electronics geeks needed, circuit board repair

[Linas]

Well, my presonus firebox got fried the other day. Rather than pay them $75 to fix it, i figure i should attempt to do it myself, and if i fail at that, they can fix what i broke even more. I am familiar with simple circuitry, but don't know the theory of it all that well. I know that I blew a capacitor, which is most likely the result of a diode that failed, but im not sure.

In the first pic, you can see the cap that blew up, and spewed fiber on everything. About 4 o'clock of that, there is a diode that looks burnt. I suspect that diode failed, allowing current to flow backwards and killed the cap. Please chime in with different theories.

[L-A]

I did a bit of tech support where I got to feel the amplitude of the failing capacitor problem:

http://en.wikipedia.org/wiki/Capacitor_plague

[warnergt]

I've seen electrolytic capacitors fail like that. Usually, it was caused by a
voltage beyond the capacitor rating or capacitors used at voltages too
close to their rating.

That diode does indeed look very suspicious. The problem is that you
don't know if it was the cause or the effect. A blown capacitor may have
presented a short to the diode and burned it out. Conversely, a blown
diode may have presented a wrong polarity voltage to the capacitor
and blew it out. Then again, that diode could still be good but just
showing some discoloration from getting hot.

What you really need is a schematic. It appears that your problem is
in a power supply circuit. These are generally pretty easy to diagnose
and fix.

Note also that your wall supply is suspect in this sort of failure. It may
have been outputting too much voltage which, in turn, took out the
power supply circuit.

[Linas]

Originally Posted by warnergt

I've seen electrolytic capacitors fail like that. Usually, it was caused by a
voltage beyond the capacitor rating or capacitors used at voltages too
close to their rating.

That diode does indeed look very suspicious. The problem is that you
don't know if it was the cause or the effect. A blown capacitor may have
presented a short to the diode and burned it out. Conversely, a blown
diode may have presented a wrong polarity voltage to the capacitor
and blew it out. Then again, that diode could still be good but just
showing some discoloration from getting hot.

What you really need is a schematic. It appears that your problem is
in a power supply circuit. These are generally pretty easy to diagnose
and fix.

Note also that your wall supply is suspect in this sort of failure. It may
have been outputting too much voltage which, in turn, took out the
power supply circuit.

Unfortunately Presonus will not give me a schematic. I was powering this unit just using a firewire cable, which I heard if plugged in at a slight angle can easily short. I also learned I probably should have powered it using the AC adapter as well as the firewire cable. I guess since the cap is easy enough to replace, i will replace that first and see where that gets me. It is currently using a 33uF 63v. Do you recommend any different values for the replacement? Unfortunately, I cant read any numbers on the diode.

[line6man]

Originally Posted by Linas

Unfortunately Presonus will not give me a schematic. I was powering this unit just using a firewire cable, which I heard if plugged in at a slight angle can easily short. I also learned I probably should have powered it using the AC adapter as well as the firewire cable. I guess since the cap is easy enough to replace, i will replace that first and see where that gets me. It is currently using a 33uF 63v. Do you recommend any different values for the replacement? Unfortunately, I cant read any numbers on the diode.

There is no need to fool around with different values, but if you want, you can try a higher voltage rating.

As far as the diode, it should be a fairly basic component, so long as it's not something special like a Zener. It's difficult to tell with SMT components. I would be tempted to try a general purpose diode.

I can't tell if that's a white band to indicate the cathode?
Can you identify the polarity?

[Linas]

Originally Posted by line6man

There is no need to fool around with different values, but if you want, you can try a higher voltage rating.

As far as the diode, it should be a fairly basic component, so long as it's not something special like a Zener. It's difficult to tell with SMT components. I would be tempted to try a general purpose diode.

I can't tell if that's a white band to indicate the cathode?
Can you identify the polarity?

The "diode" in question is in fact a resistor. But i have no clue how to determine the value. After looking around some, I came across this forum, in which a guy had almost the same problem I have.

http://www.prodigy-pro.com/diy/index.php?topic=38350.0

his image of what failed is below



He fixed his problem, unfortunately he didnt post the values for the resistor. It turns out the cap in the bottom left corner of mine failed too, just didnt notice because it didnt explode. How do I test to see if my switching regulator is fried like his?

[bongomania]

Originally Posted by Linas

I also learned I probably should have powered it using the AC adapter as well as the firewire cable

Interesting, can you tell me/us more about this? I've been powering mine just off the Firewire, but if there's a real reason to do both, I will.

[Linas]

Originally Posted by bongomania

Interesting, can you tell me/us more about this? I've been powering mine just off the Firewire, but if there's a real reason to do both, I will.

This is quoted from another forum I was seeking help on.

"The design of a FireWire 400 plug makes it such that there is a risk of shorting one of the power pins with a data pin in the connector if the connector is inserted at at just the wrong angle. Then ZAP. Voltage shorting the data lines frys things. It can end up frying things on the peripheral and/or the FireWire port on the computer. So you may have fried the FireWire port on the computer as well. If the FireWire port on the computer is fried it may still be able to supply power over FireWire cable, but data transmission may be toast. So if you replace your Presonus Firebox with another FireWire device you're going to want to confirm that the FireWire port on the computer still works, or opt for using a plug-in PCI FireWire card if you're using a desktop computer and have a spare PCI slot.

There's some info around on the dangers of hotswapping with powered 6-pin FireWire 400 cables. Two that I found with a quick Google:
http://www.wiebetech.com/whitepapers...rtFailures.php
http://www.sounddevices.com/notes/re...-hot-plugging/
I've seen other explanations that also included a graphic that showed how inserting the 6-pin cable at just the right angle can short the data and power pins. I don't see that link in my quickie Google search.

My M-Audio FW410 came with warnings about hotswapping or hotplugging with a powered 6-pin connector. The advice was to never do it. Power down the computer, plug the cable in, then boot while the cable and device are connected.

If you do hotplug with a 6-pin connector you need to be very careful to insert the plug straight and also be mindful of static electricity (discharge yourself first and stay grounded).

I power my FW410 with a wall-wart rather than FireWire bus power. My computer only has powered 6-pin ports so I use an adapter that converts the 6-pin to a 4-pin (4-pin carries no bus power) and then I can hotswap with no worries. If you replace the Firebox with another FireWire device I would suggest doing the same. The 6-pin to 4-pin adapters can be found online with a little google searching. They come in various gender changing flavors so you need to make sure you get the right gender on each end of the adapter and have a proper cable that will match."