My Surf PI Null Offset on Pin 6 of 5534 is different when measured with multimeter & oscilloscope

[Qiaozhi]

LOL The Trail of Ignorance is Indelible. Mine is Far and Wide.

Yep ... all words of "wisdom" are indelibly printed in the forum.

[homefire]

LOL The Trail of Ignorance is Indelible. Mine is Far and Wide.

[Qiaozhi]

i think not being able to edit posts doesn't help there is a few of mine i can go back and edit and correct now

If you were able to go back at any time and edit an old post, it would cause all sorts of confusion with the continuity of the forum.
Replies from other members to your [edited] post could end up completely out of context.

If you want to posts updates, you need to make a new post.

[kosacid]

i think not being able to edit posts doesn't help there is a few of mine i can go back and edit and correct now

[homefire]

Most of which could have been avoided if the Golden Rule of Build would be followed. BUILD IN STAGES ! TEST IN STAGES! AND MOVE FORWARD!

[Qiaozhi]

I see this as being a huge straddle with such a wide group, with you and other Gurus in the middle.
Surely this is not an easy task. As you were saying - where do you start?

When I am pointing something out, please don't see it as criticism.

I am just hoping to maybe find new ways with more clarity or simplicity,
from the viewpoint of a rookie.

I agree that many of the projects are somewhat messy, with information scattered around the forum. That's exactly why the GeoBara and MPP projects were placed in one location together with a Build Document. In fact, there have been far less problems and questions regarding these two projects than any of the others. If I had any spare time (which I don't) I would have liked to put something similar together for the Surf-PI. There seem to be endless problems with the Surf-PI, which is quite amazing for such a simple design.

However, it almost doesn't matter how simple you make a project, as there will always be someone who insists on making their own PCB, salvaging secondhand components, not using an oscilloscope, and incapable of reading the documentation.

Despite that, we collectively do try and help when the questions are not just plain dumb, and the person is genuinely trying to learn. After all, we were all at that point at some time in the past.

[Polymer]

It does actually say in the introduction of the Build Document: "Please note that it is highly recommended you have access to an oscilloscope during the setup procedure. Although it is not impossible to successfully build a working unit without one, you will need to be very careful and methodical during the construction. However, it is mandatory that you possess a multimeter."

As far as measuring the DC offset at the output of the preamp, it should be fairly obvious that you will need an oscilloscope to do this, unless you disable the TX oscillator first. The problem is that the signal at the preamp output varies with time, even if the coil is disconnected.

And "yes", the Build Document does assume a certain level of knowledge, otherwise where do you start? As I've stated before, it is not recommended that anyone should start with a metal detector design as their first electronics project.

You also have to remember that prior to the Geotech Baracuda REV-A, the original Baracuda details were a complete mess, riddled with design errors and reliability issues.

Hello Qiaozhi,

I am not knocking the Geotec Baracuda at all and I appreciate that the mess around the original has been cleared up.

On one side you have electronics proficient people interested in building detectors.
On the other side you have people interested in building detectors with little or no electronics knowledge.
And everything in between of course. I would take a guess that the latter group is bigger.

I see this as being a huge straddle with such a wide group, with you and other Gurus in the middle.
Surely this is not an easy task. As you were saying - where do you start?

When I am pointing something out, please don't see it as criticism.

I am just hoping to maybe find new ways with more clarity or simplicity,
from the viewpoint of a rookie.

[Polymer]

On this forum I have read that you CAN adjust the offset with only a multimeter.

yup. but... do you attentionally read mine?
i write to DISCONNECT mosfet and dumping resistor then do measurement. LOOK DETAILS.

WHERE can I find yours? I would read read it with much awareness if I could only find it. Is the coil connected or not?

I found this looking:With your multimeter, you need to adjust the offset pot to give 0v DC on pin 6 of the ne5534

http://www.geotech1.com/forums/showthread.php?18986-Newb-needs-help-with-Surf-PI-Volume-knob&p=147461&highlight=offset+adjust+multimeter#post147461

This is not about pointing fingers, but hopefully finding a better way to deal with all the different qualities of information here.

[kt315]

On this forum I have read that you CAN adjust the offset with only a multimeter.

yup. but... do you attentionally read mine?
i write to DISCONNECT mosfet and dumping resistor then do measurement. LOOK DETAILS.

[homefire]

O Scopes NEVER LIE ! LOL, Well almost never.

[Qiaozhi]

Does it make a difference if null is adjusted using a voltmeter or a scope? Does it work better if a scope was used? A scope may not be needed to build a good kit project, but probably is necessary to see how it works.

The output of the preamp periodically hits the supply rail (i.e. the output saturates) before settling to zero. When you adjust the null-offset you're trying to get the 'settled' voltage close to 0V. However, if you measure the output with a DMM, the voltage will be skewed by the flyback period. Some designs actually work better if the preamp output does not settle to 0V, but is offset by maybe 1 volt or so. You can only really understand what's going on if you use a scope, otherwise it's more or less trial and error.

Personally I wouldn't even attempt to build a metal detector design without a scope. It's like trying to work with one hand tied behind your back.

[green]

Does it make a difference if null is adjusted using a voltmeter or a scope? Does it work better if a scope was used? A scope may not be needed to build a good kit project, but probably is necessary to see how it works.

[Qiaozhi]

For me this is ill-defined in the face of a rookie detector builder. It is not defined as what to use for measuring.
An expert would know how to measure this properly. It presumes a certain experiential/knowledge base.

Since it's DC voltage I somehow automatically thought I'll use the DMM. I had nothing better at the time anyway.
This has most likely happened to others too.

It does actually say in the introduction of the Build Document: "Please note that it is highly recommended you have access to an oscilloscope during the setup procedure. Although it is not impossible to successfully build a working unit without one, you will need to be very careful and methodical during the construction. However, it is mandatory that you possess a multimeter."

As far as measuring the DC offset at the output of the preamp, it should be fairly obvious that you will need an oscilloscope to do this, unless you disable the TX oscillator first. The problem is that the signal at the preamp output varies with time, even if the coil is disconnected.

And "yes", the Build Document does assume a certain level of knowledge, otherwise where do you start? As I've stated before, it is not recommended that anyone should start with a metal detector design as their first electronics project.

You also have to remember that prior to the Geotech Baracuda REV-A, the original Baracuda details were a complete mess, riddled with design errors and reliability issues.

[kt315]

Don't know about the Surf PI but step 5 of the Geotech Baracuda (RevA) build document suggests measuring the DC volts at amplifier out when adjusting null.

let me look on this document. link. it seems I missed something important in my life.

[Polymer]

first question to you. who from the geotechers did give you the suggestion to use voltmeter for null tuning?
mine? George? Carl? Eric Foster???
WHO?

Oh dear, kt315. You seem to be in crappy mood right now or what?

I have read a helluva lot on this great website and really appreciate it, warts and all.

From my point of view the information I find here is sometimes conflicting or half right or wrong or
muddled or sloppily expressed or the question interpreted differently etc. ... such is life.

Here's one of many examples I found (I will get to the DMM after):

"Measure the DC voltage on U2 pin 6, and adjust R6 to give 0V"

Who wrote this doesn't matter.

For me this is ill-defined in the face of a rookie detector builder. It is not defined as what to use for measuring.
An expert would know how to measure this properly. It presumes a certain experiential/knowledge base.

Since it's DC voltage I somehow automatically thought I'll use the DMM. I had nothing better at the time anyway.
This has most likely happened to others too.

So, what can we do with this?

On this forum I have read that you CAN adjust the offset with only a multimeter.
Certain things have to be done first apparently. I believe this to be true. Have not tried it myself.

Since you are more experienced with electronics, may I suggest that you put a clear reply here
which leaves no amateur second guessing. Make it easy to find. I hope I did use the right keywords in this thread.

This will save a lot of repeated questions and you getting sick of them ...

for example: "How to measure and adjust DC offset of pin 6 5534 pre-amp to null"

A.) With a Multimeter
B.) With an Oscilloscope

I think you get what I mean.

I'm grabbing a beer now ... Prost!