HP 1740A Help ... Voltage Amplitude is Exactly 10x too Small

[Pork]

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.


Help

What I know:

Probe is a Tek P6122 (10x)

1V Cal has been set to .500 volts with a Fluke 177 DMM

Depressing the blue 10x button on the scope increases the width of the signal.

Depressing the tan 5x button increases the amplitude by 5x.

[0culus]

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.

[james_s]

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.

[Gyro]

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.

[Pork]

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

[Pork]

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

[helius]

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.

[tggzzz]

So I guess the lesson is don't mix different brands of probes and expect a universal behavior between scopes.

Two 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?

[tggzzz]

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.

In 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 at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[Gyro]

So I guess the lesson is don't mix different brands of probes and expect a universal behavior between scopes.

Two 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?

The HP1740A doesn't have a x1/x10 indicator (or detection). As I mentioned above, the BNC has a fixing nut on the front.

[tggzzz]

So I guess the lesson is don't mix different brands of probes and expect a universal behavior between scopes.

Two 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?

The HP1740A doesn't have a x1/x10 indicator (or detection). As I mentioned above, the BNC has a fixing nut on the front.

Whoops, my mistake; the LED is for "uncalibrated". HP expected people using their scopes to be educated and alert, and to think

[helius]

In case anyone reading gets the idea that 10X probes are "proper", ...

Yes, 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.

[james_s]

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.

[tggzzz]

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.

[Fungus]

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.

[artag]

I use a *10 probe for most things, and hate *1*10 switchable probes.

A 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.