How to find out a faulty RAM chip - Page 1
Forum section: electronics / general

author: Bondguy
author_rank: Contributor
timestamp: November 19, 2016, 09:39:44 pm
content: Whathardwaretest should I use to find out a faulty memory chip in DIMM module?My understanding is that if any RAM chip ( used in DIMM ) is faulty then the DIMM is considered ( by the system that uses that DIMM) to be faulty .I can replace ALL RAM chips on the DIMM but it is not what I want to . Also a replacement of DIMM is not what I want.So, does anyone have any experience with such repairs or where I can learn more?.

author: Ampera
author_rank: Super Contributor
timestamp: November 19, 2016, 10:10:41 pm
content: Quote from: Bondguy on November 19, 2016, 09:39:44 pmI don't think I have seen a single person ever try to do this.Often the answer would be a scope, trying to test the individual chips with it to see if the outputs do what they are supposed to do. Most DIMMs are VERY tight SMD chips, and I do not suggest replacing them, and unless it is some infinitely rare memory, you really should just buy new memory.But a scope, maybe DMM testing, just the standard stuff.

author: wraper
author_rank: Unknown
timestamp: November 19, 2016, 10:45:32 pm
content: Quote from: Bondguy on November 19, 2016, 09:39:44 pmWell, change one by one (reuse previously removed) until is starts to work or errors disappear. But, frankly I'm unlikely to repeat the process again unless I'm desperate. Also with modern modules with BGA RAM, it is even more difficult. AFAIK there is no method to tell which particular IC is faulty, unless particular IC is completely dead and heats more than others (thermal camera).

author: ajb
author_rank: Super Contributor
timestamp: November 19, 2016, 10:46:29 pm
content: Partly it depends on what level of "faulty" you're trying to detect. A marginal IC that returns incorrect data only intermittently will be much harder to detect and isolate than one that is just plain dead. If you need to isolate an intermittently faulty IC for some reason, your best bet is probably one of the utilities designed to be run from a bootable disk to repetitively test the entire memory range of a PC for hours on end. Otherwise you'd need to build some sort of test fixture to repeatedly write and read every address on a DIMM as fast as possible. Not an impossible task, but hardly seems to be worth the effort if you can just use the existing PC to run the test for you. As far as isolating the specific IC, the testing tool should report which address(es) exhibited faults, and from there you'll need to figure out which IC contains that address by understanding the DIMM interface's addressing scheme and examining how the address is decoded on the DIMM itself.

author: wraper
author_rank: Unknown
timestamp: November 19, 2016, 10:46:33 pm
content: Quote from: TwoOfFive on November 19, 2016, 10:10:41 pmNow you have, LOL.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 19, 2016, 10:54:48 pm
content: Try memtest86.

author: Macbeth
author_rank: Super Contributor
timestamp: November 19, 2016, 11:18:41 pm
content: Quote from: RoGeorge on November 19, 2016, 10:54:48 pmAlso, take note of the faulty addresses - you can map them out in Linux easily with badram and grub. Windows lets you do it too but a little more convoluted, but it's good enough to get you by until you replace the RAM (or more likely just replace the whole fucking PC/Server lol!)

author: Ampera
author_rank: Super Contributor
timestamp: November 19, 2016, 11:36:33 pm
content: Quote from: RoGeorge on November 19, 2016, 10:54:48 pmMemtest86 isn't going to help him here, and to add he also specified a hardware test.Memtest86 is GREAT for testing memory to see if it IS bad, but he already has discovered that, he wants to know what specific chip is bad.I think your SOL here, A 30 or 72 pin SIMM, maybe because they still use large enough pin pitching so that you can desolder them easily, but unless you have a commercial tester that happens to do this sorta thing, you're going to have a better time spending the hours you would need to use up trying to diagnose and repair the DIMM, and then most likely for it to not work (Longshots like this NEVER work for me, and this is beyond anything I've ever done), you could pick up a second/odd job and get the money for any memory that could possibly be thrown on a DIMM, it's just not worth it.And I know, I hate when I have crazy ideas and people come and tell me EXACTLY what I already know, and do not need to know instead of giving me the exact few sentences that will reveal what I want, but not only have we already done that, it's just a bad idea to do it. Also what was previously said, and I didn't even think of, there are newer DIMMs with BGA chips that are impossible to desolder manually, and impractical to do with something like a reflow oven, or a heat gun. These things were NEVER made to be user serviceable, and some may argue that DIMMs in their own right aren't user serviceable parts as a whole, but that's something else.I'm sorry, I hate this when I have loads of free time, and I need to spend money on part and do something boring like waiting for it to come, but it's borderline impossible to do this sorta stuff.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 19, 2016, 11:52:35 pm
content: I would estimate board constraints do limit the order the memory chips are layed out in. But I don't know if the memory is evenly and linear mapped on the chips.If they are, you *could* derive the broken chip if you know chip #0, since then you just take the address and divide it by chip size, voila, reaplace that chip.Otherwise desolder them all and get a tester?http://www.simmtester.com/page/news/showpubnews.asp?num=169Note that you're not even considering timings here. Which are crucial to the operation of dram. Finding a suiteable replacement could be hard. Not to mention you might have to manually clock that dimm form now on since the eeprom profiles don't match anymore.

author: SeanB
author_rank: Super Contributor
timestamp: November 20, 2016, 06:52:15 am
content: Use Memtest to find the bad blocks and which data lines and address lines it is at, then take the info, and use the known layouts of DRAM modules ( but beware that often the board manufacturers often swap address and data lines in layout to get trace lengths down in the layout as it has no effect on using the actual module) pinouts to get the address and data line pads, and then follow them to identify which chips have that address and data pins connected. You will then have either a chip with 4 data lines or 1 data line ( depends on the manufacturer, older were 1 but common now is 4) and which gets that address range and that particular data bit range. You are working in binary here so all address and data values are in Hex, and you simply have to determine which 4 bit nibbles are failing, and find the chip/s that handle that block. Then you can do the unsolder and replace with a known good salvaged donor chip.As an aside, I was contemplating using a faulty 4G module a few years ago ( when they were incredibly expensive) in my machine, and simply using a kernel switch to exclude that 16k block with the single flaky bit in it from use, as pretty much every Linux kernel supports excluding a block of memory from use. Just would have made kernel updates a manual thing though, instead of a package manager automatic thing. The reason for wanting to try was the module was free to me, as it was not under warranty, but I ended up not doing it.

author: _Andrew_
author_rank: Regular Contributor
timestamp: November 20, 2016, 07:09:42 am
content: You have to consider the time and effort that you would have to put in to testing and diagnosing the faulty chip. Finding the replacement. Changing it out. Verses finding a replacement module.One quick one and it may seem silly but have you tried cleaning the contacts on the dimm and the socket. It would not be the first time I have found memory errors that ended up just needing to be a clean and re-seating.

author: German_EE
author_rank: Super Contributor
timestamp: November 20, 2016, 07:35:59 pm
content: Back when computers came with 16K RAM chips I worked with a guy called John Bamber who reckoned he could find faulty chips with his finger. All he did was run a finger along a row of chips and when he came across one that was warmer than usual he knew that he had found the faulty part. Well, this worked until one day when he had a board that was a dead short, he did the usual trick until there was a sizzling sound and an OUCH! as he drew his finger back. Until the day he retired old JB had the Intel logo burned into his finger.

author: Bondguy
author_rank: Contributor
timestamp: November 21, 2016, 08:30:10 am
content: My problem is that it is a non standard computer that uses that DIMM like module. Let's call that device an industrial device. That device has a kind of Linux installed and a kind of busybox software is running when booting is successful.I can check the booting (process) via COM port but ONLY if all 4 RAM chips are working. Yes, those 4 RAM chips form a complete DIMM like memory module.If any of those 4 RAM chips is faulty the device will not start booting and I can see nothing as output (on COM port)So, I would need to test which RAM chip is faulty only by hardware means - no software solution because CPU does not boot properly.Any idea?

author: RoGeorge
author_rank: Super Contributor
timestamp: November 21, 2016, 09:24:05 am
content: For an industrial equipment with only 4 RAM chips you can try replace them all, one by one, with testings in between.The price for 3 more chips should be insignificant compared to the repair fees or to a new equipment.

author: stfsux
author_rank: Contributor
timestamp: November 21, 2016, 09:30:21 am
content: I often read that you can run a JTAG test on RAM IC but never experienced this.https://www.xjtag.com/about-jtag/testing-non-jtag-devices/ddr4/I would appreciate if anyone already done such thing with DDRx memory

author: Ampera
author_rank: Super Contributor
timestamp: November 23, 2016, 07:14:27 am
content: Perhaps it's a SO-DIMM, (that's so dimm, right?, ok i'll stop) like laptop memory. If you're in the mood of messing with strange hardware, try to find the pinout of that, get a compatible regular DIMM, and just wire up an adapter. It will be a lot easier than doing all the stupid SMD work, and ball joint work if your particularly unfortunate.Have you tried looking into replacement parts by the manufacturer? Probably will gouge you, but it's easier than attempting to test out an entire DIMM.I unfortunately have to let you down, DIMMs are never easy to take apart. SMD work, especially at that level if your lucky enough to have a human desolderable package. If it's a modern ball joint, good luck with that. You could try to find a hardware test for entire DIMMs, but I doubt they are easy to come by if it's a nonstandard part. And even if you did the chips off, it's probable it will just break, because as I said, whenever I do something that (No offense) stupid, it always breaks.Once a DIMM is bad, nobody I know would do anything but replace the DIMM, and there is really nothing you CAN do either. Sorry to let you down like that, but it's the truth.