Oscilloscope probes

[amateur_24]

Hi
I brought a 2nd hand oscilloscope without probes. I want to know how do I know what probes are appropriate?

From spec Tektronix 2215A .
Resistence 1Mohm
Capacitance 20pF

 

[Reloadron]

Your scope has an input impedance of 1 M Ω shunted by 20 pF which is pretty standard. The scope also has an upper bandwidth of 60 MHz. Just about any 10:1 probes out there should work fine as long as they meet the upper bandwidth limits of your scope. Standard Tektronix probes for that scope can run upwards of $100 USD or more and you really don't need something that good. Just shop for probes designed to work with your 1 MΩ input that give a desirable upper frequency limit.

All assuming you just want basic 10:1 passive scope probes for your applications.

Ron

 

[amateur_24]

Reloadron the 2215A would do fine for a beginner who just very interested in electronics but is still learning. BY 10:1 you mean the ones that have either 10K/1K attenuation?

 

[Dean Huster]

Oscilloscope Probe Selection and Use
by Dean Huster


Choosing oscilloscope probes is not necessarily an easy task. There are many parameters to consider: What kind of input connectors are on the oscilloscope? What is the scope bandwidth and/or risetime? What is the input resistance of the scope? What is the input capacitance of the scope? Do I need any probe attenuation? What frequencies am I going to measure? What is the impedance of the circuit under test?


System Bandwidth

If you have a scope with a 50 MHz bandwidth, you don't necessarily want to choose a probe with a 50 MHz bandwidth. Anytime you add another instrument (a probe IS an instrument) into the signal path of the instrument used for the measurement, you affect the overall system bandwidth. In fact, you will ALWAYS lower your system bandwidth. Choosing the proper probe will minimize this reduction in bandwidth.

The risetime of a system is directly related to bandwidth. The standard equation for this is:

rt = 350/BW,

where risetime (rt) is in nanoseconds and bandwidth (BW) is in megahertz

If you have a175 MHz oscilloscope mainframe with a risetime of 2ns and a 175 MHz plug-in with a risetime of 2ns, the system risetime (input, through the preamplifier plug-in, through the mainframe circuitry to CRT) will be the square root of the sum of the squares of the individual risetimes. So the square root of (22 + 22) = the square root of 8 or 2.8ns. A 2.8ns risetime corresponds to a 125 MHz bandwidth. This means that a 175 MHz plug-in installed in a 175 MHz mainframe will not have a system bandwidth of 175 MHz, but will be 350/2.8 or 125 MHz! If you add a probe, it gets even worse.

Let's assume you have a 100 MHz mainframe, a 250 MHz plug-in and a 200 MHz probe. The risetimes of those three will be 3.5ns, 1.4ns and 1.75ns respectively. This calculates to a system risetime of the square root of 3.5² + 1.4² + 1.75² or 4.15ns which calculates to a bandwidth of about 84MHz. Although the lowest bandwidth element of the system is the 100 MHz mainframe, the overall bandwidth is even worse at 84 MHz.

If you're using a portable scope with a 50 MHz bandwidth (you don't have to worry about a plug-in with a portable), you'll have a system bandwidth of 35 MHz with a 50 MHz probe, 45 MHz with a 100 MHz probe and 48.5 MHz with a 200 MHz probe. If you can outfit your scope with a probe with a bandwidth of at least twice the scope bandwidth, you'll be doing pretty good with system bandwidth.

Don't forget that the better brands of scopes (Tektronix, Hewlett-Packard/Agilent) are pretty lax in their bandwidth specifications. A 100 MHz Tektronix 465 can often have an actual bandwidth of around 130 MHz. If it actually is 130 MHz, a 200 MHz probe will give you a system bandwidth of 109 MHz, better than the scope's catalog bandwidth specification!

For a "fast" system bandwidth calculation, the equation is "the inverse of the square root of the sums of the inverse bandwidths". This method eliminates the frequent bandwidth-risetime calculations. Here's how it works:

If you have a 500 MHz mainframe, a 250 MHz plug-in and a 350 MHz probe, here's the calculator steps for system bandwidth:

500 [1/X] [X²] + 250 [1/X] [X²] + 350 [1/X] [X²] = [1/X] [SQR X]

which should get you about 188 MHz.

The bottom line with bandwidth is this: get the highest-bandwidth probe that you can if you're interested in getting all the bandwidth possible from your oscilloscope system.


Probe Compensation

Yes, probe compensation is important. A feature of all attenuator probes (not non-attenuating 1x probes) is that they have a compensation adjustment to match the probe's attenuating capacitance to the input capacitance of the scope. A properly made adjustment will keep the system's frequency response flat throughout the system bandwidth. An improperly compensated probe will cause an erroneous fall in amplitude or increase in amplitude as the frequency is increased. This could cause a scope with a normal ±2% amplitude error specification to actually be in error by ±5, 10, 15 or 20%!

Poor compensation is often noticed with poor waveshape on square waves having a frequency in the 100 Hz to 2 kHz area. Unfortunately, most non-rectangular waveforms will not show any problems at all because only the amplitude is affected, not the waveshape.

The best way to compensate a probe is to simply use the probe compensation signal available on the front panel of every scope. I say "every" here because you don't have much of a worthwhile scope if it doesn't have that signal available. Most lab-grade scopes have it available in several amplitudes and at a fairly accurate frequency. It's usually a square wave of around 1 kHz and the probe is adjusted (see probe manual) for the "flattest-topped" square wave possible.

Selecting a new probe for your scope must include the determination that the range of capacitance to which it can compensate will include the input capacitance of your scope. Beware of older and lower-bandwidth scopes. They sometimes have higher input capacitances. I've seen as high as 33pF and 47pF and many probes, especially high-bandwidth probes in the 200- and 300 MHz area, will only compensate between 10pF and 30pF. 20pF has become pretty much the standard input capacitance for most oscilloscopes.

Vertical amplifier input resistance is also very important with regard to compensation. The probe is designed to compensate such that the capacitive reactance (Xc) of the probe to the Xc of the vertical amplifier is the same ratio as that of the attenuation resistance (9M ohms for a 10X probe) to the input resistance of the vertical amplifier. 1M ohm is the standard high-impedance input resistance of any industrial-grade scope made since about 1955 or so. There are some older service-grade scopes made for the TV industry that may have an input resistance that is different, such as 3.3M ohms or 4.7M ohms. Not only will it be difficult to compensate a modern probe to an older scope such as this, but the attenuation factor of the probe will be wrong as the probe depends upon the input resistance/reactance of the scope to be 1M ohm for accurate attenuation. If the scope's input resistance is higher than 1M ohm, the attenuation factor of a 10X probe will be much less than 10X.


Passive Attenuating vs. Non-Attenuating Probes

Non-attenuating probes are available as passive probes (relatively inexpensive) and as active FET probes (get ready to spend about a thousand bucks). The FET probe will give you very high bandwidth, low circuit loading, easy volts/DIV calculation, less money to spend on other equipment, a probe that is limited in maximum voltage that can be measured and a probe that is very easy to destroy when connected to the wrong circuit. From now on, we'll only be discussing passive probes.

Passive non-attenuating probes are much more robust over an active probe. The advantage of a non-attenuating passive probe over an attenuating probe is that it allows you to have the maximum vertical sensitivity available from the scope. If your scope can go all the way down to 2mV/DIV, a non-attenuating (1X) probe will provide you with that sensitivity at the probe tip. If you have a basic scope, another advantage is the fact that the attenuator setting is the sensitivity of the scope at the probe tip. There're no calculations involved, nothing to forget to do. A third advantage is that you don't ever have to remember to check the probe compensation. There's no adjustment for that. That's the end of the advantages list.

The disadvantages of the 1X passive probe is that it has the lowest bandwidth of all probes. It also puts the input impedance of the scope right at the end of the probe where it can load down high-impedance circuits and alter the actual voltage giving you a significant measurement error. The third disadvantage of a 1X passive probe is that it presents a very high capacitance and therefore a very low XC to the circuit under test, an even more significant loading factor when you're working with AC signals.

The highest-bandwidth 1X probe that I've ever worked with is the Tektronix P6101 in a 1-meter length. It has an 11 MHz bandwidth. Regardless, it still has horrible loading factors to the circuit under test and is useful only on lower-impedance, lower-frequency measurement situations.

An attenuator probe works by putting a 9M ohm resistor in series with the scope's 1M ohm input resistance. A capacitor is placed in parallel with this 9M ohm resistance to compensate for the fact that at high frequencies, the capacitance and Xc of the resistor will begin to alter its effective value and therefore the attenuation ratio of the probe.

Let's look at the scope input itself and see how it loads down a circuit. With a direct current input, all the signal will see is the 1M ohm resistance of the scope's input. The 20pF capacitance (a pretty standard input capacitance for most oscilloscopes made since about 1970) that shunts (is in parallel with) this resistance appears as an open circuit or infinite resistance/reactance.

With an AC signal of say 1 MHz, things begin to change. That same 20pF begins to have a reactance in parallel with that 1M ohm. This reactance will have a value of just a little under 8K ohms! At only 1 MHz, the scope will present a load of less than 8K ohms to the circuit under test. At 100 MHz, this reactance will be less than 80 ohms and at 500 MHz, it will be less than 16 ohms! That's a darned heavy load to any circuit, let alone a high-impedance circuit.

A typical 10X passive attenuator probe is basically a 9M ohm resistor in parallel with maybe 2.2pF of variable capacitance (the variable nature is the probe compensation adjustment). While the 9M ohms in series with the scope's 1M ohm provides the 10X attenuation factor (at a 5mV/DIV attenuator setting, the scopes actual sensitivity will be 50mV/DIV at the probe tip), the 2.2pF in series with the scopes 20pF input capacitance takes over to keep the attenuation ratio correct at higher frequencies. In fact, the Xc of the probe/scope combination "swamps" the (1M ohm/9M ohm) resistance combination. At 1 MHz, the Xc of the scope and probe is about 8K ohms and 72.3K ohms for a total of about 80K ohms instead of the scope's 8K ohms. That's a factor of 10 less circuit loading.

If you want even less loading, use a 100X probe for more like 800K ohms of loading or even a 1000X probe for more like 8M ohms load. By the way, a 1000X probe is NORMALLY used to measure high voltages rather than to lessen circuit loading, but the lower circuit loading can be used to an advantage in otherwise normal measurement situations of lower voltages. However, the typical 1000X probe costs three or four times what a 10X probe costs, is the size of an extra-large turkey baster and often requires a filling of Freon for its operation.

Also, don't forget that if you're using a 100X probe for lower circuit loading, the scope's 5mV/DIV attenuator setting now equals 0.5V/DIV at the probe tip; a 1000X probe means a 5V/DIV sensitivity at the probe tip. So there's a definite down-side to using attenuator probes, especially the 100X and 1000X models.

The 10X attenuator probes are still the handiest to use and that's why most scopes come with a pair as the supplied accessories.


Switchable Attenuation Probes

There are dual attenuation probes available from both Tektronix and as imports. The attenuation ratios are 1X and 10X, giving you the better parts of both worlds in one probe. The cost of the Tek version will take your breath away, but then, they're high-bandwidth probes. A switchable probe has one major disadvantage: the 1X position has horrible bandwidth compared to a dedicated 1X probe. For instance, the older Tek P6062B has something like 200 MHz bandwidth in the 10X position, but only 1 MHz at the 1X setting. Except for the convenience, you're really better off having two separate probes rather than a switchable model. You can buy two separate Tek probes for the cost of a Tek switchable.

Remember that you'll be using a 10X probe for most applications to reduce circuit loading, so the ability to immediately switch to a 1X attenuation ratio isn't that big of a deal as opposed to simply swapping out probes.


Probe Voltage Rating

There is a maximum voltage that a probe can handle. As the attenuation factor increases, typically the voltage rating increases. A 10X probe has a higher voltage rating than a 1X probe; a 100X probe can handle higher voltages than a 10X probe. The old P6006 10X probe could handle around 600v while the P6007 100X probe could handle more like 1,100v. The 1000X probes are designed for the tens of thousand volts, hence their large dimensions. Sometimes the attenuation selection is not for the actual attenuation factor or the loading factor, but for the voltage rating. It's just something to keep in mind.


CONTINUED IN THE NEXT POST BECAUSE OF LENGTH RESTRICTIONS

 

[Dean Huster]

SELECTING SCOPE PROBES -- PART 2:


Type of Input Connectors

Really old, low-bandwidth scopes had banana or binding post connectors for their vertical inputs. The earliest Tektronix triggered-sweep scopes used UHF connectors for their inputs and scope probes were available with UHF ends. In the 1960s, BNC connectors became the standard input connector while in the 1980s, SMA connectors began to appear on the really-high bandwidth models.

During the 1960s, probes such as the Tektronix P6006 10X model were available with either a UHF or a BNC connector. It was still a P6006, but the Tek part number changed. In fact, the Tek part number will spell out the probe type, the connector type and the cable length. Usually, the last two digits of the nine-digit part number is the cable length assignment. A P6101 1m probe may end in a -00 while one with a 2m cable ends in a -03 and one with a 3m cable ends in -05. The "inner" four digits remain the same for each cable length.

If you have an older scope with UHF connectors, you can simply use a modern BNC probe by inserting an adaptor between the probe and the scope. An adaptor with a UHF male on one end and a BNC female on the other is all that is needed. These are available from Pomona and for the past 20 or 30 years, inexpensive imports have been available for a fourth the cost and they'll work just fine for this application.

If you have an even older scope with banana or binding post inputs, you can use an adaptor with a male banana on one end and a female BNC on the other. Beware that an older scope like this may not have a 1MW input resistance!


Probe Encoding Rings

Tektronix scopes, in models designed since about 1972, have probe encoding rings concentric with the vertical BNC connectors. That includes all of the 7000-series vertical plug-ins and the "new" 400-series portables (e.g., 434, 465, 475, 485) and many of the newer portables.

If on the 10mv/DIV setting, the 434, 465 and 475 will normally have the knob skirt under 10m illuminated. Shorting the ring with the collar of the BNC with anything from 11K ohm and under will cause the lamp under the 100m be illuminated instead.

The 485 and the 7000-series goes a step further and a 100X probe will cause a lamp under the 1 to illuminate. With the 7000-series, it's the readout that's affected.

Each type of attenuator probe has a resistor built in connected between the probe encoding pin and the collar of the BNC (ground). My mind has lost the values other than the fact that it's 11K for the 10X probes, another value for 100X, another for 1000X. It's infuriating that I don't remember those values considering I made a little test jig for use in the Service Center for checking all possible encoding ring resistances. X1 non-attenuating probes have no probe encoding pin. Tektronix used to sell an accessory "spring" that slipped onto the BNC connector of a older 10X probe so that it would actuate the probe encoding circuitry of the simpler scopes.

Switchable probes change the value of this resistor from an open to 11K when in the 10X position. Some probes have a little pushbutton on the side of the probe head that shorts the ring to the BNC collar. On 7000-series, this causes the trace for that channel to jump maybe one minor division so you can tell which trace is associated with that probe. It's called "trace identification" and the button on the probe is often labeled "ID".

Some non-Tek imported probes have the probe encoding pin as well.


Probe Circuit Loading

The resistance of the scope and the scope probe are important. But they usually aren't the culprit when it comes to the circuit loading when connecting to an AC signal. It's the probe or system capacitance that will ruin your measurements.

Suppose you connect your scope directly to a circuit though a BNC-to-BNC cable with a set of EZ Hooks at the end. Your circuit test point will see 1 M ohm of resistance. It will see the capacitance of the scope and the cable in parallel. Let's suppose that you're using a six-foot length of coaxial cable with 50pF of capacitance per foot. That's (6)(50) in addition to the 20pF of scope capacitance – 320pF. At 1 MHz, 320pF is 497 ohms of capacitive reactance. That's like connecting a 497 ohm resistor from your test point to ground as far as the AC signal is concerned. If it's a circuit of any impedance at all, it's no wonder you have a small signal. Besides that, a heavy load may alter the operation of the circuit (e.g., feedback telling another part of the circuit that the output is low to the point that stages clip trying to bring the output back up), further messing up the waveform.

That's why you want to use an attenuator probe in most measurement situations. It reduces the capacitance by a magnitude, helping to isolate your scope from the circuit under test. A 10X scope probe may only present 2.2pF to the circuit and at that same 1 MHz, would be a reactance of around 72K ohms, much higher than that awful 497 ohms!

As the frequency of interest increases, things always get worse. In the first scenario, at 75 MHz, the loading would get worse, reactance dropping to an incredible 6.6 ohms! Even with the 10X probe, expect the loading XC to drop to around 965 ohms. That's better than 6.6 ohms, but 975 ohms can be quite a load on the circuit. It's something you have to watch out for.

Knowing the specifications of your scope and probe capacitances can help you determine if loading can be a problem in your test situation.


Probe Cable Length

If you need a longer probe cable, buy a probe with a longer cable. Probes have traditionally been available from Tektronix in 3-foot (1 meter), 6-foot (2 meter) and 9-foot (3 meter) cable length and I seem to remember a 12-foot P6006. They're available and usually long probes aren't required like they used to be when you were working on tall equipment racks using scopes rolling around on carts.

If you need a longer probe, DO NOT use a long BNC-to-BNC cable between the scope and the probe connector. You'll kill risetime. You'll kill bandwidth. You'll not be able to compensate the probe. Nothing will work correctly much above 1 kHz.


Probe Care and Storage

When I was in the U.S. Navy, it was common to have a Tektronix 545A sitting on a big scope cart with two probes attached to the scope. Most of the technicians used these probes in a secondary fashion as "tow ropes" to pull the scope and cart around the work area. Needless to say, probes didn't last long around there.

Other Navy techs and later, customers of mine when I worked for Tektronix, stored their probes by wrapping them into tight hanks (hank length equals the length of the probe head) with the ground lead used to wrap the assembly up tight, the alligator clip at the end pinched to the coax to hold things in place. Also not good.

First, never use a scope probe for anything but a probe. It's not a tow rope. It's not a cable used to connect 24vdc from your 5 amp power supply to your latest project.

Scope probes are more delicate than your average coaxial cable. For one, the center conductor of a high-end manufacturer's probes is a thin, single strand of wire – not stranded wire. It is very delicate and easily broken if the probe is stretched – hence, the "no-tow-rope" advice. The coax is also smaller in diameter with foam insulation so that it's more flexible and has a lower capacitance-per-foot. It's easily damaged and pinches, kinks or dents in this cable can affect its performance. Don't clip alligator clips to it or shut it in drawers and doors.

I feel there's only two ways to store a scope probe – or any type of test cable, for that matter. The best way is to hang them from cable racks mounted to the wall. Commercial ones are available from Pomona in three cable diameter sizes and there is at least one other manufacturer of them that I've seen, although I don't remember the name or if it was nothing more than a company satisfying a military contract. You can make your own cable racks from wood, although they won't be as robust as metal racks.

The second-best method is to loop the cables into six-inch circles and loosely secure them with pieces of #22 insulated solid-strand wire or twisty-ties and store them in drawers or on cabinet shelves.

Ozone is an enemy of rubber-based products, so I'd try to keep all of my cables (and any equipment using belts or rubber tires such as tape decks or VCRs) away from ozone-creating things which include laser printers, copiers and electronic air filters.


Using a probe on a frequency counter or other device input

Nothing says you can't use a scope probe on the input of a frequency counter. An X1 probe is easy. Just hook it up and use it.

However, a 10X probe is more of a problem. If the counter has a 1M ohm input resistance, you're almost there, for a 10X attenuator probe will attenuate by a factor of 10. However, you have no way of compensating the probe. The specifications for the counter might tell you what the counter's input capacitance is so that you will know if a probe can even be compensated to it. But doing the actual compensation would require you to open the counter and connect a scope to some point where there's a linear buffer amplifier between the input and your test point and look there for a good square wave with the counter's probe connected to a 1 kHz square wave. So, it can get "iffy". You could try using the counter near its maximum frequency or the maximum frequency of the probe and if it doesn't count, try adjusting the probe compensation until it does and then check it at the low end for good operation.

But you won't damage anything by trying to use a probe with a counter. If the counter has an input resistance other than 1 M ohm, the probe attenuation factor and compensation ability will be affected.


Using a probe on a signal generator to inject a signal

Here is where there can be some real problems. Every decent scope probe (I won't include cheap imported probes here because I don't know any different) does not have normal wire for the center conductor in the coaxial cable. This center conductor as previously mentioned is a single strand of wire. More importantly, this is a resistance wire, having anywhere from 5 to 100 ohms-per-foot of resistance, even in the X1 probes. This resistance wire design is to reduce the ringing at the edges of step waveforms. Any signal you try to send down this cable from a generator to a low-impedance circuit will be reduced since that resistive center conductor will be acting as part of a voltage divider. If you try sending down a signal of a greater amplitude than is normal into a low-impedance circuit, you could actually make this center conductor heat up, melt the coaxial insulation or even burn out.

The bottom line? Don't use a scope probe as the output probe on a signal generator. Just don't. That's what standard RG-58C/U coax is for. Or RG-172. Or whatever. Not scope probes.

Needless to say (so why is this being written?), using a scope probe as the output cable of a power supply is nothing more than pure lunacy. That high center conductor resistance will just burn up with any sort of load at all.




The Author
Dean Huster was a Communications Technician - Maintenance (CTM2) specializing in PMEL (Precision Measurement Electronics ) technician (test and measurement equipment repair and calibration) in the U.S. Navy from 1970 to 1976 and was a bench technician for Tektronix, Inc. at their Dallas and Oklahoma City Service Centers from 1976 to 1982. In 1982, he took a position as Electronics Instructor at the Francis Tuttle Vocational-Technical Center in Oklahoma City where he taught for 15 years. In 1996, he moved to the Poplar Bluff (MO) area where he taught electronics at the Technical Career Center for five years. During that time, be began as an adjunct Industrial Electricity and Print Reading instructor at Three Rivers Community College. He began writing the monthly "Q & A" column for Poptronics magazine in February 2001 until the publisher and magazine went out of business with the January 2003 issue. Electronics has been his hobby for about 50 years. He's getting old.

 

[Reloadron]

Reloadron the 2215A would do fine for a beginner who just very interested in electronics but is still learning. BY 10:1 you mean the ones that have either 10K/1K attenuation?

Uh, I know that scope would do fine, never said it wouldn't. BY 10:1 I mean the probe has a ten to one attenuation ratio.

Dean, thank you for again publishing your fine article on scope probes. Even to read it again it is informative and a wealth of information.

Ron

 

[amateur_24]

sorry I meant that as a question. My bad english lol. Must check my question before i post.

Wow I feel electronics is even more complicated then programming. I'm praying my oscilloscope arrives and works.
I know I'll feel like a kid on Christmas eve when it finally arrives. That sense of anticipation as to what might be his present this yr.

Thanks for the advice guys!

 

[Reloadron]

Merry Christmas...

Yeah, a scope is one of the coolest pieces of test equipment to add to any bench. Maybe because it allows the user to see things and understand them in a new light. Best of enjoyment using the new addition.

Ron

 

[leob]

Oscilloscope probe recommendation

Dean, I'm a retired electrical engineer just starting to get back into my field with a little home lab. I bought a Rigol DS1052E digital storage scope for $400 and can't believe that such a fine instrument can be purchase for that price. However, I would like to have better probes. I don't need to get into very high frequency stuff. My Rigol model is specified with a 50MHz bandwidth but is actually a 100MHz scope (google eevblog for info on this.) At any rate, I suspect my requirements will be satisfied with a quality 10:1 probe. But price is an issue and I suspect that with Tektronix I'll be paying a high penalty just for the name. Do you have any recommendations on this or info on the quality of the various probe manufacturers?
Leo Burke

 

[SABorn]

As for sourcing probes, go to ebay and run a search, you will be supprised how cheap they are. (often out of China)

I have bought several sets now and they work just as well as a brandname set.

Dont get too wrapped up in spending big dollars for a set of probes, untill you find the work you do requires high end probes.

To spend $100.00 on a probe for a hobbiest is plain stupid, and more than likely get damaged in time to come.

Most of the ones from China are rated to 100mhz or highet with a 10:1 switch on them.
(will work fine on any lower frequency)

For the price of them you cannot lose.

Pete.

 

[mdanh2002]

I order 2 100 Mhz probes on 11 December via eBay. It was sent by registered mail from China and has not arrived until today. The last status shows it at Guangzhou on 22 Dec, departing for Singapore. Until today, Singapore Post website cannot find the tracking number (which means the probes did not reach Singapore and were likely lost in transit). The seller (who seems reputable with 99.5% positive feedback) asks me to wait for a week more, claiming that postal mail, even registered mail, is slow during Christmas/New Year period. I guess I was unlucky.

 

[leob]

Thanks for the reply. I definitely don't want to spend $100 for a probe. Actually, the specs on the probes supplied with the Rigol look pretty good (for the 10X setting) and I'll probably stay with these (for a while, anyway). What really bugs me is that the grabbers pull off of the probes if the slightest bit of tension is applied. But if they perform well electrically it won't be worth spending big bucks to fix that.

 

[leob]

OOPS. I thought I was replying to you but hadn't noticed that a reply from mdanh2002 had come in. Please look there for my reply.