I seem to have been having a run of repair-successes lately, so I figured I'd have (another) shot at a problem that has bested me for over 40 years. Seriously.
I may have already mentioned the issue in a previous post, it involved the Magnum Metal Detector's 'pin-pointing' function. In use, when a metal object is detected while sweeping an area with the detector, the 'auto-tune' will tune out the 'find', zero the meter, and mute the audible find-signal, but only when the head remains stationary over the object - exactly as it should, there's nothing wrong with the auto-tuning! However, in order to make pin-pointing more accurate, auto-tuning can be disabled, with a push & hold of the pin-point button.
Thing is, even when I built the detector, the pin-point function never worked 100% - it was closer to 90, then 80%, then getting progressively worse as the decades passed. What would happen was that, after a detection, and with the pinpoint button pressed & held, the meter would slowly start to swing from zero to fully negative, say, over the course of about 10 seconds. The faster it would swing negative, the more useless it became. This negative voltage-swing direction (labelled 'BAD' on the meter) also meant that the 'object-detected' tone, was instantly cut off - whereas a positive-swing (labelled 'GOOD'), retains the, 'object-detected' tone. Without a tone-signal while trying to pin-point a find, the pinpoint function is practically useless.
I started off by consulting with my current A.I, LLM of choice, Gemini. I explained the issue and was advised to brush down the PCB board containing the responsible circuitry with isopropyl alcohol, which I considered a, too 'lo-tech', even insulting, suggestion - implying perhaps that something that trivial, hadn't already occurred to me. But, try it I did, and no, it didn't work - but it appeared to have improved things marginally. I then started wondering if the alcohol, running up the legs of the 2N3819 FET, that enables/disables the auto-tune circuitry, could somehow be responsible for the perceived improvement? Removed from the circuit, it was apparent that time hadn't been kind to the FET in question - its legs once bright & shiny appearance, had become tired & tarnished looking, so much so, that they proved impossible to tin with solder. So, rather than keep messing, I went searching for a replacement.
All I managed to find was about half a dozen, genuine, yet equally tired looking 2N3819's, all likely bought at the same time in the distant past. These proved equally hard to tin with solder, so I was reduced to scraping 40 years worth of grime off of them with a scalpel blade. Back in-circuit, the replacement seemed to be performing a little better, than the first 'little better' of the original. It was then I tried cleaning off the underside of the FET - where the legs enter into the transistor's plastic body - and the result was profound, almost a religious experience!!! Pushing & holding the pinpoint button now, leads to practically no negative voltage drift, no matter how long it's held for (within reason). The way it was always meant to perform, but never did.
While all of this was resolving itself, I happened upon another bizarre problem. At fault was the shielded wire, that connects the upper PCB to the female socket in the case, that in turn accepts the male plug, that connects to the detector's head. One of its wires had gone open-circuit. This 'intermittent break had almost driven me to distraction in the past, exhibiting itself as wild swings of the meter, accompanied by the associated 'beeps' - I had presumed that it was down to circuit instability caused by aging components! The break wasn't near either end of the wire either, where one would expect a wire to break, but right in the very middle! Despite my best efforts, I managed to destroy the cable while trying to locate its break-point, and as I (understandably) no longer had any of the specified cable left, I opted for what was to hand, namely a bit of ribbon-cable. And it seems to work just as well with this! This turned out to be a huge discovery/fix as far as stability in use was concerned.
The only other excitement I had with the detector, concerned a poor wiring decision, again, made in the 80's, that resulted in the death of one of my Liitokala Li-ion 9V batteries, as well as a few drained alkalines, before I figured out what was happening - inadvertent short! On the plus side, though the Liitokala did suffer a fried transistor, I managed to get it up & running again. I'll probably leave that 'repair-success' for another post.
Edit.
Well, that was a short-lived 'victory' (about 2 weeks) - yep, it's back at it again, and I'm clueless as to why! I've been playing with the idea that it may be humidity-related, but that's just me grasping at straws. Grrrr.
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