Photo of GP Extreme Badness?

[bklein]

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.

SOT89 package.

[Mechanic]

Hi Bklein,

The package looks to be sot-223, but a bit smaller?
This one would work fine. If you can find one with lower capacitance for the same or lower on resistance that will be better. Breakdown voltage can be higher than 200v if the R and C are better.
http://www.farnell.com/datasheets/454180.pdf
http://www.farnell.com/datasheets/454180.pdf sot89 package

Before Putting the new fets in, try firing up the machine and test the voltage going to the gates to make sure you have your +5 to -15v gate drive signal. Also check that the coil signal is working right. They use a switched damping resistor, if something is not right there the coil will not damp and you will have large oscillations after tx turn off. This can cause the rx fets to fry.

If all looks ok then proceed to installing new fets.

Cheers Mick

[bklein]

Looks like a good part.
Then there's the small transistor across the 100 ohm resistor.
The "GS" marking may indicate a BC857C PNP. Does that make sense?

Can the unit be turned on without the coil attached without harm?
Would that be safer than having it attached and turning on without the fets?

Looks like I need to check out the coil as it may have caused all this.
Other than visual check of the coax and connector soldering, anything else? I remember thinking it had some pretty tight coiling of the coax. Maybe the shield wore through the wire insulation? Has that happened to others? The unit isn't that thrashed looking, coax generally good condition but I haven't gone over it inch by inch yet.

[Mechanic]

Hi Bklein,

If that is what the marking indicates, I see no reason why the bc857 would not work

The machine will fire up without the coil, but there will probably be no signal going to the fets because these will remain turned off until the 180v cap has charged up to its target voltage. There will be no harm in firing up the machine with the coil connected and no front end fets. This way you will be able to check the gate voltage signal.

The coil cable can go bad(open cct or near open cct), but I don't think it would cause this. If the lead was to short, there are over current protection systems in place. You will be able to see what the coil is doing before hooking up the front end again. Post a scope shot of the coil signal if you can, set the probe to X10, all going well there should be a long -5v pulse, followed by a short 180v flyback, then 3 -15v pulses also followed by 180v flyback each.

Cheers Mick

[IBGold]

Hi I think you will find the Fets in question are BSS84 and the diodes are BAV99 have fun.

Regards, Ian.

[bklein]

Hi I think you will find the Fets in question are BSS84 and the diodes are BAV99 have fun.

Regards, Ian.

Hi Ian,
I'm confused by your statement. I had been thinking the small transistor was a BC857C due to the "GS" marking on it. Your suggestion is that is could be either a fet or dual diode (both SOT23)?

I guess I should test without the parts installed and check for gate signal on the 2 fets. If none, check the small transistor spot to see if it has a signal on one of the 2 other pins. Then install it and see if this creates the gate drive signal to the 2 fets. Then safe to install the 2 fets. I have BC857C and fets on order, have BAV99 at work, don't have any BSS84's.

[bklein]

The pin diagram I've seen on this forum for the Minelab coil shows pins 1 and 5 connected and pins 2,3, and 4 connected. My coil cable connector has pins 1 and 5 connected and pins 3 and 4 connected.
With my (#1100, 10.5") coil, there are two shielded lines at the cable plug, the shields tie together and connect to pins 1-5. One coax hot goes to pins 3-4 and the other coax hot goes to pin 2.

Shields to pin 2 395uH 18.9 ohms
Shields to pins 3/4 287uH 1 ohm
pin 2 to pins 3/4 684uH 19.3 ohms

On the unit side:
Pin 1 = yellow wire
Pin 2 = green wire - goes to right fet drain
Pin 3 = blue wire - goes to left fet drain
Pin 4 = red thick wire
Pin 5 = black thick wire with small blue wire to control panel shield ground

Does this make sense??

[Mechanic]

Hi Bklein,

The pinout you have mentioned is for a mono coil.

The coil you have is a DD coil, so differs slightly.

The yellow wire is for an analog ground, pin 1 the large black wire to pin 5 is coil tx ground, the short blue is as you said a shield drain for the panel, it is surprising that they hook it up to this point, though it is very necessary that it is hooked to ground at some point.

The green wire is the receive wire for DD signal

The blue wire is for the mono receive signal, the red wire is the tx hotwire.

The resistance between the shield and pins 3&4 looks a bit high, should only be about .4ohm, did you account for the 0 error on your meter?

Have you tried firing up the unit without the fets in place yet?

Cheers Mick

[bklein]

I just wanted to check to see if something may be shorted on the coil first. I'll power it up tomorrow afternoon.

[Mechanic]

I'll power it up tomorrow afternoon.

The moment of truth! Fingers crossed and good luck.

Cheers Mick

[IBGold]

Hi Ian,
I'm confused by your statement. I had been thinking the small transistor was a BC857C due to the "GS" marking on it. Your suggestion is that is could be either a fet or dual diode (both SOT23)?

I guess I should test without the parts installed and check for gate signal on the 2 fets. If none, check the small transistor spot to see if it has a signal on one of the 2 other pins. Then install it and see if this creates the gate drive signal to the 2 fets. Then safe to install the 2 fets. I have BC857C and fets on order, have BAV99 at work, don't have any BSS84's.

Hi, the 100 R resistor goes to the Gates of the two BSS84's and the drains go through the BVA99's to signal ground there is two other mosfets prior to these in the signal chain but I have no idea what they are could be the same.

Regards, Ian.

[bklein]

A BSS84 is a SOT23 P channel. Those two fets are SOT89 N channel. Their drains are going to the coil. So are you describing other parts?

[bklein]

The moment of truth! Fingers crossed and good luck.

Cheers Mick

Ok, turned it on this morning and hey AUDIO. Cool!
I took some scope photos. Gates of the 2 fets are driven without the small transistor, so it has some other function. I show pins 1 and 3, pin 2 is -14.2V.
Favorable(?) signals on the left fet, peaks to 208V.
On the right fet, I don't know... maybe ok maybe not.
Gotta drive in the rain now to a gold prospecting lecture in Temecula.
In case these don't get labeled on upload,
Leftfet drain
Rightfet drain
Rightfet source (same as left one)
small trans left pin
small trans right pin, middle -14.2V

[mario]

Look at that, there's that famous Minelab signal train!

I'm surprised you are detecting a flyback of 208 Volts. The RX N Fet maybe rated at 230Volts? Either that, or there's something wrong with your circuit, frying your fets. The ones in the 2100s/2000s are rated at 200V. Also, the stock flyback on my 2100s was around 184Volts.

Hey Mick, is that a frequency of 1.25Khz? I maybe wrong, wasn't the 2100s around 1.4khz? or is my math wrong

[bklein]

Ok sounds like the other half of this circuit is damaged with bad P channel fets, diodes, and maybe whatever drives them. So I will have to scratch off more paint and do some component testing. I'll order parts mentioned just in case.