FISHER T 20

[danGeorge]

I HAVE AN OLD FISHER T20 THAT TAKES A 9V BATTERY THAT IS NO LONGER AVAILABLE. IS IT POSSIBLE TO CHANGE THE TERMINALS AND USE DIFFERENT 9 VOLT BATTERY.

 

[Digger]

I'm not an electronic engineer. And I'm not familiar with the specific battery you need for the T20. But I know that E=I x R. That means voltage equals the current flow times the resistance. The voltage is the electrical pressure required to push amps through that circuit. The resistance is the circuitry of the detector and is relatively constant. If the voltage (9 volts of electrical pressure) is applied, regardless of the shape of the battery, it will transport the amount of amps that the resistance demands. In other words, it takes a certain amount of electrical pressure to get those amps running through the circuit. (completing the circuit between the negative terminal and positive terminal) The circuit doesn't care what shape the battery is, as long as it provides 9 volts of pressure to allow the correct current flow (amps). A D-cell "battery" is 1.5 volts. So is the C-cell, the AA cell and the AAA cell. Any of them will operate a flashlight. The only difference is the amount of energy that they are capable of storing. (how long each one will keep the bulb lit) A nine volt battery is basically six 1.5 volt cells connected together in series. So my thoughts are that you can use whatever 9 volt battery you want. Just make sure when you connect it, that the polarity is correct. JMHO HH Randy

 

[silvernail]

How about a field test report on that baby if you can get it cranked up? As I recall that era of machinery, it might work OK for playground work and should pinpoint and separate targets rather well. If there is any ID capacity other than guestimating by the size of the target, I wouldn't know what it is. When I was playing around with a T-10X a few decades back, I didn't know enough to watch for 'broken' sounds and that sort of thing, and maybe they don't happen with the old Fishers anyway. That is a real fossil from the early-mid 1960's. HH

 

[danGeorge]

YOU KNOW I GOT IT WORKING. AND I HAVE USED IT. BRINGS BACK OLD TIMES. THE FIRST TIME I USED ONE. I WAS IN THE NATIONAL TREASURE HUNTERS LEAGUE SHOWROOM RAY SMITH ASK ME IF I HAD EVER SEEN A METAL DETECTOR THAT WOULD PICK UP PAPER MONEY. IT ACTUALLY WOULD PICK UP THE MINERAL IN THE INK. I WAS SOLD, I HAD SEEN ONE ON THE COVER OF TRUE TREASURE MAGAZINE. THE PROBLEM AS I FOUND OUT ,NO GROUND CONTROL MOST PLACES I TRIED TO USE IT. IT WOULD PICK UP THE GROUND. HERE IN TEXAS NO PROBLEM. IT IS A DEEP DETECTOR, BUT YOU DIG EVERYTHING. STILL IT'S FUN

 

[Monte]

I still have a few old-time favorites, but they just can't compare with a few of today's favorites. Great for reliving the "good old days" though.

YOU KNOW I GOT IT WORKING. AND I HAVE USED IT. BRINGS BACK OLD TIMES. THE FIRST TIME I USED ONE.

"Memories." Yep, those were some fun times back then, regardless of which unit I had, all of which are considered "ancient" or "vintage' or "out-dated" by today's standards, to be sure!

I WAS IN THE NATIONAL TREASURE HUNTERS LEAGUE SHOWROOM RAY SMITH ASK ME IF I HAD EVER SEEN A METAL DETECTOR THAT WOULD PICK UP PAPER MONEY. IT ACTUALLY WOULD PICK UP THE MINERAL IN THE INK.

Nope, not really. But many of the old units we used to use did a splendid job of reacting to the conductive salts in your hand when adjusted for a 'proper' metal response..

THE PROBLEM AS I FOUND OUT ,NO GROUND CONTROL MOST PLACES I TRIED TO USE IT. IT WOULD PICK UP THE GROUND. HERE IN TEXAS NO PROBLEM.

If you hold the search coil at about 3" or so from he ground, tune it to a slight audio hum (threshold) for metals, and then lower the coil to the ground over a metal target-free spot, the T-20 ought to null, even to the point of going silent. If you're getting an audio increase with a conventional TR model turned for metal acceptance, then you're over a metal object or some low-conductive ground, such as a wet saltwater setting.

If you have it tuned for a proper slight audio hum and keep the search coil flat on the ground (or a uniform distance from it), then lift the coil or sweep over a depression or hole, you will likely get an audio 'beep.' That's not a detection response but a response to a 'void" or coil position change such that the ground mineral influence is moved farther from the electromagnetic field.

IT IS A DEEP DETECTOR, BUT YOU DIG EVERYTHING. STILL IT'S FUN

Fun? I'm sure. Deep? Well, I never got anything with those 11" coils that I would want to boast about, but they were competitive for the times. In good ground they can do okay.

Monte

 

[wolfeeee]

T 20 was my first after building a BFO according to a Carl Von Muller schematic.
Found out 20 years later that the battery terminals were backwards in the schematic.
It took me a while to discover that the T20 11 inch coil didn't work over the whole coil.
There is a crescent shape about 4 inches back from the front, as I recall, about 1 inch deep and 3 inches wide.
Found my first Indian cent in Fairmont Park, Ft Wayne, IN some time in '64.
Think of all the items I missed making 6 inch sweeps.
Learned that year NOT to dig holes in sod when there's been no rain for weeks.
The battery for the T20 was about 1 in by 3 in and round like a D battery at the time it was for a dentist's drill.
I would use a double AA 6 pack.
Have all my old detectors and could use a handle for the T20. So called friend lost it.

Now I sort pennys for copper and now and then I find an Indian.

 

[Calnevaroy]

T 20 was my first after building a BFO according to a Carl Von Muller schematic.
Found out 20 years later that the battery terminals were backwards in the schematic.
It took me a while to discover that the T20 11 inch coil didn't work over the whole coil.
There is a crescent shape about 4 inches back from the front, as I recall, about 1 inch deep and 3 inches wide.
Found my first Indian cent in Fairmont Park, Ft Wayne, IN some time in '64.
Think of all the items I missed making 6 inch sweeps.
Learned that year NOT to dig holes in sod when there's been no rain for weeks.
The battery for the T20 was about 1 in by 3 in and round like a D battery at the time it was for a dentist's drill.
I would use a double AA 6 pack.
Have all my old detectors and could use a handle for the T20. So called friend lost it.

Now I sort pennys for copper and now and then I find an Indian.

Thanks for the bit about the reversed battery terminals in the Little Monster schematic. Never could get it to work:angry:

 

[fallscitybob]

I just came upon my old T 20 in the basement resting behind the furnace. I dusted it off, took off the side cover, and checked the battery--or what was left of it! It must have been 40 years, or so, since I last used it. The battery was toast, but I could see enough of it to tell it was 12 volts. I've decided to rewire to an external 8 AA cell holder as a 12 volt battery. Just want to try it out. Seems like technology has passed me by!

 

[Old California]

Good job getting the old T-20 going, can't remember the voltage but a regular 9-volt battery will work.

The older T-10 tube type were extreamly heavy compared with your T-20, congrats with getting it going!

Paul