My simple portable DMM has a 50 Ohm range. It has a "calibrate" push button.
Push the button, and read resistors that are as low as about 10 milli Ohms.
Even the old VOMs had a zero adjustment, but most of them could not realistically accurately measure single digit Ohms.
Get a real 4 wire Ohmmeter if you want to do low resistance measurements.
And do not forget to send it in at least once a year for calibration.
Push the button, and read resistors that are as low as about 10 milli Ohms.
Even the old VOMs had a zero adjustment, but most of them could not realistically accurately measure single digit Ohms.
Get a real 4 wire Ohmmeter if you want to do low resistance measurements.
And do not forget to send it in at least once a year for calibration.
PRR,
Thanks.
So a DMM Ohmmeter 'Zero' is calibrated with a double banana plug short that is plugged into the DMM banana jacks.
Yes?
Of course.
In all my years doing measurements, I do not know how I never thought of that.
Now you got me thinking about DC Power Supply Remote Sensing.
And that reminds me of another measurement opportunity.
Decades ago, I was giving training at the Mexican Government owned Tel Mex phone company.
I was "out on the floor" showing 15 engineers a spectral measurement.
The measurement was of a microwave link that had an attenuated sample port.
The port attenuation was known. But the 2 identical coax cables I had were of unknown attenuation at that microwave frequency.
I made the power measurement with one coax, and then connected the two coaxes back to back, using a "bullet" back to back connector, and measured the power level again.
The difference in the two measurements was the coax attenuation (plus the extra connection of the bullet).
Now we could easily determine the sample port power level, without having to bring more test equipment out on the floor.
Not precision, but good enough to establish if the microwave power was at or near specification.
Thanks.
So a DMM Ohmmeter 'Zero' is calibrated with a double banana plug short that is plugged into the DMM banana jacks.
Yes?
Of course.
In all my years doing measurements, I do not know how I never thought of that.
Now you got me thinking about DC Power Supply Remote Sensing.
And that reminds me of another measurement opportunity.
Decades ago, I was giving training at the Mexican Government owned Tel Mex phone company.
I was "out on the floor" showing 15 engineers a spectral measurement.
The measurement was of a microwave link that had an attenuated sample port.
The port attenuation was known. But the 2 identical coax cables I had were of unknown attenuation at that microwave frequency.
I made the power measurement with one coax, and then connected the two coaxes back to back, using a "bullet" back to back connector, and measured the power level again.
The difference in the two measurements was the coax attenuation (plus the extra connection of the bullet).
Now we could easily determine the sample port power level, without having to bring more test equipment out on the floor.
Not precision, but good enough to establish if the microwave power was at or near specification.
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Wow, did not think I could spark that much conversation about a basic topic! 
What I think I am understanding of what PRR said is that 0.3ohms is an accurate “zero” considering the test leads themselves create resistance being as they’re about 4’ and that length of wire at that gauge is expected to create that much resistance anyway.
Did I pass?
I do not know when the last time this scope was calibrated nor do I know where I could take it to have it calibrated. At this point the only thing I would be using it for is my voltage checks on my amp, which I believe I am now ready to do...
I have seen a few guys on the other Clarion build post up their voltage readings, is this a fair place for me to start my testing?
Does anyone have any advise other than don’t have my children running around me and don’t die?
What I think I am understanding of what PRR said is that 0.3ohms is an accurate “zero” considering the test leads themselves create resistance being as they’re about 4’ and that length of wire at that gauge is expected to create that much resistance anyway.
Did I pass?
I do not know when the last time this scope was calibrated nor do I know where I could take it to have it calibrated. At this point the only thing I would be using it for is my voltage checks on my amp, which I believe I am now ready to do...
I have seen a few guys on the other Clarion build post up their voltage readings, is this a fair place for me to start my testing?
Does anyone have any advise other than don’t have my children running around me and don’t die?
Before I go ahead and power it up to check voltages, I wanted to verify that all of my ground points have a solid connection. I am somewhat concerned. Would the distance of the ground points through the aluminum chassis add much resistance as the distance between the test points grows?
I am using one of the attachments that came with the new test leads to clamp to the ground taps and another attachment that has a fine point pin to reach between the wires to reach the points I’d like to test. I noticed that just adding these attachments and checking my “zero” I end up with 0.6 to 0.7 ohms when touching the leads together.
Now, I go to test resistance between my ground points I am seeing numbers average around 30-45 ohms. Closest points were 7.0 ohms and farthest were 80ish Ohms. I am using all stainless steel hardware to fasten the terminal strips to the aluminum chassis.
I have been concerned about the ground points and how well they’re making contact through the fasteners and through the chassis paint. What is everyone’s thoughts on this? I would assume that there will be some resistance between the contact points, but I just never thought there would be that much.
I am using one of the attachments that came with the new test leads to clamp to the ground taps and another attachment that has a fine point pin to reach between the wires to reach the points I’d like to test. I noticed that just adding these attachments and checking my “zero” I end up with 0.6 to 0.7 ohms when touching the leads together.
Now, I go to test resistance between my ground points I am seeing numbers average around 30-45 ohms. Closest points were 7.0 ohms and farthest were 80ish Ohms. I am using all stainless steel hardware to fasten the terminal strips to the aluminum chassis.
I have been concerned about the ground points and how well they’re making contact through the fasteners and through the chassis paint. What is everyone’s thoughts on this? I would assume that there will be some resistance between the contact points, but I just never thought there would be that much.
There should only be one point where the signal ground touches the chassis. There should also be a point back where the power cord comes in that connects the chassis to mains earth. Those to points need to be connected by very low resistance.
Anodized aluminum isn't the best conductor. Generally you'd want to use toothed lock washers to pierce the anodized layer to make for good continuity.
If you've built something with terminal strips and you've treated every lug that touches the chassis as a signal ground, you've likely made some future noise problems to debug.
Anodized aluminum isn't the best conductor. Generally you'd want to use toothed lock washers to pierce the anodized layer to make for good continuity.
If you've built something with terminal strips and you've treated every lug that touches the chassis as a signal ground, you've likely made some future noise problems to debug.
Earth is the chassis. Where the power cable comes in, the ground on the power cable should be connected to the chassis in very close proximity.
For the rest of the circuit, the electrical/signal ground should only touch the chassis in one spot. I tend to prefer to have that spot by the first gain stage, but if you'd like to pick a different point and run all your ground connections to that point (star ground), that's OK too.
What you don't want to do is have a bunch of different parts connected to the chassis in random places. If you have shielded cables or internal tube shields, you may earth those shields to the chassis where it's convenient.
For the rest of the circuit, the electrical/signal ground should only touch the chassis in one spot. I tend to prefer to have that spot by the first gain stage, but if you'd like to pick a different point and run all your ground connections to that point (star ground), that's OK too.
What you don't want to do is have a bunch of different parts connected to the chassis in random places. If you have shielded cables or internal tube shields, you may earth those shields to the chassis where it's convenient.
Ok, that’s unfortunate, but hind sight is 20/20... then I used the center post of some/most terminal strips as grounds for my signal ground. Can I daisy chain a wire to each of these “signal” grounds to make a single point connection?
What is the difference between multiple signal grounds vs one single, and why is it necessary?
What is the difference between multiple signal grounds vs one single, and why is it necessary?
I’ve got plenty more, but desoldering and relocating those points will be the challenge for me....
It might be easier for me to cut the center post of the strip and solder the daisy chain ground wire...
I would like to understand the reading for having one single v.s. multiple grounds. Again, I just don’t have the knowledge/education.
It might be easier for me to cut the center post of the strip and solder the daisy chain ground wire...
I would like to understand the reading for having one single v.s. multiple grounds. Again, I just don’t have the knowledge/education.
Using the chassis as a conductor for the signal path will cause noise issues.
What I do to kind of cheat around this issue is to use a #4 shoulder washer between the chassis and the terminal strip with the shoulder facing up, then use a plastic acorn washer to secure the terminal strip.
The plastic shoulder and the plastic nut will isolate the terminal strip from the chassis, then you can add your ground wires.
What I do to kind of cheat around this issue is to use a #4 shoulder washer between the chassis and the terminal strip with the shoulder facing up, then use a plastic acorn washer to secure the terminal strip.
The plastic shoulder and the plastic nut will isolate the terminal strip from the chassis, then you can add your ground wires.
One problem with circuit boards it is hard to change the wiring.
Remember, you need to take care of 2 potential ground loops:
B+ ground loop.
Connect the power transformer B+ secondary center tap Directly to the first filter cap negative lead. Connect a wire from the first filter cap negative lead to the 2nd filter cap negative lead. That is a local ground loop. Then connect the 2nd filter cap lead of to your amplifier's central ground point.
Input circuit ground loop:
The RCA phono input jacks outer connection should be insulated from the chassis.
Connect from the phono jack outer connection 'ground' tab to the bottom of the input tube's cathode circuit; if I remember correctly, that is the bottom of the negative feedback sense resistor that connects to ground and to the self bias resistor and bypass cap (not the resistor from the output transformer secondary).
Connect the bottom of the volume pot and bottom of the input tube grid resistor to the bottom of the sense resistor.
Then, connect the bottom of the sense resistor to the amplifier's central ground point.
Controlling these two loops by connecting it this way before they connect to the central ground point does wonders to reduce hum.
Other circuitry connects directly to the central ground point:
The bottom of the output tube's self bias resistor and bypass cap
The bottom of the grid resistor of the output tube.
The chassis
The power mains ground where it comes in at the 3 connection IEC connector.
I think that covers all of it.
"Grounds Are Commonly Misunderstood" - Me
Remember, you need to take care of 2 potential ground loops:
B+ ground loop.
Connect the power transformer B+ secondary center tap Directly to the first filter cap negative lead. Connect a wire from the first filter cap negative lead to the 2nd filter cap negative lead. That is a local ground loop. Then connect the 2nd filter cap lead of to your amplifier's central ground point.
Input circuit ground loop:
The RCA phono input jacks outer connection should be insulated from the chassis.
Connect from the phono jack outer connection 'ground' tab to the bottom of the input tube's cathode circuit; if I remember correctly, that is the bottom of the negative feedback sense resistor that connects to ground and to the self bias resistor and bypass cap (not the resistor from the output transformer secondary).
Connect the bottom of the volume pot and bottom of the input tube grid resistor to the bottom of the sense resistor.
Then, connect the bottom of the sense resistor to the amplifier's central ground point.
Controlling these two loops by connecting it this way before they connect to the central ground point does wonders to reduce hum.
Other circuitry connects directly to the central ground point:
The bottom of the output tube's self bias resistor and bypass cap
The bottom of the grid resistor of the output tube.
The chassis
The power mains ground where it comes in at the 3 connection IEC connector.
I think that covers all of it.
"Grounds Are Commonly Misunderstood" - Me
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