How many cells in the rechargeable pack?

[Aarong81]

I have the Pro and don't mind looking inside. In the meantime, it will take me awhile to get the pictures, Here is someone elses input on another fourum. He mentions the delta amount that the regulator requires above the operating voltage of 8 volts. Whites, however, claims the MXT works down to 8 volts. This person says it works on 8 volts PLUS the delta amount and that makes more sense just as you suggested that regulators would require. This also explains why so many swear that 9 volts is irratic and some claim to notice it at 10 volts. I cannot imagine using the MXT at 8 volts! I cannot link outside this forum....

With a detector that uses an internal voltage regulator, higher battery voltages do not affect performance, as long as the battery voltage level is higher than the regulator's output voltage setting, plus a delta amount to keep the regulator working.

For example, the MXT internals run off an internal regulator set for 8V. In order for the regulator to work properly, it needs a battery voltage of about 8.5 to 9V.

Having said that, you need to differentiate between the battery pack's "open circuit voltage" and the pack's output voltage under load (ie. the voltage when actually powering the detector).

So, going back to the MXT example, I find, on my MXT, little if any performance difference between a fresh pack and one that has drained down to around 9V.

Posted by a user named Rudy.

EDIT: I see Larrys post before this one showing Whites statement now. I think it is pointless to spectulate about the regulators existence. I do think it needs a bit extra input to provide 8 volts to the board and not only that but a battery has a different voltage at rest than it does under a load. That said, I think anytime the MXT is turned on it is under said load so the battery meter should account for that.

 

[Aarong81]

There is a Linear Technology LT1446 (source info)digital-to-analog converters (DAC). These chips seem to be used in voltage regulating but what I can tell this has an output around 5 volts but I don't really know anything about it aside from a bit of googling. Here is a spec sheet for it. http://cds.linear.com/docs/en/datasheet/1446fa.pdf
Other than that, I don't see any obvious voltage regulators. There are several possibilities but they are hard to read. Let me know if there are any in particular you want a closer look at and I'll try to get a better picture. One picture shows a chip labeled FPX8.00 which is intriguing since the MXT is supposed to have a voltage regulator with an output of 8 volts. A little google search suggests this refers to 8.00KHZ though but very inconclusive.
http://s9.postimg.org/qecuthagv/image.jpg http://s9.postimg.org/xtbddcb3z/IMG_0005.jpg http://s9.postimg.org/mfou250lb/IMG_0006.jpg
http://s9.postimg.org/sfcl5sldr/IMG_0007.jpg http://s9.postimg.org/kwoujf4n3/IMG_0012.jpg http://s9.postimg.org/bdjr3p6in/IMG_0014.jpg
http://s9.postimg.org/uyciv2s73/IMG_0001.jpg http://s9.postimg.org/iift1byv3/IMG_0004.jpg

 

[PSS1963]

Wow, the water is getting really muddy now! There's a lot to digest here and some of it seems contradictory. There shouldn't be such a large difference from one machine to the next as to when they start acting funny - unless this is one of those subjective things that depends more on the skill/perception of the user. The quote Larry posted from the manual is claiming performance down to eight volts (at least), the poster that Aarong81 quoted is saying you're good down to 8.5 or 9 volts and we have others saying it's going to get flaky if you go below 10v. This seems like a moving target.

Taking each part in turn:

With a detector that uses an internal voltage regulator, higher battery voltages do not affect performance, as long as the battery voltage level is higher than the regulator's output voltage setting, plus a delta amount to keep the regulator working.

Ok, this agrees with the part of my previous post about regulator overhead.

For example, the MXT internals run off an internal regulator set for 8V. In order for the regulator to work properly, it needs a battery voltage of about 8.5 to 9V.

Initially, I was skeptical that a regulator would have such a low overhead requirement (0.5-1v) except maybe when using a very rudimentary transistor/zener/resistor combo, 1 volt of overhead may be possible, but I checked and there is such an animal as an LDO (low drop-out) linear regulator (maybe they use MOS transistors - I dunno). Seems possible at any rate.

Having said that, you need to differentiate between the battery pack's "open circuit voltage" and the pack's output voltage under load (ie. the voltage when actually powering the detector).

This makes sense. A hand-held voltage meter (DMM) will have a really high input impedance and can measure the voltage on a battery without drawing any noticeable current from it, so you see a higher voltage than if you measure it while its under load.

So, going back to the MXT example, I find, on my MXT, little if any performance difference between a fresh pack and one that has drained down to around 9V.

Not sure what to make of this. Other people are saying 10v. Is this a difference of perception as I suggested earlier or could there be a difference among MXTs of different vintage?

The quote from the manual doesn't help to clear anything up:

If using rechargeable batteries 8 volts
is surely the end of their performance. If you are
using quality alkaline batteries you do have a
reserve after 8 volts. The MXT utilizes a low
voltage regulator.

This statement implies to me that the regulator has an output that is below 8v. How much below? I couldn't say. It also hints at something I was saying earlier about the difference between alkaline and NiMH discharge characteristics. When NiMH "hits the wall" it goes down hard and quite suddenly compared with alkaline which will continue to decline at more or less the same rate.

Larry's comment:

They do not state if a high voltage regulator is utilized but it is good electrical engineering to include some kind of overvoltage protection such as a fuse, or on low powered circuits, a high voltage regulator . . .

Where the manual says "The MXT utilizes a low voltage regulator." it may just be a case of bad wording. I think they were trying to say that the output of the regulator is low (compared with the battery). The purpose of a regulator (in most circuits nowadays) is to make sure the circuit works consistently, rather than to protect it. A fuse does protect a circuit, but from an over-current condition, like a short circuit.

Not sure where to go from here. This is usually the point I bring the thing into the lab and do some tests.

- pete

 

[PSS1963]

Wow, nice pics! There's a lot going on there. They seem to be keeping up with the times - using lots of surface mount parts saves weight. The first place I would look is where the wires from the battery connect to the circuit board (can we see a close-up of that?)- that is the most sensible place to put a regulator. It may not look like anything that recognizable (like a 78xx for instance) and because the MXT uses so little power it wouldn't have to dissipate much heat so a heat sink would be unlikely.

- pete

 

[Aarong81]

The MXT is chattery to begin with so it is likely a matter of perception whether it BEGINS to be more chattery below 9 volts or 10 volts. Exactly when that chatter increases is very hard to discern. It acts like it is less stable, as if the gain is set too high or like you are near an electrical source causing noise.

 

[Aarong81]

Wow, nice pics! There's a lot going on there. They seem to be keeping up with the times - using lots of surface mount parts saves weight. The first place I would look is where the wires from the battery connect to the circuit board (can we see a close-up of that?)- that is the most sensible place to put a regulator. It may not look like anything that recognizable (like a 78xx for instance) and because the MXT uses so little power it wouldn't have to dissipate much heat so a heat sink would be unlikely.

- pete

The battery is directly below the circuit board and the power feeds up through the only two threaded bolts near the front of the board. Each are labeled either + or -.
The LCD displays the battery status when you first power it on so the voltage is not immediately regulated. Only after it detects the battery voltage does it go through regulator.

 

[PSS1963]

Ok, I can see where the power comes in at those two bolts, but I haven't found a likely candidate yet for a regulation circuit. The LT1446 is very near the connectors for the speaker and headphone jack, so it seems likely it converts digital data into an analog audio output. Why it needs to be dual escapes me - maybe they are only using one side - don't know. The unit doesn't process audio output in stereo - right? Interestingly, this chip needs its power supply regulated to between 4.5 and 5.5v (according to the data sheet). Hmmmmm.

The FPX8.00 is a timing crystal (designated on the board as X1). Digital processors need to be clocked in order to function so a crystal tuned to a particular frequency is used to create a clock signal to step these from state to state. The large socketed chip right next to it seems a pretty good candidate for an FPGA (field programmable gate array) though I can't read the part number to look it up. These are a good way for a designer to put a lot of digital circuitry in one package and keep it hidden from the competition. Whites would buy them "blank" and program them with all the necessary logic functions in their own factory. All the outside world sees is the generic part number of the blank array - essentially a "black box".

Aside from this, I see a number of 8-pin chips that may be op-amps.

Without being able to read most of the part numbers and follow the traces, its near impossible to find what I'm looking for.

- pete

 

[vito]

Those are AAA cells, Vito? Do you happen to know the mAH rating?

-p

Those are 800 mAh ones, Pete. The new pack I made now has 900 mAh ones. The idea behind this all came to mind when a minelab rechargeable pack from the Sovereign GT was opened. In the ML standard battery pack inserting NiMh or NiCd will have the same effect as inserting dead batteries depending on their voltage regulation. So in the rechageable pack they had 10 NiMh - 1000 mAh - cells installed. But what lacks is their charger. Within 4 month I had 2 dead cells. And with a dead cell, they're solered in a row, the pack is dead. It will work for 5 mins if fully charged, but then nothing. Made a few bad hunts to me. Then I decided to create this system where every cell goes to charging for its own.
And in the Whites the 10 AAA have enough space to get working, and there are manufacturers which will have 1100 mAh NiMh.
Since I started charging each cell for it's own I had no dead cells anymore.

Just to mention, the MXT has a complex 5 Volts regulation including thermal compensation for the OP-Amps. So dropping below 8.1 Volts may give the "Low-Batt", but you're still able to do some detecting. Not a complete hunt, but will be safe for the started dig.

 

[PSS1963]

Vito,

5v makes more sense than 8v, taking into account that 12bit DAC that wants 4.5 to 5.5v. What is your take on the issue of performance degradation? Some folks are saying it starts to misbehave below 10v . . . others, including Whites give a lower number. Also, you mention it has a "complex" regulation - I'm curious how you came by this - have you seen a schematic and its your own assessment or did you read it somewhere?

- pete

 

[leslie]

These are the way to go [attachment 305362 _12.JPG]

 

[PSS1963]

The MXT is chattery to begin with so it is likely a matter of perception whether it BEGINS to be more chattery below 9 volts or 10 volts. Exactly when that chatter increases is very hard to discern. It acts like it is less stable, as if the gain is set too high or like you are near an electrical source causing noise.

You may have hit on something very truthful here, Aaron. A change in supply voltage could definitely affect a gain setting depending on how it is derived in the circuit or possibly some other level that is referenced against the gain setting. Also, its entirely possible that some parts of the circuit have regulated supply while others do not.

- pete