Just what is it doing wrong?
You might look at the Garrett patents so see if any one of those matches or comes close to your detector in design. You can find various patents at the Geotech1 website. I am not sure a link would be ok here but you can email me for a link.
I am not sure just which model you have. Can you post a pic?
Now, one thing I look for is leaking caps. There will be what appears as dried liquid on the pc board around a bad cap. Also, look for bulging ones. Usually, they will bulge at the bottom.
Next look at the IC's. If they are 8 pin dips, then they will usually be either singe or dual units. If a number is on them, then look up the part and check the power supply pins. Normally, they will be pin 4 for the - voltage and pin 8 or 7 for the + voltage. Pins 2 and 3 are usually inputs and on the dual units 5 and 6 are also inputs. Sometimes one can see signals with an old analog meter, but a scope is much better.
Take a cheap pocket radio and turn it on, turn it off channel and make sure it is on AM and then place the radio near the coil. Usually, you will get a loud buzzing if the transmit circuitry is working. If it isn't working, check the power supplies first. This can be done by checking the battery connections, then the wiring at the board where the batteries connect and finally tracing to circuitry normally used for a power supply. Quite often a zener diode is a dead giveaway.
If you are not sure or unsteady, it is best you leave even the testing to someone more knowledgeable such as Keith Wills at Broken detectors in Texas. It is easy to make things worse.
If I had time I would look at it for you but right now, that isn't possible.
Be extremely careful when checking or replacing parts. I always use a professional desoldering tool (the type with a electrical vacuum pump to assure a clean removal of solder) and a grounded soldering pencil to eliminate any possible destruction from static. It is all too easy to blow a cmos chip using the wrong equipment. A friend of mine can attest to that. Also, if I change an IC I always install a quality socket. This way any future replacements will not require soldering equipment.
Reg
