Calibrating a MD

[scrapman]

I would like to know how I could calibrate my Garrett GTAx550. I see two adjustment access's in the back of the housing. Can this be done without using test equipment?

[Geo]

Hi
A simple way is to mark the potentiometers and after it you can play. If you don't like the new adjustments, turn them again at the mark points.
My opinion is "don't turn them"

[scrapman]

Ok thanks for the info. I monkey'd around alittle with it and I was able to get nickels reading as nickels again.All the other coins read good. Nothing changed on depth,I air tested a few coins and they I get a belltone at 8".
Is there any Technical Manuals for MD's available somewhere?
I'd like to sudy up on what makes them tick. I have little electrical knowledge,I'm always fixing appliances and things around the house.

[Max]

Ok thanks for the info. I monkey'd around alittle with it and I was able to get nickels reading as nickels again.All the other coins read good. Nothing changed on depth,I air tested a few coins and they I get a belltone at 8".
Is there any Technical Manuals for MD's available somewhere?
I'd like to sudy up on what makes them tick. I have little electrical knowledge,I'm always fixing appliances and things around the house.

Hi,
uhm... not good to play with disc or geb presets unless you know what are doing. Reason is simple: factory have all informations to tune the MD at best but not you or hobby/amateur electronic people.

Now, I'd like to suggest you, if wanna try do that anyway, as Geo stated to mark the positions of trimpot cursors and the like, but better (and essential in the case of multiturn trimpots) is to actually remove them from pcb and measure resistances between cursor and other leads.

That's the definitive trick to avoid a big mess before moving the pots, cause even if you mark position often in 3/4 turn ones a VERY narrow spot is the right spot to obtain required params. Tricky thing, and also could be disappointing turning them to the "same" marked position but getting different behaviour from device... cause of small variations you could not see at cursor position that will give different actual resistance at leads.

Obviously, you need don't make a mess when desolder the trimpots from pcb, e.g. not too much heat for too time, not broke tracks etc, not always easy stuff expecially in double or multiple layer pcbs.

Kind regards,
Max

Kind regards,
Max

[scrapman]

OK thanks, I turned one trimpot slightly and I seen that nickles were reading correctly again.All other coins and metal I tested are reading good. Depths are the same. If I have any other problems I'll send it in to Garret for a proper calibration. I'm planning on buying a new MD soon,so I'm not to worried about this one,it's 10 years old.If I wreck it then I'll have more reason to get a new one. I'd still like to get a book on MD's and see what makes them work. Thanks again.

[simonbaker]

Hi,
uhm... not good to play with disc or geb presets unless you know what are doing. Reason is simple: factory have all informations to tune the MD at best but not you or hobby/amateur electronic people.

Now, I'd like to suggest you, if wanna try do that anyway, as Geo stated to mark the positions of trimpot cursors and the like, but better (and essential in the case of multiturn trimpots) is to actually remove them from pcb and measure resistances between cursor and other leads.

That's the definitive trick to avoid a big mess before moving the pots, cause even if you mark position often in 3/4 turn ones a VERY narrow spot is the right spot to obtain required params. Tricky thing, and also could be disappointing turning them to the "same" marked position but getting different behaviour from device... cause of small variations you could not see at cursor position that will give different actual resistance at leads.

Obviously, you need don't make a mess when desolder the trimpots from pcb, e.g. not too much heat for too time, not broke tracks etc, not always easy stuff expecially in double or multiple layer pcbs.

Kind regards,
Max

Kind regards,
Max

Hi Max:

Why remove pots from PCB? Wouldn't measurements in-circuit be enough to restore original setting after fooling around?

Regards,

-SB

[Max]

Hi Max:

Why remove pots from PCB? Wouldn't measurements in-circuit be enough to restore original setting after fooling around?

Regards,

-SB

Hi,
yes, in some cases could be enough but in others not. I mean, all depends on how the measure is taken and accurancy you need.

I think you mean just measuring the resistances between leads of trimpot when in pcb and circuit is off. Ok, suppose there is something like an op amp connected to the pot leads (as actually is in many disc/geb sections of common vlf mds) now... you'll read the effective resistance reading due to the pot and the op amp influence and eventually other components around.

Fine: but what's that resistance you read ? isn't something that always change with e.g. temperature ? in bipolar op amps isn't the effect of e.g. resistances and output junction of last transistor pole etc ?

Suppose the apparent resistance there is influenced by e.g. temperature: you'll need to measure resistance and also temperature then replicate , at same temperature, the resistance value when you retune device.

Now, if a semiconductor junction is in that scenario you'll also need same current flowing during the measures, so use e.g. the same meter etc

What I mean is that if you add components you could add side effects and influence on measures of external factors like temperature variations or meter current...

So, the easy case is that you don't need that precision and just measure resistance values with trimpot connected to other things; the worse case is that you need all precision you can get and so exact measure of just the trimpot resistances at leads.
That will at least free you of measuring always using same current , so same meter...

Now, have to say that also trimpots are influenced by temperature, expecially 3/4 turns ones with oxides layers BUT that in every complicate md that requires absolute precision on that values you'll find multiturn trimpots that are wire made internally and so have less troubles with stability, though even wire thing change with temperature.

So, all depends on precision required I think, I would not try to touch e.g. relic hawk pots without first desolder them from pcb and take accurate measures just on pots. Other way I will be safe enough just measuring values with trimpots on pcb on a very simple md like e.g. a tesoro compadre.

Kind regards,
Max