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How to work vintage O-Scope

kong

Sep 26, 2010
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I received a 1960's RCA WO-91C Oscilloscope from a relative as a gift a couple of months ago and just now determined it had a blown fuse. I replaced the fuse and it turns on. I'm very excited to finally have an oscilloscope.

I found a manual for the WO-91B version online as a PDF, here it is: http://pacifictv.ca/schematics/rcawo91badjustment.pdf

I have a photo of the front attached.

I was wondering if someone with any knowledge of o-scopes, old or new, has any basic start-up knowledge for me so I can start to use it as a tool for circuit testing, such as visualizing waveforms, etc.

What can one do with such an oscilloscope? How do I measure a signal? Acceptable voltage levels? Can I hook a microphone up to it and watch myself speak?

Any and all advice is appreciated.

Thanks, George.
 

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kpatz

Feb 24, 2014
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Well, start by reading the manual and getting familiar with how the controls work, what they do, etc.

Do you have a known signal source to connect to it so you can test the calibration?

Looks like the trace is tilted in your pic. Usually the CRT has to be rotated to correct this. There's probably instructions in the manual on how to do it. Be careful because (a) it's fragile and can break, and (b) it can hold a charge of tens of thousands of volts and give you a nasty jolt if you touch the wrong thing. See what the manual says. It's possible there's a screw slot in the back to rotate it and you don't have to open 'er up to do it. If it's an electromagnetic deflection system (common on TV sets but not on scopes), the yoke would have to be rotated.

Get the trace going straight across without any tilt, see if it'll focus tighter (focus knob at top right), center the trace using the position knobs, set the vertical range control and then apply a known voltage and see if the trace moves up or down the correct distance. If you have a known frequency source (say a 1 KHz square wave) hook that up and see if the horizontal timebase is accurate.

Given its age, it'll probably need calibration before it gives accurate readings.

Quick Cliff's notes on oscilloscope operation: The vertical movement of the trace indicates voltage. Higher voltages move the trace up the screen, lower voltages move it down. The vertical range control sets how many volts it takes to deflect the trace an indicated amount (usually volts/division if there's a grid on the screen, or volts over the range). Usually 0 volts is the center of the screen if it's set up properly. The horizontal movement of the trace is done via an internal ramp generator whose frequency is set using the horizontal rate controls. So, if the horizontal sweep is set to 100 sweeps per second (for example), you'd see 1/100 second worth of signal on the screen at any given moment. You'd set this based on the frequency of the signal you're measuring.

So vertical is voltage, horizontal is time. The pattern on the screen indicates how the voltage of the signal changes over time.

There are other features, such as trigger, that (if available on your scope) will be described in the manual.
 
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kong

Sep 26, 2010
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Thanks for the info. So for calibration, that's where I'll start. The "manual" doesn't really offer too much in the way of information. It talks about "astigmatism adjustment" and vertical amplifier alignment, the latter of which I don't have the other instruments required. What is astigmatism adjustment?

I can get it to focus tighter. Is the vertical range control on the lower left above the probe input? I have a 555 timer producing a 481hz square wave at 50% duty cycle for testing, but first I think I need to understand how to calibrate it. So for a 481hz signal, should I set the horizontal sweep to 1000?

As for the trace tilt, are you talking about a physical re-positioning of the CRT? I think there is an adjustment screw on the inside that I saw when I replaced the fuse. Now I'm scared to go back in after turning it on, lol.
 

kpatz

Feb 24, 2014
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Astigmatism adjustment is to get the spot circular. If the spot is elliptical in shape, it needs astigmatism adjustment. You should be able to do it "by eye". You'll need to get the spot to stop sweeping so it stays stationary. Possibly the sync/H sel knob on the right has a setting that'll do this.

The vertical range control is the knob with V-RANGE above it. The knob to the right is a fine adjustment, usually with a position marked "cal" which gives you the calibrated range.

The manual doesn't say how to adjust the tilt? Next time you open it up, take a picture of the screw you saw.

Play with it. Hook up your 555 timer circuit to give it a signal, and play with the controls to see how they affect the displayed trace. You'll quickly figure out what the v-range and sweep knobs do.

If you can post a better (sharper) picture of all the controls and the markings on them, we can help you understand what they do.

Some of the knobs say "sync". This is probably the same as "trigger" on modern scopes, which holds the sweep until the signal hits a certain voltage. It's handy to stabilize the image when the sweep rate isn't in sync with the input frequency, by causing the sweep to wait until a certain point in the waveform (such as the rising edge of your square wave).
 

kong

Sep 26, 2010
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Here are the controls. I have no idea what any of them really do, aside from the limited experimentation i've done.
 

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kong

Sep 26, 2010
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Regardless of the control functions, I have managed to get the square wave to show on the scope. The only thing is there aren't any vertical lines connecting the 'high' signal and 'low' signal. In other words, there are only opposing dashes showing. Do you know why this is? Also, how much voltage can the probe withstand? What is a common value for oscilloscopes of today? Thanks for all your help.
 

davenn

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Sep 5, 2009
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Regardless of the control functions, I have managed to get the square wave to show on the scope. The only thing is there aren't any vertical lines connecting the 'high' signal and 'low' signal. In other words, there are only opposing dashes showing.

that's pretty common for a lot of scopes displaying square waves. you may see a little more of it by adjusting the intensity a bit without making the horizontal lines too bright

Also, how much voltage can the probe withstand? .

since you have told us noting of the probe its impossible to answer that. Try and find a user manual
the pdf you linked to is the service manual or part of it.

you need the user manual to get the specifications of the scope and its probes and find out how to use it correctly

What is a common value for oscilloscopes of today?

how long is a piece of string ?
in other words, the prices for used and new scopes vary wildly depending on make, model, quality, country you live in etc etc
A new 10MHz scope will probably go for ~ $300 and upwards
a decent used 100MHz scope will go for a similar range


cheers
Dave
 

kpatz

Feb 24, 2014
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Ok, I'll do the best that I can. I don't know what all those things mean on your scope either. :)

Starting at the top left in your controls pic, the first knob appears to select two scales of voltage ranges marked 1.5MC and 4.5MC. I'm not sure what those mean or what "MC" stands for, but it appears to correspond to the different scales on the screen.

Below that is V RANGE, which selects the voltage range that will appear full scale on the screen. It looks like in the 1.5MC setting gives you a range from 0.15V P-P at the fully counterclockwise position up to 50V P-P at the fully clockwise position. The 4.5MC setting gives you 0.5V pp to 150V pp pp from counterclockwise to clockwise. So, the scope appears to be able to handle up to +/-150V peak-to-peak. Are there any switches on the probe? Some probes have a 10X setting that scales the voltage down when it reaches the scope, to allow higher voltage measurements.

V POLARITY reverses the polarity, so you can have negative voltages move the trace "up" instead of "down". It basically just inverts the image.

V CAL is a fine tuning adjustment for the V RANGE, if you want full scale to be some voltage in between the increments.

SYNC ADJ sets the threshold for the sync. Adjust this to get a steady display. It holds the sweep until the input signal hits a certain level, synchronizing it to the input signal. This is usually called trigger on modern scopes.

SWEEP controls the sweep rate, in increments from 10 to 100K. Not sure what the units are, sweeps per second maybe? Experiment with it. The TV settings are for measuring vertical and horizontal signals from a TV set, useful if you like to fix old TVs.

SWEEP VERNIER is the fine tuning for the sweep rate, so you can set a rate in between the increments on the sweep knob.

H GAIN controls the horizontal gain, or the width of the sweep.

The SYNC knob selects the sync source. Most commonly used is the +INT and -INT settings, which syncs to the input signal. You can sync to a separate signal provided at the SYNC jack as well, say if you're looking a a serial data transmission that has a separate clock line, you could sync to that. The red H SEL positions cause the horizontal drive to be controlled by an external signal (on the H INPUT jack) instead of the internal ramp generator.

PHASE, I'm not sure, but I'm guessing it adds a delay to the sync so you can adjust the phase (portion of the signal you see horizontally) in relation to the sync point.

Last but not least, the Z AXIS input lets you modulate the intensity of the beam using an external signal.
 

kpatz

Feb 24, 2014
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Regardless of the control functions, I have managed to get the square wave to show on the scope. The only thing is there aren't any vertical lines connecting the 'high' signal and 'low' signal. In other words, there are only opposing dashes showing. Do you know why this is? Also, how much voltage can the probe withstand? What is a common value for oscilloscopes of today? Thanks for all your help.
This is normal for square waves on an analog scope, since in a square wave the signal transitions from the "high" to the "low" and back so fast the beam literally jumps from one position to the next so fast it doesn't get a chance to illuminate the path in between.

You could add a simple lowpass RC filter circuit between your 555 circuit and the scope probe to slow down the transitions, making something closer to a triangle or sine wave. Then you should see the vertical transitions better.
 

kong

Sep 26, 2010
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Wow both of you have been very helpful, thank you for taking the time. As long as the behavior regarding the square wave is normal, I don't have a problem with it. I was just curious. The SYNC ADJ explanation is very helpful, that will make it easier to examine the waveform if I can slow it down or hold it steady. So I was also wondering, in the image attached, if P-P is defined between the two horizontal lines that are encompassed by the arrows and "CAL"? Meaning that if a 12v signal is coming through, and it's 15v P-P, should I calibrate the unit so that the waveform stretches between these two lines? ie. the peaks would fill that space completely?

And here are the photos regarding the adjustment of the tracer. I was thinking maybe those screws could be adjustment screws, but they could just be mounting screws.
 

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kpatz

Feb 24, 2014
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Those are mounting clamps and screws. I found a datasheet for the 5UP1 CRT that scope uses: http://frank.pocnet.net/sheets/137/5/5UP1.pdf

It uses electrostatic focusing and deflection, so there is no yoke. To adjust the tilt, you'll have to rotate the entire CRT. Start by grounding the probe and centering the trace, and mark it on a sticky note on the face of the screen so you can see which way you need to turn the tube. Unplug the scope, let it cool, then open it up.

If you loosen the screw you show in "screw 2.jpg" it may allow the tube to rotate in the mount. Is there any clamp under that metal sleeve, or one on the wide end? If there are, loosen those as well. Wear thick gloves to protect the tube from damage and yourself from any high voltage (anode 2 can be around 2,000 volts and it can act like a capacitor, so it holds a charge even when powered off). Carefully rotate the tube to get the trace level. GENTLY tighten any screws you loosened and power up the scope, to see if you got it right. Repeat as needed until it's straight.

As for calibration, does the scope show a waveform if you set the 1.5MC/CAL/4.5MC knob to the CAL position? From looking at the manual it sounds like it does.
 

kong

Sep 26, 2010
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Great, I'll give it a shot later on today if I get a chance. Yeah, when that knob is set to 'CAL' it gives a sine waveform for, I'm assuming, calibration.
 

kpatz

Feb 24, 2014
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From what I can see in the manual that calibration sine wave is from the mains power, so it's 60 or 50 hertz depending on where you are.
 

kong

Sep 26, 2010
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Yep, looks like a 60 hz wave. So I guess I'm just not sure how to calibrate it to receive accurate voltage readings
 

kong

Sep 26, 2010
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I got the tracer as horizontal as I could....on the right side it curves up a little but that looks to be a defect in the oscilloscope itself; the right side is right on with the center line.
 
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