Capacitive Effects

[UK Brian]

Can anyone who builds/modifies detectors give me a hand on 'capacitive effects' ?
Problem is I bought an old Altek Quad Mode VLF/TR off E-Bay ($2!!)thinking that getting it up and running would help me understand the electronics of detectors a little better. Everthing now works but I think I have capacative effect... VLF is fine but if TR is selected both the selection knob and the discrimination fine tune cause a left (ferrous) meter movement and a target response noise. Slight as the knobs are approached, loud when touched. I remember that it was always suggested that if a metal case was used for a detector then the control box should be earthed. However I'm using a plastic box and plastic knobs.
Anyone any idea why I'm having the problem and how to cure it ?
Many Thanks....Brian

 

[Joe in austin]

Try running a ground (ok...EARTH) wire from the metal cases of the pots to the circuit ground.

The case should be connected to the metal shaft (if the shaft is metal)--verify with an ohmmeter.

Such "hand effects" are usually more pronounced when the pots are high resistance values. Hand capacitance is not much but in a high impedance circuit, you could have problems such as you witness.

Good luck,
Joe

 

[UK Brian]

Thanks Joe
will try what you suggest. It has got a plastic shaft however (plastic everything in fact).

Brian

 

[Joe in austin]

Another thing to try, as a next step perhaps, is to remove the circuitry and put aluminimum tape (or copper tape, if you can find it)inside the front panel, then connect that to the circuit ground.

Most pots have a metal body, even if shafts are plastic.If you can get the metal case to touch the tape described above, that would ground everything and provide some shielding.

Good luck!

Joe