Surfmaster PI Tune Up

[Can Slaw]

Hi guys first post. I just purchased a Surfmaster PI, been trying to find one for some time. I bought an o ring to seal the box since the o ring is probably original. I plan on a couple of modifications and hoping someone here can help.

first one is to upgrade the 8 AA batteries, can the Surf PI handle more then 12v? Was thinking maybe a rechargeable 14.4 Li-ion or maybe one of my Excalibur batteries.

Next was to lower the pulse delay "Mr. Bill Mod" to make it more sensitive to small gold /jewlery. I ordered an IRF 9640 Resistor and a 390 ohm 2 watt resistor. Are these correct? The 200 ohm original resistor is only a 1/2 watt but Whites upgraded the PI Pro with a 2 watt resistor so I went with the 2w over the 1/2w not sure if that's ok.

Thanks in advance I hope to hear any ideas or suggestions in what direction to go.

[Can Slaw]

Here some pics of the machine.

[Carl-NC]

The parts sound right, you definitely want the 2W resistor.
The SMPI can probably run OK with 14.4V but there is a caveat. The 555 timer runs directly off the battery and some of those are limited to 15V. A fully charged LiIon pack can easily hit 16V so I would look closely at the 555 and replace it with an 18V version to be safe. Also double-check the voltage rating any electrolytic caps in the 12V domain.

[kt315]

ufff.... Mr Bill mod... i hope to see both sides of the pcb while i have them much. you are on right way CS.
ok. two-three 18650 in parallel and a converter STEP-UP tuned will give you sure 12V.

[bbsailor]

Hi guys first post. I just purchased a Surfmaster PI, been trying to find one for some time. I bought an o ring to seal the box since the o ring is probably original. I plan on a couple of modifications and hoping someone here can help.

first one is to upgrade the 8 AA batteries, can the Surf PI handle more then 12v? Was thinking maybe a rechargeable 14.4 Li-ion or maybe one of my Excalibur batteries.

Next was to lower the pulse delay "Mr. Bill Mod" to make it more sensitive to small gold /jewlery. I ordered an IRF 9640 Resistor and a 390 ohm 2 watt resistor. Are these correct? The 200 ohm original resistor is only a 1/2 watt but Whites upgraded the PI Pro with a 2 watt resistor so I went with the 2w over the 1/2w not sure if that's ok.

Thanks in advance I hope to hear any ideas or suggestions in what direction to go.

Minimize the coil seen capacitance to allow a higher damping resistor value. Will also allow guicker sampling to get lower TC targets like small gold nuggets.

Here are the places in the metal detector design to look to improve.

1. Coil seen capacitance from the MOSFET and it’s circuit.
2. Coil seen capacitance in the coil wire choice and winding.
A. Read this forum for information related to points 1 and 2above.
B. Wing the coil with stranded Teflon insulated wire.
C. Find a way to measure the TX coil and RX coil self resonance frequency
D Shield the coil properly. Look up Scotch24 a fine mesh that makes a good low capacitance coil to shield design. Only lay a single layer around the coil with no overlap while looking at the cross section of the coil. If the letter o was the cross section bundle then the letter U can represent the cross section view of the Scotch24 shield. You will get a better low delay by not allowing the top section of the U shaped shield to make mechanical contact.
E. Measure damping resistor ohm value both cold and after being warmed up through normal operation. Note any resistor change in value.

Enjoy researching your new metal detector and all the good stuff you will find on this web site to modify and improve it. Beach hunting for coins at a good depth requires using a larger diameter coil than someone looking for small gold nuggets who must use a smaller diameter coil.

Joseph J. Rogowski