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fnorkn

Varistor on the USB port blown?

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Hi

I started a thread about a busted sensor earlier, solved that problem by replacing an opamp. I think the cause was a shortcircuit on the usb port.

Now I want to upgrade my firmware by using the USB port, but it is no longer responding.

Not to worry, I have purchached an AVR programmer and successfully downloaded the firmware from the chip using ICSP1, so I can change the firmware.

But I would like to get the USB connection working again. Just for the hell of it.

I did some measurements, and want to check with you guys about assumptions. Here is the pic of the board (the blown coil L1 is bypassed with a piece of copper foil) :

IMG_20160429_225049.thumb.jpg.4683e5a8e62517daf648182c852a0611.jpg

Here is a piece of the documentation:

5a331bbb09e9a_um2usb.thumb.png.4bafee103dc17c0eeb7a9803d23d2d55.png

Now I assume that R8 and R9 are varistors, and are there to lead away spike voltages on the datalines to ground?

R9 is not conducting according to my multi-meter (good) but R8 is 33 Ohms. So D- is not getting through to the atmega 16u? So I think I should remove it, and see if the USB connection will work again?

Also, C6 seems to be busted. It is connected to the evaporated L1 coil, so I checked it out. It should be 1 uF according to documentation. The multi-meter says it has infinite capacity, which usually means it is blown. Indeed, I measure 3.4 Ohms, not good for a capacitor. I am not really sure what the UCAP port does, it does not seem critical to me for USB communtication, but I could be wrong?

Suggestions are welcome!

IMG_20160429_225049.thumb.jpg.4683e5a8e62517daf648182c852a0611.jpg

5a331bbb09e9a_um2usb.thumb.png.4bafee103dc17c0eeb7a9803d23d2d55.png

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I removed the varistor and the capacitor.

When I measured them again, detached from the board, the values were OK.

So the problem is a short circuit somewhere else. Could be the atmega chip, perhaps the USB input ports are fried...

Desoldering this chip is not something I am really looking forward to, and if I need to replace it , I probably need to flash the eeprom, program, "fuses" , boot-loader (?) etc (I am used to PIC microprocessors, atmega seems to have its own way of doing things)

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