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EEVblog #401 - Lecroy 9384C Oscilloscope Repair - Part 2
Posted by
EEVblog
on 21 Dec, 2012 01:33
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Dave.
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#1 Reply
Posted by
albertr
on 21 Dec, 2012 03:09
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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
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#2 Reply
Posted by
Psi
on 21 Dec, 2012 03:52
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Seconded.
Just because it has a short doesn't mean it wont work.
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#3 Reply
Posted by
kbs1
on 21 Dec, 2012 04:07
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that's what I was wondering as well. Does it work?
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#4 Reply
Posted by
HardBoot
on 21 Dec, 2012 04:37
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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...
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#5 Reply
Posted by
Psi
on 21 Dec, 2012 04:47
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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.
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#6 Reply
Posted by
EEVblog
on 21 Dec, 2012 06:16
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Anyone know a nice way to get those heatsinks off?
Dave.
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Thermal epoxy... Dissolves in MEK Methyl Ethylene Ketone... Nasty stuff... Do it outside. It stinks up the whole place.
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#8 Reply
Posted by
Psi
on 21 Dec, 2012 06:53
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i once tried to pull/pry off a heatsink epoxied to a BGA chip.
The epoxy pulled the die right out of the IC
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
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#9 Reply
Posted by
kyndal
on 21 Dec, 2012 06:58
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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
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#10 Reply
Posted by
nitro2k01
on 21 Dec, 2012 07:32
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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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#11 Reply
Posted by
HLA-27b
on 21 Dec, 2012 09:36
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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
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#12 Reply
Posted by
M0BSW
on 21 Dec, 2012 11:09
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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
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#13 Reply
Posted by
robrenz
on 21 Dec, 2012 12:24
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Both videos are great. Seeing troubleshooting techniques in action and then all the forum commentary is quite an education. ( at least for me)
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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.
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#15 Reply
Posted by
pir
on 21 Dec, 2012 13:47
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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.d2kthis 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 groundwell ..... 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
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#16 Reply
Posted by
M0BSW
on 21 Dec, 2012 14:08
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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.
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#17 Reply
Posted by
vaualbus
on 21 Dec, 2012 14:36
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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.
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#18 Reply
Posted by
pir
on 21 Dec, 2012 14:50
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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
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G'day Dave & others,
I've gotta hand it to ya Dave, you have at least 543% more patience than I do
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
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#20 Reply
Posted by
Oracle
on 21 Dec, 2012 15:01
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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...
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#21 Reply
Posted by
opablo
on 21 Dec, 2012 15:24
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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....
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#22 Reply
Posted by
cengland0
on 21 Dec, 2012 15:28
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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.
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#23 Reply
Posted by
pir
on 21 Dec, 2012 16:07
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The 3.3V is the Floating Ground of the DAC<---->SRAM<--->CPUThe ASICs are OK...DO NOT REMOVE THEMTHE PIN-1 of 3.3V is the SRAM Vcc..that is why it is not shorted to groundTHE 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
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#24 Reply
Posted by
gszo123
on 21 Dec, 2012 17:00
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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.pdfBTW, hello from Poland NOT Holland