Author Topic: EEVblog #398 - Lecroy 9384C Oscilloscope Repair  (Read 31915 times)

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Offline EEVblog

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EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« on: December 17, 2012, 07:23:56 am »


Dave.
 

Offline mikeselectricstuff

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #1 on: December 17, 2012, 08:23:49 am »
Here are my tips for finding supply shorts.
Using resistance measurement, even on a high-precision meter is generally fairly futile due to contact resistance variation
Instead, apply about an amp or two across the supply and measure mv levels across the rails at various places. The voltage will reduce as you get nearer the fault. This effectively gives you a 4-terminal measurement, avoiding the issue of contact resistance

I've also use the 'blow the arse out' method. Now I have thermal imager this approach should be a lot easier in future!
An IR thermometer can also be used by scanning it carefully across the board.
 
The other useful tool is the i-prober. You need to switch the supply with an external MOSFET so you're looking for an AC signal to avoid the position sensitivity.
 
This could be a game-changing tool for faultfinding : a $175 thermal imager
http://www.kickstarter.com/projects/andyrawson/ir-blue-thermal-imaging-smartphone-accessory?ref=card
« Last Edit: December 17, 2012, 08:26:03 am by mikeselectricstuff »
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Offline ceecrb1

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #2 on: December 17, 2012, 08:46:07 am »
A great video and very well timed for me..
At work, I am currently being beaten by a robotic moving light (martin mac 2000) which refuses to move its pan stepper motor but will "brake" it..
So your video just was great for a "back to basics" remind me what I am missing, remind me of the stages I am missing out and the corners I have cut.

I did find you complaining about spending an hour or so on it really funny.. As I am no where near as good an engineer, it can take me a lot longer.. If I had gotten that far with so little head scratching etc in an hour or so I would be going home very happy with myself!

Isnt it funny how self crittical we can really be!

Really hope you get to the bottom of it, and that you upload the video of showing how you got there!

 

Offline adrianblack

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #3 on: December 17, 2012, 08:56:35 am »
Silly question, but what if it was the power supply the whole time? Maybe the model you have actually users 11 amps on the rail normally? Nothing was actually wrong? The low impedance was just a red herring  .... And a dying rail might have been causing the original fault.

I was curious if you can power the board with both the original power supply and also your bunch supply for the 3.3v. You would just have to pit the caps back of course.

I'm probably way off here but thought it would toss it out there.

Sent from my Nexus 7 using Tapatalk 2

 

pir

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #4 on: December 17, 2012, 08:58:32 am »
well ....there is no way a comp not blown up with 11Amps...
also there is a big probability that a trace would blown up or heated with 11Amps...

also...
taking in mind that you connected and disconnected the power plug into the board many times ...and you put 11Amps...and this is an old horse...
IT IS THE F..... PLUG of the board !! unsolder it and inspect the boar under it
a sort under the plug ..you can't see it...you can't imagine that

it happened to me... O0

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Offline M. András

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #5 on: December 17, 2012, 09:09:35 am »
worth take a look at the ps itself too 11 amps would warm up the area around the short even if its the connector
 

Offline JoannaK

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #6 on: December 17, 2012, 09:23:32 am »
I like that people dare to show the less successfull attempts too (be it design, repair or whatever). Debugging (even Hw) can be PITA, and sometimes you'll just spend hours with dead-ends, wrong leads and none progress what so ever.

Well, at least you'll know that those tantals are OK..   8)

Mike: that IR-imager looks really nice idea.. Would really like to have one of those.
 

Offline david77

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #7 on: December 17, 2012, 09:24:31 am »
Can you run the PS without it being connected to the board? My guess is also a flaky 3,3V line on the supply.

Isn't that usually the first step: Check if it's actually plugged in?  ;)
 

Offline hans

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #8 on: December 17, 2012, 09:28:56 am »
I don't see why it would be the power supply, as it's a ATX-like power connector which has shrouds between each contact. If a short is going to exist between the two, it would be noticed at the plastic is severely damaged.
And ofcourse the power supply of the unit isn't very suspicious as there is still a short without the cable plugged in.
But I guess an inspection can never hurt.

I'm very interested whether the unit 'works' if Dave would power the 3.3V from it's bench supply and the rest from the original unit. But I guess that's one hell of a setup to achieve, and still wouldn't narrow down the search (except maybe if channel 2 or 3 is completely dead and the others are working, you know one of those ASICs there is very well done inside).

On college we used to have a short locater. It gives an acoustic beep where the tone depends on the resistance. The ranges were quite small (200 ohms to 200m ohms), so it's very audible to hear differences. It had some very very sharp probes.
Looks like this is the exact unit: http://doc.es.aau.dk/fileadmin/doc.kom.aau.dk/labs_facillities/control/manuals/polar850-sheet.pdf

I think it's probably even nicer or atleast a good alternative to a very expensive  multimeter. You don't have to look at a display all the time, so you can entirely focus on the board and how you grab the probes.

But.. if the short is ~0.16 ohms, and you get ~11A@3.3V, it means you're getting something non-linear. Although faults can be all over the place, wouldn't this suggest its something semiconductorish because would expect to get well over 20A-30A with 0.16ohms short?
« Last Edit: December 17, 2012, 09:31:54 am by hans »
 

Offline maca_404

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #9 on: December 17, 2012, 09:40:36 am »
Whilst I doubt it is related to the current problem,  When doing the visual inspection on the input you did not comment that there is a section which seems to be broken out of what I assume is one of the brown input relays might be worth a quick look.  Good idea to  run the supply and look for hot spots though if you had a thermal imager I think you may have had more luck with this.
« Last Edit: December 17, 2012, 09:42:58 am by maca_404 »
 

pir

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #10 on: December 17, 2012, 09:54:06 am »
I don't see why it would be the power supply, as it's a ATX-like power connector which has shrouds between each contact. If a short is going to exist between the two, it would be noticed at the plastic is severely damaged.

well... ask the grounded screwhole next to the 3rd 3.3V pin
also take in mind the "paranormal activity" while measuring the last cap and bending the board...@23:00
« Last Edit: December 17, 2012, 10:11:56 am by pir »
 

Offline PuterGeek

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #11 on: December 17, 2012, 10:02:50 am »
Silly question, but what if it was the power supply the whole time?

I was wondering the same thing. If the 3.3V supply in the power supply is flaky it could do weird things. Also if the resistance is really 0.16 ohms, the power supply should be drawing 3.3V / 0.16ohms = 20.6A instead of 11.5A.

I think I would start by opening the power supply for a visual inspection. Then plug the power supply connector into the board but disconnect the 3.3V wires from the connector and measure the supply power using the dummy load.

If the 3.3V supply can't generate enough amps, I would use the bench supply for 3.3V and scope power for everything else. I might do a partial reassembly to 'running' configuration and see if it works.

Of course, I would probably should solder the caps back down!  >:(

I would hate to change from chasing a power supply issue to chasing a noise issue!
 

Offline peter.mitchell

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #12 on: December 17, 2012, 10:07:06 am »
one thing I have found that helps is using canned air and freezing suspect components, powering it up and checking.
 

Offline grumpydoc

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #13 on: December 17, 2012, 10:08:55 am »
I haven't seen the video yet but the 9354 I have pulls 220-240W from the wall and a lot of that is dissipated from the motherboard so I'd expect pretty heavy current flow on the main supply rails.
 

Offline EEgalitarian512

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #14 on: December 17, 2012, 10:40:32 am »
Hi there Dave. Great video, as always. Not sure if this suggestion is doable or helpful - but what about the possibility putting things back together and substituting your 40 Amp power supply just for the 3.3V and or 5V supply lines to see if the unit operates normally. If it does, then that could possibly move the troubleshooting path in the direction of the Power Supply - with the overall assumption being that the 11 Amps is normal for the 3.3V rail. Good luck, whatever approach you choose. Peace. John Martin
 

Offline PuterGeek

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #15 on: December 17, 2012, 10:43:58 am »
Blowing the short out reminds me of a time I tried that.

Quick turn prototype PCBs in the mid 80's were very expensive so the large (about 16" x 18"/40 cm x 45 cm) 4 layer boards made with one week turn around cost about $5,000. We hand built a board, powered it up and absolutely nothing!

The external 5V power supply is shutting down so we try to find solder bridges but can't find anything. No parts are getting hot and remove a few part but still and still nothing.  :-//

Resistance between 5V and ground is nearly zero (only had a 3 digit meter). On a whim I check a blank board and it's shorted too! And another and another, all 5 are shorted!

Since the power supply we had couldn't blow it out we decide to use the highest current supply we had - a 12V deep discharge lead acid battery that can supply lots of amps!  >:D

We connect wires to the connector, hook up the battery and the insulation cooks off the wire in seconds! We use bigger wire and the insulation is gets pretty soft but the connector fails before it burns off! We solder heavier wires directly to the board and the solder melts!  |O

At this point I'm more than a little steamed! I take one of the bare boards and remove the laminate to expose about 2 square inches of the power planes and solder 10 AWG (2.5 mm) wire to the power planes and hook it up. In a few seconds the solder melts! So I loop the wire for more area and use a lot of solder with similar results!

Being extremely near sighted at the time, I had a very limited focal distance without my glasses (8" / 20 cm tops). This let me see very small detail which is good since I don't think we even had a loupe. I whip off the glasses, hold the board up to a bright light and get very close. Looking at a via, it looks like the hole is off center and maybe shorting the power planes! And so is that one! And that one! In fact every via I look at is off center so much it looks like they all short 5 volts to ground!

They delivered new boards three days later that worked fine. I don't remember if they volunteered or not but they ate the cost for the whole order.
« Last Edit: December 17, 2012, 10:48:21 am by PuterGeek »
 

Offline LaurenceW

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #16 on: December 17, 2012, 11:41:34 am »
Dave, I was just about to offer a suggestion, but see MikesElectricStuff has beaten me to it! I have used this very approach, successfully  before.

You need something like a 1V 1A Constant current power supply, that is not going to do any damage to your board. Get good quality low impedance power connections (ideally SOLDERED) onto the board, and switch on. Set your meter on millivolts, and go hunting for the lowest value - that will be nearest to your shorted component. this is MUCH easier than trying to differentiate a MilliOhm here or there.

Where this might come unstuck is if there are very low impedance power planes, with no appreciable voltage drop across them.

Hmm, suspecting the power supply is an interesting one, but consider this. If 11A is actually a normal current for this board, that means EACH 3.3V pin on that connector is carrying 6A a piece. That sounds a bit much, don't you think?
If you don't measure, you don't get.
 

Offline beretta

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #17 on: December 17, 2012, 11:49:59 am »
Johnny come lately...I just reviewed this video and all of the ideas that have been suggested are well in line as what I thought.  My first was... load the 3.3 ps and monitor, second check out the connectors, third freeze components.  Looks like a classic fault but hard to find.  Maybe the circuit is happy with the .158 reading on the multimeter ...  Just food for thought.  I think Dave should carry this to Fix!
 

Offline mikeselectricstuff

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #18 on: December 17, 2012, 12:07:30 pm »
Quote
Where this might come unstuck is if there are very low impedance power planes, with no appreciable voltage drop across them.
In which case crank up the current. Low voltage drop = low power dissipation, so unlikely to do any damage.
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Offline Mike Whitenton

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #19 on: December 17, 2012, 12:27:56 pm »
Could it be that the power supply is rated at 6A per pin, times 3 pins =18A? If this is the case 11A would be a reasonable load. In which case the power supply would be suspect. I'd open the supply and do a visual.

I was also thinking that unpinning the 3.3V wires from the plug, powering the unit and measuring the 3.3V wires for voltage. It would tell you if the mainboard is shorting the supply or the supply is dead on the 3.3V wires.
« Last Edit: December 17, 2012, 12:39:02 pm by Mike Whitenton »
 

Offline AlfBaz

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #20 on: December 17, 2012, 01:40:07 pm »
Some time ago I was fault finding a board with an FPGA on it with a very low resistance on the power rail. Not being able to find anything obvious I started reading up on VLSI chips and thier power requirements. Since they have millions of FETs, a lot of them in complementry cofiguration, with highly capacitive gates, their static on/off state is unknown. You may end up with many parallel low impedence paths to ground. If the VLSI part doesn't power up correctly or has failed some how large section of the chip may end up remaining as it was before powering up causing large current demands

EDIT: There can also be strict power up ramping timings and sequences that cause the asics not to come out of POR
« Last Edit: December 17, 2012, 01:43:16 pm by AlfBaz »
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #21 on: December 17, 2012, 01:57:08 pm »
Hmm, suspecting the power supply is an interesting one, but consider this. If 11A is actually a normal current for this board, that means EACH 3.3V pin on that connector is carrying 6A a piece. That sounds a bit much, don't you think?

Yes, that is why I don't question the manual's mention of it being a maximum of 6A capable 3.3V rail. 6A per pin does not make sense.
So I'm sure the total 3.3V supply is normally under 6A, and that would include the ASIC chips looked at, the memory card (not plugged in), and maybe the CPU card (not plugged in)
That means there is some kind of short on the board as I suspect which is drawing at least an extra 7-8A.
Yes, it could possibly be somehow flowing into another unpowered rail, but IME, never by that much.
Yes, the PSU could have failed, that needs to be checked. But that's another issue.
A 3.3V system rail should not be 0.1ohms on a multimeter, something is up, I smelled a rat and still smell a rat.

Dave.
 

Offline lewis

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #22 on: December 17, 2012, 02:19:24 pm »
Mike/Dave beat me to it - this is where a thermal imaging camera is indespensible. We use a Fluke Ti10 for this exact type of thing. Apply 100mA-2A constant current, wait a few seconds, see what gets hot.

We've found all sorts of things you wouldn't expect shorted. Works about 98% of the time, and can find most faults like this in a few minutes.

It's one of those things that once you've used one, you'll never be without it.
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Offline Monkeh

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #23 on: December 17, 2012, 02:27:41 pm »
Hmm, suspecting the power supply is an interesting one, but consider this. If 11A is actually a normal current for this board, that means EACH 3.3V pin on that connector is carrying 6A a piece. That sounds a bit much, don't you think?

Yes, that is why I don't question the manual's mention of it being a maximum of 6A capable 3.3V rail. 6A per pin does not make sense.

So far as I'm aware, those connectors are rated for up to 13A per pin. Mind you, you'd need about 14AWG for that at 3.3V.

E: Could Molex be more annoying? 9A, 13A, make up your minds.
« Last Edit: December 17, 2012, 02:33:05 pm by Monkeh »
 

Offline Geezer

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #24 on: December 17, 2012, 02:30:22 pm »
  Being a newbie, my advise may be taken with a grain of salt, but here goes.    When you first demonstrated the LeCroy in the first tear down and powered the unit, you blew your circuit breaker.  Upon reset of the breaker, you were able to power the oscilloscope and it appeared to operate albeit with several obvious glitches--errors on the DACs, inaccurate readings, dodgy signal traces, etc.  Also, when you showed us the interior of the power supply, it looked pretty crusty and full of all kinds of potentially faulty capacitors.

  To my knowledge, 3.3V CPUs are notorious current hogs.  Even today's multiprocessor CPU can draw 90 to 125 Watts of power.  For 90W @ 3.3V that's approx 27 1/4 amps.  Although, the processors in the scope don't come close to the component capacity in today's CPUs, you still have 4 processors powered by the 3.3V rail.  So, a 11.2A to 10.2A draw from the 3.3V rail seems a plausible power draw for this circuitry. 

  Therefore, like several of those who posted before me, I think there is strong possibility that the 3.3V rail in the power supply requires closer inspection.  A major portion of the scope's functions is dependent on the components powered by the 3.3V rail.  If this power rail is faulty, it could explain most, if not all, of the errors including the breaker tripping, demonstrated in the first video.
 
 

Offline cidcorp

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #25 on: December 17, 2012, 02:42:31 pm »

I have to say - I do really enjoy sitting back and reading the threads on these troubleshooting missions.  I really don't have much experience in troubleshooting devices like this
and I learn alot each time one of these threads pops up.  Wish I could offer some advice.

BTW - Those thermal imaging cameras can't be cheap...are there ones out there that don't require you to mortgage your home?  Just curious...cause it makes sense they *could*
save you alot of time searching for a fault.

Anyway, just wanted to say I'm enjoying this thread.  :-+ :-+
 

Offline lewis

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #26 on: December 17, 2012, 02:51:15 pm »
It's extremely unlikely the 3.3V rail is fine, it should not read that low on an ohmmeter even if it is designed to take 11A. I don't know what the output voltage of that Agilent multimeter is on the ohms range, but it's unlikely to be enough to want to power up everything connected to the 3.3V rail, and it's certainly not magical enough to turn on any semiconductor junctions when the probes are reversed. Semiconductors are not linear and will draw no current if the 'supply voltage' (from the ohmmeter) is low enough or negative.

Also, as someone mentioned, the low resistance reading suggests the rail should draw 20A or so at 3.3V. So if Dave's getting 11A, something's heating up fast and increasing it's resistance, reducing the current. That suggests to me it's not semiconductory, probably tantalumy, ceramicy, bonding-wirey, or circuit-boardy. Or the resistance reading is erroneous. Either way, as Dave says, there is definitely a rat there somewhere.
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Offline ecat

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #27 on: December 17, 2012, 02:53:26 pm »

...

BTW - Those thermal imaging cameras can't be cheap...are there ones out there that don't require you to mortgage your home?  Just curious...cause it makes sense they *could*
save you alot of time searching for a fault.

You could take a look at ThermoPhone (heh, this is what I would call it, or maybe iPhone uHot :) )
http://www.kickstarter.com/projects/andyrawson/ir-blue-thermal-imaging-smartphone-accessory?ref=card
 

Offline lewis

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #28 on: December 17, 2012, 02:58:57 pm »
BTW - Those thermal imaging cameras can't be cheap...are there ones out there that don't require you to mortgage your home?  Just curious...cause it makes sense they *could*
save you alot of time searching for a fault.

They're not cheap - http://www.rapidonline.com/Test-Measurement/Fluke-Ti10-Thermal-imager-82499

But they're coming down! http://www.alpha-electronics.com/Items/10930/flir-i7-educational-package.html
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Offline excess_heat

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #29 on: December 17, 2012, 03:08:04 pm »
Why not try your AIM-TTi I-Prober 520 in field mode to track down the short?
 

Offline jammit

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #30 on: December 17, 2012, 03:21:23 pm »
Here's my two cents. I posted to the Youtube video, but finally decided to get an account here.
3.2V at 11A is 0.29 ohms. The resistance you were measuring was about 0.110 ohms at the DC power plug (I subtracted the 0.060 leads resistance). The resistance almost tripled when when you went to 11A. This is telling me the traces are heating up and increasing resistance. You might want to desolder the DC plug and check for shorts under it, and you might want to measure the voltage at the tantalum bypass capacitors when powered with this 11A. An ohm meter uses a small current to measure the voltage across resistors, and the changes in resistance is small. Imagine if the ohm meter uses 11A instead.
 

Offline htassell

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #31 on: December 17, 2012, 05:01:40 pm »
Hi,

I just posted on Youtube, but I thought I'd mention it here. As others indicated on the video, you can see damage to one of the brown packages in the analog front end of the left most channel (when the BNC's are facing towards the bottom of the frame at 12:36-12:54.

I guess it could just be physical damage due to handling the board and getting it in and out of the chasis etc. rather than an electrical fault. It may well be unrelated, but it might explain some of the other weired behaviour if it isn't the cause of the short.

I thought I'd just mention it in case it was worth looking at...
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #32 on: December 17, 2012, 06:08:23 pm »
I just posted on Youtube, but I thought I'd mention it here. As others indicated on the video, you can see damage to one of the brown packages in the analog front end of the left most channel (when the BNC's are facing towards the bottom of the frame at 12:36-12:54.

Yep, aware of that, done by accident during the teardown.
Not related in any way.

Dave.
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #33 on: December 17, 2012, 06:12:25 pm »
BTW - Those thermal imaging cameras can't be cheap...are there ones out there that don't require you to mortgage your home?  Just curious...cause it makes sense they *could*
save you alot of time searching for a fault.

You won't get any change from $1500 for a bottom of the range one.
That's why I don't have one, and even if I did, I'd be a bit loath to use it in a troubleshooting video knowing it's not a tool Joe Average is likely to have.

Dave.
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #34 on: December 17, 2012, 06:25:54 pm »
But they're coming down! http://www.alpha-electronics.com/Items/10930/flir-i7-educational-package.html

AU$990 "educational" pricing.
http://www.flir.com/cs/apac/en/view/?id=56018

I'm an "educational" resource, surely?  ;D
But still, even at that price it's a tool that's hard to justify.

Dave.
 

pir

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #35 on: December 17, 2012, 07:22:45 pm »
Here's my two cents. I posted to the Youtube video, but finally decided to get an account here.
3.2V at 11A is 0.29 ohms. The resistance you were measuring was about 0.110 ohms at the DC power plug (I subtracted the 0.060 leads resistance). The resistance almost tripled when when you went to 11A. This is telling me the traces are heating up and increasing resistance. You might want to desolder the DC plug and check for shorts under it, and you might want to measure the voltage at the tantalum bypass capacitors when powered with this 11A. An ohm meter uses a small current to measure the voltage across resistors, and the changes in resistance is small. Imagine if the ohm meter uses 11A instead.

 :-+
well i'll take it a little further....
the scope is 1G so FOR SURE there is a "symmetrical tracing lengths or symmetrical impedance" all over the place (as possible..from the designers point of view..especially over the memory storage circuitry)
i bet that the "paranormal activity" over the 4Th tantalum cap measuring 0.100 Ohms @ 23:00 was also the same to the other 3 caps...board bending....
so it is a 3.3V-GND short from a trace...
and where is the bigest possibility for this to happen? ...THE SCREW HOLE NEXT TO THE PLUG(pin no3  and no2 3.3V...pin no1 is not shorted)
and why this was to happen?....because THE PLASTIC CASE of the scope is broken...it fell off !!!  :-//
and why is this also possible to be the teardown video problems with that glitches?....the 2 memory boards hung-over from their plugs and their weight put stress on the plugs and the board...
and what is the "mechanical lever" to absorb that behavior?.... the screw next to "MAIN POWER PLUG"  |O
DESIGNER'S FAIL !!!!

i suggest Mike's method with the voltage drop and interrogate that screw hole

p.s.
damn broken keyboard...that's why the editing
« Last Edit: December 17, 2012, 09:31:45 pm by pir »
 

Offline IanJ

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #36 on: December 17, 2012, 08:47:01 pm »
Hi all,

Just because it's easy to do and just to rule it out I'd hook up the Lecroy PSU but substitute your bench PSU on the 3.3v rail and check the current consumption. I suspect the current will drop down a bit. If it drops a lot and to within spec of the Lecroy PSU then there's some ideas for you.

Yes, we've all been there........and frustrating it certainly can be.

Ian.
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Offline T4P

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #37 on: December 17, 2012, 08:47:45 pm »
The resistance you were measuring was about 0.110 ohms at the DC power plug (I subtracted the 0.060 leads resistance). The resistance almost tripled when when you went to 11A.
He had his leads rel'd already.
 

Offline JackOfVA

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #38 on: December 18, 2012, 03:37:39 am »
As a rough estimate of the 3.3V rail consumption, we can work backwards from the heat sink Rth and observed temperature rise. I saw 8 ICs with heat sinks in the area identified as 3.3V rail powered and I believe Dave said the heat sinks measured around 60 deg C after a few minutes with the high current 3.3V supply.

For a typical room temperature of say 28 C, the temperature rise is 32 deg. As a really rough estimate of Rth, take  6 deg/watt -- I imagine there is fan cooling and 6 deg/watt is a guestimate of Rth without the fan running. Probably closer to 3 deg/watt with the fan operating.

(The 8 heat sinks are 4 each of two types, but for this purpose we'll assume all are 6 deg/watt.

Each heat sink is thus responsible for 32 deg rise / 6 deg/watt = 5.3 watts per device, or 43 watts for all 8 devices.

At 3.3V supply, current required is 43/3.3 = 13 amperes.

This is not wildly off the 11A observed draw.

This estimate could be refined with data on the heat sink dimensions and measured temperature rise, of course.

Jack


 

Offline M. András

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #39 on: December 18, 2012, 03:47:37 am »
btw at the beginning of the video there is tons of dust and god knows what under the little soic chips near the power connector maybe a good dusting with compressed air would help
 

Offline free_electron

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #40 on: December 18, 2012, 04:17:39 am »
video at 11:14 : you can clearly see the deformation of the pcb around the screw hole. Take an xacto knife and scrape that metal off the top and bottom layer. then carefully scratch the fr4 away.. if you hit another plane : peel the plane away as well...

i have fixed an agilent scope once. they used long smetal standoffs from the board to tthe case. the case got thumped exactly where there was a standoff. this compresses the board enough that it actually permanently dented the board around the hole. there was a short between planes. by peeling off the pad of that hole i managed to get rid of the short. i also rpelaced the metal standoff with a nylon one ( as the internal 5 volts plane was exposed after peeling off )

scope still works five years later.
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Offline lewis

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #41 on: December 18, 2012, 04:34:33 am »
But they're coming down! http://www.alpha-electronics.com/Items/10930/flir-i7-educational-package.html

AU$990 "educational" pricing.
http://www.flir.com/cs/apac/en/view/?id=56018

I'm an "educational" resource, surely?  ;D
But still, even at that price it's a tool that's hard to justify.

Dave.

Never take any list price at face value, there's always some haggling to be done even if you're not strictly an 'educational' resource (university/school)  ;) Failing that, if you know someone at a uni, get them to buy it for you. In fact, I'd bet Flir or Fluke would give you one!

It's an extremely good tool for the price, especially when compared to a $12000 scope, not just for troubleshooting but for design too. I use mine for thermal modelling and heatsink evaluation during the design phase of pretty much everything and it's very very useful.
I will not be pushed, filed, stamped, indexed, briefed, debriefed or numbered.
 

pir

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #42 on: December 18, 2012, 05:15:43 am »
video at 11:14 : you can clearly see the deformation of the pcb around the screw hole. Take an xacto knife and scrape that metal off the top and bottom layer. then carefully scratch the fr4 away.. if you hit another plane : peel the plane away as well...

i have fixed an agilent scope once. they used long smetal standoffs from the board to tthe case. the case got thumped exactly where there was a standoff. this compresses the board enough that it actually permanently dented the board around the hole. there was a short between planes. by peeling off the pad of that hole i managed to get rid of the short. i also rpelaced the metal standoff with a nylon one ( as the internal 5 volts plane was exposed after peeling off )

scope still works five years later.

 :-+
 

Offline jancumps

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #43 on: December 18, 2012, 06:03:02 am »
video at 11:14 : you can clearly see the deformation of the pcb around the screw hole. Take an xacto knife and scrape that metal off the top and bottom layer. then carefully scratch the fr4 away.. if you hit another plane : peel the plane away as well...

i have fixed an agilent scope once. they used long smetal standoffs from the board to tthe case. the case got thumped exactly where there was a standoff. this compresses the board enough that it actually permanently dented the board around the hole. there was a short between planes. by peeling off the pad of that hole i managed to get rid of the short. i also rpelaced the metal standoff with a nylon one ( as the internal 5 volts plane was exposed after peeling off )

scope still works five years later.

I feel (yes, not based on anything better than feelings) that we have a winner here.
 

Offline PuterGeek

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #44 on: December 18, 2012, 06:18:47 am »
Dave,

Don't you just love all the 'free' advice you are getting (me included)?  ;D

-Joe
 

Offline Electr0nicus

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #45 on: December 18, 2012, 06:53:24 am »
Dave,

Don't you just love all the 'free' advice you are getting (me included)?  ;D

-Joe

Means we all want to see a part 2, where Dave actually fixes that broken LeCroy.

I also suspect that hole as the culprit, as many people here also do. Maybe the 3.3V power supply layer on one of the inner layers has a very small clearance to the plating of that hole. And as you can see in the video the screw has somehow fractured the plating and then maybe pressed the splitters against this 3,3V power supply layer, shorting it out to ground. Because when you look at that screw hole closely, you can see, that the PCB is there somehow de- laminated.

That wouldn't surprise me, because if you watch Dave's first video, you can see that this lecroy has some really big damage to the side of the case. This indicates that it may have been dropped. So that would put very high stresses on the PCB, particularly at the points where it is screwed to the case.

I would recommend to drill out the plating of that hole first, and then see if something changes.

The high current experiment Dave performed also tells me, that if that screw hole has somewhat to do with that problem, then it can't be a dead short. Because then the 8 Chips wouldn't have gotten warm, which means that they were actually powered up.

Also a easy method check, if some sort of high current action is going on around that screw hole, is to test the temperature with a finger. If there are significant amounts of current flowing trough a resistive fault (because the screw hole can't be a complete short because of the above mentioned reason) then it has to get warm. A easy calculation confirms that: Let's assume the circuit consumes 6A in normal mode, and also assume that all the chips are OK. So if there are 11A going in, then trough the fault, 5A have to flow. Even at a resistance of 0.15Ohms at 5A this equals 3.75W of power- dissipation. This screw hole has to get warm, even if there are big power Planes on that PCB.
 

Offline Pat Pending

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #46 on: December 18, 2012, 08:20:05 am »
Anyone else with access to the same LeCroy and can measure the impedance of the 3v3 rail?

Just a suggestion but maybe some of that 11A is due to internal protection diodes or substrate diodes where the 3.3V and 5V systems interface to each other
within the custom chips, maybe 2.7V can be found on the 5V side?

On a side note, this particular LeCroy PSU OEM resembles the design used in the TDS420. Except in this case the LeCroy service manual
has the PSU schematic whereas the Tektronix SM does not.
« Last Edit: December 18, 2012, 08:22:42 am by Pat Pending »
 

Offline Dread

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #47 on: December 18, 2012, 09:07:50 am »
Lots of good suggestions, I also loved the video, keep these coming  ;D

Mike beat me to it in that I knew you were wasting time with using that meter and trying to find milliohms of difference, that’s futile and compounded by the fact that that you say  the meter has a high resolution but as you stated it's accuracy is in question! So at those resolutions it begs the question what is it's repeatability of reading at the same point twice or more, it seemed to be really poor. Which goes to the point that a lack of accuracy does not mean it will always be off by plus X or minus X but can drift all over the place.

My second piece of advice is don't be afraid to use that exacto Knife and cut a surface trace or two.  I use to do that when the voltage ran all over the board.  I could then isolate which side had the problem.  A second or third cut can isolate it down to a very small exact area
.




The Optimist says the glass is half full, the Pessimist says its half empty, an engineer only see's a glass that’s twice as big as it needs to be!
 

Offline merser

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #48 on: December 18, 2012, 09:36:45 am »
Quote
My second piece of advice is don't be afraid to use that exacto Knife and cut a surface trace or two.  I use to do that when the voltage ran all over the board.  I could then isolate which side had the problem.  A second or third cut can isolate it down to a very small exact area

That's ok, but I'd check there aren't jumpers to desolder first that serve this purpose of isolating sections.

I don't see how you can't follow this through and fix the damn thing in part 2 Dave. It is like I am about to climax and she says take it out.  I'm all in favour of happy endings.  ; - )
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #49 on: December 18, 2012, 09:37:58 am »
Mike beat me to it in that I knew you were wasting time with using that meter and trying to find milliohms of difference, that’s futile

No, it's not. This is a very common and successful technique to quickly locate shorts. It is also safer than powering the board up, so is a common first option.
It just happened to have not worked in this instance.

Quote
and compounded by the fact that that you say  the meter has a high resolution but as you stated it's accuracy is in question! So at those resolutions it begs the question what is it's repeatability of reading at the same point twice or more, it seemed to be really poor.

Watch the video, the repeatability of the meter and probes is excellent.

Quote
Which goes to the point that a lack of accuracy does not mean it will always be off by plus X or minus X but can drift all over the place.

The accuracy of the meter has nothing to do with repeatability, that's not how it works.

Quote
My second piece of advice is don't be afraid to use that exacto Knife and cut a surface trace or two.  I use to do that when the voltage ran all over the board.  I could then isolate which side had the problem.  A second or third cut can isolate it down to a very small exact area

Once again, a last resort method. And something that is not possible with multi-layer boards.

Dave.
 

Offline Electr0nicus

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #50 on: December 18, 2012, 10:32:22 am »
Maybe the worst possible thing happend. Maybe there is a intermittent short between the 5V and 3.3V rail. As the 3.3V rail is weaker than the 5V rail, it would have easily brought the voltage on the 3.3V rail up to nearly 5V.  If that is the case then this 3.3V chips would have been fried internally. This would explain the increased current consumption, and nothing else except these 8 chips with  heatsinks getting hot.

Just another possibility to think of.

Cheers Gregor
 

Online tom66

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #51 on: December 18, 2012, 10:42:24 am »
What about the PSU being mislabelled? "1" rubbed off, is actually 16A?

I mean, 11A at 3.3V seems high, until you consider that over 8 ICs, that's only 4.5W each... which could be pretty normal for high performance devices, they certainly have a good heatsink and would normally be air cooled.
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #52 on: December 18, 2012, 10:48:25 am »
What about the PSU being mislabelled? "1" rubbed off, is actually 16A?

It is scanned from the service manual and is very clear, 6A.

I'm amazed at how many people seem to think that 0.1ohms both directions on a multimeter could possibly be normal on a 3.3V system rail  :-//

Dave.
 

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #53 on: December 18, 2012, 11:33:53 am »
What about the PSU being mislabelled? "1" rubbed off, is actually 16A?

It is scanned from the service manual and is very clear, 6A.

I'm amazed at how many people seem to think that 0.1ohms both directions on a multimeter could possibly be normal on a 3.3V system rail  :-//

Dave.

stop posting and fix that damn old horse !!!...
we are all waiting the part 2
 :phew:
 

Offline Razor512

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #54 on: December 18, 2012, 11:53:46 am »
While not an oscilloscope, I tested 4 different old ATX motherboards and the in going from ground to the 3.3V rail, they were all around 3.2-3.8 kohms

The only system that was lower was a non ATX design compaq system from the early 90.s, the 3.3V rail was at 7.5 ohms (it still worked the last time I used it but have not used it in many years.

I wonder what if you power the device suing the stock power supply but then bypass the 3.3V raiil on the power supply and instead use your other power supply to power that raid, will the device still work?

A power supply can give above it's rated power. (eg I have an old 300 watt power supply that is rated for 11 amps on the 12V rail, but I am pulling at least 40-15 amps from it (though it is running no other load on any of the other rails (I use it to power an old car radio/ sound system since the system can do 200 watts at max volume (connected to 4 speakers, and I can max it out and none of the lights dim on the radio or the amp. (it is a cheap $12 chinese ATX power supply that came with a $20 ATX case that tiger direct was selling (and also had free shipping :) )

I have been using it for a while with no problem (I keep it away from anything flammable just in case since I do have it wired up in a sloppy fashion)

(I bet if I wanted, I could push that china power supply even further above it's limit (not sure at which point it would start smoking, but what ever the real limit is, what ever protection it has (if any) has not kicked in yet :) )

edit: Pictures yay (has been had been used in a PC for a year back in 2003, then retired until about 2 years ago for use with a speaker system)
« Last Edit: December 18, 2012, 12:53:47 pm by Razor512 »
 

Offline Greg323i

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #55 on: December 18, 2012, 11:54:55 am »
Hmm, suspecting the power supply is an interesting one, but consider this. If 11A is actually a normal current for this board, that means EACH 3.3V pin on that connector is carrying 6A a piece. That sounds a bit much, don't you think?

Yes, that is why I don't question the manual's mention of it being a maximum of 6A capable 3.3V rail. 6A per pin does not make sense.

Dave, didn't you say that you don't have the exact service manual for this unit? Could it be that the unit you have actually has a higher capacity 3.3V rail?
 

Offline Dread

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #56 on: December 18, 2012, 12:03:58 pm »
Mike beat me to it in that I knew you were wasting time with using that meter and trying to find milliohms of difference, that’s futile

No, it's not. This is a very common and successful technique to quickly locate shorts. It is also safer than powering the board up, so is a common first option.
It just happened to have not worked in this instance.

Quote
and compounded by the fact that that you say  the meter has a high resolution but as you stated it's accuracy is in question! So at those resolutions it begs the question what is it's repeatability of reading at the same point twice or more, it seemed to be really poor.

Watch the video, the repeatability of the meter and probes is excellent.

Quote
Which goes to the point that a lack of accuracy does not mean it will always be off by plus X or minus X but can drift all over the place.

The accuracy of the meter has nothing to do with repeatability, that's not how it works.

Quote
My second piece of advice is don't be afraid to use that exacto Knife and cut a surface trace or two.  I use to do that when the voltage ran all over the board.  I could then isolate which side had the problem.  A second or third cut can isolate it down to a very small exact area

Once again, a last resort method. And something that is not possible with multi-layer boards.

Dave.

Dave I never had any real success doing it that way, you may get lucky but on a complex board like that one it's a long shot.

Cutting the board can be done when the 3.3V line comes up to the top or bottom, it will often snake around the place then go back down to another layer.  You can cut it at the surface when it snakes around and that will isolate a section of the board.

I must confess that my feeling is that the power supply is the issue and not the board. I suggest you do what some previous posters have said and use your external source for the 3.3 and put the unit back together and see if it works.   The fact that you had odd behavior before makes me think the 3.3V was going south from when you did the Teardown and now it has failed.
Yes it defies the power consumption you expect but I have a feeling that consumption is going to drop once you hook up the other rails.


The Optimist says the glass is half full, the Pessimist says its half empty, an engineer only see's a glass that’s twice as big as it needs to be!
 

pir

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #57 on: December 18, 2012, 12:16:23 pm »
hey people...you are missing the point here...

3.3V pinout:
pin1 ----> good
pin2----->sorted gnd
pin3----->sorted gnd

as the man said in the video they use many pins to get the watts in...
with your logic pin1 is powering something else say low watts...and pin2,pin3 is the super-duper 10+ amps
 :o
 

Offline Electr0nicus

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #58 on: December 18, 2012, 12:21:24 pm »
As Dave and some other forum members pointed out, that 0.15 ohms on the 3.3V rail in both directions can't be the normal state. You can think through it back and forth, it is just not possible in my opinion. So the real question to ask, is not if the powersupply can deliver 6 or 16A, but why the resistance is the same in both directions. To measure the resistance the multimeter has to apply a low voltage across the 3.3V rail, so if the voltage is reverse polarity (probes reversed) than there should be at least a little bit of change in the resistance value (due to reverse biased semiconductor junctions). But in this case it is exactly the same, disregarding tolerances.  So my gut feeling says that there is maybe something fishy going on in those heavily heatsinked devices, because no other device (Chip) on this board can withstand that high power dissipation for a long time without emitting magic blue smoke. The only devices which can be faulty without actually blowing up or bursting in flames when x times the rated current is passed through them are those heatsinked devices.

I know I sound very pessimistic, but that can be a very propable scenario.

I would also recommend powering that 3.3V rail separately via the lab supply. This way Dave may be able to rule out a defect of the ADCs before digging deeper into the problem and maybe wasting hours for nothing.

Cheers Gregor
 

Offline Greg323i

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #59 on: December 18, 2012, 12:30:14 pm »
My suggestion about the service manual was mainly because I found it odd that the resistance you measured and the current that you measured don't correspond.

I watched this video again and I agree with the people that think that screw hole is probably the cause of the short, as it doesn't look deformed. Easy enough to check resistance between pin 2, 3 and the screw hole.

[edit]

I'm an idiot...

Of course you're going to measure the same short anyway if that screw hole is grounded.

[/edit]
« Last Edit: December 18, 2012, 01:17:46 pm by Greg323i »
 

Offline Monkeh

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #60 on: December 18, 2012, 12:51:29 pm »
What about the PSU being mislabelled? "1" rubbed off, is actually 16A?

It is scanned from the service manual and is very clear, 6A.

I know the service manual (didn't you say it's for a different model?) says 6A, but.. is it just me, or does the label on the PSU in the teardown video at 19:37 say 3.2V 13A?

Quote
I'm amazed at how many people seem to think that 0.1ohms both directions on a multimeter could possibly be normal on a 3.3V system rail  :-//

I know, it smells funny to me, too. Still, if that label says what it appears to..

E: Here, have a screen cap.
« Last Edit: December 18, 2012, 12:56:47 pm by Monkeh »
 

Offline Anks

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #61 on: December 18, 2012, 02:23:17 pm »
Get your AIM-TTi I-Prober 520 Current Probe out and see if you can find the short. Used to do this sort of thing with a HP 548A current tracer. Should point to the places that are eating the current without having to resort to thermal imaging. I know its not really a run on the mill everyday joe type of tool but if its at hand it would be nice to see it used.
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #62 on: December 18, 2012, 02:32:30 pm »
My suggestion about the service manual was mainly because I found it odd that the resistance you measured and the current that you measured don't correspond.

On the contrary, you would expect something like that with the difference between 11A and naff-all.

Dave.
 

Offline NiHaoMike

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #63 on: December 18, 2012, 03:25:24 pm »
Connect a function generator across the rail and set it for some low frequency like 46Hz. (Keep it distinct from the mains frequency so you don't get confused. In fact, if you keep it near the mains frequency and then trace it with an audio method, you'll probably get a very distinctive beat pattern.) Then you can use a magnetic field probe without getting false readings from magnetized bits of metal. And it's possible to use a MP3 player instead of a function generator and a hacked tape player instead of a magnetic field probe.
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Offline G7PSK

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #64 on: December 19, 2012, 03:57:41 am »
What is limiting the current to 11 odd amps as at 0.16 ohms the draw should be 20 odd and as that power supply is able to output 40 amps something should have given, is there a current limiting device on the board, I did think I saw a TO20 device in the region of the 3.2 volt area Dave was looking at. Also as Monkeh pointed out the power supply does look like its rated to 13 amps.
 

Offline robrenz

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #65 on: December 19, 2012, 05:31:20 am »
The video from this LeakSeeker 82b page gives a good idea what Dave was doing except with much higher resolution and audio feedback.

All the videos HERE

The one on the link to the site
« Last Edit: December 19, 2012, 05:55:36 am by robrenz »
 

Offline KedasProbe

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #66 on: December 19, 2012, 06:55:53 am »
What is limiting the current to 11 odd amps as at 0.16 ohms the draw should be 20 odd and as that power supply is able to output 40 amps something should have given, is there a current limiting device on the board, I did think I saw a TO20 device in the region of the 3.2 volt area Dave was looking at. Also as Monkeh pointed out the power supply does look like its rated to 13 amps.

Dave did you check the voltage on the board connector?
There will be loss over your cables but we don't know how much.(we assume not much but if calculations are done it has to be right.)
Maybe you can try a voltage below 0.7V, see what it gives.
« Last Edit: December 19, 2012, 07:00:08 am by KedasProbe »
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Offline Mr Smiley

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #67 on: December 19, 2012, 07:29:51 am »
And how to do it with a battery and multimeter  :-+



 :)
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Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #68 on: December 19, 2012, 08:21:27 am »
What is limiting the current to 11 odd amps as at 0.16 ohms the draw should be 20 odd and

It's the heating up of the short (and changing of it's resistance) due to the current. This is quite common and generally expected.

Dave.
 

Offline G7PSK

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #69 on: December 19, 2012, 08:35:17 am »
If the short is in one of those chips then the heat sinks could keep the short from blowing out forever then. So what happens if the board is frozen would the current go up enough to blow the short.
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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #70 on: December 19, 2012, 08:49:43 am »
Freeze spraying components could help in locating shorts.
 

Offline ejeffrey

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #71 on: December 19, 2012, 04:28:41 pm »
I definitely prefer the "pass constant current  from a supply through the short and trace it down with a volt meter" method to using 2-terminal resistance measurement.  Using an ohmmeter may be a standard technique, and certainly can work, but I think it is categorically inferior.  Saying that you are more likely to damage your board is a red herring.  You can use whatever current you want.  Ever meter I have seen has the same or better resolution on mV as ohms.  If you use a small current on the order of what your multimeter uses, you will get the same resolution.  You have the option to crank up the current for more sensitivity, but are in no way required to.

Even with good sharp probes, you definitely had to fuss around to make sure you were getting a good contact.  With a 4 point measurement, this is essentially not a problem.

If I were really frustrated or just being careful, I would use two meters.  One connected right near the power supply leads that would show changes in the resistance due to mechanical changes, and the other that I would move around looking for the fault.
 

Offline PuterGeek

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #72 on: December 19, 2012, 05:11:59 pm »
The video from this LeakSeeker 82b page gives a good idea what Dave was doing except with much higher resolution and audio feedback.

Too bad they don't make those any more.

The fourth paragraph from the bottom of the page said the last production run was in January and they have none left. They do say they might come out with a new design.

I'm guessing some part(s) they used were obsoleted.
 

Offline PuterGeek

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #73 on: December 19, 2012, 05:41:46 pm »
I'm amazed at how many people seem to think that 0.1ohms both directions on a multimeter could possibly be normal on a 3.3V system rail  :-//

I agree it would be strange but the symptoms aren't really adding up.

It was only drawing 10 to 12 amps and you didn't find a hot spot. I was very surprised when the bench supply was able to maintain 3.2V and apparent hotspot.

When I run into strange situation I try to think outside the box. I try to eliminate anything I can, starting with things that are easy.

Verifying the 3.2V supply is OK with a visual inspection and load test would be quick.

Since bench supply can manage the current, using it for the 3.2V and powering up the scope might show a fault in the POST.

If not, you will be able to use differential voltage measurements which as several people have said, should be more accurate.

I would also try a cheap infrared thermometer to make spot measurements.

Then would be pulling chips if you want to keep at it. In particularly I would focus on the devices with heat sinks (I'm guessing they are DSPs). Since they're getting hot, removing them would keep them from masking a short or they could be the short.

I would probably scrap it if all that wasn't fruitful. That is unless I 'needed' to get a short tracer then this was a good excu reason to buy one now.   ;D
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #74 on: December 19, 2012, 05:52:07 pm »
I agree it would be strange but the symptoms aren't really adding up.
It was only drawing 10 to 12 amps and you didn't find a hot spot.

That's because I haven't found it, yet!
It will be there. It's almost certainly not evenly spread across all the ASIC's. So you can't beat the laws of physics.

Dave.
 

Offline IanJ

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #75 on: December 19, 2012, 06:29:43 pm »
Hi all,

Reading all this it's turning out to be on of those repairs that HAS to be done irrespective of how useful the final product will be. Pure satisfaction over usefulness...... :)

Ian.
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Offline PuterGeek

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #76 on: December 20, 2012, 12:46:36 pm »
That's because I haven't found it, yet!
It will be there. It's almost certainly not evenly spread across all the ASIC's. So you can't beat the laws of physics.

I'm sure you will find it Dave, we're just trying to be helpfully impatient!  ;)

Hopefully we haven't crossed the line to annoyingly impatient (yet).

-Joe
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #77 on: December 20, 2012, 01:19:35 pm »
I have a few extra tools at my disposal today....


Dave.
 

Offline Monkeh

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #78 on: December 20, 2012, 01:59:54 pm »
Those should be pretty revealing..

While you're at it, check the label on that PSU.
 

Offline PuterGeek

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #79 on: December 20, 2012, 02:09:17 pm »
I have a few extra tools at my disposal today....

Dave.

Oooh! Pretty!

And there will be a contest to give away two of them, right?!?  ;D
 

Offline SeanB

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #80 on: December 20, 2012, 03:45:08 pm »
Now you can see the power planes in the board.
 

Offline albertr

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #81 on: December 20, 2012, 03:48:40 pm »
Out of curiosity, I've cranked open LeCroy 9374L scope, but unfortunately the power supply connector has completely different layout (has a single row of pins) as well as main board and ADC chips are not anything like of 9384. I still could try to measure the resistance on 3.3V rail if anyone let me know the pin-out of that power connector. Anyways,  speaking of 9374L these ADC chips get quite hot and I wouldn't be surprised if they dissipate some 30-40 Watts...

-albertr
 

Offline TehPepper

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #82 on: December 20, 2012, 06:07:58 pm »
I have a few extra tools at my disposal today....


Dave.

Thats the spirit!
 

Offline lewis

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #83 on: December 20, 2012, 11:25:12 pm »
I have a few extra tools at my disposal today....


Dave.

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Offline JoannaK

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #84 on: December 21, 2012, 05:31:18 am »
I'd expect some spooky videos coming up.

 

Offline deephaven

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #85 on: December 21, 2012, 05:53:50 am »
What I have done successfully before now is to spray the underside of the board with freezer so that it frosts up, then quickly apply power. Sometimes you can see the hot spot with the rapidly disappearing frost around the fault location. It's a sort of poor man's infra red imager.
 

Offline robrenz

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #86 on: December 21, 2012, 06:22:38 am »
Nice tip  :-+

Offline jerry507

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #87 on: December 21, 2012, 08:22:59 am »
I can't help but think he's going to fire up those cameras and find 6 hot spots above the asics. It's worth keeping the possibility that all the asics are blown in the back of ours minds...
 

Offline xDR1TeK

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #88 on: December 21, 2012, 10:16:26 am »
What would be the worst case scenario? Taking the components one by one and testing? even the chips?
 

Offline Psi

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #89 on: December 21, 2012, 10:56:46 am »
I think there's one thing left to try.  Try and run it anyway.

The device was working, it just had memory issues.
So the reason it stopped altogether may have been the 3.3V power supply dying.

If that's the case then it will probably run again with the 3.3V wired to the bench supply.
Also, if the memory issues were due to 3.3V ripple (because the psu was being pushed to far) then using an external psu which can handle the high current may fix the memory issues.

I guess my point is that it may go just fine (even with the fault) if you can supply the current.

My data projector has a very similar issue. One of the 3 LCD drivers has a shorted output pin inside the chip.
Luckily the pin was a simple DC 5.5V output to control LCD bias (brightness adjust) so i forced the pin back to 5.5V and that made the projector work again.
It still has the short and the chip does get hotter than the others but it runs fine, I just cannot adjust the green channel brightness from default.

It's been running like that for 2 years.
The only other mod i did was to add a bigger heatsink to the faulty IC to get rid of the extra heat.
« Last Edit: December 21, 2012, 11:08:04 am by Psi »
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Offline HammerFET

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #90 on: December 21, 2012, 11:43:04 am »
Quote
I think there's one thing left to try.  Try and run it anyway.

I was going to say the same thing but to be honest if one of those ASICs have gone wrong internally, you might as well not trust the entire chip anymore. It would be riddled with massively dropped internal voltages and the whole thing would be going bananas inside.

It was a bit gutting to see all those other devices heat up when the 3.3V rail went down. I think I saw that IR camera peak one of them to just over 100 degrees at one point :P

Another problem could be that the drop on the 3.3V rail as soon as it hits the board and spreads out is bad enough to stop things working all together.

Maybe put it back together and set the voltage so that it's 3.3v at the connector. Might just have some life left in it.
 

Offline JoannaK

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #91 on: December 21, 2012, 11:49:36 am »
yeah.. that new video #401 didin't show much promise. When the other chips (5V powered) got heated over 100C at IR camera there's high likelyhood that there's a lot broken on the board. Not just those 3.3V powered Special AD/buffer asics

That said. Taking the hottest Asic out of the board and force-feeding the 3.3V power lines real hard (so that it keeps within 5%), could *in theory* work.. Unfortunately, not likely.

 

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #92 on: December 21, 2012, 12:07:35 pm »
 :palm:

D_E_A_D....

long live to us

.....
blog suggestion.....take it apart with A HAMMER  :-DD
 

Offline mikeselectricstuff

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #93 on: December 21, 2012, 12:18:17 pm »
Certainly worth unsoldering that hottest ASIC.

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Offline Mike Whitenton

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #94 on: December 21, 2012, 12:21:58 pm »
I'm sure you called it right Dave. You've demonstrated the ASICs are shorted. Since it isn't repairable at this point, how about sucking them off the board and seeing if the short goes away? That would settle it.

So what do you think the most likely failure mode was on this thing? PSU over volted the 3.3V rail on the board damaging the ASICs. Then the shorted ASICs blew the PSU 3.3V circuit?
If this is the case, there may be 2 components bad in the PSU at this point. One that went open at the tail end and one that let the voltage go above spec.

It certainly isn't worth opening the PSU other than to satisfy curiosity about what started the failure.

Unrelated:
What's up with one of the 5 brown boxes on the underside of the board near the BNC connectors? Looks like there is a hole blown in the corner. Not related but just curious.
 

Offline Monkeh

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #95 on: December 21, 2012, 12:24:06 pm »
Unrelated:
What's up with one of the 5 brown boxes on the underside of the board near the BNC connectors? Looks like there is a hole blown in the corner. Not related but just curious.
It's a relay. He broke it during teardown.
 

Offline Mike Whitenton

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #96 on: December 21, 2012, 12:31:53 pm »
Unrelated:
What's up with one of the 5 brown boxes on the underside of the board near the BNC connectors? Looks like there is a hole blown in the corner. Not related but just curious.
It's a relay. He broke it during teardown.

Don't know how I missed that. Thanks! It's been bugging me.
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #97 on: December 21, 2012, 12:40:54 pm »
So what do you think the most likely failure mode was on this thing? PSU over volted the 3.3V rail on the board damaging the ASICs. Then the shorted ASICs blew the PSU 3.3V circuit?
If this is the case, there may be 2 components bad in the PSU at this point. One that went open at the tail end and one that let the voltage go above spec.

My best guess is that it's a two stage failure.
There was something screwy with the 3.3V rail which was causing the issues that we saw, and then it finally whent kaput and took out the ASIC chips.
Although given that I never measured the resistance of the 3.3V before it died, we will never know.

Yes, I'll probably desolder the 2nd ASIC for kicks.

Dave.
 

Offline jerry507

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #98 on: December 21, 2012, 02:09:19 pm »
So now that it's obvious the asics are busted, remember back to the first video when you were looking at the display and 3/4 of the display were solidly drawn and only the right quarter of the display worked? Kinda makes sense now doesn't it? :)
 

Offline peter.mitchell

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #99 on: December 21, 2012, 07:18:29 pm »
So what do you think the most likely failure mode was on this thing? PSU over volted the 3.3V rail on the board damaging the ASICs. Then the shorted ASICs blew the PSU 3.3V circuit?
If this is the case, there may be 2 components bad in the PSU at this point. One that went open at the tail end and one that let the voltage go above spec.

My best guess is that it's a two stage failure.
There was something screwy with the 3.3V rail which was causing the issues that we saw, and then it finally whent kaput and took out the ASIC chips.
Although given that I never measured the resistance of the 3.3V before it died, we will never know.

Yes, I'll probably desolder the 2nd ASIC for kicks.

Dave.

I'm surprised you didn't freeze the ASIC with the dust buster, eg, freeze it before power on (really freeze it and wait so the chip gets cold all the way through) and see if the current was different?

That 100+ degree device? I immediately though probably powered between the 3v3 and 5v for 1.7v, but when you take out the 3v3 and it is shorted to ground it gets a full 5v - probably bullshit, but hey, it makes sense to me so i don't care what you think!  ::)
 

Offline mikeselectricstuff

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #100 on: December 21, 2012, 07:46:03 pm »
A possibility is it's not the asics at all, but something they are driving, pulling too much current through logic outputs.
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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #101 on: December 21, 2012, 08:32:59 pm »
I'd second Mike. There must be some external condition that effects all four ASICs.
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #102 on: December 21, 2012, 08:35:47 pm »
I'd second Mike. There must be some external condition that effects all four ASICs.

The ASICs appear to bridge the ADC hybrids and the memory module slots, and that's pretty much it.

Dave.
 

Offline Electr0nicus

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #103 on: December 21, 2012, 10:56:54 pm »
Considering that second asic to be defect and all other ones working, would mean that the excess power (5A *3.3V) has to be dissipated in that second ASIC. I don't think that 16.5W additional power dissipated in that second asic, will only result in a few degrees temperature difference in comparison to these other three ASICs. I think, that all ASICs are pretty much dead, and the second ASIC only has more internally circuitry blown and shorted out, thus the lower resistance.
As there are no other hotspots on the board and the only hotspots are those 4 ASICs tells me the following:

As this low resistance can be measured in both directions it's likely that there is a real large short in those ASICs, where maybe a huge part of the Die is shorted out.
As the current decreases while the ASIC heating up tells me that there is no semiconductor material involved, so there could only be a complete short between the some bond wires and the metal Base- plate of the die. Because if a semiconductor would be involved the current consumption will increase, because of the negative temperature- coefficient of semiconductors. As the current decreases with increasing temperature this short must be "metallic" because only this would explain the positive temperature- coefficent.
Although there is a second possible alternative: The 3v3 power plane, the ASIC pins and internal bond wires are heating up, increasing their resistance and and thus lowing the current consumption, although there is a internal semiconductor short.
A possibility is it's not the asics at all, but something they are driving, pulling too much current through logic outputs.

That would be possible, but this also means that those logic outputs maybe defect forever, because of overcurrent.

Dave can try desoldering this 2nd asic, but that would be tricky, because of the large heatsink, which can't be removed, because,  as he said, it's very well glued on that ASIC. The only possibility to remove a such high pin count device, is with hot air. Because of the huge thermal capacity of that ASIC with heatsink, that could be a hopeless undertaking, except Dave will be able to remove the heat sink.

Cheers Gregor
 

Offline JoannaK

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #104 on: December 21, 2012, 11:37:08 pm »

Dave can try desoldering this 2nd asic, but that would be tricky, because of the large heatsink, which can't be removed, because,  as he said, it's very well glued on that ASIC. The only possibility to remove a such high pin count device, is with hot air. Because of the huge thermal capacity of that ASIC with heatsink, that could be a hopeless undertaking, except Dave will be able to remove the heat sink.

Well, If he wants to desolder that part, solution is quite obvious. Just heat up the entire Heatsink to 300C or so.. In no time the solder will flow and the asic separate from board.

Ok, I do admit that may not make baord any more working as it is at the moment. But (if done right) there's hope to see a cool video.  :-BROKE
 

Offline tnt

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #105 on: December 21, 2012, 11:42:56 pm »
if you don't care about the asic you can also cut all the pins with a dremel ...
 

Offline EEVblog

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #106 on: December 22, 2012, 12:17:04 am »
Dave can try desoldering this 2nd asic, but that would be tricky, because of the large heatsink, which can't be removed, because,  as he said, it's very well glued on that ASIC. The only possibility to remove a such high pin count device, is with hot air. Because of the huge thermal capacity of that ASIC with heatsink, that could be a hopeless undertaking, except Dave will be able to remove the heat sink.

Yes, it would not be pretty trying to desolder this or remove the heatsink. Classic case for just cutting the pins.

Dave.
 

Offline vaualbus

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #107 on: December 22, 2012, 12:21:43 am »
I don't belive that the asics are broken. In fact How the problem on the trigger of the oscilloscope, as we can see on the teardown, can affect the asics?
For me probably some components are partial demage, and cause the short.
Probably the best way is to try first to resolder the components and change all the bypass capicitor and than try to remove one by one.
Another think How I capicitor can became hot?
Than you could try to replace the hot chip.
« Last Edit: December 22, 2012, 01:35:17 am by vaualbus »
 

Offline M0BSW

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #108 on: December 22, 2012, 01:32:30 am »
Here are my tips for finding supply shorts.
Using resistance measurement, even on a high-precision meter is generally fairly futile due to contact resistance variation
Instead, apply about an amp or two across the supply and measure mv levels across the rails at various places. The voltage will reduce as you get nearer the fault. This effectively gives you a 4-terminal measurement, avoiding the issue of contact resistance

I've also use the 'blow the arse out' method. Now I have thermal imager this approach should be a lot easier in future!
An IR thermometer can also be used by scanning it carefully across the board.
 
The other useful tool is the i-prober. You need to switch the supply with an external MOSFET so you're looking for an AC signal to avoid the position sensitivity.
 
This could be a game-changing tool for faultfinding : a $175 thermal imager
http://www.kickstarter.com/projects/andyrawson/ir-blue-thermal-imaging-smartphone-accessory?ref=card
As someone learning this stuff for hobby use, these bits of info are really valuble.
no one would or will tell me how to delete this account
 

Offline vaualbus

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #109 on: December 22, 2012, 01:36:04 am »
I don't belive that the asics are broken. In fact How the problem on the trigger of the oscilloscope, as we can see on the teardown, can affect the asics?
For me probably some components are partial demage, and cause the short.
Probably the best way is to try first to resolder the components and change all the bypass capicitor and than try to remove one by one.
Another think How I capicitor can became hot?
Than you could try to replace the hot chip.
 

Offline peter.mitchell

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Re: EEVblog #398 - Lecroy 9384C Oscilloscope Repair
« Reply #110 on: December 22, 2012, 12:38:16 pm »
Dave can try desoldering this 2nd asic, but that would be tricky, because of the large heatsink, which can't be removed, because,  as he said, it's very well glued on that ASIC. The only possibility to remove a such high pin count device, is with hot air. Because of the huge thermal capacity of that ASIC with heatsink, that could be a hopeless undertaking, except Dave will be able to remove the heat sink.

Yes, it would not be pretty trying to desolder this or remove the heatsink. Classic case for just cutting the pins.

Dave.

For removing those heatsinks, usually you can just hit it with a blowtorch as you gently twist with a pair of pliers, the epoxy isn't designed to work reliably up to those temperatures. It usually either goes soft or it crumbles.
 


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