Garrett old school units.

[mwman]

Hi All.

My first post after many years!!

I started building up a couple of the PI kits on the market, circumstances delayed there completion so I looked at buying dead commercial units and repurposing the hardware.
I have ended up with three units, two dont work and one has no audio. I contacted Garrett hoping to obtain the diagrams so I could get at least one running again, unfortunately they would not release the info even though these are 1980's units!

I decided to draw up my own diagram, starting with the "Fortune Hunter". This model has three controls, Tuning, Variable discrimination, Power.
I will update when done, if anyone has the user instructions that would be useful.

[mwman]

I have completed my circuit diagram, I have included layout but file size limits will require me to reduce size of the main layout.

[mwman]

[Op04]

Thank you for sharing Sir Mwman, great job!

[sinclairuser]

i find these older designs a work of art.

[mwman]

I have now completed my tests on the detector, looks like one of the LM358's has become faulty.
I noticed a slight error on my diagram, R19 the 22k resistor I have shown going to the 9V rail actually goes to the 5.4V rail.
When checking opamps, the + is the non inverting input and the - is the inverting.
As a rule, which ever is greater determins the output status. So if the + has a higher voltage on it then the output will be positive, reverse the situation and the - has the higher voltage then the output will be low or 0V.

[mwman]

I have replaced the LM358 IC2 a, b. Unit now work fine!

So for less than a £1 GBP I have a working unit, the clue was in my last post about the voltage anomallies on the OPAMP.
I am now looking at the others and already I have got to the problem, the audio output device is faulty.

I noted when searching the net on these units one big issue is the battery usage, if you use duracell 9V batteries the cost is quite staggering for 45 minutes use!

I have opted to use another power source, 18V cordless drill batteries. The treasure hunter needs 9V for audio and 18V for the search electronics, by using a 18V to 9V DC to DC converter a 6.6Ah battery should last ages.
My plane is to remake the battery cover, then fit a DC power socket. I will run the battery externally in a pocket using a fly lead.

When I have it sorted I will post the upgrade.