Re: Cool cheap oscilloscope for troubleshooting

Tom, wb6b
 

Someone told me they saw an old Tektronix 465 scope on eBay for $50 (probably $150 shipping :) ). That could be a good route. Hard to imaging that those scope might have cost $20,000 or more in todays dollars, and can now be bought for less than $100. They definitely were made well.

My first scope was a Knight Kit scope. I saved all school year and when summer vacation came I spent $109, if it recall, to purchase the kit and spent the summer putting it together.

This scope was only about 2 Mhz tops. But, I learned so much from it being able to "see" how so many electronic circuits worked and then start trying out designing my own circuits and projects, and seeing how they worked, or needed improvement.

This scope was my first realization that professionally designed equipment could be improved and not accepted as correct without question.  

My first experience was when a electrolytic capacitor exploded with a loud bang when I first plugged the scope in, as I waited breathlessly for the thrill of seeing my summers's work come to life in a magic little green line.

I was able to reason out the voltage rating of the capacitor (part of a cathode resistor negative bias circuit) was exceeded momentarily, on start up, so I got on my bicycle, rode down to the local electronics store and was able to confidently buy a replacement was not the exact same value. But, higher voltage and a close, but larger, capacitance value.

My second design improvement came after I discovered the wonders of the Z axes. I was having a great time feeding the video from a TV set I was fixing for a neighbor into the Z axis, syncing up the horizontal and feeding in the vertical to the X axis, and watching TV on my oscilloscope screen.  

There was a little RCA plug that shorted the Z axis input on the back of the scope when it was not in use. 

I'd been using the scope and realized I'd not replaced the shorting plug. The Z axis input was essentially a .1uF capacitor connected to the cathode of the CRT at 1,200 volts. 

I reached over and behind the scope to plug in the shorting plug. I remember how the little plug made such a loud bang as it hit the wall on the opposite side of my room. And how stunning the jolt was traveling through me. 

My next design improvement was to solder a 1M ohm resistor across the Z axis connector, inside the scope, to let the capacitor charge and equalize to the 1,200 volts on its own. So I would not become a live demonstration of RC time constants and capacitor peak charging current again.

Tom, wb6b



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