P6563A Tek probe repair?

What exactly are you measuring? If you are measuring in the X1 mode from the tip to the center conductor of the BNC 390 ohms might be right. My elcheapo probe measures 250 ohms.

Are you really measuring from tip to shield?

What is the symptom? Mikek

cope probe? The problem is the signal line has only a 390 ohm resistance to the shield as measured with a DMM.

Hmm...you are right, I was measuring center conductor to shield. Measuring tip to shield gives about 9M (pretty darn close to the 9.5M it says on the probe).

The problem is if I remove the shield (the part that's springy and spins ar ound) and just connect the center conductor pin to the scope, it works: I s ee a square wave riding on a 60Hz sinusoid (since there's no shield connect ion). The probe tip is on the square wave cal output of my scope (5V @ 1kHz ).

As soon as I put the springy/spinny part back on (I guess Tek calls this th e "BNC Shell") my scope reads 0V. This is what lead me to believe there was some kind of internal short between the center conductor and the shield.

If you are measuring tip to shield, that means you have short between the center wire and the cable shield. See terms defined below. However, I don't think you are measuring tip to shield.

The probe tip is on the square wave cal output of my scope (5V @ 1kHz).

I think we need to define terms. >Shield< on a scope probe usually means the outer conductor of the cable which is a coaxial braid around the center wire.

The BNC connector is the part that makes a 1/2 turn to connect to the scope.

On the other end (the tip that you place on a test point) there is an accessory called the >Retractable hook tip< it slips on the end of the probe.

Sense you are using the scope internal calibrator, I would connect it and start wiggling the bnc end and see if the display corrects. If not wiggle the end connected to the calibrator point.

It seems like you have two problems, an open shield at one end or the other, and your Retractable hook tip is not making the connection to the tiny point sticking out of the end of your probe. It may be as simple as pushing it on just a little further.

Keep going, we will get on the same line of thinking and solve the problem. Mikek

OK, sorry for top-posting but a picture is worth a thousand words. BTW, thanks for helping with this.

Here is what's going on:

1. Just a shot of the inside of the probe

1. No BNC connector, not probing anything: we're just getting noise

2. No BNC connector, the tip touched to the shield

1. No BNC connector, the ground clip (next to the tip) touched to the shield

2. No BNC connector, the tip touched to the square wave output. Here is what leads me to believe that the tip-to-center-conductor path is OK as it's making it all the way to the scope.

1. But...as soon as I connect the tip AND the ground clip to the square wave output + earth, the signal goes to 0V.

2. Now the BNC connector goes on, the probe is not probing anything, and we have a solid 0V (no noise)

1. The BNC connector is still on, the probe tip is on the square wave output: still 0V

2. The BNC connector is still on, the probe tip and ground clip are on the square wave output+earth: still 0V

1. Just for illustrati> >

In a case like this I would suspect the cable itself. Over the years they get bent around all over the place, and maybe worse, like being rolled over by a scopemobile or something similr with three hundred pounds of equipment on it.

Ok, I'm not familiar with that probe, But when you put the BNC connector back on, do you measure low ohms from the BNC to the other end of the compensation housing? Since you have it apart can you measure the cable shield from end to end? Is it continuous? Do you think the cable shield makes a good connection to the compensation housing when you put it together?

You have a funny wire making your ground in the next to the last picture, what's that about? Can you post a picture of that end of your probe? Why not the normal wire with an alligator clip?

I don't think I've helped yet, but gave you some items to check.

Mikek

Not really. I've got 5 interesting points to measure:

A: The probe tip, the thing you generally touch to points in a circuit B: The probe ground clip, the wire next to the probe tip C: The BNC center conductor, makes the connection to the scope D: The BNC shield around the center conductor E: The compensation housing

Now I believe B/D/E should really all be shorted to each other. This is confirmed with a DMM.

Now, for the other measurements:

A-B: 9Mohms A-C: 9Mohms A-D: 9Mohms A-E: 9Mohms

B-C: 390 ohms

C-D: 390 ohms C-E: 390 ohms

Yes, just about 0 ohms

Yes...see above measurements.

That's the way the probe came (used...wish it had the retractable hood but it doesn't).

Picture is here:

I take what I can get! The probe came free with the scope (and to be fair, the seller told me the probe didn't work).

At least it helps to have someone to bounce ideas off of. This is really starting to stump me.

While this might go a bit deeper than you are ready for but it is a good reference to have.

After that, I think the comp board is shorted. That can happen if it gts hit with too much voltage too fast. Any chance of that having happened to it ?

Reading through your measurements, I don't notice anything wrong. However, in your latest picture you don't have the BNC connector on which completes the ground to the scope. I noticed in your older pictures sometimes the BNC was on and sometimes it was removed. It should always be on, because it completes ground the from that funny wire near tip to the scope. How about trying this. Put the BNC back on. Now measure from that funny wire at the tip (the ground), to the case of the scope with a DVM. It should be 0 ohms. Probably best to use the ground on the BNC for channel 2 to get a solid ground. (No paint, not plastic, you know it is ground) Mikek

I guess I want to know B-D

B: The probe ground clip, the wire next to the probe tip D: The BNC shield around the center conductor (this part connects to scope) Mikek

The latest picture was just to show you a close-up of the probe tip.

But that's the point...I was demonstrating that I can get a signal from the probe tip all the way through to the scope when the BNC was OFF. But when I put the BNC on, all I get is a 0V reading.

It is 0 ohms (yes, that is the B-D measurement). For good measure, I measured resistance from the probe tip to the scope case and it is 9Mohms.

Oh!

Couple more thoughts,

Sense you have two probes, can you just swap the attenuation network and see if the solves the symptom?

Mark your parts so you get them back to the same place they started.

And a longshot; Does your BNC have the pin that resets the scale by a factor of X10? I noted on your scope display-- Probe Function 20.000X Is something strange happening in that system?

Yes, this is getting frustrating.

Mikek

I suppose you've plugged it onto a bnc that's not connected to the scope??? Might be something as simple as a bad connection that shorts under torque.

That was a good idea....and it finally led me to figure this out (mostly). Yes, I did swap them and the attenuation networks both worked. The final th ing to investigate was the probe tip and cable that led to the attenuation network, and THAT was wonky. There was high impedance from probe tip to the center conductor on the other side.....which made perfect sense once I rea lized that the probe "tip" was actually intended to be the ground!!

This looks like some kind of a custom cable job. That weird-looking "ground clip" is actually the "probe tip" and the sharp tip, which you would THIN K would be the probe tip, is actually ground. I think.

Probing in this way I get my square wave! But....it's very noisy, and only looks like a clean square wave when I apply 20 MHz bandwidth limiting. Furt hermore, the probe calibration procedure fails, and finally, I only get the right square wave amplitude (500mV) when I manually set the probe attenuat ion to 4X (even though it says 20X on the probe label).

SO....my best guess is that someone constructed a custom cable, or stole th e cable from another probe and matched it up with a different attenuation b ox leading to a franken-cable.

In any case, a good debugging exercise. Thank you all for the help.

Yes, I did swap them and the attenuation networks both worked.

The final thing to investigate was the probe tip and cable that led to the attenuation network,

and THAT was wonky. There was high impedance from probe tip to the center conductor

on the other side.....which made perfect sense once I realized that the probe "tip" was actually intended to be the ground!!

is actually the "probe tip" and the sharp tip, which you would THINK would be the probe tip, is actually ground. I think.

and only looks like a clean square wave when I apply 20 MHz bandwidth limiting.

Furthermore, the probe calibration procedure fails, and finally,

I only get the right square wave amplitude (500mV) when I manually set

the probe attenuation to 4X (even though it says 20X on the probe label).

from another probe and matched it up with a different attenuation box leading to a franken-cable.

Can you fix it back to normal? Mikek

If you have a x1 probe and it's not connect to the scope (or any other piece of test equipment) then the resistance should be near infinity.

Do you have the manual? It's available on the http:

This is the Tek stock number for the cable & probe body:

It is 20X, with the ID pin.