Rev A - Stuck in step 7

[green]

I would use my scope over the voltmeter to trouble shoot the problem. If no scope, a DC meter reading might define the problem. The outputs of U4 should be either -5v or 0v. If the output is high 5% of the time, should get a reading .95*-5v supply= -4.75v. If chip is bad and floating at some value the voltmeter won't help. -5v on pins 6,8,10 and 12 suggests U4 pin14 isn't at zero volts, U4 no good, C7 bad or others. What is the DC volts on U4 pins 4 and 5?

[diegogeid]

Yeah I figured it was odd, I'll wait for the oscilloscope to do the proper readings, thank you very much for your patience!

[kt315]

I get -5 Volts at Pins 6,8,10 and 12 of U4
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sorry, you can not measure AC impulse signal by chinese tester. just oscilloscope.

[diegogeid]

I get -5 Volts at Pins 6,8,10 and 12 of U4 and the voltage at pin 6 doesn't change when I adjust R14. Voltage does not change at pin 1 of U7 when I swing a metal over the coil.

[green]

With pins 1 and 2 of the sample delay shorted. What voltage do you read at U4 pins 6, 8, 10 and 12? Does the voltage at pin 6 change when adjusting R14?

[Qiaozhi]

Here is a photo of the pins shorted, I'm sure thats not the problem.

The voltage doesn't change when I adjust r14, it doesn't move from 0V. I tried with the trimmer all the way up and down, and I checked also that the trimmer was working. What I noticed is that the speaker below the coil picks up the signal and i hear the frecuency going up and down when I move the trimmer.

R14 adjusts the minimum delay setting, so there's probably a problem with the timing circuits. You will a scope to debug that.
Does the voltage at U7 pin 1 change when you move a metal target across the coil?

[diegogeid]

That's not cheap when you have a devaluated currency like us

[diegogeid]

I'll be receiving an oscilloscope in a few days, but I don't know how to use one, can I count on you to help me? It's an old scope, hung chang 3502, do you think it will serve?

[daluka]

Maybe only a cold connection, soldering this part again.
Cheap oscilloscopes Hantek 6022BE, Instrustar ISDS205A abaut 50$.

[diegogeid]

Here is a photo of the pins shorted, I'm sure thats not the problem.


The voltage doesn't change when I adjust r14, it doesn't move from 0V. I tried with the trimmer all the way up and down, and I checked also that the trimmer was working. What I noticed is that the speaker below the coil picks up the signal and i hear the frecuency going up and down when I move the trimmer.

[Qiaozhi]

Hello, my name is Diego, I'm from Argentina. I recently bought a kit from Silverdog and started to follow the instructions step by step. I've cleared them all until I reached the dreaded step 7. Since I don't have access to a scope I've been doing it just with the help of a Multimeter and a Component tester. The test coil i'm using is made of ~30 turns of 0.5 mm ennameled copper wire with a diameter of 10 cm, I guess it's ok for testing puroposes.
In the video you can hear the high inductance speaker resonating with the coil. The thing is that I can't set the 20K Trimmer (R14) to get -2,1V in pin 1 of U7, it's stuck in 0 volts. I tryed shorting the 3 pins of the 10k delay pot insted of using the pot in the lowest resistance, but it's doing the same thing. When I take out the 10k pot connector, pin 1 goes to -3.20V and if a swing a metal over the coil it goes up to ~-3.06V so I guess it's an indication that the coil is picking up something. I don't know what to do now, I inspected the board and everything seems fine. I'll leave some photos for you to see. Thanks in advance, I know that with your help I'll have a nice metal detector at the end.
The power source i'm using is an old ATX with an output of 11.9V DC.
Board: https://i.imgur.com/PPDebq2.jpg
Board back: https://i.imgur.com/lTH6prQ.jpg
Coil tester: https://i.imgur.com/EGALenm.jpg
Video showing the problem: https://youtu.be/RP8zsKQADHo

You need to short pins 1 and 2 of PL2 (the Delay pot) together, and adjust R14. The value of -2.1V is only a guide, but you should at least see the voltage at pin1 of U7 change as you adjust R14. With pins 1 and 2 of PL2 shorted together, the sample delay is being defined soley by R14. The pins must be shorted during the test of placing a target near the coil. In your video, the external delay pot is completely disconnected. This is wrong.

[diegogeid]

Pots checked, both the trimmer and the 10k pot are working. I noticed that when I adjust r18, the frecuency that the speaker is picking up changes, but the voltage on pin 1 of u7 is still on 0. I'm seeking for a cheap scope but in the meantime if you think of something i should try, tell me. I also tried changing the 40106 chip since I had another one, same result but different voltage range when pl2 is disconnected, otherwise, with pins shorted on pl2 It's stuck on 0v.

[kt315]

A PC sound card O'scope should
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he did not solder whole board and can not hear the sound. with sound you do not need a o-scope.

[waltr]

A PC sound card O'scope should be good enough to check that clocks and pulses are working.
This can be done with a resistor and a cap in series (DC block) and free software.

What I would be checking is that the oscillator (U4a&b) is working and then there are pulses (turn on Q2 & Q3 sampling switches) from U4e & f.
Also check the resistance of the pots with your Ohmmeter (DMM) and check that the wiper to end changes resistance when turning the pot's adjustment.

Are all the proper Voltages on all the ICs?

Try wrapping some thin bare wire around pins 1 & 2 of PL2 to ensure they are shorted.

Since you bought the kit from Silverdog I'm assuming all the part are correct and good (not fakes).

[diegogeid]

Yes, I tried to short the pins with a screwdriver and the reading goes from -3.18 to 0V. The thing is that I don't know anyone that have an oscilloscope, and buying one is out of the question since I wouldn't do much with it once I finish this project. I thought about the cheap ones that come in a kit but they don't have two channels to make the adjustments, can you think of anything else to try? The components are brand new so I discard the posibility that something is broken. Thank you very much.