Firestudio mobile preamp fault.

Hey all,
Presonus FS Mobile 'developed' a fault years ago and I didn't really pursue it.
It sat in a cupboard since and for some reason I decided to have a bash at it again.

In short <sigh>, a damaged cable allowed pin 3 to sort to ground with +48 engaged - This is on preamplifier one.

The result is that both microphone preamps still work but preamp one is substantially quieter.

If phantom power is engaged, some mega fissling and cracking goes on on channel one, long term.
It eventually settles to a crackly hiss to reveal that, as with dynamics, the preamp works - Just at a much lower gain than the normal channel.


With phantom engaged it measures within tolerance on all four pins.


When injecting a signal into (touching..) the individual connectors, I get

Preamp one Pin 2 - Buzz
Preamp one Pin 3 - nothing* (This is the pin which was shorted to gnd.)
Preamp two Pin 2 - Buzz
preamp two Pin 3 - Buzz

So presumably something in the path of pin 3 has been compromised and is preventing the balanced circuitry from operating correctly?

No issue is seen when using an instrument via TS jack - Both preamps have similar amplification here.


Also relevant, if I plug a microphone into input two, crank the gain, and create a noise enough to cause clipping, the signal will bleed into channel one, showing on the meters, then, for both channels.
This happens like a precise switch - There is only bleed when channel two is overloaded.

Heh..Long shot, but I wondered if anyone might be able to speculate about specifics here.
I've been unable to find a schematic for this unit and troubleshooting has been tricky because there are two PCB layers stacked.
The unit wont operate with the top one removed, and the preamps are on the bottom one.

Also, finally, there's a pair of 4403 transistors per channel.
Voltage measurements for channels 1+2 are mirrored.

Many thanks in advance.
 
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I'm looking at the TL082.

My logic is that balanced mic input signal passes through here, but unbalanced inputs do not. The unbalanced inputs work fine.

V+ and V- at the chip must be ok as they're common to both channels and channel two works.
Outputs and paths following must be fine as inverting* input works for channel one.

So this issue must be with non-inverting* input or the circuity that precedes it which is minimal.

Live voltage checking is out, as described, but I wasn't able to pick up any discrepancies in resistance and capacitance comparisons between preamp one and two in this area,
so I'm thinking replace the TL082?

*May have these reverse - I don't recall.
 
Now wait a minute. That opamp is completely isolated from DC conditions on the input itself unless they're so bad as to draw down the whole supply, and even that's not going to hurt anybody.

It is possible that the series resistor from pin 3 burned out. There shouldn't be much current flowing from the phantom itself at that point, but that cap if suddenly discharged just might do it. That cap itself may have also been damaged, but I'd start with the resistor.
 
Hey,
Thanks ashcat.

This is going to be such a pain to describe but I have found a discrepancy.
Those amphenol combo sockets don't have any components inside, do they?
 
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