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Mic preamp repair help!

Discussion in 'Troubleshooting and Repair' started by Fangsstl, Apr 10, 2018.

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  1. Fangsstl


    Apr 10, 2018
    Hi folks,

    I have a 2 channel microphone preamp that no longer receives signal from the input jack. Neither channel works anymore. The unit does power on, but no audio goes in or out. I am a total novice to electronics. I know kinda how to solder and I have changed pickups in my guitar before. I think I may have found some bad capacitors in the circuit board, but I'm not sure. I could just be seeing adhesive. Here's some pictures.

    The first 2 pictures are capacitors from both sides of the board. I assume 1 for each channel. The third is a reference image to show a healthier looking capacitor on the board. Not sure if the brown stuff is guts from the blue cap leaking out, or if its just adhesive. The last picture is a chip on the board that has some shiny stuff around it. Again I'm not sure if that's just normal adhesive or if it's a problem. Those were the only visually suspect things I could find on the board.

    Any input would be appreciated!

    Attached Files:

  2. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir Fangsstl . . . . .
    • 1 . . . . .Glue/goo
    • 2 . . . . .Glue/goo
    • 3 . . . . .Glue/Goo
    • 4 . . . . . Superfluous . . . . .heavy on the fluous . . . . rosin flux
    Now give us the units brand and model number, as should have been done initially.

    73's de Edd
  3. Fangsstl


    Apr 10, 2018
    Thanks. ART Digital MPA Gold.
  4. KJ6EAD


    Aug 13, 2011
    The residue on the top of the capacitor, C300 in image 1726, is a potential problem. It's out of focus, but looks more like electrolyte than glue. Does the PCB hang upside down in use?

    The glue around the bottoms of the various capacitors is no problem.

    The residue around U410 is solder flux. It's not a problem but it indicates the component has been replaced post manufacturing since the other visible components are clean of flux residue.

    A high resolution, well lit picture of the whole PCB would be helpful.

    There is a schematic for an earlier revision (-103; yours is -104) online but it is missing the last page (4).
    Last edited: Apr 11, 2018
    73's de Edd likes this.
  5. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir Fangsstl . . . . .

    Is this unit being "new" to you . . . . and thus possibly, not all of its operation, being all familiar to you ?
    Or is this a long time owned unit and you being as familiar with it as the back of your hand ?
    If the latter . . . . . do all of the " Christmas Tree " of front panel LEDS, seem to light up and respond to switching commands.
    Am I correct in guessing that you have only tried the 1/4 plug in jack receptacles, and not tried the Amphenol jack inputs ? And that being on both inputs and outputs ?

    With inputs attempts being made to all of the possible inputs and monitoring all of the possible outputs . . . . . .do the VU meters make any respond to input signals?

    Considering that this unit has run for a decade, after 5 min or so of run time, do the glass envelopes of the units two 12AX7 tubes feel warm . . . .. OR you might see their filaments orange glows within the tops of their central cathode sleeves, IF vacuum + gettering action has not silvered / or / blackened their insides . . . . to impede any viewing.

    If then, by unplugging each tube , one at a time, and then immediately plugging it back in again, do you get a pop from the audio output jack . . . OR . . . a comparative response on the VU meter needle?

    KJ6EAD de W5VR . . . . .tnx 4 schema !

    73's de Edd
  6. nepow


    Jul 18, 2011
    It copuld be flux, but however if it's glue then remove the cap and clean it all away. The adhesive often used in the past was similar to impact adhesive and after time reacted to electrical voltage which not only turned it a brown colour but became conductive as well. I've repaired many a pcb with similar glue around components that created all kind of circuit faults including low resistance current paths.
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