HP 1740A Help ... Voltage Amplitude is Exactly 10x too Small - Page 1
Forum section: electronics / testgear

author: Pork
author_rank: Contributor
timestamp: May 29, 2020, 03:05:59 am
content: So my HP 1740A is displaying the voltage exactly 10x too small.If the volts per div is set to .1 and the probe is connected to the 1V Cal, I get a one division reading (same for both A & B).The amplitude of the reading is exactly 10x smaller than it should be and I can't seem to correct it.HelpWhat I know:Probe is a Tek P6122 (10x)1V Cal has been set to .500 volts with a Fluke 177 DMMDepressing the blue 10x button on the scope increases the width of the signal.Depressing the tan 5x button increases the amplitude by 5x.

author: 0culus
author_rank: Super Contributor
timestamp: May 29, 2020, 04:34:53 am
content: It's because you're using a 10X probe? I'm not sure I see a problem here. A 10X probe is a 10:1 voltage divider. On a tek scope the proble's little pin will contact a separate ring around the BNC and the scope will correct this for you on the readouts, but on your 1740A you need to mentally correct for it. So if you are seeing 1Vpp, for example, the signal is really 10Vpp when using that probe. Another way to think about it is, again for example, the 1V/div vertical setting is really 10V/div with a 10X probe.

author: james_s
author_rank: Super Contributor
timestamp: May 29, 2020, 05:30:28 am
content: On the Tek analog scopes without a digital readout all the little pin does is select which of two lights is illuminated behind the numbers on the V/div knob. I'm a bit surprised that the HP scope doesn't have something similar but it's easy enough to do in your head, just multiply the value on the V/div knob by 10.

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2020, 10:12:46 am
content: Nah, on the 1740A they had a front fitted BNC mounting nut instead. The pin graunches and locks semi permanently with it if you're unlucky enough to accidentally plug in a Tek probe!Actually it used to be a pretty effective way of holding onto your probes in the lab - nobody was prepared to go through the hassle of getting the damned thing off again.

author: Pork
author_rank: Contributor
timestamp: May 29, 2020, 01:44:18 pm
content: Thanks, I've been multiplying it my 10 to compensate. It's been a while since I've needed to use this scope and I don't remember the Tek probe not compensating for the 10x. Anyway I appreciate the input.Cheers

author: Pork
author_rank: Contributor
timestamp: May 29, 2020, 01:59:16 pm
content: Just to tie this up for anyone that may have a similar issue in the future. I attached the proper 1x probe and the scope works as expected. So I guess the lesson is don't mix different brands of probes and expect a universal behavior between scopes.Much appreciation

author: helius
author_rank: Super Contributor
timestamp: May 29, 2020, 03:02:13 pm
content: In case anyone reading gets the idea that 1X probes are "proper", a passive 1X probe has low bandwidth and very high loading capacitance compared to a 10X probe. It will distort many types of signals and even disturb the circuit under test if applied to a sensitive component.

author: tggzzz
author_rank: Super Contributor
timestamp: May 29, 2020, 03:53:30 pm
content: Quote from: Pork on May 29, 2020, 01:59:16 pmTwo better lessons are "think" and "sanity check what your instrument is telling you".The 1740 has a *1/*10 LED indicator, so why wasn't it illuminated? Faulty LED? Faulty interconnection to the LED? Probe doesn't have an indicator spike, or it wasn't making good contact?

author: tggzzz
author_rank: Super Contributor
timestamp: May 29, 2020, 04:00:39 pm
content: Quote from: helius on May 29, 2020, 03:02:13 pmIn case anyone reading gets the idea that 10X probes are "proper", a bog-standard high impedance 10Mohm passive 10X probe has an impedance that is much lower than a "low" impedance 500ohm resistive divider passive 10X probe. Its capacitance and bandwidth and ground lead inductance will distort many types of signals and even disturb the circuit under test if applied to a sensitive component.And then we get onto safety near mains circuits, for which neither are "proper".FFI, have a look athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2020, 04:37:28 pm
content: Quote from: tggzzz on May 29, 2020, 03:53:30 pmThe HP1740A doesn't have a x1/x10 indicator (or detection). As I mentioned above, the BNC has a fixing nut on the front.

author: tggzzz
author_rank: Super Contributor
timestamp: May 29, 2020, 05:50:00 pm
content: Quote from: Gyro on May 29, 2020, 04:37:28 pmWhoops, my mistake; the LED is for "uncalibrated". HP expected people using their scopes to be educated and alert, and tothink

author: helius
author_rank: Super Contributor
timestamp: May 29, 2020, 11:09:19 pm
content: Quote from: tggzzz on May 29, 2020, 04:00:39 pmYes, horses for courses, as they say. But a 10X passive probe is the proper tool in a much larger number of situations than a 1X, and their relative ubiquity speaks to that point.

author: james_s
author_rank: Super Contributor
timestamp: May 29, 2020, 11:57:34 pm
content: It's very, very rare that I have ever wanted a 1X probe for anything. There are niche applications where they have uses but I would call 10X probes "proper" for the vast majority of general purpose scope use. They should be the default unless you know you need something different for a specific application.The lesson here is knowing how to use your tools properly and understanding their limitations.

author: tggzzz
author_rank: Super Contributor
timestamp: May 30, 2020, 06:41:48 am
content: I use a *10 probe for most things, and hate *1*10 switchable probes.*1 probes are highly beneficial when looking at noise on PSU lines, due to the low amplitude of the noise. That's also true, with limitations, to other forms of noise.

author: Fungus
author_rank: Super Contributor
timestamp: May 30, 2020, 06:55:59 am
content: Quote from: helius on May 29, 2020, 03:02:13 pm

author: artag
author_rank: Super Contributor
timestamp: June 02, 2020, 01:05:20 am
content: Quote from: tggzzz on May 30, 2020, 06:41:48 amA bit of tape around the probe body to keep the switch safely in the x10 position is beneficial.Glue, even.Better still (as I'm sure you know), only buy probes without the stupid switch.