General I'm going to try to help you with the detector.
Try this.
Turn it on. Get it running and the threshold steady, laying on a TOTALLY metal-free bench at least 4 feet from any metal the size of a corn can lid, or larger. Remember, THIS is a Fisher!
Put it into discriminate mode, with it humming along..
Then tap your hand on the bottom of the coil. If there is no change in the noise in the speaker then there is probably no loose coil winding inside the coil casing. If the noise becomes harsh or broken up, the coil may have been dislodged, or just part of it. If all seems to be still the same after checking it for looseness, move on to this next step.
Wiggle both ends of the cable, at the box, and at the coil. If the sound changes, you may well have a bad or loose connection there.
Wiggle the discriminate knob or switch, or both. If it changes the noise coming out of the detector, you likely have a faulty or really filthy or corroded potentiometer (the discrimination dial), or a bad switch. It can be cleaned or replaced with a new (IDENTICAL) value pot or a similar switch.. If you elect to try to clean it after having found it to be faulty somehow, DO NOT use any oil-based spray cleaner in it, or you will really have a bad day and may wind up stabbing it right through the ten-ringer. Use ONLY the compressed air type you can buy at Rad Shack or TV repair shops.
Now, if the above does not solve the problem, then you will have to tap on the outside of the main component housing (the box). If this causes the detector to make any additional noise besides the threshold hum, you may very well have either a broken or cracked PC board, or a bad internal connection. Right now I'm thinking you have a bad discrimination switch or pot, either one, and that's because the all-metal mode is up and working but the split-off on the disc is not. It's all a process of elimination, just like diagnosing an old car or doing simple subtraction.
If none of all the above helps, then it is more-likely-than-not that you have a partially disconnected path to and from the discrimination pot, or high resistence somewhere near there. If so, then open the detector, get it up and running and laying on the bench in all-metal, and (g-e-n-t-l-y) tap on each and every electronic component and part with a plastic (NEVER METAL) piece of something about the shape of a handle on a Lady's hair comb, the size of around 6"X 1/4". This will allow you to locate any other changes in sound which can often help you locate where the partial or half connection is that you want to repair. Just tap here and there, especially where there are little connections or where there are little transformers with coil windings in them, they being about the size of 5/8" X 5/8". If you get any odd changes, you may well be on your way to fixing it, or showing a TV repairman where you discovered a possible fault.
If your hand being near the circuitboard but not touching it causes the threshold to drift, you may well have a bad or faulty condensor or capacitor somewhere in the vicinity.
The disc pot (may) be the likely culprit though.
Hope this helps.
