TVS Diodes - Help diagnosing

[sidetrack11]

I have 2 Samsung HD204UI 2TB dives together in an external enclosure that have both died. I've taken them out and tried to connect them to the computer separately, and neither will spin up when the power is plugged in. So I'm assuming there's been a power issue with the PCB, and have been doing some reading on the TVS diodes.

I've found the diodes on the PCB according to this picture
http://www.users.on.net/~fzabkar/HDD/HD204UI_TVS.jpg

I've tested both diodes using a multimeter. When I set the multimeter to 200ohms, on the 12V diode it reads 1.1ohms. The 5V diode needs the multimeter to be on 2000ohms to get a reading, and that reading is 1310ohms.

I was under the impression that if the diodes had blown they would read close to 0hms. Is the 1.1ohm reading on the 12V diode suggesting that diode is blown? If so, do I just remove it from the board?

I have some really important data on the drives and would love to save it before spending alot of money. Any help is appreciated!

 

[sidetrack11]

Anyone??

 

[McHenryB]

Are you sure that you tested the 12V diode in both directions? The 12V one might be unidirectional (I believe that's what the bar on it means) which means it would read like a normal diode. Also, are you sure that you did not exceed the diode voltage?

Note that you should be using the diode range on your multimeter to do the testing.

 

[fzabkar]

@sidetrack11, the 12V diodes are shorted (1.1 ohm is probably the resistance of your meter leads). You will also find that the zero-ohm resistor (a "fuse") will be open circuit.

The first thing to do is to verify the integrity of your PSU (the diodes were killed by an overvoltage). You can then flow a blob of solder over the zero-ohm resistor and remove or replace the diode. Just be careful not to overvolt your drive in the absence of the diode, as you will no longer have any protection.

Best of luck.

 

[arcitech1]

Anyone??

Testing diodes in circuit is problematic. you could be getting readings from other components in parallel with the TVS. The problem with TVS diodes is that they use the breakdown voltage to pass transient voltage spikes. They can fail open or short, or somewhere in between. To test the proper operation, you need to know the breakdown voltage of each diode and have the proper rig to measure at that level. If you are comfortable doing your own repair/shortcut as fzabkar describes, be ready for good or bad results. Good luck and I hope you get your data back!

 

[sidetrack11]

fzabkar it worked! Thankyou so much. I removed the 12V diode and put solder across the 0 ohm resister and both drives spin up and are recognised by windows.

But now I've run into a second problem.

The drives were in a Buffalo LinkStation LS-QVL in RAID, and windows doesn't know how to read them. I'm pretty sure they were in RAID 1.

I've looked at many guides on how to use Ubuntu Live to mount the drive, but I'm having no luck. Can anyone help as to how I can mount the drive(s) in Ubuntu (or windows if possible) to pull the data off?

Thanks in advance

 

[fzabkar]

Try Linux Reader for Windows (freeware):

http://www.diskinternals.com/linux-reader/

Otherwise ...

Reiser FS - mounting in Windows:
http://www.hddoracle.com/viewtopic.php?f=36&t=1132&p=5127