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Here's my 2014 present to the Geotech forum members ->
Over the Christmas period I managed to acquire a non-working Pulse Technology Minipulse PI Detector. The guy who was selling it said "I do not know the fault of this detector so just selling as spares or repairs".
The detector was in generally good condition for the age, but someone (who doesn't know what they're doing) had attempted a repair. Here's a list of the faults that I found:
The battery pack was missing.
A few PCB tracks were damaged.
Someone had tried to replace the MOSFET, but put the new one in back-to-front.
The series resistor between the MOSFET drain and the coil had been removed and not replaced.
The TX oscillator (555) was kaput.
There was a track short at the preamp output.
The audio (555) was also kaput, as was the transistor that varies the audio frequency.
All the trimmers had been tweaked.
And finally, the connector that goes to the headphone socket was in backwards. That had me confused for a while.
In the end I decided to back-engineer the PCB, and now I have a complete schematic drawn in EasyPC.
The coil is 543 uH (1R0) with a damping resistor of 310R.
TX is running at 80pps (unbelievably slow, but true) with a TX pulse width of 234us.
Main sample can be varied (REJECT pot) between 58us and 150us, although the coil will allow a minimum sampling time of 25us.
Secondary sample is at 204uS (max = 296us).
Both main and secondary sample widths are 40us.
There is no SAT, so it's a bit unstable. Depth is average for a coin, but is hopeless with low conductivity targets due to the 58us minimum sampling time.
The Reject knob is labelled as "min" (position 0), "foil" (position 4), "pull tabs" (position 7) and "max" position 8. However, foil is rejected even at position 0 (again due to the timing delays).
Note that PL5 is fitted, but unused (and is labelled "unknown").
There are a few locations near the bottom of the PCB where additional components have been left out during manufacture, and R46 and R47 do nothing except connect between BATT+ and -5V.
Here's the back-engineered (final) schematic, and photos of the top and bottom of the PCB. The BATT+ net is labelled as 0V in the schematic.
Please also see the modified / updated version called MiniPulse Plus, that comes complete with a PCB layout, Gerber files, etc.
Over the Christmas period I managed to acquire a non-working Pulse Technology Minipulse PI Detector. The guy who was selling it said "I do not know the fault of this detector so just selling as spares or repairs".
The detector was in generally good condition for the age, but someone (who doesn't know what they're doing) had attempted a repair. Here's a list of the faults that I found:
The battery pack was missing.
A few PCB tracks were damaged.
Someone had tried to replace the MOSFET, but put the new one in back-to-front.
The series resistor between the MOSFET drain and the coil had been removed and not replaced.
The TX oscillator (555) was kaput.
There was a track short at the preamp output.
The audio (555) was also kaput, as was the transistor that varies the audio frequency.
All the trimmers had been tweaked.
And finally, the connector that goes to the headphone socket was in backwards. That had me confused for a while.
In the end I decided to back-engineer the PCB, and now I have a complete schematic drawn in EasyPC.
The coil is 543 uH (1R0) with a damping resistor of 310R.
TX is running at 80pps (unbelievably slow, but true) with a TX pulse width of 234us.
Main sample can be varied (REJECT pot) between 58us and 150us, although the coil will allow a minimum sampling time of 25us.
Secondary sample is at 204uS (max = 296us).
Both main and secondary sample widths are 40us.
There is no SAT, so it's a bit unstable. Depth is average for a coin, but is hopeless with low conductivity targets due to the 58us minimum sampling time.
The Reject knob is labelled as "min" (position 0), "foil" (position 4), "pull tabs" (position 7) and "max" position 8. However, foil is rejected even at position 0 (again due to the timing delays).
Note that PL5 is fitted, but unused (and is labelled "unknown").
There are a few locations near the bottom of the PCB where additional components have been left out during manufacture, and R46 and R47 do nothing except connect between BATT+ and -5V.
Here's the back-engineered (final) schematic, and photos of the top and bottom of the PCB. The BATT+ net is labelled as 0V in the schematic.
Please also see the modified / updated version called MiniPulse Plus, that comes complete with a PCB layout, Gerber files, etc.