Ohm meter with variable test conditions?

[Raccoon1400]

Has anyone seen an ohm meter with variable test conditions? (Current/Voltage) I find this may be useful, sometimes my meter won't detect a certain fault because the test current is higher than the current that needs to pass under operation. For example, for testing RCA cables for use with phono.

This would be most useful for checking continuity in wires, traces, etc. It would be useful to be able to start at a higher test current and decrease it until the wire can't carry the current.

I'm wondering if anyone has seen something like this, or if anyone has any comments on how useful it could be if there was. If it doesn't exist, I may have to build one.

 

[Ttech]

What kind of fault are you hoping to detect? Voltage and current for Ohm function on most meters are already very low. Do you want to measure resistance(DC), or impedance(AC)?

 

[Hyperion]

What you describe sounds like a low resistance ohmmeter? - something that will measure tiny resistances?

You can make a very simple one of these out of just a few components. (and a standard multimeter).

 

[whoaru99]

What kind of fault are you hoping to detect?

Yes, more detail is necessary about the desired testing.

 

[Raccoon1400]

What's the test current of my meter compared to phono and line audio signals?

I'm hoping to detect continuity faults in traces, phono cables, etc. I'm pretty sure I've had phono cables that tested fine on the meter, but cut out with the phono signal.
Knowing the exact resistance isn't as important as seeing if the resistance jumps once the current decreases past a certain point. If one cable did this before the other, I'd know there was an issue.

Does this make any sense?

 

[Ttech]

What's the test current of my meter compared to phono and line audio signals?

Current is a function of the resistance(or impedance) and the voltage. A quick search online indicates that the average output voltage of a MM cartridge is 3mV. The voltage from your meter for the Ohm function is probably 1 to 2 volts, but the only way to know for sure is to measure it with another meter.

I'm hoping to detect continuity faults in traces, phono cables, etc. I'm pretty sure I've had phono cables that tested fine on the meter, but cut out with the phono signal.

I would suspect bad connectors or problems with your test procedure. For a basic continuity test you can't beat an analog meter. It's easy to detect small movements of the needle. A DMM may not have a fast enough response time to show you small variations in the reading. Also, are you flexing the cable while you test it? If there is a break or weak spot in it, it may not show while you are holding the cable still.

Knowing the exact resistance isn't as important as seeing if the resistance jumps once the current decreases past a certain point. If one cable did this before the other, I'd know there was an issue.

As I said above, current is a function of the resistance(or impedance) and the voltage. The resistance isn't going to change in a cable or circuit board trace unless there is a break in it. These are just conductors, not semiconductors. I think oxidation on connectors might explain your problems too.

Does this make any sense?

Not really. I get the feeling that you think the resistance is going to change because the current changes. It doesn't work that way, at least not for simple conductors like cables and circuit board traces.

 

[Raccoon1400]

Current is a function of the resistance(or impedance) and the voltage. A quick search online indicates that the average output voltage of a MM cartridge is 3mV. The voltage from your meter for the Ohm function is probably 1 to 2 volts, but the only way to know for sure is to measure it with another meter.

I'll try measuring the test conditions.

I would suspect bad connectors or problems with your test procedure. For a basic continuity test you can't beat an analog meter. It's easy to detect small movements of the needle. A DMM may not have a fast enough response time to show you small variations in the reading. Also, are you flexing the cable while you test it? If there is a break or weak spot in it, it may not show while you are holding the cable still.

Yes, I'll flex the cable.

As I said above, current is a function of the resistance(or impedance) and the voltage. The resistance isn't going to change in a cable or circuit board trace unless there is a break in it. These are just conductors, not semiconductors. I think oxidation on connectors might explain your problems too.

It is the case where there is a break that I'm interested in. Oxidation could be a problem too, and in that situation larger currents would pass, but there would be a threshold where it couldn't pass, right? I've also noticed phono switches can be harder to clean than line input switches because they have to pass a smaller current.

Not really. I get the feeling that you think the resistance is going to change because the current changes. It doesn't work that way, at least not for simple conductors like cables and circuit board traces.

Here's an example of a situation I saw. I was working on a small sherwood receiver, and the power supply for the outputs was dropping about 10V, then climbing back up, then dropping again. The cause was the mica was damaged, it was leaking voltage. The voltage would drop, it would drop below the point where the insulator breaks down. Then it would climb back up again to the point where the insulator breaks down again, and the voltage drops...
This example involves higher currents/voltages, but it seems like a situation where a device like I described would detect the fault when an ordinary meter wouldn't?

 

[Ttech]

Oxidation could be a problem too, and in that situation larger currents would pass, but there would be a threshold where it couldn't pass, right?

Wrong. Oxidation on a contact doesn't function as a transistor or diode with a threshold, it's just resistance. There may be a threshold caused by the sensitivity of your meter.

Here's an example of a situation I saw. I was working on a small sherwood receiver, and the power supply for the outputs was dropping about 10V, then climbing back up, then dropping again. The cause was the mica was damaged, it was leaking voltage. The voltage would drop, it would drop below the point where the insulator breaks down. Then it would climb back up again to the point where the insulator breaks down again, and the voltage drops...

This example involves transistors or other semiconductors that were shutting down because of excessive current or heat. This doesn't apply to simple conductors like resistors, cables or traces on circuit boards.

 

[petehall347]

for testing cables you might be better off with a megger

 

[Ttech]

A megger is used to measure extremely high resistances. I don't see how this applies to testing RCA interconnect cables.

From Wikipedia, the free encyclopedia

Megohmmeter (sometimes referred to as a megger) is a special type of ohmmeter used to measure the electrical resistance of insulators. Insulating components, for example cable jackets, must be tested for their insulation strength at the time of commissioning and as part of maintenance of high voltage electrical equipment and installations. For this purpose megohmmeters, which can provide high DC voltages (typically in ranges from 500 V to 2 kV) at specified current capacity, are used. Acceptable insulator resistance values are typically 1 to 10 megohms, depending on the standards referenced.

 

[petehall347]

yes it will test the breakdown of cables

 

[Ttech]

Yes, it will test the breakdown of the insulation of cables, especially high voltage cables, which could result in short circuits or electrocution.

 

[petehall347]

maybe just test the cable in a known working system then ..if it works it passes the test .if not throw into the scrap pile .. or get technical and get the scope switched on and fire some signals through the cables .

 

[ConradH]

I don't think such a tester would be very useful. There are some specially meters like the HP 4329A High Resistance Meter that allow you to select the voltage applied, but I've never needed that for audio. It could test capacitor leakage at high voltages, but a capacitor tester is a better tool for the job. There are also ground bond testers that measure very low resistance values at high currents, say up to 20 amps, and are adjustable, but again, not that useful for audio work. With classic CGRL (LCR) meters, signals are often adjustable, but not as low as you'd like. Finally, I've seen something similar to what you're talking about with cables. Connections carrying very low currents can indeed be subject to oxidation, and go open. The instant any "significant" voltage is applied, the connection is remade. It stays remade until the joint oxidizes again, making troubleshooting difficult. I've seen this on digital recording heads where somebody neglected to specify gold on gold connectors. Again, not much of an issue if connector pressure is good and the connectors are clean. OTOH, a lot of systems seem to sound better if you unplug everything and plug it back in. It's the same effect as the specification for a relay concerning minimum permissible load or similar terms.

 

[I LIKE MUSIC]

Actually the oxidation can have a voltage breakdown threshold and a non linear E I curve. That is it can cause rectification of the signal.

In the real world, those of us that have been in involved in high power RF systems know what happens when there is a bad connection located in a high RF field. All one has to do is look at the RF field with a spectrum analyzer. There will be many spurious frequencies be cause the bad connection is non linear, similar to a diode.

This issue comes up in the use of relays. Oxidation builds up on the switchable contacts and blocks the signal until it reaches a certain level. The can be seen in our hobby in the instance where there is no output from an amplifier until the volume control is turned up past a certain point and then there is sound and it may work for a few times until the oxidation builds up again. Right at the point that the voltage is just about at the break through point, the voltage and current curve is non linear. This effect may result in the generation of spurious signals.

In terms of any significant voltage, companies that manufacture relays for automotive use have done research in the the amount of voltage needed to punch through the oxidation encountered and have found values around 3 volts, depending on the thickness of the oxidation. Technically this is called the A-frittering voltage.

This puts the voltage of an Ohm meter into perspective per the OP. Again, this voltage and the current involved will depend on the material and the thickness of the oxidation.


Again, this may be something that is difficult to measure reliably outside of the lab. The mere act of testing a cable may make pass the test and yet it fails in use.



Back in the day, the telephone company had issues with oxidation build up in their mechanical switching networks resulting in diminished sound quality and loss of signal.

For reliable switching of small signals, relays with mercury wetted contacts may be used.

I'm pretty sure I've had phono cables that tested fine on the meter, but cut out with the phono signal.

Back to the OP's point, poor physical connections, poor solder connection, and oxidation can all result in a non linear contact patch. The amount of voltage "punch through" the oxidation will vary depending on conditions.

This is not something that is easy to measure outside of the lab. If oxidation is present, the mere act of making a connection may penetrate the oxidation and or physical movement may change the quality of the connection.

We have some rather low voltage signals in our hobby, MC phono cartridges come to mind. Some vintage audio equipment had input sensitivities of around 150 millivolts for full power and in regular operation might see signal levels much smaller that that.

In our hobby it is not unusual to encounter oxidized switches (call we all say "tape monitor"). The symptom for this is not only no signal or a reduced signal, at times it is a distorted signal because the oxidation on the contact patch causes it to be non linear.

Contact patch oxidation may be less of a problem with more modern equipment where line level signals may approach a volt or more during normal use.

The OP raises a good point. There are any number of threads on AK relating to RFI (radio frequency interference) and in some case contact patch oxidation is causing rectification and adding the the problem.

 

[Raccoon1400]

I think this might be worth experimenting with. If it proves not useful then I'll still end up learning a few things.

I'm thinking it might be useful for testing grounds too. So far I haven't had a useful test for many grounding issues.

Seems I would need a voltage source that is very accurate and can go very low, as well as a very sensitive current meter. Any ideas?

 

[petehall347]

testing grounds is pretty easy .just measure voltage between ground points . should be zero volts .

 

[I LIKE MUSIC]

A good contact for ground connections is important.

This first step in trying to measure very low resistances (good connections) is the use of a 4 wire Ohm meter, not your usual DMM. This is done to make sure that the connection that you just made is as good as you think it is.

Turntable grounding wires can suffer from oxidized connections. Even though the grounding wire is used to make sure that the grounds are at the same potential, the potential difference may be very small.

And something else that may complicate your testing. It you are using a test set up that uses direct current, the polarity of the test current may be important.

The fritting voltage for oxidized copper may be as low as 0.1 millivolts and up, depending on the thickness of the oxidation. When we get in to connections we must consider what other contaminants may be present.

At some of the levels were are into the signal levels of moving coil phono cartridges and even moving magnet phono cartridges.

The voltage that you use will depend on what you are testing. Some moving coil phono cartridges may have output voltages in the 0.2 millivolts range as just one example. You would want your testing voltage to be less than this in this case. Then you would need a sensitive way to measure current. If you used 0.1 millivolts for your test voltage the current would be around 500 micro amps once the connection was made guessing at common values for the resistance in the a common lash up. YMMV

You need to take care of all of the variables in the test lash up, so you know what is a good connection. Just the act of testing a cable may cause it to pass the test. Where is the oxidation located? Is it on the shell or center pin of the RCA connector. What effect does connecting the test lash up have. If the oxidation is in the wire connection, there may be flexing of the connection during the test setup and this may change the cable.

As I mentioned in a previous post, this may not be all that easy to do and achieve accurate and repeatable results outside of a lab.

And an anti static work station may not be a bad idea. Even a very small static charge may be above the A-fritting voltage.

I wish you luck with this if you decide to try it, but it may be more trouble than it is worth if you want to achieve meaningful results.

 

[ConradH]

Armchair rambling, but if the connection is bad, it's going to be obviously bad, so long as you don't subject it to any significant voltage. Good point about ESD above. Anyway, if you made an AC measurement with some very low voltage, say by applying 10 uVpp, and used a tuned detector, it would be very obvious whether 10 uV appeared on the other side of the connection, or if it were open. Narrow band measurements eliminate all sorts of otherwise difficult issues. No idea about the best frequency but it's probably not critical. Maybe a simple 1 kHz oscillator and a voltage divider to get down to the needed level. There's all manner of tuned detector circuits around.

 

[petehall347]

have just realised i have a seaward oscar 250 here its a test meter i forgot i had . it was made for british telecom . i also think these are used in sound reinforcement set ups for testing the wiring .
it has 2 send settings . 800hz and 1600hz .