Thursday, September 8, 2016

Cisco 3500 XL fan subsystem failure.

It is quite old, yet still useful. I'm talking about an old Cisco 3500 XL series switch, I use every day in the lab of the company I work to. I don't want to talk about all the good points this switch has, but rather I'd like to talk about a failure it had, after almost 20 years it worked continously.
After all this years it broke its fan system.
It's summer, and if I want it last other 20 years, I had to fix it somehow.
The very first thing to do is understand how fan subsystem works.
Not the hardest task in the world, you just need to follow the tracks on the PCB from fan backwards to components.

Doing this you you'll identify the two main components the subsystem:
HP4410D: the  MOSFET used to command the power 5V circuit
TC648: the fan speed PWM controller.

One of those twos must be the broken one.
Roughly, the fan subsystem should look something like:

Both devices are packaged into a SOIC8 package, people skilled more than I am, are able to remove this package without damage the PCB nor the component using just iron. About me, I'm not that good and I had to use my cheap rework station.
Once the PWM controller is removed, it is possible to probe Vin. By using the TC648 datasheet you know that at this point tension should range from 1.25V to 2.65V, according to enviroment temperature.
In my case it was 2.4V which is a resonable.
With TC648 removed, the base of the mosfet is exposed to manual drive, I could therefore short Vout to Vdd and GND and observe fan spin at maximum rate, and fan to stop when Vout is put to GND.
This leaded me to the conclusion that  the MOSFET HP4410D had to be ok.
As counterproof, test the TC648 was a little more difficult since its package is not breadboard friendly.

Using this I could be sure the TC648 was broken, and than changing it I could fix my switch.
I hope someone else could found this article useful.

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