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How to find out the faulty SDRAM

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Jane1

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From time to time I must repair a Linux embeded equipment that has its own BIOS, CPU and SDRAM.
This SDRAM has 256MB together and consists of 4 SDRAM modules(IC).
Sometimes equipment does not work.Then there are several possibilities for that.
Faulty CPU or BIOS or SDRAM.
Is there a method how I can find out that SDRAM is faulty? If so which module of those 4 SDRAM modules might be faulty?
Each SDRAM module has 66-pin Plastic TSOP(II) and is soldered on PCB.
Or what method shall I use to check that the processor sends signal( e.g. address signal) to SDRAM?
Thank you for help
 

If a faulty SDRAM bit, or bit loss, is the problem you could write a memory test program and execute it for some time. The simples pattern is to write first all zero's verify the zero's(maybe several time), then write and verify all one's. Another option is to write its own address into SDRAM word with parity bit and verify that, then run a checksum and compare it with the checksum you have generate by the CPU. This way you have a chance to identify at least the faulty word.
 

What You mean under "Sometimes equipment does not work."

Do not work at all, or some particular thing? Not turning on,....?
 

does NOT work AT ALL
 

I check BIOS and it works.Also CPU seems to be working.Only problem seems to be either SDRAM or a PCB fault.(e.g. paths from CPU to SDRAM).It is Dreambox 800 PVR satelite receiver equipment. Do you have any idea? Thanks
 

Check is there proper and continuous voltage on board.

Check voltages on IC pins.

Inspect electrolityc capacitors on board.

How do you check BIOS?

Check cables.

Did you inspect PCB and other small smd parts visualy?

How do you check cpu if "does NOT work AT ALL"?

Do you consider under "does NOT work AT ALL" no any picture on screen but device working (led lights is on, fan is on,...) or something else (dead complete) ?
 
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Thanks for the reply

Check is there proper and continuous voltage on board.
I do.

Inspect electrolityc capacitors on board.
Yes, I do

How do you check BIOS?
With oscilloscope


Did you inspect PCB and other small smd parts visualy?
Also under microscope

How do you check cpu if "does NOT work AT ALL"?
With oscilloscope

By saying "it does NOT work AT ALL" I mean the Dreambox does NOT start, that is the program from BIOS does NOT start.
So if CPU is OK( tested before soldering too in test jig), and BIOS works too, only SDRAM or PCB can be faulty, I would say.
Do you agree?
 

Sometimes solution of problem are very amazing, and often not logical.

Sorry I tryed to find some better and detailed picture of that board but without success.

If You can make detailed photo (7Mpix and Up @ Macro) of it will be useful.
 

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