There's a really common way to kill your
firewire-devices, which is to forcefully plug in the
6-pin connector, carrying both supply power and data signals, rotated by 180 degrees (which normally should be prevented by the asymmetrical shape of the connector). That way you'll send +12V, the most common supply voltage, into the
chip that connects to the cable. This kills the chip pretty reliably.
This audio-interface (Alesis IO26) was damaged using this method:
Thankfully the internal layout of the device is pretty clean. There's a stack of two PCBs (only the lower one shown) on the left side inside. The lower one has the DSP, Firewire controller and a ARM based controller. It connects with flat cables to the "analog" boards. On top, it carries a switching power supply generating the internal voltages (+3.3V, +5V, +/-15V and +48V from a single, galvanically isolated 9..30V input) which is missing in the photo.
A good way to diagnose these kind of errors (according to some forums and FAQs) is to look at the idle-voltage on the firewire-bus lines (TPA0+...TPA0-...). They are supposed to be biased to a internally generated 1/2 Vcc (~1.8V) via 56 Ohm resistors from the twisted-pair lines to a bias pin (TBIAS0/1). It's described in the data sheet of the chip, here it's a
TSB41AB2 from TexasInstruments In my case one bias-voltage-regulator spat out 3.3V, the other 0V (there's one for each of the two firewire ports). One of the ports had one receiver line stuck at 0V (chip-internal short to GND).
The only way to repair it is to desolder the chip, and replace it with a new one. Then the interface should be functioning again (I did not populate the mechanically damaged connector and associated terminating resistors, one port is enough for me).
The two shorted pins #1 and #2 are both GND, so I did not bother to clean up the blob.
