help with surf pi kit no sound

[Toros]

If you use the analogy of MPSA13, as KSP13 it should know that they are with different pinouts=

[methanolcat]

I did like you said and with the scope on TP2 the waveform changes in the presence of metal can, so that part seems to be working.

I do not get an audio tone from (point 3) with +5v applied, I do get a noticeable pop/ crackle when making connection but no tone.

[Qiaozhi]

Let's debug this logically:

1) Monitor TP2 with the scope (coil disconnected) and adjust P1 to get the DC voltage close to zero.
2) Still monitoring TP2 ... connect the coil and confirm that the output reacts to the presence of metal. Use something large like a cola can
3) Take out U3 and connect the junction of U3a-1 and R23 to +5V. Check for a loud audio response.

Did you get a response at the output of U2 (point 2), and an audio tone (point 3)?
If yes, then the input and output circuits are ok, and the fault lies in the middle section, which means there something broken in either the sampling circuits or the integrators.

[methanolcat]

Hello guys, I'm new here on the forum. I've been reading threads for a few couple months now. Long story short, I bought 2 detector kits from silverdog, surf-pi and idx-pro. I have some electronics background (analog), I put them both together (still need to make coils for idx-pro, so its untested), the surf-pi was working fine and then something happened while I was putting it all in a plastic box, normally this is something I wouldn't have much trouble with but this really has me stumped.

When I turn it on, U5 (regulator) gets warm and I have almost no sound anymore, very faint. I have read through this thread as it was similar to my problem. I checked all the supply voltages on all the IC's positive and negative and they all seem right but something must be overloading U5 somewhat causing the heat.

I did notice that somehow P1 got broke so I replaced it, didn't help. I thought maybe a problem with U5 so I replaced it, didn't help.

I also changed out D6 and Q1 with no change. I have an oscilloscope and have confirmed tx and rx to be working with the waveform changing as metal nears the coil and thats when I hear the faint beep.

Since I have an idx-pro board some of the IC's are the same so I swapped out U2, U3, and U7. I also had another U4 (4093BE) in my IC box, so changed it too. Still no changes.

I have checked all of the solder joints, checked both external mounted pot's.

I'm running between 12 and 13 volts on my power supply and the circuit is drawing 50ma idle with coil hooked up. Here is something odd, with the voltmeter (set on 20vdc range) black on GND, red on pin 11 of U7, I get a more audible tone from the piezo, which makes me think the sound stage is all working.

Any help in getting this working would be greatly appreciated.

Matt

[Luuk]

Thank you for your dedication and problem solving on this project

[alcartraz]

thank you very much cant see how i got the resistors mixed up who knows but its working cant thank you enough

[Koala]

Nice work Qiaozhi. Never easy when there is more than one fault. Alcartraz will be a man.

[Qiaozhi]

Alcartraz - Your Surf PI board is working!

To start with I probed the signal across the coil with a scope, and this was found to be as expected. Next I looked at the collector and emitter pins of Q1, and discovered the signal to be low in amplitude. Since you had already replaced Q1, and it appeared to be inserted correctly, I examined R4 and R5. These were found to be 100k instead of 100R. After replacing these resistors, the waveforms were correct, but there was still no sound from the speaker. The sample pulses at pins 5 and 13 of U7 looked correct, so clearly at least 3 of the gates of U4 were OK. I then looked at pin 7 of U7 (which is the audio tone) and that was also good. Hmmm, but still not sound. The signal at TP2 was behaving normally, as were the signals at pins 2 and 4 of U7, which meant that sampling was taking place. Pin 7 of U3 was providing a good strong response to a metal target, but pin 1 was very weak. Then I noticed that R22 was 100R instead of 100k. But, after replacing R22, there was still no sound; even though the output at pin 1 was giving a good strong response. Now, suspecting that both the 100k and 100R resistors had been swapped over, I looked at R35. Ahah! ... it was 100R. And .. guess what? ... after changing to 100k, still no sound. The bias voltage on the base of Q6 was being pulled up in the presence of a target, leading me to think that Q6 was kaput. I didn't have an MPSA13 in stock, so I replaced it with a BC517, which is also a darlington pair. The pinout is different on this device, so be aware that it doesn't match the PCB silkscreen. Lo and behold ... still no sound! There was only one last thing that could be stopping it working fully, and that was U7D. To prove this I shorted pins 11 and 12 of U7 together and the detector produced a loud audio tone. Replacing U7 (4066) fixed the final fault.

Lastly, I adjusted the null offset of U2 to zero volts, and set the delay to match the test coil.

Working perfectly!

[alcartraz]

hi i just built the second kit and it works straight of the mark just need to tune it it up a bit detects small gold ring about 7 inches from coil but seems very erratic outside think the delay is to slow
i will post the other one tomorrow ! thank you for all your help

[Qiaozhi]

hi iam in england and the other kit should hopefully be here tomorrw i ordered it tuesday could just swap it over

if that works i proberly will not build the other one i will whant to get out and play do i run the risk of blowing the new chip if i just swap it ?

I am also in the UK. If you would like to post the non-working board to me, I'll gladly fix it free-of-charge.
Send me a PM if you're interested.

[alcartraz]

hi iam in england and the other kit should hopefully be here tomorrw i ordered it tuesday could just swap it over

if that works i proberly will not build the other one i will whant to get out and play do i run the risk of blowing the new chip if i just swap it ?

[Qiaozhi]

ok i wish mine was that simple new q1 resistor back in place and still the same made no difference and tried a new coil as well

If you're sure that Q1 is OK, then the next thing to do is replace U4.

Are you located in Europe?

[alcartraz]

ok i wish mine was that simple new q1 resistor back in place and still the same made no difference and tried a new coil as well

[Qiaozhi]

I have been closely following this thread because I have the exact same problem with my surf pi 1.2 since the beginning.
Now that Qiaozhi has narrowed it down to Q1 I took mine and had a good look at it with the schematics in hand.
The leg of Q1 that goes to R4 are right next to each other and were soldered seperately I now bridge soldered them and..........UREKA!!!!
now my detector works like a charm and even the delay which never made any difference now works.
I have to thank Qiaozhi for his dedication and patience with this matter. It really shows a lot of character. I salute you sir.

I just did a continuity test between Q1 emitter and R4 on a blank PCB, and the connection is good. Therefore, it sounds like you either had a break in the track, or there was a dry joint.
At least that's one Surf-PI which is now working, and helps to confirm that Acartraz's problem must be a similar issue.
Thanks for the feedback.

[Luuk]

I have been closely following this thread because I have the exact same problem with my surf pi 1.2 since the beginning.
Now that Qiaozhi has narrowed it down to Q1 I took mine and had a good look at it with the schematics in hand.
The leg of Q1 that goes to R4 are right next to each other and were soldered seperately I now bridge soldered them and..........UREKA!!!!
now my detector works like a charm and even the delay which never made any difference now works.
I have to thank Qiaozhi for his dedication and patience with this matter. It really shows a lot of character. I salute you sir.