Is something wrong with my oscilloscope? - Page 1
Forum section: electronics / beginners

author: sincspecv
author_rank: Newbie
timestamp: August 06, 2019, 03:56:28 am
content: I just got my first ever oscilloscope, a HP 56400B, that is showing ~5.5v with or without a probe attached on ch 1. Ch 2 is also showing voltage, but not as high as ch 1. Is this a misconfiguration somewhere or is it a bigger issue. I've gone through every menu to try to find a setting that would fix this but have not found anything.

author: Psi
author_rank: Super Contributor
timestamp: August 06, 2019, 04:12:14 am
content: Did you mean to say HP 54600B instead of 56400B ?Are you aware that you can move the trace up and down with the vertical position knobs for each channel?The 'ground' level is not fixed to the centerlineSorry if you already know this, i'm just not sure where your skill level is at.It shows a little marker on the right to indicate where the 'ground' position is for each trace.See here

author: Brumby
author_rank: Unknown
timestamp: August 06, 2019, 04:49:21 am
content: Like Psi said - if you are new to oscilloscopes, it could be that simple.Look for "Vertical position" adjustment.

author: sincspecv
author_rank: Newbie
timestamp: August 06, 2019, 01:43:08 pm
content: Sorry, yes it is an HP 54600B. I am completely new to oscilloscopes other than watching a couple of "how to" videos. I have the position set at 0.0V for both channels and it is still showing voltage. Possibly worth noting is that on Ch1 the voltage drops to ~2V when I change to 5V/div and lower.Here is an image of what's going on on Ch1:And here is Ch2 (it's actually showing negative voltage):Thank you for your help.

author: ledtester
author_rank: Super Contributor
timestamp: August 06, 2019, 02:26:55 pm
content: Some things to try out:- Set the channel coupling to GND- With the probe tip grounded (i.e. the ground clip attached to the probe tip) set the coupling to AC- Same as above with the couple set to DCAll three should give the same trace - i.e. the same horizontal line. (Don't change the vertical position during these tests.)Then put your probes on the scope's calibrating signal output and it will be interesting to see what you get with AC coupling vs DC coupling.While you are at it you might as well adjust the compensation on your probes:

author: sincspecv
author_rank: Newbie
timestamp: August 06, 2019, 03:24:30 pm
content: Ground coupling was at 0, but DC and AC coupling looked just liked the pictures I posted above.Here is the calibration signal on AC coupling:And DC:

author: ledtester
author_rank: Super Contributor
timestamp: August 06, 2019, 04:05:54 pm
content: It seems there is a DC offset being added in the front-end.I presume you see the same thing on all of the channels?I would try the calibration signal tests with the BW limit filter on. No particular theory on why it would make a difference, but the BW filter is part of the front-end and it would just be interesting to see if there is any change. For slower speed signals you usually have the BW filter on anyway.In any case, your scope is still very usable. Most of the time only the shape of a signal or change in voltage is important.

author: sincspecv
author_rank: Newbie
timestamp: August 06, 2019, 06:04:59 pm
content: Ch2 has the same symptoms but the voltage is at -2v instead of +5v. I've turned on the BW filter and it made no difference.Is there any way to figure out what's causing this? I understand it's not absolutely critical, but it's quite annoying and I'd like to fix it if I can.

author: MarkF
author_rank: Super Contributor
timestamp: August 06, 2019, 06:49:44 pm
content: There is a calibration procedure. However, you probably don't have the tools necessary?Starting on page 86:http://web.mit.edu/8.13/8.13d/manuals/hp-54600a-54601a-oscilloscope-service-manual.pdf

author: rstofer
author_rank: Super Contributor
timestamp: August 06, 2019, 06:57:29 pm
content: Quote from: sincspecv on August 06, 2019, 03:24:30 pmWhy do the images say DC Coupling?

author: sincspecv
author_rank: Newbie
timestamp: August 06, 2019, 07:05:53 pm
content: Quote from: MarkF on August 06, 2019, 06:49:44 pmYou would be correct. I guess I'll just live with it until I can afford a nicer scope.Quote from: rstofer on August 06, 2019, 06:57:29 pmI'm sorry, which images are you referring to? The calibration images are set to the appropriate coupling.

author: james_s
author_rank: Super Contributor
timestamp: August 06, 2019, 07:20:32 pm
content: I don't see anything super exotic in the calibration procedure, I'd probably just have the thing calibrated rather than replace the whole scope, assuming there's nothing serious wrong with it.

author: rstofer
author_rank: Super Contributor
timestamp: August 06, 2019, 09:26:36 pm
content: Quote from: sincspecv on August 06, 2019, 07:05:53 pmThe images in Reply 5 which are purported to show AC Coupling but the image shows DC Coupling. I think...

author: sincspecv
author_rank: Newbie
timestamp: August 06, 2019, 09:34:33 pm
content: Quote from: rstofer on August 06, 2019, 09:26:36 pmThe first one is AC and the second is DC.