TGSL-EDU

[Daren]

I'm working on getting TGSL-EDU to run. I use Tesoro 9x8 concentric web coil (white, 5 pin). After initial problems with some of the transistors installed backwards (did not check the datasheet as should have) the board came up. I checked the signals with the scope again the very helpful pdf "TGSL 101" (thanks Don Bowers!). All looked good, just I had pots ends swapped, corrected it.
Setup GB, seems to work.
Here are the problems:
- after turning the board on, it will take about 15 seconds before the board will detect anything, the TX/RX looks good from the start
- the board will go into a 5Hz beeping after 15 seconds, waving a metal in the front of the coil gets it out on the "beeping" but after some time it will start again
- with the sensitivity very low it will not go into the beep mode
- with the sensitivity in the middle it will not go into the beep mode after some time
- with the sensitivity turned very high boards goes into the "beeping" mode right away
- it seems that changing settings will take about 15 sec to take effect


I have spend a lot of time trying to debug this,
So far I have:
-double checking values of installed parts
-solder joints
-washing board
-shorten RX connection cable
-removed U101 (beep still there)
-removed U102 (beep gone)
-board takes about 30mA while running (35mA with piezzo sound)

I used Mylar 220nF caps since they were missing from the silverdogs kit. (polyethylene box type will come, just ordered them). I don't think that will make any diff anyway....

Any pointers how to approach this will be appreciated! What should be behavior of the board as you go higher with sensitivity, should there be a constant beep on the 5Hz freq on/off 400Hz (or whatever the freq is) beep.
Thanks!

[Davor]

Which FETs do you use, and what are U104 and U105?

[Daren]

U104/105 are TL071. FETs are what schematics calls for: J107, (I will double check it when I get home... but I'm 95% sure...)

[Silver Dollar]

The first thing I'd try is make the leads to the speaker longer and move it as far away from
the coil as you can.

The coil picks up the magnetic field and that can cause an oscllation. Can you eliminate the
problem by turning down the Sens?



My Bandito echo's, oscillates and beeps on occasion too but I asume it is "normal". I'm trying to
push the detection limit and so the ultra high gain with narrow bandwidth can do some funky
things.I've seen 104 hz oscillation and 10 hz. I saw 5V ac on my skin so I know there is a bit
of EMI around.

Running on a battery outside is the best way to test your MD. Also using a metal case might help a bit too...

[Daren]

U104/105 are TL071. FETs are BF245B

[manny77]

Bf245 in my silverdog kit was a pain in the a.. I discovered that those fet has to be reversed from the design on the board.

Check the 2 back to back capacitor too. 22 microfarad with minus in common...

[Daren]

I rechecked BF245 s; both look correctly installed, Gate is in the correct place (facing "down" when you look at PCB); Source/Drain can be interchanged (I didn't know you can do this, but that what the datasheet says). Once the board starts beeping the setting sensitivity to min does not stop the beep, but if you move a metal in the from of the coil the beeping stops. Very annoying.... I don;t see a "quicky" solution/ I think I will have to learn how the whole circuit works and work through it.

[manny77]

BF245 in silverdog kit are swapped respect silkscreen of pcb. Anyway for clarity i show how they have to be correctly soldered:



Anyway if you said that gate is facing down it should be correct...

Try to re-check every component, diodes direction etc.
Seems to me a capacitor problem...due to the long times of 15 sec...but really i cant figure it.
I know very well your situation...it happened to me when i firstly made my tgsl...everything was correct according to silkscreen...but damn BF245 are swapped internally! When i found the error was very happy!
Hope you find yours very soon!

[Davor]

Perpetual beeping can also be a problem of Tx instability.

[Silver Dollar]

Hmm I could swear my 245's went in according to the silkscreen and worked OK.

I had some prloblems with oscillations on my Bandito and figured it was an unsoldered pin
somewhere. I resoldered everything and it worked!

In trying to get higher gain I ran into oscillations a few times. I decoupled all opamps with
10 ohms and 4.7UF and now it will oscillate sometimes but turning the sens down stops it.

The whole circuit is a tuned one where everything affects everything else. It's tough to
gain much without running into problems. The Bandito is almost ready to go in a case but
doing a few tests last night it suddenly got very quiet. Full sens and no noise but it would
still detect at 4". I think I traced it to a loosely soldered cap on the transmitter (I hope so!).

Just unsoldering and resoldering the TX transistor affected my TX amplitude a bit. I was messing
around with the 20k resistors feeding the CD4066 (FETs on TGSL) and only 20K worked! 100k and
lots of noise showed up, 10k and lost depth. Weird! I think there might be a problem with that 4066
maybe some decoupling would help?

[manny77]

Silkscreen of silverdog boards is correct!
BF245 instead are produced from several manifacturer in different pinouts.
Mine, furnished by silverdog kits, are inverted internally...

[Daren]

The 2n2907 that came with the kit had pinout reversed; I installed them per PCBs silkscreen initaly but it was wrong... The board did pull 100mA then before I fixed it maybe 2n2907 got damaged?? I should be able to put in any general purpose PNP transistor; correct? .The FETs seem fine, the pins are even labeled G S D...
I'll reflow all the joints tonight just in case....
Thanks!!!

[Daren]

Silver Dollar:
>>The coil picks up the magnetic field and that can cause an oscllation. Can you eliminate the problem by turning down the Sens?
1. increase sens until beeps, 2. go sens min, still beeps, 3. wave metal in the front of the coil, beep gone
>>Running on a battery outside is the best way to test your MD. Also using a metal case might help a bit too...
I'm running on battery all the time, tried taking it outside, no change....

[Davor]

Can you check Tx amplitude on coil?

[Daren]

its 15V
Davor, I used is your mod (as posted by Silver Dollar):
>>Remove D8, D9, C20, C23, R33 and put diodes at C20 and C23 reversed from D10 and D11 and change R29 and R31 to 1 meg.
>>sens pot from GND to V-

The beeping is magically gone now. All is stable, BUT: after switching board on you still need to wait 15 sec before anything is detected, and the detection range is only 4 inches (10cm) for US quarter in all metal mode (any mode for that matter). I think it should be better than that. I use Tesoro 9x8 "web" coil. The Sens pot does not have any effect that I can see.
Certainly a step in the right direction anyway

I'll upload two images; first signals straight from the coil (yellow is the TX, blue RX) the second is the RX signal from U101 Pin7

the TX freq is 14.3KHz not 14.5KHz; I'll try different caps to get closer to 14.5 once they arrive..

I looked at the signal path. What is expected voltage signal at the gate of TR4 and TR5? There are the signal on my board;
Yellow is RX from U101 pin 7; blues Gate of TR5. Is this enough to open/close TR5?