Need advice on Oscilloscope probes

[ral-clan]

I was given an old Tektronix T912 oscilloscope. This is the first scope I have ever owned so I am learning with it. This once was a high quality scope. I realise since it is old, it might not be entirely accurate or calibrated anymore, but I think it will help me understand how to use a scope.

Image of T912 scope:
http://www.co-recyclage.com/photos/r...1421745930.jpg

However, no probes came with it, so I need to order some new ones. The manual says it originally came with a "General purpose 1X voltage probe" with an input capacitance of 54pF. Tektronix also made a 1X - 10X switchable available for it. I can find a bunch of probes at affordable prices on ebay, but am a little confused because all the ones on eBay have a MHz rating in the title. Like this one, which says it is 100MHz:

http://www.ebay.ca/itm/P6100-DC-100M...YAAOSwVFlUBDrz

What does the MHz rating mean? My scope is a 10MHz scope, so would any probe rated above that be fine for it? If so, I think that's a match for my scope...and it's switchable, which is a bonus. It's also very affordable.

So can I buy this (or two since my scope is dual channel)? Does any probe work with any oscilloscope, or do I have to be careful and watch for something in the specs?

[eslapion]

See:
http://w140.com/tekwiki/wiki/T912

This scope was made and sold from 1976 to 1984. Not sure you'll want to invest anything in this piece of museum. Not sure any company can still calibrate it.

As for probes, usually, direct connections (X1 probes) are good up to 6MHz. Any probes higher than that must be X10 probes and they usually begin at 60 MHz. X10 meaning that in order to be sensitive to higher frequencies, the probe has to divide the amplitude by 10 and so when you see pk-pk or RMS measures on your scope, you must calculate 10x to get the real value.

BTW, your links seem damaged.

My own TDS1002 has a bandwidth of 60MHz and is considered antiquated even with the full digital system built in it.

[Mike]

I was given an old Tektronix T912 oscilloscope.

Nice gal!

What does the MHz rating mean?

It specifies the bandwidth of the amplifiers in the scope and of the transfer function of the probe, when both are regarded as signal low-passes.

That means, for signals with low frequencies the signal is not changed in amplitude and phase (besides the 1:1 or 1:10 attenuation of the probe, of course). The bandwidth frequency commonly is defined where a 3 dB attenuation occurs, there then the (displayed) signal only has 70% amplitude (and a 90° phase difference, BTW) - and then follows a 20 dB attenuation for each frequency decade - so the signal as seen on the scope quickly approaches zero amplitude.

When you combine several bandwidth limited systems, it's normally the system with the lowest bandwidth that dominates the whole frequency response. That means for your 10 MHz scope, a probe with 100 MHz bandwidth is just about right.

...

Just one hint on trying out the different display options on your scope: before you use [X-Y] (which makes a horizontal deflection depending on the signal of channel 1, and a vertical deflection from channel 2), make sure you have a non-zero signal on both channels by checking in the other dual channel modes. Otherwise you'll only get a very bright spot in the middle, that quickly burns a hole in the phosphor!

[ral-clan]

Thanks for the replies.

Yes, I know I don't want to invest too much in this old scope....but since it was free, and since I am entirely new to scopes, hopefully it will help me learn enough in order to make a better decision if and when I buy a new scope.

I've already learned enough in the past 24 hours that scopes have changed (for me) from a mysterious device to something I understand the use of.

[eslapion]

....but since it was free

Free is always a very good deal. The T912, just like Commodore computers may actually have a pretty high historical value no matter what it's value as an electronics instrument may be.

It's just, I expected this machine to be difficult to learn for someone without a lot of knowledge.

I've already learned enough in the past 24 hours that scopes have changed (for me) from a mysterious device to something I understand the use of.

Then again, the fact that it is more complex to learn to use than modern scopes may allow you to actually learn more than if you had a fully automated model.

You could probably use your VIC-20's user port and a 5V reference source to generate a square wave of the proper amplitude and frequency to calibrate it.

I remember a trick on page 232-234 of the Programmer's reference guide to generate square waves off the CB2 line. The information provided there tells you how to set the exact frequency you want.

You can then use the 9Vac on the user port to generate a slightly higher voltage which you clip down with a reference such as the LM336Z-5.0 and then a simple NPN transistor connected as open-collector output will use the CB2 to invert the square wave into a perfect rail-to-rail 0-5V signal.

[Gyro Gearloose]

I think the important thing to note here is that it's a *storage* scope. Storage CRTs have limited life compared to non-storage tubes and are much easier to damage; you can burn in a waveform that won't erase. Use it in the non-store mode for now unless you are confident you get it. RTFM and believe it when they say it's fragile.

I had a HP1741 with a 100MHz storage tube, I got quite good at using it but the beam intensity just started dying and it became just a standard non-storage scope with a somewhat dim trace. I ended up donating it to a hacker space.

[eslapion]

@Gyro Gearloose
That's an important caveat.

My own scope is a digital storage scope. How does a scope offer storage function with no digital technology ?

[a4000bear]

@Gyro Gearloose
That's an important caveat.

My own scope is a digital storage scope. How does a scope offer storage function with no digital technology ?

Analog storage scopes use a special storage CRT.
http://ecomputernotes.com/computer-grap ... orage-tube

[eslapion]

Analog storage scopes use a special storage CRT.
http://ecomputernotes.com/computer-grap ... orage-tube

Whoa! Impressive... the ultimate flicker fixer!

Conceptually the Direct View Storage Tube (DVST) behaves like a CRT with highly persistent phosphor. Pictures drawn on there will be seen for several minutes (40-50 minutes) before fading. It is similar to CRT as far as the electronic gun and phosphor-coated mechanisms are concerned. But instead of the electron beam directly writing the pictures on the phosphor coated CRT screen, the writing is done with the help of a fine-mesh wire grid.

Judging by the given description, these can get damaged quite easily if the electron beam consistently scans exactly the same spots.

[Gyro Gearloose]

Tek and HP came up with various CRT technologies over the years, the storage CRT and the MCP CRT are the most impressive to me. But they are both fragile and short-lived. (We can argue over who invented the storage tube, it was Hughes AFAIK, but Tek and HP perfected it. Maybe HP a bit more than Tek.)

I am convinced that if digital technology with cheap RAM didn't exist, they'd have perfected those monsters. Maybe some kind of "The Fly" type of fusion between the Aiken thin CRT and storage. Who knows?

http://www.earlytelevision.org/geer_color_crt.html

My HP had a "store" function, it would trap the picture in the grid but not show it, it would keep it for hours then you just pressed display and BAM!, waveform.
Or maybe a digital scope with one of these:
http://www.electricstuff.co.uk/glassadc.html

Mmmm, tubes. Delicious.

The MCP was a special etched plate behind the phosphor...

http://w140.com/tekwiki/wiki/Micro-channel_plate_CRT

Single shot 1GHz bandwidth with a visible trace from one trigger. Visible, but not stored. You better have a camera for that. I never heard of a MCP storage tube...

http://w140.com/tekwiki/wiki/7104

Monsters. Don't get me started on the 519.

Sorry for the diversion, but I enjoy the 1960s and oddball technology.

Almost any old probe will do for the little Tek.

[Gyro Gearloose]

..and if I may add to my little wool-gathering post above, when I was in college, my version of a digital storage scope was a Tektronix 531 with a Digi-View system pointed at the CRT. The Amiga stored the waveform digitally, right?

I can't believe I didn't preserve that document. I was the only one, if not the first, to hand in lab coursework with printed waveforms at that school. If anyone else was doing it, I didn't know about it. But I was, as usual, punished for not doing work in-class, and I think there was an anti-Amiga bias with the faculty back then. "Game machine".

I got the same grief in high school when handing in physics reports printed out at home with GEOS on my 64... You see, the 64 was a game machine and the teachers didn't like that either. That one, I kept ...

[ral-clan]

Just updating this thread from September. I finally got some proper (not expensive, but proper) probes for this oscilloscope. So that was nice. Unfortunately, now that I was able to test the scope with proper probes, I can see it seems to have problems syncing to a signal - including it's own square wave test signal. Then after a white the power supply seems to have failed. It no longer powers up (well, the fan powers on but the power light does not come on and there is only a faint glow in the centre of the CRT.

So now I have two dead T912 Tektronix 1970s era scopes. I'm guessing both have bad power supplies. Since these have CRTs in them, I'm not brave enough to fool around inside, as I don't want to be electrocuted. I don't mess around with anything involving AC line voltage or CRTs.

Fortunately, both of these were given to me free, so I haven't lost any investment.

So...after having a taste of what oscilloscopes can do, I would like to get one - a basic one. I would only probably ever use if for fixing old 8-bit computers and doing drive alignment, etc. So would anyone have any advice for me on a basic, but reliable learner's scope? Should I even be looking in the used market? I think I'd be fine with a basic, analog scope if only it was reliable. They seem to be affordable to get, but I don't know if I can count on them. I feel that if I were to get an analog scope, I should get the most recently made one I can find (1990s?).

Or should I just buy a new digital scope? The problem is, they seem to be very expensive and are probably overkill (feature wise) in terms of what I would use them for. I hear the cheap Chinese made ones are terrible.

I would like a stand-alone scope instead of one that relies on a laptop ---- unless I can get one that will work with one of the several old WIndows 98 to Windows 2000 laptops I have that were given to me and that aren't doing anything. In this case I could actually use the laptop as a dedicated display for the multimeter.

[groepaz]

i'd look for a used 2-channel 20Mhz (or so) scope on ebay.... good enough for fooling around with this 8bit stuff

[FD22]

I've just bought one of these: https://www.amazon.co.uk/gp/product/B00 ... UTF8&psc=1

Not exactly cheap, but good value for money.

[eslapion]

Not exactly cheap, but good value for money.

Considering the specs, it seems like a very good deal.

I bought a TDS-1002 back in 2003 and it cost approx. 1500$CAD and had similar capabilities with a B/W display.

I purchased the serial/parallel/GPIB communication board for an extra 600$CAD so the total cost for similar specs was in fact 2100$CAD.