mp-1

The modifications detailed below include:

These modifications are to be performed only by qualified electronics repair technicians. They involve working with surface mount components. Please read the entire modification before proceeding. These modifications or any other internal modifications will void the warranty.

Modification: Changing Low-Cut Frequency (1 hour est. time)

The low-cut frequencies of the MP-1 can be changed by opening the unit and adding or replacing one through-hole capacitor (at J3). The only caveat with this modification is that the original ratio between the frequencies of 2:1 must be retained (the original frequencies are 80 Hz and 160 Hz). The slope of the filters will remain at 6 dB/octave.

  1. Disassemble the unit by removing the four corner screws on the back side of the unit. Remove the front panel knob using the proper sized Allen-wrench. Unscrew the nut holding the rotary switch to the front panel.
  2. Carefully slide the unit apart by pulling on the rear panel while gently pushing the rotary switch through the hole. This will expose the pc board.
  3. Locate J3 on the pc board. It is a jumper (empty) which is adjacent to the low-cut capacitor (which looks like a blue box). Depending on the new capacitor value (determined below), the blue capacitor will or will not be removed. In either case, the new capacitor will be soldered into J3.
  4. The value of the capacitor must be determined: C=36.9/f, where: C is in uF, and f is the lower frequency in Hertz
  5. If C is more than .47uF, then subtract .47uF from C. This is the value of the capacitor which must be soldered into J3. The capacitor (blue box) which is immediately adjacent to J3 must remain in the pc board after the modification.
  6. If C is less than .47uF, then the value of the capacitor which must be soldered into J3 is C. In this case, the capacitor (blue box) which is immediately adjacent to J3 must be removed.
  7. The new capacitor needs to be non-polarized and 50V or higher. A high quality film capacitor is recommended. Ceramic types will work although they will yield a higher THD near the corner frequency.
  8. Re-assemble the unit in reverse order paying close attention to the rotary switch: The rubber washer should be assembled to the switch, and then the metal flat washer, and then the switch through the hole. It is easy to strip the threads on the switch if the nut is not threaded on far enough before tightening it.

Modification: Adding an external DC jack (2 hour est. time)

An external DC jack may be added to the MP-1 to facilitate running the unit off of either AC power (via a wall wart and the DC jack) or off of batteries. After this mod is completed, the unit can operate off of a regulated 3V wall wart or the batteries. The MP-1 will run off of whichever of these sources has the higher voltage. For example, if the batteries are at 2.8V total and the wall wart is at 3.0V, the wall wart will power the unit and the batteries will not discharge. If at this point the AC power disappears, then the batteries will take over and power the unit automatically. Important: using a higher voltage or non-regulated wall wart will permanently damage the unit.

Parts needed:

  • DC power jack (Radio Shack # 274-1582 or equiv.)
  • DC wall wart, 3V regulated (Radio Shack #273-1663 or equiv.)
  • 2 Schottky diodes, 30V or higher (Digi-Key #1N5819GICT-ND or equiv.)
  1. Disassemble the unit by removing the four corner screws on the back side of the unit. Remove the front panel knob using the proper sized Allen-wrench. Unscrew the nut holding the rotary switch to the front panel.
  2. Carefully slide the unit apart by pulling on the rear panel while gently pushing the rotary switch through the hole. This will expose the pc board.
  3. Remove the four screws holding the back panel to the XLR connectors. At the front of the battery tube, there is a connector connecting it to the pc board. Slide this connector apart, and then remove the battery tube/rear panel assembly from the pc board assembly.
  4. Find a DC input connector of your choice, and drill a hole for it in the rear panel of the unit below the battery inlet. You may have to cut away some of the pc board to make the jack fit.
  5. Wire the negative (-) of the jack to J5 on the pc board (this is the contact clip riveted to the pc board. Solder a 30V (or higher) Schottky diode to top pin of S2 (power switch). The stripe on the diode must be the end which is soldered to the switch. Solder the other end of the diode to a wire and solder the wire to the positive (+) of the DC jack.
  6. Turn the pc board over and find the very short trace which connects J4 (positive battery connector) to S2 (power switch). Cut this trace. In place of the trace, solder another Schottky diode from the connector to the switch lead. The stripe on the diode must go to the switch.
  7. Insulate the new wires, connections and parts properly so that they do not short out to anything on the pc board.

Re-assemble the unit in reverse order paying close attention to the rotary switch: The rubber washer should be assembled to the switch, and then the metal flat washer, and then the switch through the hole. It is easy to strip the threads on the switch if the nut is not threaded on far enough before tightening it.

Modification: Changing 12V Phantom to 12V T power (2 hour est. time)

The 12V phantom position on the phantom switch on the back of the MP-1 can be changed to 12V "T" power. This is a difficult modification which involves removing parts, cutting several traces and adding some parts. Please note that "T" power can permanently damage dynamic and phantom powered condenser mics! "T" power can only be used with "T" power mics. With this modification, the 48V phantom position will be unaffected.

  1. Disassemble the unit by removing the four corner screws on the back side of the unit. Remove the front panel knob using the proper sized Allen-wrench. Unscrew the nut holding the rotary switch to the front panel.
  2. Carefully slide the unit apart by pulling on the rear panel while gently pushing the rotary switch through the hole. This will expose the pc board.
  3. Unscrew the rear panel from the XLR connectors. Pull the battery tube connector out of J4 (at front of tube) carefully - use a screwdriver if necessary to pry apart. Remove the rear panel/battery tube assembly from the pc board.
  4. Remove J1 and J2 (XLR connectors). Be verrrrry careful not to lift any traces. Make sure all solder is removed from the holes before removing the connectors. In our experience, it is best to use a hot iron and a "Soldapulit" solder sucker. If you are not extremely comfortable removing parts like this, do not attempt this operation, as the traces are on the top of the board and are easy to destroy.
  5. Remove the two surface-mount resistors near S3 which have the marking "6810" on the tops of them. Replace each of these with 182 ohm, 1%, 1206-size surface mount resistors (Digi-Key # P182AACT-ND or similar).
  6. Cut the trace connected to the resistor shown below. Scratch away the soldermask on the copper ground fill next to the resistor as shown. Connect the resistor to the ground fill with solder.
  7. Remove the gray ferrite bead between J1 and T1 (near the middle). Solder the two leads of a new capacitor to the pads from the ferrite bead. The capacitor needs to be a 220uF x 50V cap (Digi-Key # P5277-ND or similar). The negative on the capacitor must go towards S3, and positive towards J1. Insulate the leads of this capacitor so that they do not short to the transformer T1 or the connector J1. Glue the capacitor to the top of T1 with epoxy so that it does not move after installation.
  8. Re-install the XLR connectors.
  9. Cut the trace on the bottom of the pc board going from the middle two pins of the transformer T1 to the switch S3 (See picture).
  10. Solder a jumper wire from the middle pin of switch S3 to pin 2 of J1 as shown in the picture.
  11. Make sure to mark the outside of the unit clearly that the unit has been modified for "T" power, as "T" power can damage non-"T" power microphones.
  12. Re-assemble the unit in reverse order paying close attention to the rotary switch: The rubber washer should be assembled to the switch, and then the metal flat washer, and then the switch through the hole.

It is easy to strip the threads on the switch if the nut is not threaded on far enough before tightening it.

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