EEVblog #398 - Lecroy 9384C Oscilloscope Repair

[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.