Steenamaroo
...
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.
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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