Metal Detector Repair Part 2b

This is a continuation of my previous post regarding 4 Metal detectors which I have purchased that need some attention to get them working.

Continuing on with the Micronta 3001 Metal Detector repair, I have received some of the parts that I have ordered, the Electrolytic Capacitors have arrived but sadly it seems the Transistors will be coming from China and will take a week or two to come.

I have also purchased a new piece of kit a portable digital Oscilloscope the DS213

As previously stated this unit just beeps when it is switched on so there is some kind of fault in the circuitry. I did manage to find a schematic of the unit which is shown in the next picture.

Looking again at the schematic there are 3 wave forms shown so I used my new piece of kit to have a closer look.

Firstly I looked at the square wave near to the supply, it peaks at 7V and has a 2ms frequency.

So this looks fine, so next I looked at the waveform at the junction of R10 and C8

Again this looks fine although the delay looks a bit more like 0.5ms rather than the 0.2ms as stated on the schematic, I will make a note of that in case there isn't anything else that is obviously wrong.

Now we come to the wave form near to the coil and here we do see a problem.

Here is a definite problem, the wave form is not a steady sine wave into the coil but instead is varying, this is a one shot capture when displaying continuously you can see it rises then resets to zero. Thinking about it if this is a varying signal into the search coil rather than a steady sine wave that would cause the unit to think it had found something.

So the problem is nowhere near where I had initially thought it was, it also looks like it won't be any of the Electrolytic Capacitors that are at fault but one of the components around the coil circuitry, either Q1 (unlikey) or one of the Capacitors, C1 or C2 looking the most likely.

C3 seems to be on the detector side of the coil and is the same value as C2, it may be an idea to change that over to see if the wave form is fixed?

From the videos I have seen on metal detectors there are two coils, the search coil and the detector coil, on this unit C3 and C4 are on the detecting side so I am pretty confident they won't be the problem here.

I did say earlier I don't think it will be the Electrolytic Capacitors but I am not sure what C12, C5 and C11 do in this circuit.

I changed out the Electrolytic Capacitors since I had them but it made no difference, I also swapped round the 0.01uF Capacitors C3 and C2 in case the problem was with them but still the same problem.

So my only hope now is that Q1 is the problem so will have to wait for those to come to try that, alternatively I am completely wrong and it could be C3 or C4 that is the culprit.

If anyone has any tips or tricks for fault finding these types of circuits I would be most interested to hear them.