Tom_in_CA said:Ozark, I've never heard this concept of the 77b wearing out, and loosing depth "for each time you turned it on". If this is true, then I suppose some old 77b's should show up, now and then, on Ebay, that were ones stuck-in-the-closet way back 35 years ago. I mean, that's no different from now, where someone puts a machine on ebay, that basically was used 2x, and stuck in the closet for 10 yrs. Things like that turn up at estate sales, etc.... I have seen 77b's get sold on ebay before that had absolutely perfect paint jobs, and not a scratch on them (indicating they saw very little use back then).
As far as the 10 or 12" depth you say you got back then with a 77b, I have a few observations: yes, it probably is possible, in an air test, and therefore by extension, some ground tests (perfectly white dry sand, etc...), that a person might replicate this depth. But you would admit you're talking VERY faint whispers. Unlike today's motion machines where there seems to be a "trigger point" where, the target is either "there" or it's "not", the old all-metal TRs seemed to have a never-ending decreasing fading signal, that wasn't just magically "on" or "off" like today's machines. So it was sort of like a person whispering from the end of a big long hallway, eh? The whisper never goes away, but just gets softer and softer and softer, etc....
The other thing I would add is that depths like 10 or 12" on the 77b was for clean signals. When the 77b has a nail above the coin, the nail will produce a null, as you recall. So subsequently, any coin underneath the nail has to over-come the nail above it. That was why the trick only worked to about 3 or so nails. By the time you got to the 4th or 5th nail, they would eventually overpower the coin, and the see-through trick stopped. This was dependant on the relationship between the items too. So for example, a nail at the same depth as the coin, would be different than if the nail was 3 or 4" above the coin, etc....
I just know that I dug coins that were sitting under square nails back then. I never measured the depth of the nails versus the coin, so that I don't remember.
As for the deterioration of the power in the 77B, call Keith Wills in Texas under broken detector. He will verify. Keith will change out the capacitors in a 77B if you ask and pay him. It will work much better.