Modifying old White's units?

[coinnut]

Hi, I remember in the late 80's a White's dealer having a 5000 D Series II (like I did), but he could detect a quarter at 12"+ in discriminator mode while I was barely able to get 5". This kind of depth was unheard of in the 80"s. He would only tell us that it was modified. Others we talked with thought it had something to do with a current limiting resistor in the coil or unit itself. Any ideas? - George

 

[tatumj]

George,
I played with an old Whites back then. I got a copy of the Whites patent for their motion detectors and was designing circuits based on the information in the patent to build one. Then I happened to find one in a salvage store. It was called a 6DB. The shaft was missing but I got the box and the coil for a few dollars and it worked. Amazingly, when I looked inside, I found Whites had used mostly the same components (IC chips and transistors) I was using in my attempt to build one. I had several of the circuits built and working but stopped when I found the salvage unit.

Regarding your question, yes it was possible to up the sensitivity on those units. They built them so they could be used by anyone who picked them up so they had to be kept within certain limits of stability. I found that increasing the sensitivity also meant that the machine would be noisy. To use it you had to learn to recognize the false signals and interference from random sources (power lines etc) and still be able to pick out the real ones.

As I recall, the unit had a low noise operational amplifier for the receive input. Changing a resistor could increase the gain of that chip greatly. I found that you could up it a little and be OK but if you increased it as much as you indicated in your question it probably would be great for bench testing but not very satisfying to use in the field. It likely would become a little too flakey to be fun.I am pretty sure that I put mine back as original. I used it for quite a while and found a lot of coins with it. I still have it in a box somewhere. This talking about it makes me want to fish it out and try it again.

Joe T.