The Bootleg MD-6250

[GDTUKSCOT]

So being technical doesn't provide all the answers but I bought the above machine on an auction site, spares or repair. It seemed to be very stable when tested in the house but next day outside the display and audio went haywire. I found a partial schematic and decided that the output of the 4016 chip being fed from the coil and oscillator, was lacking signal level on one side. I changed this and everything became stable as far as operational control is concerned, but there is now a constant false random signals.

What could be causing this?
a) Is the oscillator frequency slightly wrong?
b) What do the two variable resistors do? They seem to be both set exactly midway which is a bit suspicious.

Any pointers would be gratefully received.

[waltr]

So being technical doesn't provide all the answers but I bought the above machine on an auction site, spares or repair. It seemed to be very stable when tested in the house but next day outside the display and audio went haywire. I found a partial schematic and decided that the output of the 4016 chip being fed from the coil and oscillator, was lacking signal level on one side. I changed this and everything became stable as far as operational control is concerned, but there is now a constant false random signals.

What could be causing this?
a) Is the oscillator frequency slightly wrong?
b) What do the two variable resistors do? They seem to be both set exactly midway which is a bit suspicious.

Any pointers would be gratefully received.

Ramdom operation could indicate a bad connection somewhere. Could be bad/cold solder joints or connectors wire, etc.
good luck.

[WM6]

What could be causing this?

Did you perform factory reset after repair work?

[GDTUKSCOT]

I'll get the microscope on it. Thanks, that sounds hopeful.

[GDTUKSCOT]

Yes, factory reset done. Thing is it was stable for a while but started to emit false random signals. I'm going to re-solder the board. The self measuring, clever scope that I used, said the oscillator frequency was 6.67kHz from memory so I wonder if some of the passive parts are a little out as spec as I think this machine should be 6.5kHz. Would that make a difference?
All suggestions welcome. I'm electronics trained but metal detector internals are a new thing to me.

[WM6]

Yes, factory reset done. Thing is it was stable for a while but started to emit false random signals.

You mean with coil lift in air (and moving)?

Detector should be stable with coil near to ground.
In air coil can sometimes act as some sort of antenna for environment EMI.

Probably you can take some video on your MD behavior.

[Koala]

Yes, factory reset done. Thing is it was stable for a while but started to emit false random signals. I'm going to re-solder the board. The self measuring, clever scope that I used, said the oscillator frequency was 6.67kHz from memory so I wonder if some of the passive parts are a little out as spec as I think this machine should be 6.5kHz. Would that make a difference?
All suggestions welcome. I'm electronics trained but metal detector internals are a new thing to me.

within a few kHz is close enough. Yours is well within spec.

[Koala]

without a spare coil its going to be difficult to know whether its a problem with the coil or control box.


if its got a coil cover fitted its worth removing cleaning.


if you point the coil at the sky and jiggle it up and down does it beep ?


batteries and there contacts are a common source of problems as is the coil conector

[GDTUKSCOT]

I'll get back with more info but I soldered flexi leads from the puny pins on the battery side to the corner of the gold pads where they connect in the PCB, just so you could easily run the unit on the bench, its also more robust in use.

[dbanner]

within a few kHz is close enough. Yours is well within spec.

I know you meant to say within a few tens of Hertz(hundredths of khz). Surely not within a few kilohertz.
Constant false random signal? Inadvertently messed up threshold trimmer. Or ground balance trimmer, or bad connection from coil shielding to ground. Or bad coil, or dry soldering.
Where can I view partial schematic.

[GDTUKSCOT]

https://metaldetectingforum.com/showthread.php?t=42195

or

https://metaldetectingforum.com/atta...1&d=1265007112

Now I might be prematurely overwhelmed here but indoors, anyway, we have complete stability. I was never sure what the two trimmers were doing and using a scope one seemed to control the oscillator level, circa 28v p-p and the the other one, not sure,( but I confess to tweaking them a bit). Fortunately I made notes of their original settings so between strengthening the coil plug connections and putting the trimmers back HOUSTON, WE HAVE STABILITY!!! Not a peep out of it until you present the search head with an object. Thank you everyone for diverting me back to the simple obvious things. We may be back on track. Have a good weekend, my daughter and I are hitting the beach tomorrow. xxx