help with surf pi kit no sound

[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]

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

[alcartraz]

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

[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]

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]

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/showthread.php?20699-Fault-finding-on-surf-PI-with-simple-386-audio-amp

[alcartraz]

tried that no difference

[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]

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

[alcartraz]

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

[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]

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

[alcartraz]

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 ?

[Qiaozhi]

been playing some more the volts on tp2 goes from 0 volts to -1.9 when near to touching a metal object made of steel and yes the two links marked with white lines are there and good

Can someone check this on a working Surf PI?
Looking at the schematic, I would expect the signal at TP2 to go positive when a metal target is present.

Alcartraz - tell me about your coil. What is the diameter, and how many turns?

[alcartraz]

been playing some more the volts on tp2 goes from 0 volts to -1.9 when near to touching a metal object made of steel and yes the two links marked with white lines are there and good