Photo of GP Extreme Badness?

[bklein]

Hi all,
So I bought a GP Extreme needing repair (no sign of working).
It was about to be repaired by Minelab but the owner had second thoughts and had it returned and I bought it. I opened it up and see that it looks like a resistor (100 ohm) got hot and possibly a Fet/transistor/whatever shorted. Looks like Minelab had gotten started on it by scratching off the white paint. This circuit area is near where the battery wires come in to the pcb. Anyone know for sure what this part is?

Edit: just found this mention on a Google photo search: http://detectormods.maxforum.org/201...ed-gp-extreme/ Looks like this is a Vregulator and perhaps the one next to it is fried too. So what are they? No other discoloration - maybe I'm luckier here?

[Mechanic]

G'day Bklein,

What you have there is one difficult problem. Unlike other detectors, the boards as you can see are painted with white paint and all of the numbers scratched off..... Without having it working it is very difficult to figure anything out.

Having said that, the area you are looking at is the front end receive. The 100ohm resistor is most likely in series with the gate pin of the 200v N fet. That other little part could be a diode, npn, pnp, or fet of some sort??????
I don't know what fet they use in the extreme, but basically you want low rds on and LOW capacitance. The breakdown voltage needs to be 200v or higher. Look at the sd2000 schematic and see the n fet they use there for a rough guide on specs. There is also the chance that whatever drives the fets is fried too..... There should be 15v driving the gate of the fet....

Good luck man

cheers Mick

[golfnut]

What I see there is the coil drivers.

Id guess there is a ptype and an Ntype mosfet.


If they are frying it could be the coil with a short.

Or the driver cct isnt firing correctly so one fet is being told to stay on.


If you scope the fet driver transistors (the 4 at the lower half of the image you should see if the uP is driving them correct)

[bklein]

What is with the black gunk under the transistors? There are what looks to be solder tabs on the tops of them. Why weren't they soldered down to the pcb without this gunk? Layout error? Then perhaps they go because poor heat dissipation...

[bklein]

I am told by a good source these are mosfets related to a DC/DC boost circuit that feeds a storage capacitor for 184V TX coil pulse sourcing. I thought that the GP was somewhat similar to the SD2000 but looking at the SD2000 schematics on this site it looks like the SD2000 has low voltage sourcing. I'd say that is a huge difference between the two models!
(I still don't know what the parts actually are (N or P channel))

[BillNEWAST]

bklien, The description is lacking on the history of your detector.
If it was me, I would need to know the Fiddiling and Diddling that was done to the unit BEFORE you got it. ie
Was the Condition of the Board this way When you opened it up?

Did you try to remove any of the paint?

What did the Minelab folks do to it?

What did the Previous Owner Try to do to it?

What did the Previous Owner use for a Power Supply?

Where did the Previous Owner get the machine from, the owner before him/her?

Do you see where I'm comming from as to history? I would try to find out WHO Broke it first.
I know that this is a lot of leg work in tracing down the history, but it could tell you What was done with it. Who worked on it before it was sent to ML for repair.

I would try to find What the cost of having Mine Lab repair the unit to Factory Spec. with what their Warrenty period would be for it.

I would contact Woody in OZ and find out his estimate with some pics for to see of the WHOLE circuit board out, and in High Resolution. HE Warrentys His Work for 5 years and probably would give it a clean bill of health AND Do the GP Ex UPGrades for the upgrade price.

I know this isn't what you wanted to hear, just some easy parts repalaced and Then you get a Good Machine to use or sell.
I bought a SD2200V2 in 2009 BrandNew and used it for 6 months and it wouldn't Calm Down so I could hear anything (I have Bad Hearing), I sent it to Woody and he fixed it with his upgrade for $750.
I haven't regreted it since, it works Great Now and I can hear the targets from the Noise very clearly.
Sincerely
Bill Adams

PS check the SD noiseless thread for the link to the N channel Mosfet Link. don't want to post here and there also.

[Mechanic]

G'day Bklein,

What you have there is one difficult problem. Unlike other detectors, the boards as you can see are painted with white paint and all of the numbers scratched off..... Without having it working it is very difficult to figure anything out.

Having said that, the area you are looking at is the front end receive. The 100ohm resistor is most likely in series with the gate pin of the 200v N fet. That other little part could be a diode, npn, pnp, or fet of some sort??????
I don't know what fet they use in the extreme, but basically you want low rds on and LOW capacitance. The breakdown voltage needs to be 200v or higher. Look at the sd2000 schematic and see the n fet they use there for a rough guide on specs. There is also the chance that whatever drives the fets is fried too..... There should be 15v driving the gate of the fet....

Good luck man

cheers Mick

So, they run the power in on the opposite end of the board and have a charge pump right over the other side using tiny little fets with no large caps nearby also with tiny little wires attaching the charge pump fets to the coil? This would be very poor layout technique, Candy does not do silly things like that on his layouts.

Those 2 little fets, with the black gunk ARE the receive input fets. The equivalent part on the sd2000 schematic is the vn2410L . There are 2 there because they run 2 preamplifiers, unlike the 2000 which only had one.

Re-read the quote above, the info I give you is VERY good.....

Cheers Mick

[bklein]

I'll try and answer both you guys...
Bill Adams - I did talk to the previous owner, he told me an accurate history, I talked with the guy at Minelab that started work on it. I got the best info I could hope for. I was willing to take a huge risk for the price I got it for. Minelab is still an option for repairing even after I try and fail. Woody is an option too. In fact he sells a repair kit of parts. I have contacted him but am told he is on holiday now. His GP Extreme performance mod sounds great but I can't justify that kind of expense right now.

Mick, what you say about layout had me doubting both the first two responses. This area is right near the battery input. Would make sense for the TX circuit there but the TX connection is on the other side. Wouldn't likely be where RX stuff would be either. Just as it is uncool to bring HV pulses along the whole pcb it is uncool to bring RX signals all the way over to this area. I'm told it is for voltage conversion of the batt voltage to 184V. This makes sense for the layout seen. I still have to do some circuit buzzing to try and determine the local schematic and what the fets may be.

[Mechanic]

G'day Bklien,

Can you post a complete photo of the board, top and bottom, showing where the coil wires connect to the board. The bottom of the power input section does not look like that.

The 180v is built up from the flyback from the coil pulsing.

In fact I will bet you that the photo of the parts you originally posted is of the front end receive input fets.....

Does the machine make any sort of noise through the headphones?
Do you have +-5v present at the relevant pins on the ad797's(look up the datasheet) just above the burned out input fets?

Also bear in mind that Minelab will not repair a machine that has been fiddled with by an unauthorized repairer.

Cheers Mick

PS Pm sent....

[mario]

Hi Mick, wouldn't it be better to remove all that white paint first.

Even if we manage to identify the burnt components, there's no way the thing will work in case of open traces.

The damage seems to be extensive.

[Mechanic]

Hi Mick, wouldn't it be better to remove all that white paint first.

Even if we manage to identify the burnt components, there's no way the thing will work in case of open traces.

The damage seems to be extensive.

Hi Mario,

If there are any burned tracks, they will be obvious through the paint. So at this stage, leave the paint. Bklein also needs to decide if he is going to send it for repair to ML before getting too excited. Once the machine has been fiddled, they will not touch it, leaving you at the mercy of a third party repairer. If the uC has failed(which I doubt it normally would) NO third party repairer can fix it, regardless of any warranty offered. They would have to buy you another machine to honor their "warranty".

The thing is with the detector not currently working it makes reverse engineering extremely difficult if not impossible. ML tend to interlock their ccts, so if one part is not working correctly, other parts will not work either. A lot of time could be spent chasing ones tail, even with a schematic.

Cheers Mick

[bklein]

I've just got to hope for the best when replacing the parts shown in the photo. I refuse to go beyond that level as then the Minelab tech is going to add $300 to the repair estimate just to get back at me I just need to do my Columbo detective work and do my best to swap out the two larger fets and one smaller transistor - if it tests bad. My first problem has been to identify even just the package type. I haven't taken the parts off yet but it looks like a DPAK package but they don't use the center pin like used on these. Then there is a TO-263-3 that's a possible... Just not sure til I take em off.
I'm keeping my hopes up that nothing beyond this part has fried. Over the last year I've been debugging several WD Sentinel motherboards that have fet switching power management all over the place. Usually just the fets have gone there but you never know - this is more of a discrete design. The tech did say that replacing these 3 parts is not an uncommon repair because owners are messing with homebrew battery configurations. My advice to you electronics types is to test your battery system with an electronic load for turn on overvoltage issues. On the Pocket Rocket, I might make the 3 position switch a 2 position one if you have a GP (lower voltage).

[mario]

Sometimes the DIY coils are not up to spec and wreck havok on these machines. The .4 ohm resistance may be difficult to achieve without the correct wire thickness. If this was the case then you're looking at the wrong circuit.

Can you post a picture of your board?

[bklein]

No, this has a stock coil, stock design, Coiltek Pocket Rocket battery system.

[bklein]

Aww mannnn... You guys are right. This is the coil side. That's what I get for working on this stuff at 3am.
I got the parts off with no trace damage. The left big fet is shorted, right one ok.
Right one my gadget says is NMOS, Vth=3.46V, 1.41mA.
The little transistor is shorted. One of its leads went to both gates of the two big fets.
The other two leads go across the 100 ohm resistor. Pinout on the big fets= GDS. Perhaps this now matches up with your SD2000 schematic.
Drain of left fet measures 484 ohms to red coil wire.