TGSL Tuning & troubleshooting

[sdesic]

porkluvr,

in my last post with graphs is TR4 vs TR5

https://www.geotech1.com/forums/showthread.php?16122-TGSL-Tuning-amp-troubleshooting&p=173608#post173608

audiocard scope does this kind of graph...
I am going to have scope for use this next week so I can do another
it is one channel oscilloscope

[simonbaker]

I think what may be the problem for sdesic is that the Silver Dog schematic has an extra-high gain amplifier compared to standard TGSL. That can make instabilities more likely, even involving local EMI noise.

Maybe someone has already suggested this, but assuming the circuit is built correctly (in spite of the puzzling TR4 gate signal phase), I would recommend the following:

1. Change feedback resistors R29 and R31 from 1 Meg to 470K. If that is difficult, consider piggy-backing an additional 1 Meg resistor in parallel with each resistor on the circuit board (avoids unsoldering).

2. Replace capacitors C14a,b and C17a,b with single 4.7u bipolar capacitors. If bipolar are difficult to obtain, use back-to-back 10u capacitors, carefully observing the polarity in the schematic.

This will slightly reduce the gain and increase the center frequency of the filter section, same as the standard TGSL. It should be a more stable MD.

3. Take the circuit and coils outside, far away from power lines and computers. Use an AC voltmeter to null the coils by minimizing the AC voltage at LF353 pin 7. Maybe choose point slightly on either side of minimum.

Let us know if it still beeps.

Normal alkaline batteries should be fine -- you shouldn't need big lead-acid batteries.

-SB

[golfnut]

When you attach the leads via the potetial dividers, you also need a ground connection - or screen gound - this will take the huge mains hum away that is modulating the traces
S

[simonbaker]

measured 30mA on power up ... when it starts to beep after 60s 35mA
both TX and RX oscillations are there in under a second
when I touch gate of TR4 it measures 30mA

Thanks, your current looks perfect. We'll have to ponder what causes the 50s delay, but perhaps not relevant if this beeping problem is very borderline and affected by very small changes.

Actually, I will guess that the 50s delay is related to capacitors C15 and C12 reaching steady state.

With a DC voltmeter, it might be interesting to check the DC voltage on LM358 U103b non-inverting input (pin 3) and also the output (pin 1). I believe they should be about equal after steady state operation is achieved. And do the same for U103a (GB channel).

But first we must understand you scope pictures of TX signal vs. gate of TR4 -- something not right.

Regards,

-SB

[porkluvr]

Can we see the gate signals to both FETs, maybe at the same time? What you should have is two approximate square waves that overlap depending on the DISC and GB controls.

If there is any question of your scope's ability to display signals because of bandwidth, the input to the CD4024 should also be an approximate square wave with the same frequency. CD4024 pins 12, 11, 9, 6, should have square waves at f/2, f/4, f/8, f/16, respectively. Compare the edges of those signals.

The amplitude of gate signals should be approximately 3/4 that of the CD4024 outputs (but the signals will not have the same DC level, and they might not look quite so perfect as the 4024 outputs should look).

If you do not have a frequency meter but can measure the period of any of these signals, you can calculate the inverse of period (1/P) and multiply by the appropriate ratio to determine transmit frequency (in case you're wondering).

[sdesic]

Do you have a current meter? Can you measure how much current is drawn from the battery during power-up and during steady operation?

-SB

measured 30mA on power up ... when it starts to beep after 60s 35mA
both TX and RX oscillations are there in under a second
when I touch gate of TR4 it measures 30mA

[sdesic]

Would you describe more what you mean by "delay 50s"? What is delayed?

Can you put a voltmeter on the V+ rail (after the voltage regulator) during power-up, and tell us how fast it reaches 8 volts?

Can you watch the TX signal and RX signals during power-up and tell us what happens? How soon does the oscillator start oscillating?

-Regards

delay in detecting - device powers up and it has oscillations but there is no signal of detecting metal
if I move coils then I get signal ... in first 50s
after that it is normal

[sdesic]

Would you describe more what you mean by "delay 50s"? What is delayed?

Can you put a voltmeter on the V+ rail (after the voltage regulator) during power-up, and tell us how fast it reaches 8 volts?

Can you watch the TX signal and RX signals during power-up and tell us what happens? How soon does the oscillator start oscillating?

-Regards

posting two more screenshots


probes connected with 8M ohm + 200k ... yes MEGA OHMS
to gates of TR4 and 5
point of contact are J3 2 middle point of switch and LM393 U102B pin 7

and another screenshot when I touch wire connecting to TR4

voltage gets to 8 V instantly
will look into TX and RX signals ... I think that is also instantly

[simonbaker]

"oscilloscope" is connected with tx and gate of tr4
all metal mode - beeping - probe has 8 M Ohm + 200k resitors (both channels)

The phases look wrong to me -- 180 degrees wrong. I was measuring same signals on a TGSL I have and when the TX signal is high, the gate of TR4 is low.

Are you sure you are measuring the correct test points?

-SB

[simonbaker]

Davor,

I replaced all 220uF capacitors with 3x 470uF 16V and two new 220uF 25 V
Changed also ... my battery ... now it is 12V 1-2Ah lead acid battery showing 12.8V when loaded with TGSL EDU
Changed original C6 15nF

unsoldering two sided PCB is a mess ... placed them on other side of PCB
same beeps on normal overlap... same delay 50s ...

Do you have a current meter? Can you measure how much current is drawn from the battery during power-up and during steady operation?

-SB

[simonbaker]

Davor,

I replaced all 220uF capacitors with 3x 470uF 16V and two new 220uF 25 V
Changed also ... my battery ... now it is 12V 1-2Ah lead acid battery showing 12.8V when loaded with TGSL EDU
Changed original C6 15nF

unsoldering two sided PCB is a mess ... placed them on other side of PCB
same beeps on normal overlap... same delay 50s ...

Would you describe more what you mean by "delay 50s"? What is delayed?

Can you put a voltmeter on the V+ rail (after the voltage regulator) during power-up, and tell us how fast it reaches 8 volts?

Can you watch the TX signal and RX signals during power-up and tell us what happens? How soon does the oscillator start oscillating?

-Regards

[simonbaker]

new configuration stops beeping (it doesn't start with more resistance)
[ATTACH]25228[/ATTACH]

EDIT - tried one more combination - one channel scope 4 M + 100k ...
stops it and until it is disconnected it don't start beeping again

That is unexpected to me! You are saying that your probe with only 100k stops the beeping, and with 2M + 100K stops the beeping, but with 1M + 100K it continues to beep. I don't know why higher resistance probes would stop the beeping, unless you have wrong resistors or are touching something -- or the effect is quite subtle.

Well, it is food for thought. I would still like to see traces of TR4, TR5 gates where the beeping continues until you touch with your finger, using the 1M + 100K probes. (Correction -- I see you did with 8K + 200K probes -- thanks)

-SB

[simonbaker]

arranged probe like this and still doesn't stop beeps from MD...
[ATTACH]25227[/ATTACH]

maybe 1M should be between GND and signal ?

Hi sdesic:

Exactly -- I did not expect it to stop the beeping -- I wanted exactly the opposite. The purpose is to see the scope trace during the beeping, and also when you stop the beeping by touching with your finger, at gates of TR4 and TR5, to compare them.

If you can show photos of those cases, it might help us understand what type of signal is associated with the beeping. (Sorry, I see you showed photos with 8K + 200K probes -- thanks)

-SB

[sdesic]

Davor,

I replaced all 220uF capacitors with 3x 470uF 16V and two new 220uF 25 V
Changed also ... my battery ... now it is 12V 1-2Ah lead acid battery showing 12.8V when loaded with TGSL EDU
Changed original C6 15nF

unsoldering two sided PCB is a mess ... placed them on other side of PCB
same beeps on normal overlap... same delay 50s ...

[sdesic]

"oscilloscope" is connected with tx and gate of tr4
all metal mode - beeping - probe has 8 M Ohm + 200k resitors (both channels)

with finger on tr4 gate and without