goldscan IV problem

[sadeg-f]

Hello dear colleagues
Please guide me
The following three questions:

1.Part oscillator output frequency must be how much?(u3 ne555)

2.ADJUST TO ZERO potentiometer, How should be set?(rv3-rv5-rv7-rv6-rv1)

3.Role of RV8 and R62 is this circuit?

[Reg]

Hi Sadeg-f,

I haven't built the GS IV so I don't know the frequency of the timer.

Now, the pots, rv3, rv5, etc are connected to an input of an amplifier. These pots should be adjusted so the output of the amplifier they are connected to is adjusted to 0V.

As an example, rv3 should be adjusted such that U12:A output at pin 1 is 0v. Use the most sensitive setting on your scope or meter to make this adjustment.

There is an exception and that is rv4 and that is adjusted so the amp it is adjusted to, u12c switches to a negative output. Set for the least voltage that does this and the output remains negative.

When adjusting Rv1, rv3, rv5, rv7, switch the reset switch to reset. This will minimize any output signals so it is easier to make the adjustments.

Rv8 should be adjusted so the amp it is connected to, u11, output is 0v when measured with a scope.

R62 is needed to keep the voltage divider it is part of to a very low voltage. This makes it easier to adjust rv6 so the output pin on u13;c is 0V when the switch is connected so the adjustment is connected to the input of that amp.

Reg

[sadeg-f]

thank you Mr.Reg

What type winding is L4?


Again, thank you

[sadeg-f]

thank you Mr.reg

Can you explain a little about the HOLD and RESET keys?

[Reg]

Hi sadeg-f,

Sorry, I do not know the inductance value. It is there to decouple the piezo so there is no interaction between it and the circuitry.

Reg

[bchampig]

Hello,

I'm a little confused about the adjustment of RV4 in the schematics posted. Should it be adjusted so that the output of U12C is 15mV when there is no target? And when there is a target, the output of U12C should go negative(opening the switch Q10)?

Also, the schematic I h ave seen shows R50 at 10K, and R49 at 470K. is this correct?

Thanks in advance!

Brian

[Reg]

Hi Brian,

RV4 should be adjusted for about 6 mv at the center leg of the RV4 pot. Make sure the output of U12C is negative with no signal. The best way to adjust this is to hold the reset button to the reset position to eliminate external signals affecting U12C output.

If using a precision IC for U12 that has a very low offset voltage then the setting of RV4 can be reduced to as little as a mv or two. However, on most IC's, there can be that much drift so a little extra voltage is needed to assure U12C doesn't change states with no real negative signal from a target causing the change.

The idea of this circuit is to eliminate all negative signals out of U12A and B by quickly resetting the cap voltage to 0V. In other words, the FET's will turn on and become a low resistance which causes the cap to discharge very quickly when the signal goes negative.

Reg

[bchampig]

Hi Reg,

Thanks for helping to explain that portion of the circuit. Did you mean -6mV rather than positive 6mV? The schematic I'm looking at has RV4 connected between Gnd and -5V, so I'm not sure how you can get +5V here? Perhaps the schematic is wrong and RV4 needs to go between +5 and Gnd?

Thanks,
Brian

[Reg]

Yes, it should be -6mv.

[Alexismex]

Thank you Reg for your precise and clear explanation about adjustment of RV4
for me it is more difficult English translation to really explain this sensitive case of adjustment.
Thank you again
Alexis.

[Reg]

The hold and reset keys perform different functions. With the addition of the R49 and R50, the hold key doesn't work like it used to and can be removed. In other words, the hold key is not needed.

At one time R49 and R50 were not part of the schematic so the capacitor would drift. Pressing the hold key would reset the audio to the preset level. With the two resistors in place, the hold key is no longer needed.

The reset key turns on the two FET's and basically shorts out the caps. This reduces any signals to a minimal value and quickly resets the cap voltges. This control isn't needed any more now except to calibrate the 0V outputs mentioned earlier. By pressing the Reset, all signals are minimized so the adjustment of the different pots is easier.

Reg

[bchampig]

Hello Reg,

Thanks for the explanation. As for R49 and R50, is the schematic correct in showing two different values, 10K and 470K. With the 10K for R50, I imagine the high pass filtering for the Gold channel would be affected. Perhaps this is the intent?

Thanks again for the help,

Best Regards,
Brian

[Reg]

The schematic is not correct Thanks for pointing that out.

Both resistors R49 and R50 should be the same, 470K. The two channels should match to assure identical parts, meaning the cap's before the two mentioned resistors should match very closely also. In fact matching caps are the best.

I measure my caps with a LCR meter and pick caps that are better than 1% matching.

Reg

[Reg]

I need to make a correction for the adjustment of RV5. This pot should be adjusted for 0V out of U13B rather than U13A. This is quite critical and should be measured down to the 1mv accuracy if possible. Otherwise the gain for high or low conductors may be compromised.

To make the adjustment easier, you might parallel R57 with a 1K to increase the gain of the amp. The 1K can be removed if necessary if the gain is too much when operating the detector.

Reg

[Reg]

Alex,

Did you build this detector? If so, did you have the pc boards made? I would be interested in one or two pc boards if they are available.

Reg