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This is a DIY of all mods to the SD2000. This first article will describe the GPM Mod & 4 Position Frequency Mod.

First of all when you hunt around the web you will find lots of mods that are said to improve performance but take it from me that only a few actually do anything. The rule of thumb here is that anything to do with changing the Transmit side of the 2000 is mostly a waste of time. The exception is only applying a higher voltage which will alow more depth but too much voltage will produce more noise and less performance. Typically the 2000 works well at no more than 8.5V and optimal seems around 8V.

Before we describe the various mods certain common precautions should be taken before working on the detector and any electronic devices. These are to make sure you have a static free environment. This can be done by earthing yourself out by an anti-static strap or simply a piece of wire tied to your body and to an earthing point that goes to ground. A good ESD protection soldering iron/Station, Desoldering equipment although not necessary in some cases that I will describe as we proceed. So Below is a list of required tools.

1. Phillips head Screwdriver
2. Flat bladed screwdriver (preferably a jewelers set)
3. ESD Protected Soldering Station
4. Solder Flux Paste
5. Solder (I use 2 types 1 x .5mm and 1 x 1mm)
6. Solder sucker or desoldering station
7. Solderwick (desoldering braind)
8. Zinc(?) SMD desoldering wire (this is also known as SMD rework kits in shops)
9. Some ribbon cable or hook-up wire & cable ties
10. Selleys Paint stripper (although not necessary)
11. Cotton buds or small artist's pain brush
12. Electronic PCB Cleaner (spray)
13. Various electronic components as described in the procedures below.
14. Toothbrush or similar
15. Plenty of patience!!!

Removing the PCB's from Detector Case

Let's first describe extracting the PCB from the case. Firstly (and this is how I do it), take of the knobs on the coil side by using a small blade screwdriver and undo the grub-screws until they do not grip in the plastic knobs and slide off. Undo the nuts on the balance control pots, if they start to spin stop and wind the control until you can hold them so the pot itself cannot rotate and continue undoing the nuts. Do the same for the switch, but first check the switch for it's position. This can be done by looking at the thread of the switch and you will notice a small slot that runs done the length of the threaded part. Take a note of this(see fig.1). The switch normally is the hardest to take the nut off as it wants to spin all the time. If it is too difficult to hold it wait for the next procedure. Now with that done undo the six screws on both front and rear end plates and gently let the ends come off. Sometimes the ends are stuck and you may have to use a flat screwdriver to prise them off, be gentle as you don't want to harm the rubber dust/water seals between the plates and the body. Once done if you haven't taken the GB selector switch off you can do that now by holding the switch from the inside and removing the nut. I generally put the nut on so that the washers don't fall off. With that done just place the rear plate back on to the case and rest the detector on it so you can see inside the power side. Make sure that you do not place strain on the wires from inside the case to the end plates as these can come off and unless you know where they come from you can be in trouble. If you look inside the case towards the arm rest side you will see a T0220 FET on a small block with a screw and a wire attached to it. (See Fig 2.) Undo this screw and make sure you do not lose the plastic insulator that is on the screw. Sometimes this stays on the FET just make sure it doesn't drop out. Like wise there is a flat plastic or rubbery insulator between the back of the FET and case again do not lose this or damage it. Once the screw is removed put it with the rest of the screws and nuts for safe keeping. Now hold the case and tilt it downwards towards the power end gently letting the power end cap fall down. take the rear end plate and pull it out so the the inner coil cables take the strain of the PCB, gently twist the coil end cap so it can enter the case letting the pcb's slide out (Fig.3). Once the PCB's are out place them on a table or bench and lift the top most PCB and let it lie flat by the hinge effect of the 2 joining ribbon cables. You should end up similar to Fig 4. That's all there is to disassembling the SD2000.

Fig.1
 
Notice the Notch in the Balance Switch.

Fig.2


FET Attached to case

Fig.3

 
Slip the Coil end plate in so the PCBs can slide out

Fig. 4


PCB Laid out on bench

GPM Mod.

The first mod is the GPM and can be done a number of ways, that is to say you can have a fixed system that you turn on & off with a switch or the preferred method with variable control pots. I will describe only the variable method. For this you will need the following components:

2 x 500k Linear pots
2 x 100k 1% resistors
4 lengths of approximately 6 inches hook-up wire
9mm Drill

With the boards laid out as previously described look at where channel 2 balance pot wires enter the PCB you will see 2 resistors. One a barrel type and one a 1206 SMD type. Using a cotton bud place a little selley's paint stripper on the 1206 part ONLY try not to get it anywhere else. after 10-15 minutes gently scrape off with a small flat jeweler's screwdriver. After this spray the PCB with the PCB cleaner and use the toothbrush to scrub the area. Make sure no bits of selley's or paint chips are on any other components especially IC's pins. I do not use the stripper method but gently scrap the paint off with a screwdriver blade, see image below:
Fig. 5

 

Once cleaned as above using a small screwdriver place a little force on the left of the first resistor on the left board to be removed and quickly apply heat with the soldering iron to each end pad of the resistor whilst applying pressure to make the resistor slip off the pads. With the second resistor I usually get a very small blade screwdriver and slip it between the MKT Cap and the resistor to apply pressure and do the same with the soldering iron as previously. Once removed clean the area and apply a little solder to the exposed pads as in the below image.

 

Next you get the 2 x 100K resistors and bend 1 end of each at right angles to and snip the lead off so you have approximately a 2mm right angle piece of leg. Tin both ends of the resistor near the body. DO NOT CUT the other ends yet. Solder each of the right angles to the pads as shown below:



The reason for not cutting the resistor is to make it easy to hold in position when soldering to the Pads. Now get your 500K pots and bend the legs back as in the picture below. Also you will need to remove the locking tab on most pots (some don't have it) This is so it will lay flat on the case when assembled. Your pot should look like the image below. MAKE sure you check which pins to join otherwise you will have the pot turned on when to the left and off when to the right!



Now get your ribbon cable and strip the ends a little (around 2mm will do) and tin them. Solder each pair to the pots (BTW, you can cut off the excess from the 2 resistors now) and the resistor and pads. Check that they are in pairs and not mixed see photo below.



Now the last thing to do is mark the case for the holes. I alway place one of the balance pots in the top hole and draw around it then pace a pot on with the legs towards the coil plug and then centre mark them making sure of 2 things:

1. They are as close together as possible in the centre otherwise they will hit the circuit boards and make it impossible to re assemble.
2. The Balance pot can go in without interference from the 2 pots.

The image below shows the marks I made:


Okay gentle using a 9mm drill (check your pots first for the exact size), drill the 2 holes. I generally after drilling run a 12mm drill on the holes to smooth off the edges and remove any burrs present. Also it is a good idea to wrap the 2 pots in Insulating tape so they cannot come into contact with any parts on the detector boards. I have blown the receive front end a couple of times doing this when assembled. So that's it for the GPM sensitivity controls. Next we go to the 4 position frequency mod.

4 Position Frequency Mod


The 4 Position Mod or Hotchip or "too many names to mention" Mod that is done by almost everybody that has anything to do with Minelab PI Detectors has been around for as long as the SD2000. First invented by Gary Robinson of WA when the SD2000 was only a few months old. This mod gave the SD2000 an edge over most detectors. Some say that it made the SD2000 equivalent to an SD2200. Personally, my opinion is that it definitely gave extra performance dependant on what coils and frequency combination was used and possibly made it as good as a SD2100. > >The SD2000 came with 2 inbuilt frequencies with only one being used. There is a jumper on the main PCB that selected which frequency was used. Minelabs directed suppliers to change the jumper setting when they sold 2 to a family to eliminate the effect of the detectors interfering with each other. The frequencies in the SD2000 came in 2 varieties, 1.92Mhz/2.073Mhz and 2.073Mhz/2.2Mhz. The most common being the former. Now the 4 position switch gives the ability to switch between the 2 inbuilt and 2 additional frequencies (generally 2.4Mhz & 3.0Mhz). It is quite simple for the modestly competent electrical hobbiest to do this yourself and following is a DIY step by step instruction.
For this you will need the following tools:

A soldering Iron (not too hot)

Some hook up wire (6 pieces approximately 4-6" long)

A 2 pole 5 position Switch

1 x 2.45760Mhz and 3.0Mhz crystals

Solder

Phillips head and Flat blade screwdriver

Some heatshrink to fit over crystal assembly (1") and over the wire ends (dependent on size of wire generally 2-3mm diameter)
a small piece of Veriboard

Look at the left most board to the right near the top connecting cable, you will notice 2 crystals and a 3 position jumper. Generally this jumper will be on position 1 right most jumper selected. Remove the jumper. See below



Get a piece of veriboard pcb and cut it leaving a board with 4 strips by 4 hole wide and long. Place a 2.4xxx and 3.0Mhz crystals on the board so that they go across the strips with each pin of the crystals laying in parallel as below:

 

A good practice is to put the crystals so you can see the frequency in case you forget what crystal is what. Solder the crystals in (do not use too much heat as crystals are heat sensitive also do not drop as crystals are very fragile and you could render them useless if you apply a sudden shock to them). Once you have done this on one strip cut the strip (or track) between the 2 crystals at one end only.

Turn the detector PCB's over and locate the crystal pins on the PCB.Scrape away (carefully) the paint from the pins of the crystals that are closest to the outside of the PCB Apply some solder to one or both of these pins and solder the remaining wire from your extra crystals PCB to one of the pins. Make sure this is secure. Also make sure that this wire goes around the boards over towards the panels as if you do it near the pins you will be unable to slide the boards back into the case!

(I use selleys to remove the paint) as in the photo below:


Now solder a wire to each crystal (on the pads which are separated) Remove the power switch on the end panel and de-solder the wires Check your 2 pole 5 position switch and solder together using tinned wire 2 to 4 position tabs together on Pole 1 leaving the very first position open (This will be the new ON/OFF switch) Solder a wire to each of the pins that had the Jumper on so you now have 3 wires from it. The centre pin goes to Pole 2 pin (Common) on the 2 pole 5 position switch Solder the right most pin wire to Pole 2's position 2 and the remaining pin (left most) to position 3 on pole 2 Now the 2.45760 crystals wire goes to pin 4 on pole 2 and the 3.0Mhz goes to position 5.



Put the crystal package in the larger heatshrink and VERY CAREFULLY apply a little heat to shrink it over the assembly. Turn the boards back over to how they were previously. Locate the wires that you desoldered from the power switch and solder 1 to the Pole 1 common pin and the other to anywhere on the 4 linked pins You should end up with something that resembles the below image.




Place the switch in the hole you removed the power switch Check all solder joints and make sure the earth lead is not touching anything. Plug in a coil and a power lead. Turn the 5 position switch to position 1 and the detector should fire up as normal.If it doesn't then you have done something wrong! All going well you have just added the 4 position Mod & the GPM mod. You can now re-assemble the detector following the reverse of the disassembly procedure.

The finished product should look similar to the 2 photo's below.





Next article will be for a mod that improves signal quality and threshold stability, this is a mod that Woody experimented with.


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