Phantom power short - preamp repair - fire studio mobile

Hi folks.
Steen had an accident. :(

A duff cable allowed phantom to short to ground on preamp one of my presonus fire studio mobile.
The result? Preamp two works perfectly with and without phantom, but preamp one has low signal throughput and a lot of very active noise.
By active I mean not a consistent hum or buzz. More of a wishy washy wind noise.

Given that both preamps work, technically, and the phantom supply is clearly still live, I was hoping I'd burned out a 6k8 on channel one,
but opening it up and inspecting revealed nothing obvious.

Does anyone have any suggestions for testing, troubleshooting, or repair?

If you need any more info just ask. :)

Thanks folks!
 
HI Arcaxis.
Thanks for the reply.

I have some more details, although I can't test for 48V with a mic plugged in at the minute, because of the way the things designed.
Might be able to get at it with a bit of hoking.

Anyway, for now.

Channel 2 is perfect.

With phantom off channel 1 seems perfect, but the gain is very low.

With phantom on and no mic plugged in, channel 1 crackles and whooshes - Like the sound of voltage across a pot.
With phantom on and a dynamic mic plugged in, channel 1 crackles and whooshes. The signal is there and is audible, but is low as before.
With phantom on and a condenser mic plugged in, channel 1 crackles for a moment or two, then settles and seems to work at a good volume - Clean signal.

Phantom power measures the same on both preamp channels.
About 45V open and 35V with a mic connected and working.

If I turn off phantom power, with or without mic it crackles its way down to silence.


Yes, there are a lot of surface mount components, but also a number of 'normal' resistors.

In the first row of 12 smd components above the combo jacks in pic one, reading from the left 3,5,9, and 11 appear to be 6.8k resistors, and read as such.
They're black and marked '6811'

The row of caps above that - None of them is open to ground.

I presume these images will be HUUUUGE on some browsers. Apologies, but detail is important here.
If you save-as and zoom, you should be able to read everything I can read! ;)


Main Board.
1.JPG

Top Board.
2.JPG
 
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The only thought I have is since there is some symmetry in the components of each channel behind the XLR's, maybe some voltage comparisons on the components that appear to have the same function on each side. Beyond that not much I can suggest.
I saw a few posts elsewhere about repairs on Presonus gear that in the US it may be a flat rate charge for a repair. Perhaps contact Presonus where you are and see what they might charge for a repair and shipping costs.

Hey,
Thanks for the reply. I've done about as much point by point voltage comparison as I can and nothing stands out so, like you, I'm not really sure where to go next.

Given that only one channel is affected, though, your logic is perfect. I'm disappointed nothing showed up.

Maybe I'll power it down and check resistance to ground at each point, like above.
 
Hi each, bit late on parade but a couple of observations if I may?

Firstly, any kind of short on the mic cable should not cause any damage to the pre amp circuitry, the designers should know that ***t'appen.

Looking at the pre amp photo it seems to be a variant of the bipolar transistor+chip mic amp and I suspect a transistor has been slightly buggered.

WTGR! If you are not kitted out for SMT work leave it alone! Presonus might wash their hands if the board is mangled.

That input diagram shown should not use bipolar 22mfd caps IMHO. The 48V will polarized them. Further into the pres amp there should be bipolars to DC isolate the input stage when 48V is off., unless of course it is one of the (daft IMO! ) DC coupled jobs.

Dave.
 
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