Garrett TR with constant audio signal

[Ken H.]

I am troubleshooting this late 1980's ? Garrett TR/Discriminator detector with a constant audio tone.

Cannot be tuned by depressing the tune button and adjusting the tune control as you would normally.

I have a good Tx signal of 62.5khz with 11 v p-p on the Tx coil. When a 2" square of foil is placed on the search coil, I have a good Rx signal of 62.5 khz 3.5v p-p

Circuit uses 6 transistors, one FET, two dual op amps, one dual voltage comparator, one quad switch, and one single op amp.

I am presently drawing out the circuit, but wanted to know if anyone here would know what triggers the audio to the speaker?

[Ken H.]

Re: Garrett TR with constant audio signal

Have tracked down the problem to a missing signal voltage from the Rx because of a bad 4016 bilateral switch.

[proscan]

Re: Garrett TR with constant audio signal

Judging on your description it could be a detector described in patent:

US4303879 Metal Detector Circuit with Mode Selection and Automatic Tuning

or

US4334191 Metal Detector Circuit Having Momentary Disabled Output

Both available at geotech, I believe.

have fun,

Peter