HELP! SurfPI Constant tone - but hacked to work 'backwards'?

[fo0bar]

Hi Everyone!


After lurking here for a while trying to find a solution, and hammering at this until 5am this morning I've decided I need to swallow my pride and finally ask the pros for help.


I've built the Surf PI from silverdog - everything's come together very nicely and seems to be functioning as it should be except for one part - everything's pulsing correctly on the scope and I can see some very clear changes in the returned waveform when I wave anything near the coil - however something just seems to be completely wrong with the balance/offset, and I just can NOT get the opamp to output any useful voltages - the signal at the input is far, far stronger than the opamp output!


With R11 and the 100K pot in place I cannot pull the offset down to 0 - on a scope or multimeter. Removing the pot completely has negligible effect. Turning the offset pot seems to have near zero effect, and I can only get an extremely feint resonse from the NE5534. It sits at around 600mv, with very feint, small spikes where the pulses should be appearing.


All resistors have been checked, all capacitors, transistors, IC's and pots have been replaced; I've cleaned the board down with alcohol and checked for shorts and cold joints with an illuminated jewellers loupe, despite having over 20 years of soldering/EE experience... I've constructed the variable damping resistor and got it spot on.... I'm now at my wits end and almost pulled out all my hair. The opamp is definitely seeing the input OK - with the scope on pin 1 (BAL), there's a sawtooth waveform which heavily modifies towards square when metal approaches the coil. What this signal actually is, I have no idea as the datasheet doesn't say - but it's showing me that there's a definite, very strong response to bringing metal near the coil. The coil measures about 320uH and is in a rounded-rectangle form. Same effect with a flat coil and round coil. I've used all these test coils very successfully on a home-baked arduino-based PI circuit.


The absolutely wierd thing however, is that I've got it working backwards!? By placing an 8n4 capacitor between the output (6) and COMP/BAL( 8 ), it sounds constantly, and then goes silent when I bring a coin near. when removing the coin, it sings a little 'wooooo' and then goes back to constant tone. Every video and demonstration I've seen has the detector usually silent and it sings when metal is near, and it seems like I've unwittingly enabled a pseudo "motion" mode for this detector.

I'm achieving a free-air range of around ~5 inches on a 925 Aussie shilling, ~8 inches on an aussie 20c coin, ~4 inches on a 10c coin, around 1 inch on a computer screw, and a 1.5kg Fire extinguisher at 1.5 foot.


If anyone has any ideas which could help me get this thing performing normally, I'd really appreciate your help... While it does seem to be functioning in a way, I'd prefer it not to be such a hackjob (as a software developer I disdain 'hacky patches' like this!)... If you want any voltage readouts or pictures from my (crappy el cheapo) scope, let me know and I'll take some for you. If you want videos to see how it behaves I can do that too. Happy to answer any questions anyone may have which could get me closer to a solution.


Thanks in advance to anyone who might be able to lend some insight!

Mark

[homefire]

Super Clear Photo of the PCB would help.

[homefire]

Not being mean here but you should verify ALL Resistors and Caps as per Values and Location one more time ? Chit Happens. Not that I ever do that stuff.

Verified all Voltages supplying IC's ?

Hope you socket-ed all Ic's ?

[6666]

Are you useing -bat as ground ?

[fo0bar]

Thanks for your replies, I'm in the process of taking some photos and will need to go out in the sun to get some suitable lighting.

Yes, all caps and resistors are in the right locations and are the correct values. All IC's are socketed and were inserted after soldering all other components and then grounding myself. As I didn't want to be wasting anyone's time here, I've actually spent around the last two weeks double checking each and every value and location against the schematic.

Previously I was using some ceramics and greencaps and have now replaced them with MKT simply to match the kit - despite them all having the same tolerances anyway. I was previously using a TO-220 L7805, and have also changed that to a TO-92 78L05, which made no difference other than for some reason the TO-92 cased reg doesn't even get warm. As you could imagine, I've double, triple, quadruple checked each component as I've replaced them to ensure I've got it right. I've even removed every resistor to test out of circuit to ensure they're within 5% tolerance.

All voltages to the IC's are reading out correctly as would be expected per the schematic (with exception of course being the opamp's output). I will also list voltages at every pin when I post again with some clear photographs.

I know you're not being mean by suggesting I may have it wrong as there are many novices who attempt this project too - who may not completely understand the component markings or how to test in/out of circuit - But in terms of assembly this is certainly one of the least difficult or complex projects I've assembled. I use the finest tip available for the Weller, with 0.5mm solder - I'm not the usual noob with a hot end

I've tried using the GND rail (BAT-) as well as the -5v rail as GND when testing - just in case I'd misread something. Neither of them allowed the expected zero voltage at pin 6.

Anyway, will reply again shortly with some photos. Thanks again!

[edit] I've just now come to realise that the detector only performs as I described in my first post when I've got my scope grounded on BAT-, regardless of whether the probe is connected. Without the scope neg connected to GND I get an extremely weak signal at 2cm on a small spanner. Not sure if this offers any other clues in the meantime.

[homefire]

Are you using a Bench Power supply to test the PCB ?

[fo0bar]

Are you using a Bench Power supply to test the PCB ?

I did briefly during the troubleshooting process use an AC benchtop supply, though I'm primarily using a car battery to ensure stability.

Sorry I haven't got photos yet - my phone's a bit of a potato with photos so had to find the camera charger.

I'm also now dabbling with the possibility I've bought some counterfeit NE5543's - along with the 7660's. gotta love ebay. I've ordered some of both of those from RS this evening and I'm hoping they'll be the root of the problems I'm having...

[kt315]

I've built the Surf PI from silverdog
----
i did read much posting with 'help' on bara pcb. so i decided to help them and to do new board for them.
i did. what people? they do not want to buy my board. they do NOT HELP me as in back.
they ask me only to give gerber files 'for free' while i waste my time and resourses.

thats MAIN problem my friend Mark in 'help'.

[fo0bar]

I've built the Surf PI from silverdog
----
i did read much posting with 'help' on bara pcb. so i decided to help them and to do new board for them.
i did. what people? they do not want to buy my board. they do NOT HELP me as in back.
they ask me only to give gerber files 'for free' while i waste my time and resourses.

thats MAIN problem my friend Mark in 'help'.

I'm sorry for any misunderstanding, kt315 - I'm simply asking if anyone's seen the same issues as me on a SurfPI board.

I'm not placing blame or expectation on anyone, the board evidently works. I'm not asking anyone for gerber files. If I wish to redesign the board I'd use kicad and do it myself.

While I certainly respect your feelings towards people who tend to take and never give in return, I feel that your comment towards me as a new user simply asking for a little support on an existing, proven circuit is slightly misguided.

Nonethless, I fully respect your opinion and will happily leave this thread at this point if you would prefer.

[kt315]

I'm sorry for any misunderstanding, kt315 - I'm simply asking if anyone's seen the same issues as me on a SurfPI board.
---
i am sorry too. no special wishes to dig aliens' board aside just they are always only trash layout. THAT MAIN problem.
if i do my board i know it best. i can answer on my board. i hope its clear for understanding.
i can to begin to do new board for surf. who will appreciate that if i did meet hard unwillingness to evaluate my job on Barracuda.

[fo0bar]

I'm sorry for any misunderstanding, kt315 - I'm simply asking if anyone's seen the same issues as me on a SurfPI board.
---
i am sorry too. no special wishes to dig aliens' board aside just they are always only trash layout. THAT MAIN problem.
if i do my board i know it best. i can answer on my board. i hope its clear for understanding.
i can to begin to do new board for surf. who will appreciate that if i did meet hard unwillingness to evaluate my job on Barracuda.

Ah crap, did I post this in the Barracuda section somehow? If so, sorry for that - I thought it was general tech.

[Qiaozhi]

I've built the Surf PI from silverdog - everything's come together very nicely and seems to be functioning as it should be except for one part - everything's pulsing correctly on the scope and I can see some very clear changes in the returned waveform when I wave anything near the coil - however something just seems to be completely wrong with the balance/offset, and I just can NOT get the opamp to output any useful voltages - the signal at the input is far, far stronger than the opamp output!

With R11 and the 100K pot in place I cannot pull the offset down to 0 - on a scope or multimeter. Removing the pot completely has negligible effect. Turning the offset pot seems to have near zero effect, and I can only get an extremely feint resonse from the NE5534. It sits at around 600mv, with very feint, small spikes where the pulses should be appearing.

OK - obvious questions out of the way first:

1) What voltage are you using at the battery terminals?

2) Have you checked the supply voltages are correct? i.e. +5V and -5V. Note that you should be referencing the GND connection, not the negative terminal of the battery.

3) The output signal of the preamp needs to settle around 0V (referenced to GND). You cannot measure this with a multimeter because the transmit oscillator is running. To measure with a multimeter you would need to remove U1 (ICL555) to stop the oscillator.

If all the above are correct, then you may have a faulty NE5534.

[Qiaozhi]

Ah crap, did I post this in the Barracuda section somehow? If so, sorry for that - I thought it was general tech.

You have not made a mistake. The Tech Forum is the correct place to post your questions.

[fo0bar]

OK - obvious questions out of the way first:

1) What voltage are you using at the battery terminals?

2) Have you checked the supply voltages are correct? i.e. +5V and -5V. Note that you should be referencing the GND connection, not the negative terminal of the battery.

3) The output signal of the preamp needs to settle around 0V (referenced to GND). You cannot measure this with a multimeter because the transmit oscillator is running. To measure with a multimeter you would need to remove U1 (ICL555) to stop the oscillator.

If all the above are correct, then you may have a faulty NE5534.

Hi Qiaozhi, thanks for your help.

1) The voltage at the terminals is 11.88v when under load.

2) The 5v supply is correct, and the -5v supply is also "near enough" at -4.86v under load. The -5v does however show a slight oscillation which is what's led me to replace the 7660's 220uF and then repurchase the 7660's from RS when that didn't improve it. I'm also using the GND terminal/s nearest the power input on the PCB as a reference. I've also tested with the GND near the mosfet just in case - no difference there, as expected.

3) The output never, ever settles down to zero - it just remains at 600mv. I've been using the oscilloscope to read the voltage/waveform and its "zero" point between pulses is consistently near 600mv - adjusting the pot moves it down to around 589mv.

As I've tried all 10 of the new NE5534's I've got in my collection, I'm now getting increasingly suspicious they're counterfeit as they surely couldn't all be faulty!

You have not made a mistake. The Tech Forum is the correct place to post your questions.

Ah, thanks! I was a bit confused

[Tibuck19]

might help or not, but when I made my first surfpi kit from silverdog I didn't have a component tester and I had place the red cap values in the wrong spot on the board it worked but was not working well once I correctly placed them all it worked perfectly. A clear photo of the board could help others with more experience see the problem better.