HP Oscilloscope question

[timford]

I'm looking at an old second hand HP 54600A or 54601A locally on ebay.

Reading the manual they have a max voltage of 400 dc+ac peak. Having not owned or used an oscilloscope before, does this mean that if I am using the 10x probes, when reading from a 500v source the oscilloscope is seeing 50v?

For a little bit extra I could get the 4 probe unit (54601A), but I'm assuming that there is really no need for it? (and can only see 2 readings on the screen at any one time anyhow - just a convenience with setup and being able to flip between more points).

Does this sound correct?

[Noel Grassy]

[IMG:236:192]http://i145.photobucket.com/albums/r222 ... stGurl.gif[/img]


No dude! You never want to put your probes on the B+ voltage! There are
other instruments one needs on their bench to confront the big DC voltages.

[jelle]

Yes, your scope will see 50V. I use 10x probes with my Tektronix scopes too, and yes I measure the B+ directly. It is a great tool to look at ripple/filter cap performance. Just set the input of the scope to AC.
With a voltage of AC+DC 4000V at the probe, the scope sees 400v. The only place where you should see those kV size voltages is at the output transformer due to flyback voltages from the speaker coils. But I trust that you will use a resistive load on the bench. Just make sure that the actual probes are rated for the voltage that you are working with. Most are around 500V but many new ones are rated at lower voltages.

Have fun, and always put one hand on your pocket...

Jelle

[vibratoking]

I probe B+ all the time with my 10x probes. No issues with the probe handling that voltage. FYI, there can easily be an issue with the user. First, be damn careful not to become the current path. Also, I do not recommend actively probing a live circuit. It is very easy to cause shorts anywhere there are tight quarters, such as the tube sockets. A probe that becomes a short can carry enough current throught the tip to melt it, which will destroy the probe cap or tip, whichever tried to conduct all the current. Some guys turn the amp off, disharge it, attach the probes and then turn it on. I don't have the patience for that and feel it takes too much time. So I probe live, but I have caused shorts which destroyed probe caps and parts in the amp. YMMV.

[jelle]

Also, two probes is enough for most people. Four probes become annoying real quick on a bench.

[Noel Grassy]

[IMG:194:147]http://i145.photobucket.com/albums/r222 ... okin_1.gif[/img]






I feel like a heal for pontificating so vociferously! I had a bad bit of info re:
Your probes and their abilities to handle the big juice. I have a dandy
Tektronix O'scope but few logable hours due to a novice's apprehension. I
found multitudes of books that speak on restoring the old O'scopes but no
one's written a manual for the "solder jockey" with no one to apprentice under. I'd hate like Hell to damage a scope in the process of gaining some skills.

Even the stock manual for my 'scope assumes the user is well versed in it's capabilities.
There's a manual from Tektronics devoted to probe choices but it doesn't shed enough light on this very subject for me to feel confident about my ill-informed prodding methods.

If anyone can direct me to a well written book about a 'scopes limits I'd be eternally grateful. I'd gladly buy you a beverage of your choice when you were in my area too. Maybe even some of the Nation's finest Glaucoma preventative would be in order. Consult your physician.

I despise not knowing this invaluable bit of diagnostics when I'm so close.

Thank you Jelle for speaking up and clearing the confusion that's doomed to persist amongst the solder jockeys. [IMG:292:240]http://i145.photobucket.com/albums/r222 ... madcat.gif[/img]

Noel Grassy.

[xtian]

Off-topic, but--Dude. I am mesmerized by that tiger attack video. He launches out of a dead run, and his parabola at first seems to have him dropping shy of the elephant's rider, BUT THEN HE DEFIES GRAVITY and INVENTS A NEW APEX of his leap. That's really freaky.

Found the video: http://www.trutv.com/video/most-daring/beast-wars.html

[Cliff Schecht]

Noel I'll dig through my e-library and see what texts I can find. I've had the same apprehensions in the past but oscilloscopes are stupid easy to use once you get the basics of the triggering circuit and how to adjust/interpret the X/Y adjustments and scaling knobs. Like others said the 10X attenuation means your 400V rated scope probe can handle 4000V. A 100x probe can see 40,000V!

I work with a lot of people who are new to working in a lab and time after time I see people banging their heads over stupid mistakes (scope set on 1X instead of 10X for a 10X probe being a seemingly perpetual issue). To me one of the most aggravating things in the world is walking up to a spectrum or network analyzer and seeing clearly labeled 10 Meg scope probes hooked up to 50 Ohm inputs. My point is there is a certain common sense that one has to develop in the lab and once you learn a bit about how your equipment works you will become a lot less apprehensive towards using it.

To me a truly competent engineer understands how their equipment functions and the downfalls of their equipment and can design testing plans accordingly to quickly but accurately take measurements in the lab. You are writing up a full testing plan before you start probing around right?

[Aurora]

" A 100x probe can see 40,000V......" yes indeed, provided the probe is built for 40.000V !
When using a scope over 4-500V, please make sure that your probes are up to it! Probes also have a max. working voltage.
Without having searched myself, there must be lots of www pages about " how to use an oscilloscope..."..... no?

I totally agree, Cliff - the scope is actually( well, maybe not always..) a fairly simple instrument, once you get used to it, but you have to get up to the basics, at least.
As for the "new guys at the lab", this seems to be an international problem. Students of today get far to little lab time, compared to what used to be. General component knowledge is also a victim of all sorts of new topics being crammed into the curriculum. There is a reason for the 10 different ( or more) types of capacitors..... and so the story goes on....

[timford]

Thanks guys. Exactly what I wanted to know.

I have done a fair bit of reading/browsing so far, but sometimes I just want a definitive answer to what often appears to be assumed knowledge. Manual seems to be very good none the less.

And that tiger was frightening