RME Fireface 400

This sound card came in for repair with issue reported as not being visible to the computer and oddly freezing up. Customer also admitted he was “poking around with a voltmeter and may have shorted something”

Initial evaluation of the PSU section discovered few things. He reported initially it was working fine and lately it was only working when powered from the FireWire interface. External PSU wouldn’t work.

The way the PSU circuit works is there is a LM2596 5 switch mode converter which makes 5 V out of the external PSU 9-21V. At this stage selector switch decides if this power is used or the 5V from the FireWire interface. This 2596 was running on very odd frequency way lower hen the specified 150kHz. Also the associated filter cap was totally gone. ESR of 100Ohm and the same for the other cap after the second coil. This 5V Is then converted to +-12V by yet another switch mode regulator LM2588.

This one had better caps used (Nichicon specified for high ripple application) and these were all ok.

There are few more regulators one tiny switch mode making 48V phantom and two more linear regulators making 3.3 and 5V for the ADC and DAC chips.

Once the PSU was all sorted with new LM2596 and new low ESR high ripple caps, all voltages were stable and without ripple. Following this repair sound card was stable without any freezing up.

The FireWire HOST issue was caused by ESD which zapped the ESD protection on first port as well as it took down the FireWire controller IC.

I am really not well equipped for SMT. All I have is $30 hot air station, simple basic soldering iron, microscope, flux and solder wick.

It took fair amount of heat to get the controller chip off. It is a 4 layer PCB with extensive cooling ground planes. I slowly warmed the whole PCB to try to lower the heat expansion stress on the board and then focused the heat on the controller chip. I usually set the hot air station to 335 Celsius for 40/60 leaded solder or to 350 for unleadded. This time however I had to raise the temperature to 370 to be able to melt it and remove the controller chip. I was worried it would damage the PCB but it lasted well without any damage.

Main culprit

Soldering the new one back in was even harder. Its on the edge of my hand soldering skills.

Professional equipped for SMT would probably diss this but for me I’m happy with the outcome. Its soldered on, there’s no damage to the traces and the card worked ISH.

When I swapped the controller I noticed only one FireWire port works. Further evaluation and measurements narrowed this down to the ESD protection device. When I removed it for the purpose of testing both ports worked just fine. I ordered new part and it’s going back where it belongs. ESD is a serious problem for these. This time the protection wasn’t enough and it took down the controller as well.