Author Topic: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2  (Read 29947 times)

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Offline EEVblog

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EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« on: December 21, 2012, 01:33:52 am »


Dave.
 

Offline albertr

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #1 on: December 21, 2012, 03:09:24 am »
Dave, good show, man! But why not to put everything together (main board, CRT board, CPU board and memory boards) and try to fire it up while powering 3.3V rail externally? I know you mentioned that it already went kaput, so there's nothing to lose?

-albertr
 

Offline Psi

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #2 on: December 21, 2012, 03:52:19 am »
Seconded.

Just because it has a short doesn't mean it wont work.
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Offline kbs1

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #3 on: December 21, 2012, 04:07:54 am »
that's what I was wondering as well. Does it work?:D
 

Offline HardBoot

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #4 on: December 21, 2012, 04:37:54 am »
When you have a short all of that current is just going straight to ground instead of powering anything...

I wonder if the thing works with that hot chip completely ripped off and the 3v powered... if the firmware is smart enough it'll at least give an error, if you had the right firmware for the 2 channel version it'd then work... if that chip is for the extra channels and not the primary channels...
« Last Edit: December 21, 2012, 04:49:18 am by HardBoot »
 

Offline Psi

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #5 on: December 21, 2012, 04:47:28 am »
When you have a short all of that current is just going straight to ground instead of powering anything...

In this case the short isn't too bad. It's only drawing a few amps more at rated voltage.
(9A vs the normal 6A)
So it's not a dead short, It's a bit of semiconductor somewhere which is eating up power and producing heat.
There will be a voltage drop but it should be possible to overcome that by providing 3.4 or 3.5V instead of 3.3V.
The chips will run from that just fine.

« Last Edit: December 21, 2012, 04:49:09 am by Psi »
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Offline EEVblog

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #6 on: December 21, 2012, 06:16:26 am »
Anyone know a nice way to get those heatsinks off?

Dave.
 

Offline free_electron

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #7 on: December 21, 2012, 06:48:16 am »
Thermal epoxy... Dissolves in MEK Methyl Ethylene Ketone... Nasty stuff... Do it outside. It stinks up the whole place.
Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline Psi

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #8 on: December 21, 2012, 06:53:08 am »
i once tried to pull/pry off a heatsink epoxied to a BGA chip.
The epoxy pulled the die right out of the IC :palm:
It wasn't anything important so i just laughed :)


hehe, i just had a look and found the PCB still in my junk draw.
Pretty sure there is supposed to be a die in the center :P


« Last Edit: December 21, 2012, 06:59:23 am by Psi »
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Offline kyndal

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #9 on: December 21, 2012, 06:58:07 am »
i second the idea of hooking it back together
and "give it" the 10 amps it wants.

and hell.. might work just fine

might give some indications as to where the failure is located
(maby channel "second chip" is debunked ?)  snip off the 3.3v pins
and you have yourself a handicapped but functional scope ?

/Kyndal
 

Offline nitro2k01

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #10 on: December 21, 2012, 07:32:25 am »
I think the "chip 2" theory may be a red herring. I think it may just be that that chip happens to be opportunistically closer to the current return path when you're using the screw hole as the negative voltage supply. 3.2 V * 8.6 A = ballpark 30 W. This is what you got when connecting the external and original power supply in the same time, or ballpark 8 W per chip. This doesn't seem all that off for that kind of heatsink. The short when measuring with a multimeter would indeed indicate a dead short, probably inside the chips, but it can't hurt to reassemble it and try it with an external 3.3V supply. Worst thing that can happen is that it doesn't work at which point you can say OH WELL and chuck it in the recycling bin.
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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #11 on: December 21, 2012, 09:36:13 am »
Anyone know a nice way to get those heatsinks off?

Dave.

Machine them off on a mill. The only "safe" option that comes to mind  :-//
 

Offline M0BSW

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #12 on: December 21, 2012, 11:09:30 am »
 I thought it was a brilliant Video, helping me to pick this up, learning loads of stuff,I like the IR thermonitor, you can buy those fairly cheap, the flir , I think its called , that's way to expensive for a hobbyist, I have seen one of those been used buy a ghost hunter, with way to much money, back to the real world,I guess the IR therm just gives you a good indication if something is way to hot
« Last Edit: December 21, 2012, 12:17:37 pm by M0BSW »
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Offline robrenz

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #13 on: December 21, 2012, 12:24:48 pm »
Both videos are great. Seeing troubleshooting techniques in action and then all the forum commentary is quite an education. ( at least for me)

Offline DeepSubMicron

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #14 on: December 21, 2012, 01:45:23 pm »
Hi I am just a wannabe lab rat. I followed the two thrilling repair videos.
My impression is that you tried in the second video to show it is not repair able rather than actually trying to repair it. For example, you have successfully narrowed down the search area to the 4 ASICs. But then you didn't go deeper into the rabbit hole and analyzed the hot spots in more detail.
I was most disappointed about the TTI current probe. If this is not the perfect use case then such a probe is completely pointless.
You are probably right with the conclusion it is not repair able but it is hard to accept that.
 

pir

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #15 on: December 21, 2012, 01:47:10 pm »
hey Dace.....
i had a little time and went trough the Service Manual of 9374 cause was easy to find:
https://www.google.gr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDcQFjAB&url=http%3A%2F%2Fwww.ko4bb.com%2Fmanuals%2Findex.php%3Fdir%3DLeCroy&ei=m2jUUK7IFo3s0gX904GIDA&usg=AFQjCNGw2RFNWNshbCCT1cmM1D94s2fStw&bvm=bv.1355534169,d.d2k

this is VERY SIMILAR to the 9384 topology go and look at PCB layout

there are 2 custom chips (ASICs):
HSH423 ___> the ADC-Buffer ASIC from the fronted
MDX416___> the SRAM ASIC driver connected to the HSH423 ASIC  <--------the 4 ASICs shorted to ground

well ..... the 3.3V power is not powering the ASICs....it is the floating ground controlled by the 2pins cable feedback to the power supply !!!!

so maybe there is no problem with a short and it is a power supply failure on reading the feedback from the 2cable feedback connector from the main board
page 294

also there are (SMD , 3 of them) some regulators with reverse protection diodes over IN-OUT that they provide all the floating-to-the-ground power to the chips

so my theory is this:
the regulators provide the Vcc...Power Supply control over the floating ground with the 3.3V rail

....maybe i am wrong , but should take a look of that 2 pins  feedback to the power Supply or the Power Supply itself  ???
....so maybe there is nothing wrong with that ASICs  :-//

George
« Last Edit: December 21, 2012, 01:51:23 pm by pir »
 

Offline M0BSW

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #16 on: December 21, 2012, 02:08:31 pm »
Anyone know a nice way to get those heatsinks off?

Dave.

Machine them off on a mill. The only "safe" option that comes to mind  :-//
Are they that well stuck on then.
no one would or will tell me how to delete this account
 

Offline vaualbus

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #17 on: December 21, 2012, 02:36:32 pm »
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.
 

pir

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #18 on: December 21, 2012, 02:50:07 pm »

well ..... the 3.3V power is not powering the ASICs....it is the floating ground controlled by the 2pins cable feedback to the power supply !!!!

....maybe i am wrong , but should take a look of that 2 pins  feedback to the power Supply or the Power Supply itself  ???
....so maybe there is nothing wrong with that ASICs  :-//

George


just watch the video #1 @2:10
 

Offline Classicmacintosh

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #19 on: December 21, 2012, 02:56:15 pm »
G'day Dave & others,
I've gotta hand it to ya Dave, you have at least 543% more patience than I do :D
I would have just stuffed 30v at god knows how many amps through the blighter by now!
On a more serious note have you tried to give the PCB a *bloody* good whack or two? I suspect yanking the second ASIC won't do anything really useful sadly.
The damage to the scope housing must have been the result of a nasty fall though.. could have caused an internal power plane short.

-Ed
-Ed Neil
<ed@ukrnet.co.uk>
 

Oracle

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #20 on: December 21, 2012, 03:01:22 pm »
maybe the short is +V pin of the second chip.... Don't know... it should be helpful excluding some part of circuit such as the 4 chips whit heatsink and see if the current change or is still in the 10A range...
 

Offline opablo

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #21 on: December 21, 2012, 03:24:06 pm »
I would love to see what happens if you assemble it back together and start feeding as much juice as it wants to the 3.3 rail with your external PSU.

And start cranking it up until you are able to overcome the voltage drop produced by your short... 3.4, 3.5, 3.6, etc.

Maybe it works !...

Or maybe the chips are really and deeply dead and nothing happen....
 

Offline cengland0

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #22 on: December 21, 2012, 03:28:25 pm »
I think it was misunderstood how to use the FLIR.  The heat you thought you saw coming from the BNC connectors was not heat.  In fact, you can look at the screen and see the heat scale on the right hand side.  Notice that the maximum temperature on the scale is much less than when you have it pointed at one of the heatsinks.
 

pir

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #23 on: December 21, 2012, 04:07:33 pm »
The 3.3V is the Floating Ground of the DAC<---->SRAM<--->CPU
The ASICs are OK...DO NOT REMOVE THEM
THE PIN-1 of 3.3V is the SRAM Vcc..that is why it is not shorted to ground
THE PIN-2,PIN-3 is the FLOATING GROUND..and the circuitry related to that  has the problem...the feedback to PS?
THAT CHIP THAT WENT TO 100'C ....is fried because from 15V-3.3V=11.7V .. got 15V+

did anyone bother to go through an "allmost the same scope" schematic except me ?  :-//

VEE is the 3.3V floating ground of pin2,pin3
see pics
 

Offline gszo123

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Re: EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
« Reply #24 on: December 21, 2012, 05:00:20 pm »
I think it was misunderstood how to use the FLIR.  The heat you thought you saw coming from the BNC connectors was not heat.  In fact, you can look at the screen and see the heat scale on the right hand side.  Notice that the maximum temperature on the scale is much less than when you have it pointed at one of the heatsinks.

Actually I think Dave was right. There are several factors to consider when using IR camera. First is of course the scaling of colors you mentioned.

Second is that FLIR doesn't measure temperature direcly but IR radiation that can be subject to phenomena any radiation is, namely: transmission and reflection, so measuring temp of shiny surfaces can be tricky (the same is when you are trying to take a photo of a mirror, you not only take photo of mirror itself but also of objects reflecting from its surface). And this happened there on BNC. Camera was showing effect of combined radiation from the BNC itself as well as radiation reflected from eg. Dave's hand.

Third factor is that different materials have different emmissivity coefficients (dependent on color, roughness, etc), so that when measuring its temp you should first set proper coefficient in the camera to make accurate measurements. Thats why, probably, absolute values measured in the video were somewhat off. But in this case it was all about relative temp not absolute.

See more info here: http://assets.fluke.com/manuals/Ti30____umeng0000.pdf


BTW, hello from Poland NOT Holland :)
« Last Edit: December 21, 2012, 05:08:43 pm by gszo123 »
 


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