Goldscan Pulse IV

Allodium,

Interesting.  There are three possibilities.

1) It is not needed and can be omitted.
2) It is needed and there is a mistake on the circuit layout.  In which case R79 could be changed to 470R 2W.
3) It is needed and this R14 is mounted on the coil assembly.

To answer this hopefully someone familiar with the design of having built it can validate/ eliminate (3) and help decide between (1) and (2).

Otherwise its trial and error!

Chudster


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Allodium,

I think I can answer your question!

If you look at the GIF schematic R14 appears to be located at the coil.  This appears to be to "balance" the impedance of the cable to the coil transmission line style.

In other words it appears that the 1K resistor is located close to the coil (option 3 in my last post) and this is validated by the fact that this also shows the use of a second resistor arrangement for the probe.  The probe coil has an inductance of 1mH and it mentions a "rod core".

In other words the schematic is right.  R14 is not located on the circuit board and instead on/ close to the coil.

So maybe some of the metal detector experts can validate the use of a resistor near the coil for PI's is a common/ normal arrangement.  Now also with the details of the probe coil it may be possible to also build one- if a probe is useful.  Anyone with any ideas of what a "rod core" is?

Happy circuit hunting!

Chudster

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Allodium, thanks.  That second link is helpful.

I have looked at the ICs and one by one:

5534.   This is an old IC but after looking at posts on audiophile forums (commonly used as an audio pre-amp) about potential replacements, there seems little reason to change it.  I note that it has been upgraded between the GIF schematic and the PDF.  In the GIF it is a UA709 so it was already upgraded to this low noise part.

TL064 and TL074.  Equally these are pretty good op amps and I can't see a reason to change them.

4538 is CMOS and digital.  I wouldn't expect expect changing it would make difference to noise.

The 78L05 and 79L05 are still workhorses and probably don't warrant changing.

The 7912 might be changed for a lower dropout regulator- might extend battery life.

All the parts seem commonly available and that is a bonus for people making this though.

Without a major redesign (Goldigger is right that there are now simpler ways to generate +-5 volt rails) the main change suggested to the ICs is to use the 7555 timer to replace the 555s as this may reduce switching noise on the supply and will reduce power consumption.

Changing/ upgrading the op amps probably won't change performance too much but I am open to wiser opinions than mine.  Any suggestions/ opinions?

Using good quality capacitors is probably a good upgrade.

So that leaves the 10uS pulse delay, Minelab connector, DD search coil and the better potentiometer to work out.

Hope this all helps.

Chudster

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Hello
What's the meaning of 3R5 in the Goldscan IV coil ?

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		Quote:			Posted by Allodium
	If I'm not mistaken, 436uH-3R5 is the inductance and the resistance of the coil. 436 micro-henry and 3.5 ohms

Thanks, but one of my friends told me that 3R5 means 5/3 Ohm .
I think also that 3R5 means 3.5 ohm in resistors standard, but I want to be certain about this matter here.

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