Disclaimer........These plans are to be used at your own risk.  I will accept no responsibility for any damage to your detector, loss of warranty from the modifications,  or failure of the completed meter to work in the manner expected.

The first step is to remove the coil cable plug cover exposing the wiring connections.
See figure 1 & 2
  Carefully slide the rubber boot over the connector back far enough to expose the metal case of the connector(some liquid soap applied to the wire may make this easier).
There are two small screws holding the cap and one holding the metal cover to the plastic part of the connector.  Remove them and slide the metal cover back (there is a slight twist needed to do this) exposing the solder connections.

You will need to solder two wires to the connector, use the thin bell wire (3"). The face of the plastic connector has pin assignments molded in it, use these to find pin 5 and 6.  Connect one of the wires to pin 5 and the other to pin 6 marking the pin 5 wire so you can identify it later (it is the voltage lead).  Route the wires through the plugs metal cover alongside the coil wire and through the rubber boot, replace the metal cover on the plastic plug and replace the cap. Tighten all screws and slide the rubber boot back over the metal cover.
Cut the plug off one end of the RCA patch cord and strip enough of the insulation from both leads to solder to the wires from the plug.   Use tape or shrink tubing to cover the wire connections.  This completes the modifications to the Minelab for the meter connection.

Figure 2

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On to the meter mount.

The meter mount is cut from the kitchen sink-dishwasher waste tee. See figure 3
  The blade of the mount has to be 3" from the tip to the ridge of the pipe at the tee.  It is 5/8" wide at the tip and 1 1/8" wide at the ridge. The dishwasher tee is cut to create a flat mounting surface.

The following meter modifications create a flat surface for the face allowing you to put a chart on it and make room for a lead jack and power switch.

You should still have the back of the meter off, remove the battery.  Looking at the back of the meter notice that the circuit board extends down along the battery on the left side. You will need to cut the heavy wire loop on this extension and cut the board extension off.  Remove the two screws holding the circuit board to the front cover. Carefully remove the board.  When removing the board be careful not to lose the two ball bearings that make the click stop for the dial, they are under the dial and will probably be dislocated when removing the board. ( Figure 4). On the back of the pointer there are 6 metal clips used to make the connection to the board, remove them before making any changes to the dial (they can fall off while grinding).  Grind or file the pointer of the dial flush with the outer ring of the pointer marking the pointer for the top first.  Replace the metal contactors on the dial.
Figure 4

Before cutting the circuit board notice where the foil runs from the common socket to the main board and where the volt ohm amp foil runs from the socket to the main board.  Find a solder point on each of these foil strips to connect the leads from the jack.  You will have to solder a wire to each of these locations later for the RCA jack. Cut the heavy wire loop off at each end near the bend and remove the center section (see figure 5). Use a knife or hack saw blade to score the circuit board on the front and back of the board straight along the line of the bottom of the main part of the circuit board and snap the board extension off. The figure 6 & 7 shows the board and wire cut away.

Now you need to make room inside the case for the jack and switch. Use needle nose pliers to break away the two lower lead socket covers flush with the inside of the front cover. (Figure 8)

Locate a position on the bottom of the meter for the RCA jack (center of the opening left after removing board extension) and drill a 1/4" hole.  (Figure 9) Mount the jack to the cover.  Locate for best fit a position for the switch on the left side of the meter case (looking at it from the back) and drill another 1/4" hole for the switch knob.  Position the switch on the outside of the case to align the mounting holes for the switch, be sure to allow for the movement of the switch.  Mark the mounting holes and drill 1/8" holes for them.  Position the switch on the inside of the cover and use two screws and nuts to fasten it in place. Figure 9 shows the location of the switch and jack as well as wiring positions.
 

Solder one of the leads from a two inch piece of the twin lead wire to the circuit board location identified earlier as the volt ohm amp connection of the board and solder the other lead to the board at the common location. Solder the volt ohm amp wire to the center conductor of the RCA jack and the common to the shield, see figure 10.

Remove the red battery wire from the circuit board and solder one of the two leads of a 1.5" piece of the two lead wire to the board in its place. Solder the other lead to the red wire, tape the wire joint or cover with shrink tubing. Solder the other ends of the two wires to the switch, one to the center the other to either of the outside connectors.

Make sure the metal contactors on the dial back are in place, turn the dial over and put the ball bearings in their sockets.  Holding the dial stationary, put the cover over it and apply slight pressure to the dial from the inside to keep the bearings seated. Turn the cover over and carefully replace the circuit board using the two screws from the removal in the top two holes on the board. Position the new on off switch in the off position and replace the battery.  Before replacing the back cover locate the small calibrating pot at the top left of the circuit board.  Locate that position on the back cover and drill a 1/8" access hole in the back cover. Replace the cover and secure with the two screws. Fill the old test lead holes with RTV silicone to make the unit more water resistant.

Position the meter in the Minelab mount on the top of the shaft, connect the RCA plug from the coil wire to the jack and connect the coil to the detector. Turn the dial pointer on the meter to the 20 volt scale (it will be left there) and turn the meter on with the new switch.  Calibrate using a quarter over the coil. Use a jewelers flat blade screwdriver to adjust the calibration pot through the access hole drilled in the back of the case.  Set the meter to read 1.80 with the quarter.  Use readings from various coins and objects to make a chart for the front of the meter and tape it over the dial pointer (this is why it was ground flush).

If you have any trouble with this project you may contact me here.