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How to Fix Memory - Part 3

Friday, October 20, 2000 In Part-2 of the troubleshooting memory-guide, we discussed about the basic technique on how to use memory diagnostic software and hardware to detect memory failure.

For those who have missed part-2, Click here

For those who have missed part-1, Click here

I have to apologize again to all our loyal readers who have been waiting for this long overdue article and I give no excuse to make you wait any longer.

This section is written with the assumption, that you already owned a SIMM/DIMM memory tester and you are capable of performing repairs and rework on Memory PCB assemblies.

I had countless conversation over the years with many seasoned PC technicians and many said they would normally discard the faulty memory into waste paper bins and some just simply keep them in a drawer.

Upon further probing most noted they do not have the expertise and rework equipment on how to replace faulty memory chips, and some even said that it is not worth the trouble.

Yes I do agree with them if the faulty memory module is using older technology such as FPM and EDO Dram, it is definitely is not worth the effort. In today'ss high performance PC system, memory are more expensive and starts at a minimum of 64MB capacity and the demand for higher memory capacity requirements are on the rise. It makes no sense to throw away a defective 128 MB modules, if you know it can be repaired.

Lets get on with the initial step to locate the bad ram module in your PC system , using DocMemory software as an aid. Refer to part-2 for details on how to use the diagnostic software to isolate the bad DIMM module. Click here


Defective DIMM had been identified by Docmemory diagnostic software and removed from PC System - What is the next actions ?

From the picture above - if this module is defective - would you be able to locate the defective chip ?

Without guessing which sdram chip is faulty - you would need at least a :

SIMM/DIMM Memory tester or a ram diagnostic card to help pinpoint the failure

Memory diagnostic software such as DocMemory has limitation in it ability to pin-point defective data bits- which are tied to the sdram chips. So in order to perform a proper diagnostic - you would still need a hardware memory tester to do the job right.

For those folks who cannot afford a hardware memory tester which would cost at least $1500 for the most basic configuration , CST Inc is currently developing a memory diagnostic card which plugs into your Motherboard PCI slot and converts your PC into a memory tester.

The new product will be called "DocRam" Test Card which will be price at US$299 and will be available before Christmas this year. Do check back www.simmtester.com for updated details on the "DocRam" test card.

For readers who needs to know more about Memory Tester , CST have created a FAQ sections that answers all your question regarding memory tester vs memory software dignostics.
Click Here

In this example we would use a CST memory tester to identify a defective 64MB PC100 memory module which causes a blank screen on the PC monitor during boot-up.

Using the SP3000 Memory tester, a failure was detected on one of the 8 DRAM chips on the 64 MB SDRAM module, this was the failure results.

Configuration: 8M x 64 - 8ns
No of Chips: 8 chips
Bank: 1 bank

Passed: Walk Data, Walk Address

Failed: March Test at Loop 1
Failed Data Bit: 8, 9, 10,11

What is a relation between a dram chip failure and a data bit failure?

Different Jedec Standard module have different sdram component placement location, for example a 64 Megabyte DIMM modules can come in two different configuration :

- 8Mx64 DIMM which uses 8 pieces of 4Mx16 SDRAM chip on a double side PCB.

- 8Mx64 DIMM which uses 8 pieces of 8Mx8 SDRAM chip on a single side PCB

In this two configuration - the component placement are completely different however the databit assigned on the DIMM modules are identical,if you need the component placement data sheet for other type of Jedec DIMM , please send me an email: Contact DocMemory

Coming back to the memory tester, you may be wondering why the SP3000 memory tester only indicates which data-bit failed and not which chips had failed. Memory tester need to be configured for different type of memory modules and since there are million of modules in circulation ,its difficult to be able to provide a standard configuration.

CST Inc has developed a reference table which are used to configure the tester to pin-point exactly the chip placement location. See illustration :

The above table shows a configuration for "8Mx64 DIMM" which uses 8 pieces of 8Mx8 SDRAM chip on a single side PCB

The memory tester was able to detect a failure data bit 8,9,10,11 which was tied to chip D1. Replacing the defective SDRAM chip at location D1 would solve the problem.

There are several other causes of bad data-bits on a DIMM module, they are :

- excessive solder bridges on the chips-shorting the leads
- poor soldering joint between the chip leads and PCB pads
- Crack resistor chip which is connected to each individual data bit
- open trace on the module PCB board
- worned out gold tabs
- open or shorts on the dram chip address pins will result in a bad data bit

Different data-bit failure will appear at different chip location.The next difficult part in replacing a defective memory chip is using the proper tools to desolder the defective chips while preventing further damage to the delicate memory module PCB copper trace.

Conclusion to Part-3

In part-4 of the concluding chapter on "How to repair Memory" - , we will further discuss how you could rework the faulty modules and make some money out of repairing Memory modules.

By: DocMemory
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