Surf PI problem again!! Please help

[TonyC]

HI Guys ……Ok I have a pi that will only respond to magnetic screw drivers ,large tin cans and a leather man multi tool at close range MAX 60 mm with the threshold turned up to a noisy level….l.I get no response from aluminium or coins at all!.....The board performs slightly better with earphones plugged in(I can turn down the threshold and get a slight improvement on detection)
Voltages on board:
U1 pin8 12.4
U2 pin 7 4,97
U2 pin 4 4.35-
U3 pin 8 4.93
U3 pin 4 4.33-
U4 pin 14 4.98
U4 pin 7 4.37-
U7 pin 14 4.98
U7pin 7 4.75-
The 100R and 100k resistors are in the right place.I have tried many coils ranging from 1.5 ohms to 2ohms
TP2 can be adjusted to 0 v,Delay makes no difference at all .I replaced the IRF9640 with an IR434C as the 9640 was getting very.All IC have been replaced MPSA13 replaced and I have the mod to change the TX timming fitted as per Geotech

Any help would be appreciated …..I have about 200 hour on this dammed thing

[Waikiki_Sweep]

What a power consumption of your circuit?

If it is in range 25-35mA your TX is not working so find why IRF9640 not opening by 555 timer output.

If it is in range 55-75mA than TX is working and you have some problems with amplification or sampling signal on the way.

If it is more 75mA probably your IRF9640 broken or shorted or it is in "on" state all the time.

IRF9640 works is this schematic perfectly as hundreds kits have been assembled successfully. If you have it hot so something was wrong. Check pinout by datasheet, replace with new IRF9640. Check PCB near IRF9640, clean with isopropyl alcohol.

[TonyC]

Thanks Waikiki_Sweep.....I will do as you say and comeback to you...TonyC

[TonyC]

OK I have replaced mosfet with IRF9640 and cleaned board with alcohol:
Voltages as follows:consumption 54 mAh
u1 pin 8 12.4
u2 pin 7 4.97
u2 pin 4 0.68
u3 pin 8 4.98
u3 pin 4 0.69
u4 pin 14 4,95
u4 pin 7 0,69
u7 pin 14 4.96
u7 pin 7 0.69
u3 pin 7 3.7 all positive no negative voltages No response from piezo.I just get a short squeek when I connect the test leads to test consumption

[TonyC]

Tp2 voltage is now at 4.54V and i cannot adjust it with offset pot with coil connected or disconnected

[Waikiki_Sweep]

u2,u3,u4,u7 opamps and logic powered by +5v and -5v. -5v converted from +5 by voltage converter ICL7660.

In your detector ICL7660 is not working to convert +5v into -5v so you have no -5v power supply now.
"u2 pin 4 0.68" has to be -5v as well as all that "0.68" on every IC are -5v power lines.

Check everything near ICL7660 including polarity of electrolytic capacitors C14, C15 or they broken.
Check ICL7660 installed incorrectly or may be counterfeited part from China or broken.
Re solder connections, clean with isopropyl alcohol.

If nothing helps cut the line after C15 and check if ICL7660 can provide -5V without load at all. May be -5v lost somewhere by the way.

[TonyC]

I have replaced capacitors, resoldered all connections replaced 7660 and cleaned board.Still don't get -5 v so it must be lost further down on the circuit....How do I go about isolating this problem?

[Waikiki_Sweep]

I have replaced capacitors, resoldered all connections replaced 7660 and cleaned board.Still don't get -5 v so it must be lost further down on the circuit....How do I go about isolating this problem?

Remove u2,u3,u4,u7 from sockets and check -5V again. If you see it working insert IC's back one by one.
If no results cut -5V line in different places and check where you can have -5V working and where it lost.

But I think more probable that something wrong in or around 7660.

[porkluvr]

OK I have replaced mosfet with IRF9640 and cleaned board with alcohol:
Voltages as follows:consumption 54 mAh
u1 pin 8 12.4
u2 pin 7 4.97
u2 pin 4 0.68
u3 pin 8 4.98
u3 pin 4 0.69
u4 pin 14 4,95
u4 pin 7 0,69
u7 pin 14 4.96
u7 pin 7 0.69
u3 pin 7 3.7 all positive no negative voltages No response from piezo.I just get a short squeek when I connect the test leads to test consumption

In frame #1 you correctly showed negative voltages on the various IC negative supply pins.
Then you replaced IRF9640 and suddenly you are WITHOUT negative voltages. (Wah!)
Did you not see that??
Please figure out what went wrong between frames #1 and #4.

[TonyC]

In between frame 1 and 4 I managed to short the board out.....I now have - 4.5 on all pins, draw on circuit is around 46 mAh and the sound is back!.....If I connect the coil I get a constant (buzzing or interferance sound )with no change when metal object is passed near coil....do you guys think it may be the coil?.....thanks for all the help so far!

[Waikiki_Sweep]

In between frame 1 and 4 I managed to short the board out.....I now have - 4.5 on all pins, draw on circuit is around 46 mAh and the sound is back!.....If I connect the coil I get a constant (buzzing or interferance sound )with no change when metal object is passed near coil....do you guys think it may be the coil?.....thanks for all the help so far!

Set 0 voilts on the preamp output U2 pin 6 using pot P1.

Than measure changes on that pin if you put big metal piece straight on coil. At least you can check that first amplifier working.
http://www.geotech1.com/forums/showt...604#post184604


Get a $0.25 coin and check a voltage changes if you put it on coil

Output of Preamplifier NE5534 PIN 6 (for my machine output -0.1V change for a quarter ($0.25) and -0.5V change for a pliers if I put it straight on the coil.)

Output of Intergator LM358 PIN 7 (GO HIGH from 1-2V to 3.75V)

Output of LP filter LM358 PIN 1 (VOLTAGE JUMPING UP DOWN)


Also

Keep coil away from PCB and speaker.
Try to adjust "delay" pot to get some reaction on metal.

[TonyC]

Ok....Inow have problems around LM 358,power consumption with LM 358 removed is50mAh and voltages on board are U6 pin 4=-3987,pin8 =4.98......U7 pin 14=4.98,pin 7 -3.98.....U4 pin 14 =4.98,pin 7=-3.987 ....U1 pin 8 =12.37,U2 pin 7=4.98......U2pin 4=-3.98......U3 pin 4 =-3.98.....U3 pin 8 4.98......If I putLM 358 back the minus voltages drop to -1.3 and the board warms up around 78L05an consumption goes up to 100mAh.......pin 6 on U2 drops from 0 to -0.8 withpliers direOk

[Waikiki_Sweep]

So LM358 is broken or inserted up side down or PCB has shorts in soldering around LM358.
It shorts -5V supply as you can see.
As usual - Re solder, clean, check PCB, replace LM358, check its pinout.

[kenny444]

Just wanted to say that you put a lot of effort into your support, I rate you as a first class guy. Thank you.

[ZX12R]

Hi TonyC,

Did you get your board sorted?

I am getting constant sound from speaker but cannot adjust the delay down to 0V, it constantly stays at 3.47v.

I started reading your thread and checked all the pins that you did in #1 but where you are getting "-" on pins u2 4,u3 4,u4 7,u7 7 I am getting something like 0.23, so I am not getting any negatives at all, I am very very new to electronics so any help would be great.

Everything looks ok, I followed a guide on a website adrianandgallese.com or something like that and all my components look to be soldered the same as that one.

Jason