Oscilloscope Probes

[dbishopbliss]

A co-worker who used to be into HAM radio is giving me a oscilloscope.  He mentioned he no longer has the probes for it.  How do I know which probes to get for it?

I know, the first question is going to be what make and model?  However, I don't know and he doesn't remember.  I was hoping to have probes before it arrived next week.  I noticed that MCM Electronics carries probes with different MHz ratings (100MHz, 200MHz, etc).  Are the probes specific to a scope, or dependent upon what frequency I want to measure?

 

[Maxwell_E]

The probes are rated to what frequency they can measure. The scope will have a rating too, anywhere from 20MHz to 1GHz. The only brand I've ever used is Tektronix probes, 500MHz, 10x. The 10x means they divide the voltage by 10 and then the scope has to multiply it again for viewing.

 

[Grainger49]

I did this quite a few years back.  I got a good pair from MCM Electronics.  They calibrated up fine and I have used them for 18 years with no problems.

 

[2wo]

You only need one. Later, if  your doing 2 Chanel work, you'll know what you want.

The 76-102 looks good...John 

 

[JC]

I can only add that you will need to make sure of the connector the scope uses for its probes, believe it or not.  Scopes have pretty much standardized on BNC connectors, but an older scope may use a PL-259.  At one time Radio Shack had adapters for this, but I wouldn't be too sure they still do, so if you need one, you may need to order it with your probe.

 

[Grainger49]

Most probes use a BNC connector.  The Tektronics I have used from the 50s versions forward have all used BNC connectors.  Your friend will know what connector it uses and probably the manufacturer.

One other thing, if the scope is a 50M Hz scope and you get a 200M Hz probe there is no problem.

 

[JC]

Don't want to take it for granted, though; I have a B&K from the 70s that still used the PL-259.

 

[rif]

I was on partsexpress looking for some other stuff and noticed they sell probes too.  I have no clue on the quality or connection type.

 

[dbishopbliss]

This thing is a monster (green too).  I wasn't expecting it to be quite so big.  I have a non-working Eico 460 that I thought was big until this thing was dropped off. 

The scope uses BNC connectors, so that is good.  I can get cheap probes for $20, more expensive ones for closer to $40.

I don't have a manual for the 'scope, they seem to run around $35 with shipping (I can't find an electronic copy anywhere).

Now, here's my dillema...  I don't really know if this thing works.  They guy who gave it to me said it did one year ago, but the guy who gave it to him kept the probes so its not like he really used it.  Then, even if it works could it be too out of spec to be useful? 

Should I make the $50 to $75 investment, or save my money for a more modern 'scope.  There is a goldstar that is listed near me that has new leads, tested and callibrated for $115.  What do you think?

 

[Grainger49]

First, since you now have it in hand, try turning it on and see if you see anything on the screen when it warms up.  If you see a dot in the middle you turn knobs, the "time base" is a good one if it is flickering.  The time per division, time base, will increase, allow you to see higher frequencies, as you turn it clockwise.  There is a "Position" control for each channel that moves the trace up and down.  And one I can't remember that spreads the trace across the screen.  

If you get a green light for each channel that is a good start.

Then, where you live you might have a place that sells a BNC to two alligator clips cable.  This is far less sophisticated than a real scope probe.  But it will tell you if you can see a signal.  Try clipping onto the AC of your heater wires.  It will be AC floating on DC so AC couple the lead.  There is usually a 3 position switch that is labeled AC/Gnd/DC.  That is AC coupling (blocks any DC), grounded (gives a flat line across the screen) and DC coupling where you can see AC signals that float on DC.  

Using the AC coupling first allows you to see the AC wave that should be centered around zero.  Adjusting the "Volts Per Division" control for the channel you are looking at will make the wave larger.  Since it is 60 Hz you will need to turn the time base down, CCW.

 

[Doc B]

I will vote for the Goldstar - unless part of your plan is to learn how to fix an old scope when it breaks. I've had maybe 10 different scopes around here over the years from old Tek tube monsters to 70s era 7000s to the current Tek TDS420, and every one of them eventually broke down in one way or another. Like anything else it's usually the filter caps that fail, although the current one also required the replacement of around 30 surface mount tantalum caps a few months ago, quite a PITA. Luckily that seemed to be the definitive fix in a chain of repairs made to keep it running over the past three years and so far, so good. But it would have gone to the recycler like the rest of them if that hadn't fixed it. For most audio measurements a decent sound card and some software on a laptop is plenty good.

 

[2wo]

You can probably kluge something up from a cheep interconnect or something. ether get a BNC connector or a little creative stickingthewiresintheconnectorheldonwithtape action 

 

[JC]

I generally agree with Doc on this, I have an newer "off-brand" scope.

Still, in this case, you should be able to learn something of its condition before making an investment.  Just turning it on should tell you something.  Then, see if you can get a trace on the screen with the input(s) set on "Ground".

Does it have a little jigger on the front labeled "Cal" or "Calibration" or something like that?  This is usually a square wave of a certain amplitude and frequency you can use to check the scope.  With the scope on, that output should be present, and a piece of 20 Gauge solid wire will fit into the center hole on a BNC to help you get it into an input.

 

[Paul Joppa]

BNC to RCA adapters are readily available (even Radio Shack sells them, or used to anyway) and inexpensive. Then you can use regular RCA interconnects, which you may have a few of lying around. I had a long one which I cut in half and soldered a couple alligator clips to; comes in handy pretty often.

 

[dbishopbliss]

Here are some pictures of the green monster.  I placed a ruler on the top so you can get an idea of the size.  Perhaps this is how big most scopes are.  I don't know since I never used one before.

I'm going to stop by the shack on my way home and I'll make some DIY probes to start with... figure out if this thing even works.

 

[JC]

Just so you know, the length is due to the length of the CRT, and the length of the CRT is as long as it is to keep the deflection of the cathode ray as low as is practical.

Deflecting the CRT in order to direct the trace to the screen naturally introduces some error in the accuracy of where it lands; the less you have to do, the better.  Same reason that CRTs used in serious broadcast monitors for TV stations were as long or longer than your scope.  We are more used to CRTs for home use, where they trade off accuracy for making the depth of the product shallow enough to be acceptable in your living room.

Take heart, though!  When I was in school, we had "lab" scopes that came built onto their own wheeled carts and took two guys to lift!  Yours would have been considered to be a compact model!

 

[Grainger49]

That looks like the 1950s vintage Tektronics that I used in school.  No, they were old scopes when I was in school.

The key for us to get good measurements was to plug it in and get it warmed up immediately.  Also, we calibrated the probes every time we started to take measurements.

 

[dbishopbliss]

I turned it on and I get a green dot on the display.  Actually, it didn't start out a dot, but I turned the FOCUS and ASTIG knobs until it became a dot.

Then I was able to use the DC BAL POSITION knobs to move the dot up and down and another knob (LEVEL I think) to move it horizontally. 

I will try making a lead and clipping it to the heaters tonight.  I assume I clip one lead to ground and the other to one of the heaters. 

 

[JC]

Hopefully, you can turn that dot into a horizontal line by playing with the sweep or "Time Base" controls.  

It also looks like you may have a "Cal" signal you can play with, up in the upper right quadrant; can't quite read it, between the "Scale Illum" and "Astig" controls.

 

[dbishopbliss]

I made a diy probe from an interconnect that came with an old dvd player, an rca to bnc converter and some alligator clips. 

I clipped it between ground and one of the heaters and was able to see a sign wave.  There is no scale so I'm not really sure what it was telling me by looking at the wave.

I also connected it to the output of a cd player and got lots of pretty waves.  I'm guessing that is good.

Finally, I clipped the leads to each end of a 1000uH axial choke and held it near a power transformer to see if I could induce some current... I did.  I had to have the 'scope on the most sensitive setting to see anything, but it went from a straight line to a wavy line when I got near the laminations of the transformer.  The waves got smaller when I was on the bell side of the tranformer.  I also noticed a change if the size of the wave depending upon how I oriented the choke.  I guess I can use this to determine where to place my chokes, although I'm not sure how a axial choke (size of a 5W resistor) corresponds to a larger hammond-style choke. 

I went ahead and ordered some 60Hz leads.  I figure I can always use them on another scope if I decide to upgrade later.