TGSL Tuning & troubleshooting

[sdesic]

traced back from speaker this beeping and it comes through lower LM308 ... to me that is TL071 ... not the upper one
there is no beeping signal on upper one on PCB (closer to sens pot) - only detection of metal signal

[sdesic]

getting close to problem !
when I connected my "scope" of two resistors per channel on to middle point of disc switch - connecting to GATE of TR4
beeping stops and can be managed by sens pot and I have better detection depth (before there was none or was covered with sounds from beeps) on "nulling position"
looks like there is resitor to GND mising or something ...
disc. switch is now connecting points marked K1 and K2

[simonbaker]

Can I reverse coils ?

Even if coils are phased wrong (reverse), a good circuit will not beep when nulled. So I think problem is something else.

What kind of cable are you using to connect your coil to the PCB? How long? Can you try some different cables to your coil? For example, simple shielded audio cables, etc., just to see the difference. I once built a PCB that beeped and I never knew exactly what caused it, but I played with cables and other wires (such as pot wires) and eventually got it stable. ** Try stretching the cable out full length distance, separating coils and PCB maximum, and tell us what happens.

By the way, Don Bowers loaned me a very nice TGSL he built which only gets about 16 cm air depth in my workshop, but about 34 cm depth when used far into the mountains. I believe the difference is the EMI noise in my area. I think it really affects the depth. Now, Davor has made some mods to his designs that may be less sensitive to EMI noise (particularly changes in the "noise gate" after the last comparators), which would be interesting to test.

Although the phase of the "null" signal can indicate your circuit is not normal in some way, I do not believe the null phase is that important to basic operation (my opinion) -- however, if the null phase is in the detectable range and is modulated by some feedback, then you would have problems, so we don't want the null phase abnormal if possible. The coil shielding and type of cable can affect the null phase quite a bit, but the MD can still work well.

I don't understand something -- it seems you were getting 16 cm in the beginning. Then the beeping problem started. What changed?

Regards,

-SB

[sdesic]

Simon,

I am using USb cable - Pc to printer - which has 4 wires and gnd and something like mylar or aluminium over it.
I had 16 cm detection of 1 EURO coin - coils are overlaping more than should be (more overlap than in nulling position)
Beeping was never gone I used coils in more overlap and got less range... I was not able to null my coils so I ignored that.

Can you please read my last post about stoping of beeps.

Thank you.

[Davor]

I don't know what you refer to with K1 and K2, but you must be close.

[dfbowers]

Davor,

I will try reversing coils and report again, don't know why then we have basic coil wiring ground in head of coil presented in PDF documents.
It should be deleted from there ? Who is using that if it is not helping ?
I can replace my USB cable too - if it can help me.

Don,
thank you for your thoughts, will try that measurement. Soundcard scope can take 8V ? And other channel I will place audio so we can see.
Don't have experience with reading or using "scope"...

The reason I was asking about the 8v was because I commonly see that symptom when my batteries are nearly dead, so maybe something wrong with regulation. Or.. if you were to accidently drive the speaker off of the regulated 8v it will draw it down and cause motorboating.
About the ground connections in the search head.. I have built almost 20 coils now and have experienced improved stability in wet grass by not connecting the grounds together inside the head but at the connector where the coil attaches to the PCB.
Don..

[simonbaker]

I also didn't understand what K1, K2 are --Do you mean J3-1, J3-2 of Disc switch? Are you in ALL METAL or DISC mode? Do the beeps occur in both modes?

traced back from speaker this beeping and it comes through lower LM308 ... to me that is TL071 ... not the upper one
there is no beeping signal on upper one on PCB (closer to sens pot) - only detection of metal signal

You say you traced beeping back to only one channel, and the other channel has normal signal. However, the MD requires both channels have signal in order to beep. If you remove the a signal in the upper channel (no metal detected), then the beeps should not come through the speaker. Do the beeps appear only when detecting a target? I'm just trying to make sense of your observations.

Can you use your scope and show us the outputs of the two LM308/TL071 op amps when there is beeping?


Regards,

-SB

[sdesic]

K1 and K2 is disk switch wires markings on silverdog's pcb ... sorry about confusion


I am not in all metal mode I think
I have now removed beeping by connecting scope to Gate TR4 from this part
but if I switch to disc. after xxxxx seconds it comes back
Same source ?

Will try to post two channel scope graph so we can all see ... tonight

[sdesic]

sorry again - I am in all metal mode

[Davor]

Can you probe the 12V rail when beeps occur? Could it be a problem of high resistance of your power source? I know you can't measure DC oscillograms at the moment, but observing those glitches in 8V rail is disturbing. What is your exact battery voltage?

Trouble is that 78xx regulators require a lot of headroom to perform well. It is usually a symptom of a run down battery when you get perpetual beeps, as the 7808 is not capable of supplying a well regulated voltage any more.

You could seek some LDO instead, such as from LM2930 or LM2937 family, but be amazed how uncommon these parts tend to be in electronic shops.

[sdesic]

I will post screen capture in few hours when I come home ... 12 V and speaker output.
Using 12V lead sealed battery ... 7.0Ah for testing
it is over 12V ... I will measure it while loaded with MD

Don't know if I corrected value of amplitude on DC 8V ... it is 7x less than output from speaker ?
lower one (screen capture) is not in sync with audio channel, first one is ... compare how low it is ...
I think that is there even with nothing connected to scope - audio card is no oscilloscope
you think that my 8V is not stable so it triggers all of this ?

I don't understand how my experiment with Gate (middle point of disc. swich) helped with stopping beeping

I still have 50s delay in powering up

If I switch to disc. mode I have less detection - must use sens pot at 3/4 to gain few cm (2-3 max) and after 50s (maybe) it starts to beep too
with or without scope connected to middle point in disc. switch

Don't have any usable function of disc. pot

[sdesic]

I have tried scope on pin 6 of both TL071
upper channel is upper IC

first one is big overlap in coils no beeps


second one is
close to nulling - smaller overlap of coils
no metal present - beeping


third one is me waving coin across coils and detector beeping


I have 12.23V with detector ON for 10 minutes

next will be scope screenshot of 12V and signal from speaker
but do I really need to ?

I have 100k resistor on one channel and 22k resistor in parallel GND to signal

[sdesic]

here is screenshot of audio in green and open channel - nothing is connected

[sdesic]

graph 12V and sound output with beeps
magnified disturbance so you can see what it is

[sdesic]

Two gates TR4 and TR5 on two channels ... total silence - no beeps ....
(only if sens pot is on 3/4 smaller beeps as it should be)
coils close to nulling point but no beeps ... only difference is connection with scope ... ??
please explain this to me if you can

gates of silence


and waving metal over coils