CS6PI

[charleskerr]

Charles, people go in forum for THE INFORMATION. you ignored my request. WHY? you went in forum to fix only your problem and come out while people wait info from you.
this is very mutually and two sided.

Reg, if you have info that absent on the DVD please add it by your info. I promise you everybody will get DVD with your info and info from everybody.

Regards

Hi KT315

Unfortunately I left my board at home today, (I have a deadline at work to complete). and all the information is with the board.
I will bring it in tomorrow and set up the scope pc link to capture some traces. I am also learning about detectors as I go about this, my specialty is embedded micro design and my analog skills are a bit lacking. I have built a surf pi 1.2 and have been using that a a test bed / learning tool.

[charleskerr]

Hi Charles,

I am not familiar with the board version you mentioned, but I will try to answer your questions and respond to your concerns.

First, are you measuring the output of the preamp with your scope and not with a meter? What opamp is in your preamp stage? Eric's early designs had a 709 and later designs used a NE5534. If using the NE5534 be careful, some are better (faster) than others. Some TI versions are not as fast as some Philips types.

The third pot, if I understand you correctly, does balance the signal between the first and second sample. The reason is to minimize any false signal that is the result of the lack of equal signals. This false signal would be because of the earth field effect portion of the signal not being equal. Too much droop in the rise time of the preamp signal can also cause this false signal as well as poor sensitivity.

Now, have you replaced the 4017? These fail easily. If you replace it I recommend you install a socket to make changing it easier in the future.

I am trying to figure out what you are saying and which signal you are referring to when you discuss "signals".

The 4017 is driven by pulses from a 555 timer on which the frequency control adjusts the timer (which sets the pulse frequency). If everything is normal, this pulse should be continuous regardless of the frequency control setting.

When you refer to the three diodes I am assuming you are referring the ones connected to the 4017 which drive the pulse width. The diodes are nothing more than a primitive "or" gate, meaning the output of the diodes is high when any one of the diode input signals is high. This makes the first three pulses out of the 4017 the pulse on time.

If you look at the pulse drive signal at the gate of the mosfet ird113, you should see a relatively square wave. The critical part would be the shutoff of the mosfet and should be quite fast, maybe a microsecond or two at max.

As for your pulse spike, this can vary with the pulse frequency. What FET are you using? The IRD113 clamps at 80V I think? Yes, the shorter the pulse on time, the less of a spike you might get. Remember, the on time is short and the length of the on time determines the maximum current generated in the coil. The maximum current will determine spike height.

You should get signals out of the 4017 throughout the frequency range. If you don't, check the input signal from the 555 time to make sure it is working. If it isn't at some position of the frequency control, then the pot may be bad.

Did you build your own coil? If so, what did you use for wire and shielding? Can you post a pic of the output of the preamp? You should have a fast rise time and the signal should level off at 0V by the time you sample. If the signal is still drooping, it indicates you may have too much shielding.

This should get you started to a solution, I hope.

Reg

Hi Reg

Many thanks for the input. When I get home, I'll put a more detailed description down. I know my explanation may have seemed a bit garbled. I didn't replace the 4017 as the outputs are normal when they are not connected to external circirty. only when the outputs are connected to the 3 diode OR gate, do they start acting up. It could be that the 4017 is on it's way out and not able to supply the output current.