EEVblog #398 - Lecroy 9384C Oscilloscope Repair

[mikeselectricstuff]

A possibility is it's not the asics at all, but something they are driving, pulling too much current through logic outputs.

[HLA-27b]

I'd second Mike. There must be some external condition that effects all four ASICs.

[EEVblog]

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.

[Electr0nicus]

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

[JoannaK]

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. 

[tnt]

if you don't care about the asic you can also cut all the pins with a dremel ...

[EEVblog]

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.

[vaualbus]

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.

[M0BSW]

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.

[vaualbus]

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.

[peter.mitchell]

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.