My First Oscilloscope - A question about calibration

[JoeNewberry]

I got my first oscilloscope today. Nothing particularly fancy, but at $25 the price was right. I figure this will be a good scope to learn on. It's a 50Mhz Hitachi V-550B.

Now, the problem is that when I touch the probe to the .5V calibration tab, that's supposed to generate a nice square wave, I instead get the waveform I have attached. Does anyone happen to know what part of the scope I need to adjust to correct this? Maybe a simple pot or set screw I could turn to fix it? I found the operation manual here:

http://www.hdeg.tzo.com/004%20%20%20...ion_Manual.pdf

It has schematics of the whole unit, but no mention of what to adjust to bring everything into proper alignment. Someone is selling the service manual on eBay for 40 dollars and that seems a bit excessive. Any ideas?

[bandersen]

Perhaps you have the vertical input in AC mode ? You'll need to have it in DC mode to get a proper square wave. Another possibility is a trimmer capacitor in the probe itself, but it usually doesn't have that much effect.

[JoeNewberry]

I wish it were that simple, but that's the waveform I get in DC mode. In AC mode it looks like the attached picture. I get these forms with both inputs.

My probes shipped out today, so in the meantime I cobbled one together with a coax cable and some resistors. I did consider that it might just be an issue with not using a real probe, but I get the same waveform if I connect a wire direct from the input to the calibration point or if I connect a 10K resistor from the input to same. Could not having a real probe be what's throwing it off that bad?

I apologize for my ignorance. I would prefer to teach myself and not have to bother anyone about it, but I thought someone might recognize this slope where there should be a plateau issue, saving me from poking around trying to find the right thing to adjust or having to shell out 40 bucks for the service guide.

[zapro]

Just looks like you need to adjust the small trim cap in the probe, or maybe the probe cable is worn or damaged...

It is important that the probe is adjusted for your specific scope to be able to represent a square wave correctly.

// Per.

[bob91343]

You may have an issue with the calibrator in the 'scope. Check it on a known good one.

[JoeNewberry]

I should have more carefully read the entire manual. Bandersen and Zapro were right on the money about it being the probe. There was a convenient diagram showing this exact waveform as what you'd get if your probe has too much capacitance. Hopefully, all will be right when I get my proper probes in the mail this week.

Thanks for the help.

[JohnHacker1]

Glad the problem is solved.
You got a pretty decent scope for $25.
I was a calibration specialist in the Army (MOS 35H) for 20 years-calibrated hundreds of scopes.
Hopefully you got probes as close to the input impedance and bandwidth of your vertical amplifiers as possible.
It helps to "stretch out" the leading edge of the square wave as much as possible using the V/div. and time-base controls when adjusting. You'll then see small irregularities in the waveform-just try to smooth out all the ripples as flat as possible-you'll never get it perfect.
There are adjustments inside for both the calibration signal AND in both vertical amplifiers for squarewave response-you'd likely need the service manual, or know an experienced tech. to help you adjust it.

[JoeNewberry]

The probes are 100MHz, 10x, 10MEG, 7.7pf capacitance.

The variable capacitor on the probe has allowed me to smooth out the curves, but I'm still getting sharp slopes instead of flat peaks. If I'm reading it correctly, it seems like the voltage rises where it should be a steady .5V across.

I've noticed that with no probes attached and the V/div setting at the lowest point, I can see a moderate sine wave. I assumed it was unavoidable background interference, since just getting my hand near the inputs is enough to generate a waveform. Could that small wave be getting added to my test signal and causing this rise?

I think I'll try testing on some actual circuits, on the off chance the calibration point is what's off. If I get the same results there, I may have to knuckle under and buy the service manual.

[JohnHacker1]

Is it a 60hz signal? Ground the input and see what happens. Just connect a piece of wire from the center conductor to the outer shell of the vertical's BNC connector.

[JoeNewberry]

So here's what's happening with this. I still haven't sat down to properly measure out the frequency of that background signal. When I ground the inputs to themselves I get a nice flat line, though.

The other day I tested the scope again while waiting on something else and found that all of a sudden Input 1 gave me a proper square wave and Input 2 was giving me the slanted square above. So that seemed like a positive that at least one channel works. I was thinking it must be a problem with calibration on Input 2. I swapped the probes to be sure, and the situation stayed the same. With that I was confident I'd come back to it and figure out the issue with Input 2 and we'd be good to go.

Then, last night, I tested it again and now Input 1 gives me the slanty square and Input 2 gives me the proper square wave. I noticed too that Input 1 started out with a real square wave with just a bit of bend at the left of the crest, then as I was adjusting the variable cap on the probe it suddenly jumped back to the familiar slanted pattern above and wouldn't return to what I'd seen. I don't know what's going on now. Any thoughts on what would make the issue jump from one input to another like that?

I offered the guy with the service manual 10 dollars plus 10 dollars shipping, and he told me that wasn't high enough for him to process a sale. He's had it up for auction for nearly a year. Some people have more patience than sense.

I don't think I'm the only person to ever have this happen. I had found this thread before but wasn't sure it applied to me. Now I'm pretty sure it's the same issue. A poster there suggests it might be a cap or transistor in the main Y amplifier:

http://world-designs.co.uk/forum/showthread.php?t=115

[JohnHacker1]

Just a quickie;
Make sure you have good solid and clean connections from your probes to the vertical input BNC connectors.
If you haven't had it out of the case yet, you may want to clean the switches.
You may also want to turn it off and work the switches manually a little bit-may have oxidized.
Try to go back a little into the calibrator circuit and look at it on the scope.
If you have ALT or CHOP mode, put both probes on the cal. output and see what happens-if both channels either work or not, it may be in the calibrator circuit or the horizontal. Hopefully we can save you $ for now.

[Tube Radio]

Clean all switches with Deoxit or your favorite electronics contact cleaner. Dirty switches can give you all kinds of troubles in a scope.