Micronta 4003

[Pleb]

Hi Peeps, I've just joined the forum. I've seen a Micronta 4003 for sale in a second hand shop and he wants £20 for it, so last night I was on the net to see if his price was justified, when I came upon the Geotech forum. Not a popular machine it seems. Anyways I read a few posts and got the modification details down concerning the changing/adding resistors. If I can knock him down to a tenner will it be worth it considering it also needs a decent search coil? Can anyone suggest a decent coil to purchase?
My kids never visit me anymore so I've lots of time on my hands. Just trying to find summat to occupy my time.
Regards,
Pleb.

[Qiaozhi]

Hi Peeps, I've just joined the forum. I've seen a Micronta 4003 for sale in a second hand shop and he wants £20 for it, so last night I was on the net to see if his price was justified, when I came upon the Geotech forum. Not a popular machine it seems. Anyways I read a few posts and got the modification details down concerning the changing/adding resistors. If I can knock him down to a tenner will it be worth it considering it also needs a decent search coil? Can anyone suggest a decent coil to purchase?
My kids never visit me anymore so I've lots of time on my hands. Just trying to find summat to occupy my time.
Regards,
Pleb.

The Micronta 4003 is a really old low-end machine. If it needs a new coil, then it's probably not even worth a tenner. It depends what you plan to do with it. I have a Micronta 4001 in my collection (I think I paid £6.50 for it on eBay) and to be honest it's rubbish. The detection range, even in an air test, is extremely poor. It's really only a toy.

[technos]

I have a Micronta 4001 in my collection (I think I paid £6.50 for it on eBay) and to be honest it's rubbish. The detection range, even in an air test, is extremely poor. It's really only a toy.

.

The question I ponder is could you take the same exact parts list and make a better detector with a different circuit?

Beyond that, though I have not used this detector, I do not have much faith in the modifications I've seen

[pebe]

.

The question I ponder is could you take the same exact parts list and make a better detector with a different circuit?

Beyond that, though I have not used this detector, I do not have much faith in the modifications I've seen

I have a Micronta 4003, that I bought as a test bed, and for some of the components to be used as the basis for a new detector.

It has very poor sensitivity and at this stage I don’t know whether it’s in the design (most likely), or if a fault has developed. I shall do some checks, as soon as I have got my ‘scope down from the loft (where it has been, not yet unpacked, since I moved house),

The component layout is much the same as most other detectors of that type – it’s probably the way the circuit has been designed. More gain could have been obtained from the two op-amps. Then there is the question of why that particular input circuit was chosen. A grounded base amplifier has an inherently low input impedance which would load the receive coil and it begs the question, so why use that method when an input fed into an op-amp would give virtually no loading? I think with some small modifications it could be quite a useful detector.

I don’t yet know how the coils are oriented or if they have been properly nulled – all my attempts to get into the housing with a knife and without damaging it, have so far been unsuccessful.

I will let you know what I find.

[Qiaozhi]

I have a Micronta 4003, that I bought as a test bed, and for some of the components to be used as the basis for a new detector.

It has very poor sensitivity and at this stage I don’t know whether it’s in the design (most likely), or if a fault has developed. I shall do some checks, as soon as I have got my ‘scope down from the loft (where it has been, not yet unpacked, since I moved house),

The component layout is much the same as most other detectors of that type – it’s probably the way the circuit has been designed. More gain could have been obtained from the two op-amps. Then there is the question of why that particular input circuit was chosen. A grounded base amplifier has an inherently low input impedance which would load the receive coil and it begs the question, so why use that method when an input fed into an op-amp would give virtually no loading? I think with some small modifications it could be quite a useful detector.

I don’t yet know how the coils are oriented or if they have been properly nulled – all my attempts to get into the housing with a knife and without damaging it, have so far been unsuccessful.

I will let you know what I find.

Here's 2 photos I have of the inside of the search head that were posted by another Geotech member. As you can see, it's an omega configuration, and the coils are unshielded. You need to ignore the ferrite core in the top right. Apparently someone added that as a modification, but goodness knows why.

[pebe]

Here's 2 photos I have of the inside of the search head that were posted by another Geotech member. As you can see, it's an omega configuration, and the coils are unshielded. You need to ignore the ferrite core in the top right. Apparently someone added that as a modification, but goodness knows why.

Do you know the knack of removing the base? The top housing has a lip that ovelaps the base and I can't seem to get at the joint.

[Qiaozhi]

Do you know the knack of removing the base? The top housing has a lip that ovelaps the base and I can't seem to get at the joint.

I have a Micronta 4001, and I simply levered it off with a pocket knife. It appears to be glued all the way round. Luckily the glue was not very strong.

[pebe]

I have a Micronta 4001, and I simply levered it off with a pocket knife. It appears to be glued all the way round. Luckily the glue was not very strong.

I've tried to lever it off with a screwdriver - but without success. I've looked at the joint with a powerful magnifying glass and I cannot see any seam. It looks like the plastic has been thermally welded.

I'll try different solvents in the groove in case it's glued, but if I have no luck I'll have to give up on it.

[Qiaozhi]

I've tried to lever it off with a screwdriver - but without success. I've looked at the joint with a powerful magnifying glass and I cannot see any seam. It looks like the plastic has been thermally welded.

I'll try different solvents in the groove in case it's glued, but if I have no luck I'll have to give up on it.

Perhaps the 4003 and 4001 are using different plastic shells. You could try hacksawing around the seam.

[technos]

The component layout is much the same as most other detectors of that type – it’s probably the way the circuit has been designed. More gain could have been obtained from the two op-amps. Then there is the question of why that particular input circuit was chosen. A grounded base amplifier has an inherently low input impedance which would load the receive coil and it begs the question, so why use that method when an input fed into an op-amp would give virtually no loading?

From the circuit that I am seeing for the Micronta 4003, pickup coil L1 technically does not feed into a grounded base, but looks more like common emitter type (and might be what is wrong with the circuit!).

There are a couple of reasons why a designer might use a gounded base transistor at the input. It will have a very low impedance that it presents to the coil (also seen this way in radio circuits), which will match that of the coil more readily. This prevents an impedance mismatch which prevents the signal from beong reflected back into the coil instead of being sent into the circuit. Another is that it will provide very good isolation of the coil from the opamp. The opamp will not see any of the stray capacitance and hence any of the stray signals and voltages from the coil.

The reason that you see coils fed directly into a differential like opamp circuit is that for all intense and purposes (as you noted) the impedance of the opamp is so high that it does not load the coil. Plus the newer high performance opamps will reject a lot of the common mode noise from the coil. But, when the day is done, a carefully designed grounded base transistor preamp to the opamp will be a higher performing circuit because it will add a fair amount of voltage gain to the circuit and therefore it will also be more sensitive. But depending on the circuit following it, the transistor may not be needed. I.e, the circuit following the preamp needs to take advantage of the additional gain.

I would like to see what the successor circuit is to this one that they are using in the Famous Trails detector today. An interesting challenge would be to take the same parts and see if a higher performing detector could be made.

As to the coil, there was another thread that noted it was not nulled very well. If it was me, I'd hesitate before adding shielding to this. Depending on how you do it it could decrease sensitivity by a significant amount, and may not add a much benefit to the circuit.

Just my .02

[pebe]

From the circuit that I am seeing for the Micronta 4003, pickup coil L1 technically does not feed into a grounded base, but looks more like common emitter type (and might be what is wrong with the circuit!)................

The circuit I am using is the one provided in the thread ‘MICRONTA MODS’ by amtech2005 on 05-28-08. The components numbers on the PC board match that circuit so I assume it is the correct one.

It shows the Rx coil being input to the emitter of Q1 via a 47nF cap, C2. The base is grounded with a 47nF cap, C1, so the circuit is in a grounded base (common base) configuration. I measured the Tx and it runs at 14kHz. If C2 tuned the Rx coil as a series resonance tuned circuit, then it would be a low impedance source and that would be acceptable. But the Rx coil inductance is 11.5mH and that would need C2 to be 11.24nF for resonance. The coil’s resistance is 63.4ohms compared with the TX coil’s 20.9ohms, so there must be a lot of turns on it. So the source must be inductive and I can’t see that it’s a very good match.

But until I can get into the coil assembly I can’t be sure.

[technos]

The circuit I am using is the one provided in the thread ‘MICRONTA MODS’ by amtech2005 on 05-28-08. The components numbers on the PC board match that circuit so I assume it is the correct one.

It shows the Rx coil being input to the emitter of Q1 via a 47nF cap, C2. The base is grounded with a 47nF cap, C1, so the circuit is in a grounded base (common base) configuration. I measured the Tx and it runs at 14kHz. If C2 tuned the Rx coil as a series resonance tuned circuit, then it would be a low impedance source and that would be acceptable. But the Rx coil inductance is 11.5mH and that would need C2 to be 11.24nF for resonance. The coil’s resistance is 63.4ohms compared with the TX coil’s 20.9ohms, so there must be a lot of turns on it. So the source must be inductive and I can’t see that it’s a very good match.

But until I can get into the coil assembly I can’t be sure.

Yes, you are correct on this. But I do not think you will find much in the coil case. The circuit is not a tuned circuit. If you look on in the archive there is the "Magnum Metal Locater" article by Andy Flind he briefly mentions this circuit as a common one used in manufactured machines (which ones though?) and that it is "simple and works well." However the 15K resistor goes against my personal philosophy of never sticking a q-killing resistor in series with a coil. But I think this is going to be one of those circuits that I need to breadboard to get a better idea of what its performance is.

It should also be noted that it seems that a CD4069 is pressed into service substituting for LM339s. Perhaps (not so good imho) to save a dollar on circuit cost.

[pebe]

I think the problems with my Micronta may be in the preamp stage. I don't know how well the coils have been nulled, but the Ground setup is very fussy and the stage may be overloading. I intend to rebuild the preamp and feed the received signal into the base, and design a suitable circuit to inject a waveform into the emitter that negates the possible poor coil nulling and also nulls out ground effect. Properly adjusted, the output should then be zero. It will be in circuit permanently so it will still be effective in Disc mode.

I got my scope down from the loft, dusted it down, and found it was intermittent on one channel. It was a good scope in its day, but it's getting pretty old now. So I decided to ditch it and go in for a new one. I expect to get it in about 3 weeks, then I can get down to some serious checks.

[technos]

One of the other threads on this circuit mentioned that the coil was poorly nulled - which makes more sense to me than overloading preamp. The reason being that even if the preamp is overloaded, it's not headed to a mixer pe se, but a phase detector i.e. the phase detector is sensitive to the phase differences of the signals being compared rather than being overloaded by a signal that is being clipped because of too much amplitude.

Perhaps you can use the new scope to troubleshoot the old. When I worked on them for a while on a regular basis 80% of the problems were due to dirty pots and oxidized contacts in the switches. I suspect that electrolytic capacitors drying out are becoming more of a problem now too.

[pebe]

I figured that the received signal is the vector sum of the wanted radiation from the target and the 'leakthrough' signal from poor nulling of the coils. So if the coils have not been properly nulled, the phase change due to the wanted signal would be much smaller than it would have been with no leakage. Further, the amplifier is in class A but I doubt it would be linear with wide swings at its output - one side or the other of the waveform would flatten. As the stage is AC coupled to the phase detector, the detector would sample the middle of the waveform - which would not be at the zero crossover point. But in any case, the performance should benefit from good nulling.

My scope is an old Telequipment D67. In the past I have replaced multipole slide switches because their latches had stuck and cleaned tarnished transistor leads (they plug into sockets). About every 6months I have had to do a complete setup, as per the manual, because the pots had drifted. The sync is awful - it triggers on the mid-amplitude of a waveform - and it was pot luck whether I could get a TV waveform to lock. It's over 30years old now, so it's well past its 'sell-by' date.