I can’t compensate my probe - Page 1
Forum section: electronics / beginners

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 18, 2018, 11:49:05 pm
content: Hi everyone,I’m on my phone with Tapatalk so I’m having trouble uploading photos. I’ll provide a link to Imgur where the image is located.https://imgur.com/qmtz4qQ?My issue is that I cannot compensate my p6137 probes to my Tek 2465B scope. I’m thinking the probe capacitance is out of range of the scope input capacitance. The image shows the square wave at the squarest I can make it.Do you guys know what the capacitance range for the probes for this scope?If I’m out of range that’s fine, but I’ve heard so many of you say how theee are the probes used on this scope.I also have some p6156 50 ohm probes which work great. Basically I can compensate all my basic probes to this scope just not my p6137 probes.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 19, 2018, 12:42:47 am
content: The P6137 is designed for the 2400 series and the P6137 manual states 12pF to 18pF compensation range. Input capacitance for the 2465B is 15pF +/- 2pF so the probe compensation range is spot on for the scope. You may have to take the cover off the "compensation block" at the scope and adjust the LF compensation trimmer. The TEK documentation, link below, leaves a lot to be desired though.https://www.tek.com/manual/p6137-10x-passive-probe-2400-series-oscilloscopes

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 19, 2018, 02:38:23 am
content: Hey thanks for the reply. My p6137 probe states that it has 10.8pf of capacitance. I did take the cover off and adjust the other trim pots in the probe box per the manual to no avail.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 19, 2018, 05:39:42 am
content: I have noticed that if you use a probe beyond it's voltage rating for even the slightest moment, the compensation circuit will be damaged permanently. Maybe this is what happened?

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 19, 2018, 12:08:22 pm
content: Hmm if I’m reading the manual correctly, anything under 50vdc is safe for these probes at frequencies above 1MHz. I can safely say I’ve never probed anything above 12vdc with this probe. Do you think I have a bad compensation box? Should I replace the probe or attempt to repair it?

author: macboy
author_rank: Super Contributor
timestamp: October 19, 2018, 01:48:08 pm
content: Quote from: SteveRosenlund on October 19, 2018, 02:38:23 amThat is the capacitance from the tip to ground, which is not the same as the scope input capacitance specification.I hope that you noted exactly where those other trimmers had been set before you fiddled with them. If possible, put them back where they were, as it is non-trivial to adjust the mid- and high-frequency compensation properly (that's what those are). To properly trim them, you need a << 1 ns rise time square wave and follow the procedure in the manual. (A forum member,Leo Bodnarmakes and sells an incredibly fast < 40 ps rise time pulse generator at an affordable price, and it's perfect for doing this. There's a lengthy thread and a mention in a recent mailbag video).Did you try the other channels? This should help narrow the issue down to one bad input on the scope, or a bad probe. The tip containing the input divider RC network can be replaced (it unscrews from the body, it's #3 on page 10 of the manual from Tek.com) but a new one might cost more than a working used probe. Replacing components in the compensation box might be possible but is not guaranteed to yield proper response. If you suspect a bad probe, and have another good one, you can try swapping the tip and/or the compensation box (loosen the nut completely then just pull the cable out) to further narrow down the issue.

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2018, 04:49:02 pm
content: Something is broken. Does the probe compensate correctly on a different channel or a different oscilloscope?

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 19, 2018, 06:27:18 pm
content: Quote from: macboy on October 19, 2018, 01:48:08 pmThanks for the clarification on the capacitance. I won’t be confused again.Yes I marked and returned the pots to the original settings on the adjustments of the board.I have not tried other channels yet. I will do so when I get home from Dallas tonight. Good troubleshooting tip. I should have known that beforehand. I guess I was tired.Yeah I bet you’re right that replacement parts would be more expensive than a used working probe.I have severa other probes but that is my only p6137. I have 2 x working P6156 probes with the 10x tips that produce 50ohm square waves with tops flat as can be. I also have some other switchable Tek probes that compensate well on channel 1 at high impedance. It’s just my p6137 that won’t compensate. I will try channel 2 tonight then order new p6137 probes if it is broken as we expect.On a side note, I was reading the manual for the p6156. Are these things really rated up to 3.5ghz? That’s insane.David Hess, thanks for your input. I will check the probe in another scope.

author: David Hess
author_rank: Super Contributor
timestamp: October 20, 2018, 02:12:30 am
content: Quote from: SteveRosenlund on October 19, 2018, 06:27:18 pmThey sure are but a coaxial connection to the probe tip is required to take advantage.

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 20, 2018, 03:52:37 am
content: Ok so when I got home I tried the p6137 on channel 2 and another scope with the same result. I ordered a replacement from flea bay.Thanks again for the input. I’ll keep you posted.

author: james_s
author_rank: Super Contributor
timestamp: October 20, 2018, 04:46:13 am
content: Does the compensation adjustment do anything? It should have a very noticeable effect on the edges of a square wave, even if it is completely wrong for the scope.

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 20, 2018, 10:30:58 am
content: Quote from: james_s on October 20, 2018, 04:46:13 amYes. It goes from the image I posted to an even droopier left edge.

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 21, 2018, 02:33:03 pm
content: Ok I figured out how to get images here from my phone using imgur.This photo shows the best compensation I can achieve on my p6137 probe.This one shows the probe box with the additional adjustment ports opened up after my fiddling with things. No change.This photo shows the same probe on channel 2. No change.This photo shows the same probe on another scope.And this photo shows the same p6137 probe on the other scope but with the compensation screw turned all the way the other direction. In case you thought i didn't know how to compensate a probe.Here are a couple photos of different probes on my 2465B that compensate just fine.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2018, 05:35:21 pm
content: The other adjustments are for high frequency transient response and will have no visible effect on the relatively low frequency source used for the compensation adjustment. Adjusting them requires something like a tunnel diode pulser.

author: rf+tech
author_rank: Frequent Contributor
timestamp: October 24, 2018, 06:39:34 pm
content: Steve,Remove the probe tip hybrid. According tohttps://www.tek.com/accessories/oscilloscope-probes/replacement-parts, the tip hybrid should should have a black tip and natural tail insulator color. Also, measure the cable length to ensure it is 1.5 meters.Based on prior experience, there is a strong possibility someone has interchanged parts. The sum of the current components may no longer be as Tektronix originally shipped, creating this compensation problem.RF+ Tech

author: SteveRosenlund
author_rank: Regular Contributor
timestamp: October 26, 2018, 04:03:11 am
content: Quote from: rf+tech on October 24, 2018, 06:39:34 pmYou are a genius and I am an idiot. I got my replacement probe today and before reading your response figured out the same thing as the new probe compensates just fine. Turns out one of my p6156 probes has a black tip and I wanted it to have a blue tip. Swapped them over and it works just fine and dandy. Great call on your part and boy do I feel sheepish. Oh well, at least I now have 2 x p6137 probes.Here are the probe tips. Offending probe in blue and new probe in black.And here is my new comp from the old probe after switching for a black tip from a p6156.