. (mechanical)

#3) Electrolytic capacitors. (evaporation, electrolyte degradation accelerated by heat)

#4) Ultra VLSI circuits such as microprocessor and chipsets simply due to laws of probability and unbelievable numbers of transistor elements. CMOS latchup possibilities & metastability on poorly designed synchronous circuits. Alpha particles flipping bits in sensitive SRAM and on and on..

 

[Styx]

I dissagree Tant caps can handly temperature, You can get then that operate at 125C with only a voltage derate. Electrolytic that cannot handle temperature

125 yes but no one really knows how hot it was with faulty fan. And even tantalum caps rated for that will have there useful life severly de-rated.

Fair point abt not knowing what the temperature was, bu tI dissagree with the shortening of life for a component rated for that temperate. Fair enough if it was an industrial temp part run at 125C, that would shorten its life (since artificial ageing test are effectively temp cycle) But a component rated for that temp will last for the legth of time it was manufactured for

 

[Optikon]

I dissagree Tant caps can handly temperature, You can get then that operate at 125C with only a voltage derate. Electrolytic that cannot handle temperature

125 yes but no one really knows how hot it was with faulty fan. And even tantalum caps rated for that will have there useful life severly de-rated.

Fair point abt not knowing what the temperature was, bu tI dissagree with the shortening of life for a component rated for that temperate. Fair enough if it was an industrial temp part run at 125C, that would shorten its life (since artificial ageing test are effectively temp cycle) But a component rated for that temp will last for the legth of time it was manufactured for

I agree if it is rated for it.. how does he know it is/was? I was refering to any amount over the rating (whatever it was) for an effective de-rating of performance and/or life.

Also, say the part is rated for 125C for 4000 hours. Does that mean this part is guaranteed to meet performance and life for 4000 hours at 125C. Not necessarily. All these parts are specified under certain operating conditions for intended use. Temperature alone doesnt tell the whole story about the lifetime one can be guaranteed. I do not know of any company that generates capacitor specifications with Max ripple current, max voltage at max temp for x hours. These are usually different tests under different conditions. Maybe some companies do this??

I strolled over to AVX and took a look at their tantalum surface mount caps. They provide ripple current ratings at a few temps. Maybe in the laptop case, the temp by itself was fine (rated at 125 and operating there) but the ripple current spec at that temp was exceeded (but not exceeded at say 85C) This would reduce the life. Also avx says this about the reliability..

"1% per 1000 hours at 85C, Vr with 0.1/V series impedance at 60% confidence level"

what does _that_ mean? 60% confidence level to me kind of says well, if operated within these specs and under these conditions there is still a chance it will fail to meet performance which is usually the case with reliability statements.. nevertheless it makes it very tough to judge actual life expectancy of these things

 

[HandyMan]

Hi,

I wonder if anyone fancies perusing this little bit of a circuit I've drawn, as it's from something I'm trying to repair but there seems to be no voltage on the line I've marked 'Output'. I'm sure that there should be.

Both transistors read okay with the meter. There is 3.3V on the input, 3.3V present on the base of the lower transistor and the same, therefore, on the emitter of the top one. And 3.3V at the base of the top transistor. I'm assuming that the voltages are the same everywhere due to no current being drawn, so even though the resistors are 10K, they aren't making much difference as I removed the transistors from circuit and found only 2V from the IC connection.

There is 0.18V on the output of the lower transistor. When the device is first switched on, that jumps from 0V to sometimes 2V but then very quickly decays to 0.18V (a split second, higher voltage only just shows on meter...).

Funnily enough, if I stick my finger over the area of the two transistors (they are tiny SMD devices), the voltage will rise to 0.50V and higher.

Any idea what I should expect to be wrong?

Thanks,
James

**broken link removed**

 

[Nigel Goodwin]

The transistors are PNP, so the output from the IC needs to go LOW to turn them on.

 

[Optikon]

Hi,

I wonder if anyone fancies perusing this little bit of a circuit I've drawn, as it's from something I'm trying to repair but there seems to be no voltage on the line I've marked 'Output'. I'm sure that there should be.

Both transistors read okay with the meter. There is 3.3V on the input, 3.3V present on the base of the lower transistor and the same, therefore, on the emitter of the top one. And 3.3V at the base of the top transistor. I'm assuming that the voltages are the same everywhere due to no current being drawn, so even though the resistors are 10K, they aren't making much difference as I removed the transistors from circuit and found only 2V from the IC connection.

There is 0.18V on the output of the lower transistor. When the device is first switched on, that jumps from 0V to sometimes 2V but then very quickly decays to 0.18V (a split second, higher voltage only just shows on meter...).

Funnily enough, if I stick my finger over the area of the two transistors (they are tiny SMD devices), the voltage will rise to 0.50V and higher.

Any idea what I should expect to be wrong?

Thanks,
James

**broken link removed**

IS this a different repair job than what was discussed in this thread earlier?

 

[HandyMan]

Yes, it is. I added it to this one as I didn't want to clutter the forum with lots of topics from me.

I got the laptop (the original repair job) for my parents and this latest is a TFT LCD monitor, 15", for myself. It's a Proview BM568 monitor using an Acer L150X2M-2 LCD panel.

The fault is a white screen showing permanently. I found that there was no voltage coming from the output of the transistor in the circuit I included. That output voltage powers up the majority of the PCB I'm working on - the source driver PCB, it's the one attached to the glass matrix itself by plastic flex cables. The gate driver PCB has no components on it, it's all run from the source driver PCB, also made by Acer.

I wonder if the voltage not going low from the IC to switch the transistors on is because the IC is faulty. It's one of those square SMD ones with a hundred legs or so, branded Acer. Who knows, it's not easy to tell, is it.

Is there a simple way I can force the 1st transistor on, just to see what happens? Would the output voltage of the lower transistor be altered by the voltage on the top transistor, i.e. I can't guarantee that by bodging it just to see what happens that I'll be pumping the correct voltage around the circuit as the IC may be able to change the output voltage by altering the base voltage of the top transistor?

And lastly, since the base of the top transistor isn't turned on and so I presume no current is flowing, would that explain why there's 3.3V everywhere, even though the resistors are 10K? I suppose the IC, when working properly, should drag the whole lot down?

Thanks again,

James

 

[Nigel Goodwin]

Is there a simple way I can force the 1st transistor on, just to see what happens? Would the output voltage of the lower transistor be altered by the voltage on the top transistor, i.e. I can't guarantee that by bodging it just to see what happens that I'll be pumping the correct voltage around the circuit as the IC may be able to change the output voltage by altering the base voltage of the top transistor?

Connect a resistor from the base of the top transistor (in the circuit) to ground, that should turn them both on - try a 240 ohm for a start, you may need to go lower, I can't be bothered to work it out!.

And lastly, since the base of the top transistor isn't turned on and so I presume no current is flowing, would that explain why there's 3.3V everywhere, even though the resistors are 10K? I suppose the IC, when working properly, should drag the whole lot down?

No, the base of a PNP transistor can only be 0.7V lower than the emitter, so the base of the lower transistor would be 2.6V, and the upper one 1.9V (roughly).

 

[HandyMan]

Thanks for that help. It's interesting, as I removed the 750 Ohm resistor which couples the IC connection to the base of the top transistor and did a voltage reading. It read 2.0V, which I say is interesting because you say the voltage on the base of that transistor would be around 1.9V. Would this imply the transistor base is getting the correct voltage from the IC, so the IC may be working correctly?

The thing is, when all components are in circuit, there is no output from the lower transistor to the rest of the circuit and voltage measurements show 3.3V on most points, even the base of the top transistor. This 3.3V is present when the transistor base and 750 Ohm resistor are independently isolated, so it seems to be coming from the main 3.3V supply through the 10K resistors.

Would it seem, then, that the 2V from the IC (when it's isolated) is too high anyway since voltage is passing through the 10K resistors from the main supply?

Thanks

 

[Nigel Goodwin]

Would it seem, then, that the 2V from the IC (when it's isolated) is too high anyway since voltage is passing through the 10K resistors from the main supply?

The output from the IC needs to go down to pretty well 0V to turn the transistor ON, 2V isn't likely to be low enough. Assuming it's a digital IC, and runs off the same 3.3V rail, 2V shouldn't ever appear there anyway - unless the pin is pulsing, and you're reading the average voltage?.

 

[HandyMan]

It's an IC branded by Acer with markings: Acer AD30302. It's the only large IC on the board. It may be pulsing, I'm afraid I have no way of telling.

It may well be that the IC has become faulty... I was hoping that wasn't the case, since I would never find a replacement IC.

I'll try connecting the base of the top transistor to ground via a 240 Ohm resistor as you recommended and see what happens. Would you recommend disconnecting the IC output to the base before doing that with the transistor, i.e. I remove the 750 Ohm resistor currently coupling the IC to the base and instead use it to couple the base to ground, so the IC output is going nowhere but the 10K resistor still remains coupled to the base, and hence also to ground via the 750 Ohm resistor?

At least this would ascertain the transistors are okay and switching, and also I'd see what happens to the screen - if the IC is knackered, it may be that no picture will display even when the voltage line is powering the rest of the circuit.

 

[Nigel Goodwin]

I'll try connecting the base of the top transistor to ground via a 240 ohm resistor as you recommended and see what happens. Would you recommend disconnecting the IC output to the base before doing that with the transistor, i.e. I remove the 750 ohm resistor currently coupling the IC to the base and instead use it to couple the base to ground, so the IC output is going nowhere but the 10K resistor still remains coupled to the base, and hence also to ground via the 750 ohm resistor?

If you're disconnecting the 750 ohm, use that to connect the base to ground - that's exactly simulating what the chip should (presumably) do.

 

[HandyMan]

That got the rest of the circuit powered up with voltage, so the chip wasn't doing what it should do. Unfortunately, the screen didn't display properly, it just had a band appear and rapidly fade away until the whole screen was white again.

Looks like the IC is no good, so nor is the monitor.

Thanks for your help, it's appreciated as always and invaluable for my learning.

James