tests to do on used oscilloscope?

Jason Bowers wrote: ================

** You need to buy or borrow a test oscillator, function or sine /square. Then you check each setting of the controls, one at a time. Square waves should look "square".

Vertical calibration can be checked with the help of any DMM on AC volts and a 400Hz sine wave. Horizontal calibration really needs a frequency counter, but the 50/60 Hz supply is very accurate .

Make sure the "trace rotation" controlactually works.

..... Phil

Does it offer combinations of two channels, e.g. A+B; A-B. Latter differential could be checked with identical inputs and seeing how accurately you get zero!

Thanks to you both. Well, all is not paradise and beginning to think that I may return it. I could be overlooking something so looking for some suggestions.

It seems that both frequency and voltage readings are off. I have an old URM-25 RF signal generator that I have been feeding into it by sampling off of the side of a t connector with one side having 50 ohm dummy load and the other into ch 1 of the scope. Anything I've tried in frequency and it seems like I'm reading at least double that frequency on the scope if I count divisions from P-P. A 28 Mhz signal appears as

50 Mhz or plus on the scope. I just wonder if I'm overlooking something here. Also, with the maximum output from the generator, 100,000 uV, I am getting around 300,000 uV or more if I could divisions.

I don't yet have my 1/10x probes. They are due to arrive this week. I will then immediately check the 2V P-P calibration signal on the scope and see what happens.

Jason Bowers wrote: ================>

** Using an old, tube RF gen is not the way to calibrate a scope. You can started with known DC voltages, checked with your DMM. Just switch the vertical amps to DC coupled.

A square wave test shows if there are response errors - in one go. If it looks square, the response is as it should be.

** Scope have internal trim controls for calibration. You scope clearly works OK, it may need some adjustments to be spot on.

This would all be described in the owners manual.

..... Phil

Your signal generator is probably set to give RMS voltage and you are reading peak or peak to peak voltage on the scope. There is a conversion factor of some number I can not recall but it is around 1.4 or 2.8 times. Also some generators will be open circuit voltage and some into the 50 ohm load with is 6 db or someother number I can not think of right off.

my signal generator outputs some wonky voltages unless you have a load resistor across the outputs. It's 600ohms in a box with 1" spacing banana plugs and receptacles. I could never figure out why that's not handled by a pushbotton.

Also, make sure you don't have any knobs pulled out putting you in zoom mode. My signal generator has one of silly scope knobs too for some reason.

Signal generators are designed to put out a certain voltage into a certain load. Think of them as having a signal generating source of zero impedance in series with a resistor. Any load will devide between the internal impedance and the load impedance.

For RF generators there were two standards. One was an open circuit Or very high impedance) voltage and the other was for a 50 ohm load. My service monitor has a software setting to do either one. It is from the old analog cell phone days and 30 years ago the cost was about $ 50,000. When the cell phones went digital the service monitor was put out for surplus and are selling for under $ 1000 now on the used market.

When using a scope you have to allow for the peak or peak to peak ratio to the RMS value of a sine wave. As mentioned before I don't recall the ratio as I seldom need it and the scope I have now will calculate it and put on the display. If your RMS meter shows abot 120 volts AC out of the wall socket a scope will show around 170 volts peak.

Well, I've been using a coax T. On one end, there's the 50 ohm dummy load, other side is input from the URM 25 signal generator, and the final side goes into channel 1 of the scope. I wonder if I should use some sort of a buffer here, on the side leading to the scope, like a voltage divider, etc, rather than direct? Or, perhaps actually check from the "open circuit" output on the generator as the URM has that too? Yes, I'm now compensating for PP by converting to RMS after I see the waveform. It may be closer than it was, but will know with more certainty after the probes arrive today.

A small 1x 10x probe arrives today at last so I can at least check the scope's square wave output signal peg. See what happens then.

The frequency still seems to be showing higher than it actually is though. Even with a 30 Mhz scope like this, should I be able to "zoom" in to see just one or two waveforms? So far, I can't seem to get closer than 4-5 waveforms at 28 Mhz even with the 10x magnifier on.

Instead of the RF generator have you tried a transformer that converts the line voltage to something between 6 volts and 50 volts AC ? Then you can do the conversion from the RMS to peak voltage to see if the calibration is correct.

One other thing if it is a dual trace scope go to the X-Y mode and feed both chanels the same signal and see what the trace looks like.

As someone else mentioned feed the same signal to both inputs and use the invert function and see if they cancel each other out.

I am thinking that URM 25 has several matching networks that go on the end of the cable. Do you have the correct one for what you are trying to do ? Sometimes those networks get lost over the years.

==================

** Completely normal. 28MHz is at the limit of the scope's range so no detail is visible on a sine wave. Also, the amplitude will by down by 30% or so from that at say 5MHz. 30 MHz scopes are are not "flat" to 30MHz but will be at least 3dB down.

...... Phil

Thank you and the others here! The scope seems fine. I was finally able to find out what happened to the square wave test peg. It dislodged during shipping. Reattached and a nice 2V PP 1 Khz square wave.

Phil, what you said was confirmed. If I input the signal generator and set for say 10 Mhz, the frequency and amplitude are spot on.

Jason Bowers wrote: ===================

** Thanks for the confirmation.

FYI: Analog scopes are one of my favorite things. I currently own 5 of them.

One I built from new parts when I was 17 - way back in 1970. Uses 5 tubes ( twin triode and triode pentode types ) plus is fully calibrated.

I also own a modern Rigol DSO but don't like it much ....

..... Phil

Glad the scope checked out for you.

As far as the scope showing 2 or 3 waves at 30 MHz instead of one, that is probably normal. I checked out my Tecktronics 465B scope which is a

100 MHz scope and with the 10x button pushed in it shows several waves instead of just one at 100 MHz.

I think Phil mentioned it tht a scope is usually spected at a 3 db or some other figure down at the rating. Just like the 30 MHz scope has mostly full specs at 10 and not 30 MHz but is usuable to 30 and maybe some what more to look at waveforms. Then there is that square wave vers sine wave where you need a lot higher rated scope to show a true square wave than a sine wave.

Phil does your Rigol DSO work well in the X/Y mode ? I have a Hanteck that is similar to the Rigol DSO and for the most part the X/Y mode is not usuable. The one I have is a low end one that is a 200 MHz version.

I am like you, for many things I perfer the Techtronic 465B scope I have over the DSO, but for some things the DSO is better for me.

I did build a 2 inch scope in high school. I think it only had 3 tubes that handled the signals. One for the horizontal, one for the vertical, and one for the sweep generator. Couple of more were rectifiers. It probably topped out not much above the audio range.

Well, I know the thread has been kindly answered and discussed now, thanks mostly to Phil and you, I did want to tag on another question while I was at it: is there any way to determine when this scope might have been made? As mentioned, it is the Elenco S-1325. To my surprise, it apparently is still being made but who knows for how long. Just a curiosity more than anything else. Certainly seems clean enough.

Thanks once again guys.

Well done. Me too, in the early 1960s, but it was from 2nd hand bits, especially the VCR97 CRT. And definitely not calibrated! I still have it, and it works just as badly as originally...

Ralph Mowery wrote: ===================

** Nope.

Does Lissajous patterns bout as well as a dog waking on its hind legs.

** Yep, I drag it out only for special jobs like one shot events. ** Still better than having no scope. My 3 inch tube scope does 2MHz.

..... Phil

My signal generator is an all valve (four) unit and it works from 5 Hz to 600 KHz.

Need some equipment.

I take a DC voltage source and then adjust a square wave generator to that voltage and go up seeing if the amplitude stays with the changing of the V/ div. control. And move the position control to see if the waveform is linea r at all portions of the screen.

Keep going up in frequency, I happen to have a generator that stays stable at any frequency, it is a Wavetek and operates on a different principle tha n most. Most do keep a square constant, but not necessarily in sine wave.

Next you need a freq counter, set the generator accurately to a 1,000Hz squ are, put the scope on 1mS/div. Adjusting the horizontal position to where t he first cycle lines up with the graticule, check the rest. They should als o line up and the vertical parts of the square wave should be on graticule lines all the way across. If they run off, the time base or H size is off. To determine which, switch to different frequencies ad time bases, always w ith the freq counter on it. If they vary it is the time base, if they const antly run off one way or the other it is the H sweep size - or more profess ionally put - deflection.

How accurate you need it depends on price. I got one I don't pay much atten tion to, the time base or deflection is slightly off but I am not measuring frequency with it. The amplitude reads right and that is enough because I use it for everything. Even DC, it is much quicker because usually I only n eed to know a source is there.

If you need a scope that is totally accurate all the way you need an old Te ktronix.