help with surf pi kit no sound

[Qiaozhi]

Check twice where you soldered R3, connected to pin 4 instead of 3 will cause exactly same symptom and similar voltage readings you have. Voltages you measured are strange, indicating transistor is nonconducting, or maybe faulty (possible B-E breakdown, despite you measured it on Hfe test, peculiarity of some DMM types, intended to measure only working component). Certainly not failure type I described earlier.

I agree. The fault definitely must be located in this area, despite what the meter indicates.

Alcartraz - Please try replacing Q1 with a new transistor and move R3 to the top side of the PCB.

[Tepco]

Check twice where you soldered R3, connected to pin 4 instead of 3 will cause exactly same symptom and similar voltage readings you have. Voltages you measured are strange, indicating transistor is nonconducting, or maybe faulty (possible B-E breakdown, despite you measured it on Hfe test, peculiarity of some DMM types, intended to measure only working component). Certainly not failure type I described earlier.

[alcartraz]

my board hope this is clear enough please note r3 is underneath but is there !

[Qiaozhi]

collector - 4.36 bias +11.22 emitter + 11.11

Measuring these points with a voltmeter is not very informative, as the TX pulse width is very short compared to the TX period.
Can you post a clear in-focus image of your PCB, just in case someone can spot the bleeding obvious?

[alcartraz]

collector - 4.36 bias +11.22 emitter + 11.11

[alcartraz]

ok so i will try the volts to collector and see what they are

[Qiaozhi]

q1 tests 255 on meter check and was in the right way ?

In that case the problem is either located in the circuitry of U4, or it's the same problem that Tepco experienced.

[alcartraz]

q1 tests 255 on meter check and was in the right way ?

[Tepco]

Most probably this is the case. I mentioned another possibility just because under some (unfortunate) conditions, that is, 555 chip way out of spec or output stage partially damaged, 2N3906 may stay in on state all the time, completely blocking signal to timing stage and producing exactly same symptom. Actually happened to me, just before that i blew 555 output by mistake, signal is still being sufficient for MOSFET drive, but this stage around Q1 failed to operate. Again, oscilloscope issue, as usual...

[Qiaozhi]

This also may be relevant, happened more than once:


http://www.geotech1.com/forums/showt...594#post171594

I think Alcartraz's problem is a different one, as he basically has no audio output. Unfortunately he doesn't have a scope, otherwise we could have debugged this much more quickly and easily. With a large metal target near the coil, the preamp output is changing on the voltmeter, but the outputs of the sample gates are not. This points the finger of suspicion at the sample timing pulse circuitry (based around U4), which also generates the audio tone. However, U4 is driven by the TX oscillator via a 2N3906. My educated guess is that this transistor is inserted incorrectly.

[Tepco]

This also may be relevant, happened more than once:

http://www.geotech1.com/forums/showthread.php?13810-Whites-Surf-PI-schematic&p=171594#post171594

[Qiaozhi]

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

On the schematic (from Silverdog's website) the reference designators are jumbled together, and I assumed that Q2 was the BJT, and the MOSFET was Q1. However, looking at the PCB silkscreen, I see that it's the other way round.

Have a look at the partial schematic below where I've marked the transistor that has most likely been inserted incorrectly. This is Q1 on the PCB silkscreen.

[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 ?

[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]

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 .