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Re: [Discuss-gnuradio] USRP1 USB I/F failure & repair


From: Matt Ettus
Subject: Re: [Discuss-gnuradio] USRP1 USB I/F failure & repair
Date: Mon, 18 May 2009 16:00:55 -0700
User-agent: Thunderbird 2.0.0.21 (X11/20090320)

Paul Mathews wrote:
About a month ago, our USRP1 Rev. 4.5 (Ser. #2728) became erratic in it USB
connection and communications. A short while after, USB failed altogether. I
put scope probes on the USB + and - data lines and observed that one of the
signals had a much different sort of signal than the other. Following advice
from Matt Ettus given to others who had experienced USB failure, I ordered a
replacement USB chip along with a Chipquik SMD removal kit. Replacement of
the chip had no effect, so I was forced to look more carefully for a cause.
It turns out that one of the 0603 capacitors installed near the USB
receptacle (C73, C74) was shorted, and removal of these 2 capacitors
restored USB function. I'm posting this description of my experiences for
the benefit of others on the list.

I suspect that C73 and C74 (which I can't find in various schematics and
BOMs for USRP1) were added in the hope that they would provide a path for
electrostatic discharges, which is ironic. In my experience, low-capacitance
avalanche breakdown devices are more suitable for this purpose.


Paul,

Thanks for your analysis of the situation. C73 and C74 are not actually capacitors, they are, as you say, electrostatic discharge protectors, part number PESD0402-060. I have not seen any fail in the manor you mention, but I suppose enough static could cause that.

Has anyone else seen this failure mode?

Matt




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