So last night had one of these vintage tanks sitting on my table. Bought from someone dirt cheap, looked almost new. So tinkering with it, it would not turn on. Found that the battery tray had a problem and fixed it. Fired right up, super stable threshold, Wow I thought just like back in the 1980's when I had a similar model. Grabbed a quarter and started bench testing, not that it means much, did some air testing. Incredible, it was responding to the quarter about 18" from the coil. Told my wife about it and said, just have to go out and try it, see what it can find. Then I woke up from last nights dream.