Topics

P6137 scope probe repair


Jared Cabot
 

Hi all,

I recently picked up a P6137 scope probe for a song and thought I'd try my luck.
Predictably, the center conductor on the cable is open circuit..... Is there any way to repair or replace this cable with a new one? I have it apart now, but is there some sort of cable I can use as a replacement?


Thanks!
Jared.


Tom Lee
 

The cable is replaceable (I think you've seen this already, since you've disassembled it). The hard part is finding a good cable at a reasonable cost.

Does the cable show any obvious signs of damage?

If the break is at or near one of the ends, that's fixable. If it's somewhere near the middle, it's probably a lost cause, if you want to maintain a clean response.

If there are no visible cues as to where the break is, you've got a little extra work to do. If you have access to a TDR, you can pinpoint the location of the break easily. In the absence of a TDR, you can still find the location with surprising accuracy from two good capacitance measurements: Suppose the total cable length is L, and one segment is of length x1 and the other is x2. If the capacitance looking into x1 is C1, and that into x2 is C2, then x1 = L [C1/(C1 + C2)]. If you're lucky, you'll find that one of the capacitances is very much smaller than the other, indicating that the break is indeed at or near the ends.

All this assumes, of course, that you have definitely isolated the cable in determining that the cable, and not something in the compensator box or connections to it, is truly broken.

Good luck!

-- Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/16/2021 00:48, Jared Cabot via groups.io wrote:
Hi all,

I recently picked up a P6137 scope probe for a song and thought I'd try my luck.
Predictably, the center conductor on the cable is open circuit..... Is there any way to repair or replace this cable with a new one? I have it apart now, but is there some sort of cable I can use as a replacement?


Thanks!
Jared.




Jared Cabot
 

I have a small TDR unit, but it i mainly for longer runs, I'll give it a go though. I also have a 4276A LCZ meter I can use though.

Using the 4276A, I get 3.3pF at the scope end, and the probe end is not really able to be read properly, the 4276A freaks out a bit but shows 0.02nF, and my 34461A goes out of range.
I remember reading that oscilloscope probe cables are deliberately resistive or something, maybe that could be causing the weird reading. If that is the case, I would assume that the break is near the scope end of the cable as there isn't enough of the cable there to throw readings off.
It would make sense too, if the cable was pulled while attached to the scope, that is where the strain would likely be concentrated.

Sound like a reasonable assumption?


Lawrance A. Schneider
 

On Sat, Jan 16, 2021 at 03:48 AM, Jared Cabot wrote:


Predictably, the center conductor on the cable is open circuit.
I believe you could use a nanoVNA to locate the break.

larry


Jean-Paul
 

Hello again

Doubt if TDR or bridge gives any useful information

Just look carefully and try to flex every inch, you can find a mechanical or visible flaw, often near the ends.

It is easier to buy some new ones than to find the cable.

Bought many over the years via epay about $35-70 ea, mostly in liked newsletter condition.

Bon courage

Jon


Roy Thistle
 

On Sat, Jan 16, 2021 at 04:07 AM, Jared Cabot wrote:


probe cables are deliberately resistive or something
The "coax" is not normal 50 ohm coax... on high performance/high frequency probes.
It is usually a kind of "lossy" coax, which uses a shaped mono-filament resistance wire for the center conductor. That wire is somewhat brittle (compared to copper)... and shatters or cracks... when bent, mutilated, or stapled.
Usually if you buy a high performance probe for a song...without testing it... the song you might end up singing is from the blues.
Clueless sellers tie them up like the ribbons or bows on a Christmas presents... often using twist ties.


Dave Wise
 

?To my knowledge, the only Tek probe that uses standard coax instead of lossy resistance wire coax is P510 sold with the 535 in 1953. Rings like crazy but beyond vertical amp cutoff. The next model, P410 for the 545 and type K plugin, is lossy, and AFAIK every HF probe since.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Roy Thistle via groups.io <roy.thistle=mail.utoronto.ca@groups.io>
Sent: Saturday, January 16, 2021 7:43 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] P6137 scope probe repair

On Sat, Jan 16, 2021 at 04:07 AM, Jared Cabot wrote:


probe cables are deliberately resistive or something
The "coax" is not normal 50 ohm coax... on high performance/high frequency probes.
It is usually a kind of "lossy" coax, which uses a shaped mono-filament resistance wire for the center conductor. That wire is somewhat brittle (compared to copper)... and shatters or cracks... when bent, mutilated, or stapled.
Usually if you buy a high performance probe for a song...without testing it... the song you might end up singing is from the blues.
Clueless sellers tie them up like the ribbons or bows on a Christmas presents... often using twist ties.


Tom Lee
 

Hi Jared,

You've interpreted those measurements the same way I would. That doesn't mean that they're right, but in the worst case you at least have a comrade in wrong. The 20pF reading you got from the probe end is probably not quite quantitatively reliable, but we'll take it as "much bigger capacitance than 3.3pF", which is all we really need. Your break is indeed at the scope end, so that's good news.

The low-frequency capacitance per length of scope cable is typically (and very roughly) around 100pF/m, so a 3.3pF reading implies that the break is somewhere within about an inch of the scope end, give or take. You should be able to fix it without having to cut off more than about that much cable, which should not materially degrade the probe's response.

--Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/16/2021 04:07, Jared Cabot via groups.io wrote:
I have a small TDR unit, but it i mainly for longer runs, I'll give it a go though. I also have a 4276A LCZ meter I can use though.

Using the 4276A, I get 3.3pF at the scope end, and the probe end is not really able to be read properly, the 4276A freaks out a bit but shows 0.02nF, and my 34461A goes out of range.
I remember reading that oscilloscope probe cables are deliberately resistive or something, maybe that could be causing the weird reading. If that is the case, I would assume that the break is near the scope end of the cable as there isn't enough of the cable there to throw readings off.
It would make sense too, if the cable was pulled while attached to the scope, that is where the strain would likely be concentrated.

Sound like a reasonable assumption?




Tom Lee
 

Your doubts, fortunately, are misplaced. As a former boss of mine used to say, "An ounce of data trumps a pound of opinion." I have ounces of data.

Probe cable is lossy by design, to be sure, but that does not preclude good enough measurements to locate (approximately) a break in scope cable. It's a lossy structure, but so what? Good bridges will successfully measure the capacitance of a lossy capacitor. Within limits that vary from instrument to instrument, they'll give you the dissipation factor or Q, too. TDRs will show a small, but discernible bump of a discontinuous lossy line. Remember: We don't need to make an NIST-traceable measurement of the impedance per se. We're just trying to do well enough to find the location of a discontinuity. Don't conflate the two tasks.

Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/16/2021 05:37, Jean-Paul wrote:
Hello again

Doubt if TDR or bridge gives any useful information

Just look carefully and try to flex every inch, you can find a mechanical or visible flaw, often near the ends.

It is easier to buy some new ones than to find the cable.

Bought many over the years via epay about $35-70 ea, mostly in liked newsletter condition.

Bon courage

Jon





Lawrance A. Schneider
 

On Sat, Jan 16, 2021 at 10:43 AM, Roy Thistle wrote:


The "coax" is not normal 50 ohm coax... on high performance/high frequency
probes.
It is usually a kind of "lossy" coax, which uses a shaped mono-filament
resistance wire for the center conductor. That wire is somewhat brittle
(compared to copper)... and shatters or cracks... when bent, mutilated, or
stapled.
After I wrote my missive about finding the break with a nanoVNA, I thought about the problem further. Last year, I started to explore why Tek Probes use t-coils - boy oh boy was that a fun project. And I then remembered the above. Why am I bothering to note this. I discovered the Probe is 'tuned' within and shortening the lead and reconnecting would most likely result in a semi-worthless probe. I believe Alan Wolke has a video about the wire within a probe. The wire looks like a mess.

larry


Tom Lee
 

Lawrance and Roy, and a couple of others:

//Warning: Rant mode on. You'll want to power up your shields, 'cause photon torpedoes are headed your way.

What the hell's wrong with helping Jared *fix *his probe? Posts with vague or even explicit suggestions that his endeavor is doomed to fail are unhelpful, and technically just plain wrong. Adding irrelevant and sometimes wrong trivia, though perhaps interesting, just adds noise and distortion to the discussion. Here's my nth attempt at raising the SNR and actually point Jared to a solution.

It's been previously noted -- multiple times -- that probes use special resistance wire. If you've studied transmission lines at all, you will know that lines need to be terminated in their characteristic impedance in order to prevent reflections.

Probes are magic because they allow one to look at nodes whose impedances are uncontrolled, somehow without producing artifacts due to the inevitable reflections that must be occurring. How is this even possible? John Kobbe and Bill Polits had to solve this problem, or else Tek's efforts at making high-bandwith scopes would be the equivalent of inventing the phonograph without also inventing phonograph records. Kobbe and Polits understood that preventing reflections from misterminations is impossible. But they also understood that resistance is good at providing damping. In a 10:1 probe, you have a large attenuation anyway, so adding damping is relatively non-intrusive. Kobbe stripped some ordinary coax, connected resistance wire to the center conductor. As he pulled out the original lossless conductor, it pulled in the resistance wire. They tried it, and it worked. Thus was born the high-bandwidth probe. See US patent 2,883,619 (1959).

If you read anything at all of what I wrote to help Jared, you'll note that I made explicit mention of how his chances are a function of where the break is, the correct amount of damping being a function of length, among other factors. But contrary to what you implied, it's not a hyper-sensitive function of length. If you chop off an inch from the end of a 3ft cable, that 3% length reduction isn't abruptly going to trash the probe. Why would it? Suggesting that shortening would "most likely produce a semi-worthless probe" is a prime example of one of my pet peeves: Asserting an opinion without technical foundation (I'm guilty of it myself, but at least I'm aware that I have this problem). Worse, it's completely wrong. It only adds noise and distortion to the discussion. The "tuning" you speak of is not a sharp function at all. You can remove a small bit, and the effect will be a small bit. Think logically: Do you really believe that removing a nanometer instantly trashes the probe? Ok, how about a micron? A millimeter? When you acknowledge that maybe the deterioration is an analog, not binary, function of length, you will have made a cognitive leap forward. And if you have no clue at which point the amount removed is too much, that's a sign that you ought to keep your fingers away from the keyboard. The resistance per inch of probe conductor is typically  under an ohm. Jared has narrowed the general region of the break to about an inch from one end. If you think the removal of an ohm's worth of loss out of a 100 or 200 ohm total is going to produce a semi-worthless probe, show me your calculations or measurements that support such an assertion.

Better yet, how about not asserting unfounded discouraging things at all, and just let Jared try to fix his probe, instead of telling him he's going to fail? Or to "go buy another probe", as some Mr. Obvious suggested (do you really think that Jared is so thick-headed that he needs someone to tell him that)?

Next, your trivia about T-coils makes it sound like many probes use them. As far as I am aware, the total number is two: P6047 and the oddball 1kilohm P6048. If you are aware of any others, I would very much like to know their model numbers so that I can update my spreadsheet. T-coils are very much the /exception/, rather than the rule, in probes.

//Rant mode off. Photon torpedoes reset. For now.




--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/17/2021 06:29, Lawrance A. Schneider wrote:
On Sat, Jan 16, 2021 at 10:43 AM, Roy Thistle wrote:

The "coax" is not normal 50 ohm coax... on high performance/high frequency
probes.
It is usually a kind of "lossy" coax, which uses a shaped mono-filament
resistance wire for the center conductor. That wire is somewhat brittle
(compared to copper)... and shatters or cracks... when bent, mutilated, or
stapled.
After I wrote my missive about finding the break with a nanoVNA, I thought about the problem further. Last year, I started to explore why Tek Probes use t-coils - boy oh boy was that a fun project. And I then remembered the above. Why am I bothering to note this. I discovered the Probe is 'tuned' within and shortening the lead and reconnecting would most likely result in a semi-worthless probe. I believe Alan Wolke has a video about the wire within a probe. The wire looks like a mess.

larry




Roy Thistle
 

On Sun, Jan 17, 2021 at 01:11 PM, Tom Lee wrote:


What the hell's wrong with helping Jared *fix *his probe?
Well since no one that posted... as far as I can tell... explicitly said, "Jared, don't try" ... it's your claim that some people that did post are implying it. Is there data I missed to confirm that?


Tom Lee
 

Read the posts, Roy.

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/17/2021 13:44, Roy Thistle wrote:
On Sun, Jan 17, 2021 at 01:11 PM, Tom Lee wrote:

What the hell's wrong with helping Jared *fix *his probe?
Well since no one that posted... as far as I can tell... explicitly said, "Jared, don't try" ... it's your claim that some people that did post are implying it. Is there data I missed to confirm that?




Roy Thistle
 

On Sun, Jan 17, 2021 at 01:11 PM, Tom Lee wrote:


attempt at raising the SNR
I can't say with confidence that "rants" raise the SNR... perhaps they do... but, I'd need data.
In any case... it might be the case, one man's noise is another man's music... but, I don't have data on that.


Tom Lee
 

Simple, Roy. If your post isn't helping Jared fix his probe, it's noise. Your constant irrelevant comments and meta-comments are prime examples of raising the noise floor.

As did Chuck, I am now pretty much putting you on my ignore list. My rant contains specific actionable information to help Jared. If you don't see it, I suggest that the problem lies with you.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/17/2021 13:54, Roy Thistle wrote:
On Sun, Jan 17, 2021 at 01:11 PM, Tom Lee wrote:

attempt at raising the SNR
I can't say with confidence that "rants" raise the SNR... perhaps they do... but, I'd need data.
In any case... it might be the case, one man's noise is another man's music... but, I don't have data on that.




John Gord
 

Tom,
The p6131 2m and 1.3m probes used a T-coil in the compensation box. (Schematic in manual 070-4210-01)
--John Gord

On Sun, Jan 17, 2021 at 01:11 PM, Tom Lee wrote:
Next, your trivia about T-coils makes it sound like many probes use
them. As far as I am aware, the total number is two: P6047 and the
oddball 1kilohm P6048. If you are aware of any others, I would very much
like to know their model numbers so that I can update my spreadsheet.
T-coils are very much the /exception/, rather than the rule, in probes.


Tom Lee
 

Thank you for that information, John! I deeply appreciate it. Are you aware of any other examples?

—Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Jan 17, 2021, at 3:03 PM, John Gord via groups.io <johngord=verizon.net@groups.io> wrote:

Tom,
The p6131 2m and 1.3m probes used a T-coil in the compensation box. (Schematic in manual 070-4210-01)
--John Gord

On Sun, Jan 17, 2021 at 01:11 PM, Tom Lee wrote:
Next, your trivia about T-coils makes it sound like many probes use
them. As far as I am aware, the total number is two: P6047 and the
oddball 1kilohm P6048. If you are aware of any others, I would very much
like to know their model numbers so that I can update my spreadsheet.
T-coils are very much the /exception/, rather than the rule, in probes.




Tom Lee
 

Hi John,

I see that the 070-4210-01 manual isn't on tekwiki, and after an exhaustive 5 second google search, I can't seem to find one on the web. Do you know of an online source for it? (I'll keep googling, but being the lazy SOB that I am, I thought I'd ask you first.)

And if it isn't online, could I perhaps prevail upon you to scan it (even crudely) and upload it to tekwiki? My database is limited largely to what I've been able to get from online sources, plus whatever reverse engineering I've done, which means that my database is pretty complete until I get to the era when Tek began to reconsider their documentation philosophy. The P6131 is right at the 3dB corner...

Thanks again!

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/17/2021 15:08, Tom Lee wrote:
Thank you for that information, John! I deeply appreciate it. Are you aware of any other examples?

—Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Jan 17, 2021, at 3:03 PM, John Gord via groups.io <johngord=verizon.net@groups.io> wrote:

Tom,
The p6131 2m and 1.3m probes used a T-coil in the compensation box. (Schematic in manual 070-4210-01)
--John Gord

On Sun, Jan 17, 2021 at 01:11 PM, Tom Lee wrote:
Next, your trivia about T-coils makes it sound like many probes use
them. As far as I am aware, the total number is two: P6047 and the
oddball 1kilohm P6048. If you are aware of any others, I would very much
like to know their model numbers so that I can update my spreadsheet.
T-coils are very much the /exception/, rather than the rule, in probes.




Roy Thistle
 

On Sun, Jan 17, 2021 at 02:00 PM, Tom Lee wrote:


I am now pretty much putting you on my ignore list.
Yes, very good.


Jared Cabot
 

Well inamongst all the shouting, I found the problem.
It seems the cable was pulled and the center conductor has retracted into the inner insulation, pulling away from the end-cap at the scope end.

Unfortunately I have no way to satisfactorily uncrimp and them recrimp the end connector to enable me to cut half an inch off the end of the cable to reterminate it, so I'll be returning it to the seller as faulty for a refund.

Thanks all for the info and help! :)