help with surf pi kit no sound

[alcartraz]

67mm radius 0.560 enameled copper wire 22 turns but just checked the calculator it says 31 for 0.3mh is that right the last calculation ?

just made the second version of the coil and i can measure the indutance at 0.274 mH 31 turns 0.560 24awg

[Qiaozhi]

67mm radius 0.560 enamaled copper wire 22 turns but just checked the calculator it says 31 for 0.3mh is that right the last calculation ?

That might be part of your problem. A 134mm diameter coil will only have an inductance of around 172uH.
I would suggest winding a 10" (254mm) diameter coil of 20 turns to give 311uH.
Alternatively, a 134mm diameter coil with 31 turns should work OK.

I assume you are jumble winding the coil, and not making a flat spiral?

[alcartraz]

yes jumble winding and made the new coil 134mm 31 turns measured inductance 274uh still no change ?

[alcartraz]

i have give up on that kit completely and ordered a new one maybe with luck i might get the new one to work !!! and i will have a comparison to work with thanks for your help i will just wait and see what the post man brings

[Qiaozhi]

i have give up on that kit completely and ordered a new one maybe with luck i might get the new one to work !!! and i will have a comparison to work with thanks for your help i will just wait and see what the post man brings

While you're waiting for the postman, here's a small experiment:
The output of U3A (pin 1) needs to go positive to provide a bias voltage to the audio stage. Try unsoldering one leg of R23 (the one connected to U3A pin 1) and temporarily connect this resistor leg to the +5V supply. Does this produce a loud audio tone?

[alcartraz]

tried that no difference

[Qiaozhi]

tried that no difference

What do you mean by "no difference"?
The object of this experiment is to determine if there is a loud tone from the speaker when the audio output stage is biased on. If you are actually saying that no tone was produced, then there must either be a problem with the U4 circuitry, U7D, or the audio stage.

Since you do not have a scope, then this thread might be useful ->
http://www.geotech1.com/forums/showt...-386-audio-amp

[alcartraz]

right from the beging i have sound but so so quiet it would be unuseable we are talk turn every thing of in the house to just here it with the buzzer in my ear the only time i get reasonable sound is when the coil is near touching the buzzer or headphones but i think that is interference form coil to speaker ?

[Qiaozhi]

right from the beging i have sound but so so quiet it would be unuseable we are talk turn every thing of in the house to just here it with the buzzer in my ear the only time i get reasonable sound is when the coil is near touching the buzzer or headphones but i think that is interference form coil to speaker ?

What type of speaker are you using?

[alcartraz]

the supplied buzzer and a pair of xp light head phones that came with xp metaldetector

[alcartraz]

if i put the coil near the right ear of the head phones i get sound in that ear and the same with the left witch i find strange as its a mono audio circuit thats why i think it just interference

[Qiaozhi]

if i put the coil near the right ear of the head phones i get sound in that ear and the same with the left witch i find strange as its a mono audio circuit thats why i think it just interference

I agree. That sounds like interference from the coil.

We might be closer to finding the cause of the problem, so let's battle onwards.
Previously you mentioned that TP2 goes to about -1.9V when a large metal target is placed near the coil. This is actually correct, and shows that the transmitter and the preamp are working. However, if the fault is with U4, then the sample gates may also be affected. With the large metal target near the coil, can you also measure the voltage at U7 pin 2, U7 pin 4, U3 pin 7 and U3 pin 1.

[alcartraz]

right all the pins u7 pin 2 and 4 u3 pin 7and 1 have 0 volts with coil hanging on steel candle holder tp2 still dropping to 1.25 volts bats are getting a bit flat still goes back to 0 volts when not near metal u7 pins 1-4 all 0volt and pins 10 and 11 allso 0volt all the pins on u4 have voltages between 4.75 and 4,98 positive and negative respectively .

[Qiaozhi]

right all the pins u7 pin 2 and 4 u3 pin 7and 1 have 0 volts with coil hanging on steel candle holder tp2 still dropping to 1.25 volts bats are getting a bit flat still goes back to 0 volts when not near metal u7 pins 1-4 all 0volt and pins 10 and 11 allso 0volt all the pins on u4 have voltages between 4.75 and 4,98 positive and negative respectively .

My conclusion is that the problem is centered around U4, as this generates the sampling pulses, and also creates the audio tone. Since you have 0V at the outputs of the sampling gate in the presence of a metal target (and no audio), then I strongly suspect that the sampling pulses are disabled.
U4 is driven from U1 (555) via Q2 (2N3906). Firstly I would check Q2 to see that you have this inserted correctly, as TO-92 transistors can have different pinouts depending on the country of manufacture. Even if you've inserted it according to the silkscreen, it may still be wrong.

If your multimeter has a built-in transistor tester, then I would remove U2 and check the pinout.

[alcartraz]

My conclusion is that the problem is centered around U4, as this generates the sampling pulses, and also creates the audio tone. Since you have 0V at the outputs of the sampling gate in the presence of a metal target (and no audio), then I strongly suspect that the sampling pulses are disabled.
U4 is driven from U1 (555) via Q2 (2N3906). Firstly I would check Q2 to see that you have this inserted correctly, as TO-92 transistors can have different pinouts depending on the country of manufacture. Even if you've inserted it according to the silkscreen, it may still be wrong.

If your multimeter has a built-in transistor tester, then I would remove U2 and check the pinout.

do you mean q2 on my board and parts list it irf9640 or do you mean 2n3906 q1 ?